Note: Descriptions are shown in the official language in which they were submitted.
132691G
:
.~ NUCLEAR REACTOR COOLING ~Y~TEM
Bac~ground of the Invention
This invention relates to a cooling system for
particular class of water-cooled nuclear power reactor said
5 reactor having (a) a high-temperature (limited by boiling
: considerations) primary heat transport circuit which
dominates the reactor cooling process under normal
operating conditions; (b) a large pool of cool water
surrounding or otherwise in the vicinity of the reactor
~:. 10 core region and forming a component of the shutdown cooling
circuit which may be said to be passive; which is to say
that, in the post shutdown phase of reactor operation,
adequate core cooling is assured for an extended period of
-i time, independently of any sensing devices, externally
15 energized components, or operator intervention, and (c) a
shutdown cooling circuit which is itself passive in the
initiation of its operation; which is to say that the
:i shutdown cooling circuit initiates its function without
reliance on sensors, externally energized components,
20 mechanical actuation, or operator action.
More specifically, the invention relates to a
cooling system for a nuclear reactor of the type in which
the coolant liquid is normally circulated around the
primary heat transport circuit which includes the reactor
25 core, various heat exchangers, and a primary circulation
, pump. Located in this circuit and at points not far above
.~ and below the core are the very important aspects of this
invention, namely, three-way flow branching devices herein
referred to as "hydrodynamic ports", or just "ports". The
:~ 30 role of the ports is to provide free flow-through passage
f of the coolant in the primary heat transport circuit as
i long as normal reactor operating conditions prevail, and to
3 provide at all times for sizable branch-flow when the
: passive shutdown cooling circuit, which includes the pool
35 liquid, comes into operation under various abnormal
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13~691~
operating conditions.
It is a feature of the ports that they perform the
above functions automatically, and do not involve any
sensors, externally energized components, mechanical
actuation, or operator action for either the maintenance of
normal operation with high performance, or for the
transition to, or sustaining of, the safe shutdown
condition.
The ports, when functioning in support of normal
~ 10 operation, involve hydrodynamic principles applied in an
-` integrated way to the overall thermal-hydraulic
configuration, but require minimal complexity of associated
equipment to sustain normal as well as shutdown operation.
Moreover it is characteristic of the invention that it is
applicable tc a variety of reactor plants to be deployed
under unusual combinations of constraints of space, weight,
mobil~ity, etc.
Although the invention relates in the first
instance to water cooled nuclear power reactors, the
invention can be employea to advantage in other types of
reactor, including those cooled by media other than water
'l and those which may be used predominantly for research
;~ purposes.
Prior art reactors have been described (though
they have not yet been constructed or operated) in which
! passive shutdown cooling is available to coveE a range of
potential accident situations, and in which the primary
heat transport circuit, under normal operating conditions,
is maintained in thermal-hydraulic isolation from the
shutdown circuit. These reactors are known as the PIUS*
(described in Canadian Patent l,173,5~0 (Hannerz); the
SECURE*(described in Canadian Patent l,070,860 (Blomstrand
et al.); the TRIGA* power reactor, described in R.W.
Schleicher, "TRIGA Power System: A Passive Safe Co-
Generation Unit for Electric Power and Low TemperatureHeat", in: Small Reactors for Low Temperature ~eat
Applications, IAEA-TECDOC-463 (International Atomic Energy
Agency, Vienna, 1988) pp. 45-55; and the GEYSER*heating
* Trade Mark
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1 32691 ~
reactor, described in G. Vecsey and P.G.K. Doroszlai,
"GEYSER, A Simple, New Heating Reactor of High Inherent
Safety", Nuclear Engineering and Design, 109 (1988) 141-
~ 145. The PIUS, SECURE and TRIGA reactors rely on elaborate
parameter-sensing and control systems to continuously
maintain the integrity of normal reactor operations in the
.,
face of the inevitable adverse influences of the passive
;j cooling system on normal operations. Such active control
``~ measures detract from the overall reliability and the
simplicity of operation of the reactor, and aggravate the
space and weight requirements. The GEYSER reactor, while
responding to passive cooling and normal operating
requirements in ways that avoid the use of active systems,
has a physical plant size which is consequently extremely
large. Also, the system has very long response times in
; respect of load following and compensation of inadvertent
changes in operating conditions which, along with the large
j physical size, render the concept impractical in many
application environments, including that of a submarine.
.' .
8ummary o the Invention
The invention provides a cooling system for use in
association with a nuclear reactor plant of the type having
a primary cooling circuit comprising a reactor core, an
inlet duct and inlet plenum for conducting coolant into the
! 25 core, an outlet plenum and an outlet duct for conducting
~, coolant out of the core, and means for circulating coolant
through the primary cooling circuit. The cooling system
~' comprises: (a) a reserve coolant tank for containing
coolant: ~b) a first port means in the inlet duct upstream
of the core, having a primary flow channel for conducting
, coolant through the inlet duct and having exchange flow
, channel means for conducting coolant between the reserve
¦ coolant tank and the primary flow channel, the primary flow
channel being open to said reserve coolant tank through the
exchange flow channel means; and (c) a second port means in
the outlet duct downstream of the core, and spaced
generally above the first port means, having a primary flow
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132691~
channel for conducting coolant through the outlet duct and
having exchange flow channel means for conducting coolant
between the primary flow channel and the reserve coolant
tank, the primary flow channel being open to the reserve
coolant tank through the exchange flow channel means.
During normal, unimpaired operation of the primary
- cooling circuit, coolant circulates through the primary
- cooling circuit and only minimal volumes of coolant flow
- into the reserve coolant tank from the primary cooling
.:
circuit or into said primary cooling circuit from the
reserve coolant tank through said exchange flow channel
means. During impairment of the primary cooling circuit,
or other circumstance of mismatch between the energy
generation and adequate dissipation rates of the nuclear
reactor plant, a natural convective flow of coolant is
initiated without mechanical or operator intervention,
' wherein coolant in the reactor core flows by convection
,, through the outlet plenum and outlet duct and through said
exchange flow channel means in the second port means into
the reserve coolant tank, and coolant in the reserve
coolant tank flows into the exchange flow channel means in
the first port means, through the inlet duct and inlet
plenum and into the reactor core, whereby a natural
'7 convective flow of coolant results.
, 25 The invention also provides a cooling system for
,! use in association with a nuclear reactor plant of the type
~ having a primary cooling circuit comprising a reactor core,
',7 a plurality of inlet ducts and an inlet plenum for
conducting coolant into the core, an outlet plenum and a
plurality of outlet ducts for conducting coolant from the
core, the inlet ducts and the outlet ducts being configured
with substantially radial symmetry about a vertical axis
through the center of the core, and means for circulating
coolant through the primary cooling circuit. The cooling
system comprises: (a) a reserve coolant tank for
containing coolant; (b) a first port means in each of the
inlet ducts upstream of the core, each of the first port
means having a primary flow channel for conducting coolant
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132~91~
through the inlet duct and ha~ing exchange flow channel
means for conducting coolant between the reserve coolant
tank and the primary flow channel, the primary flow channel
;being open to the reserve coolant tank through the exchange
flow channel means; (c) a second port means in each of the
;outlet ducts downstream of the core, and spaced generally
at higher elevation than each of the first port means in
the normal orientation of the reactor plant, each of the
second port means having a primary flow channel for
` 10 conducting coolant through the outlet duct and having
exchange flow channel means for conducting coolant between
the primary flow channel and the reserve coolant tank, the
` primary flow channel being open to the reserve coolant tank
through the exchange flow channel means. During normal,
unimpaired operation of the primary cooling circuit,
coolant circulates through the primary cooling circuit and
only minimal volumes of coolant flow into the reserve
,~coolant tank from the primary cooling circuit or into the
primary cooling circuit from the reserve coolant tank
through the exchange flow channel means. During impairment
of the primary cooling circuit, or other circumstance of
mismatch between the energy generation and adequate
dissipation rates of the nuclear reactor plant, a natural
convective flow of coolant is initiated without mechanical
or operator intervention, and irrespective of the
orientation of the reactor plant with respect to gravity,
wherein coolant in the reactor core flows by convection
generally upward through at least some of the outlet ducts
and/or inlet ducts positioned at a higher elevation than
the reactor core and through the exchange flow channel
means in the second and/or first port means in the ducts
and into the reserve coolant tank, and coolant in the
reserve coolant tank flows into the exchange flow channel
means in the first and /or second port means in at least
some of the inlet and/or outlet ducts positioned lower than
the core, and into the reactor core, whereby a natural
convective flow of coolant results.
An objective of the present invention is to allow
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132691~
the operation of a nuclear reactor having a primary
circuit, a shutdown circuit, and, especially, a manner of
coupling the two circuits such that:
A. The performance of the primary circuit under
normal operating conditions is not significantly
`~ compromised by the presence of the (passively coupled)
shutdown circuit.
B. The shutdown circuit performc as indicated
above, namely, it adequately cools the reactor core on a
lo passive basis in the face of a variety of accident
scenarios following self-initiation of the passive cooling
function, such initiation being also on a passive basis.
C. The passive shutdown system, in both its
initiation and sustained operation, is free of encumbrances
15 which could conceivably invalidate the assumption of
-~ passive behaviour.
D. The primary circuit operates in the prescribed
manner but with a bare minimum of active devices (e.g.,
sensors, pumps, actuators, valves, hydrodynamic shims,
20 feed-back circuits, central processing units, operator
; monitoring and intervention) being required to maintain
operation at the prescribed performance level. This
requirement is in the interests of: (a) minimizing the
complexity and costs associated with the engineering,
i 25 detailed design, construction, operation and maintenance of
the plant; (b) maximizing reliability, safety, and
licensability of the nuclear plant: and (c) freeing the
basic passive safety design from additional complexities
that would restrict the range of application of a reactor.
' 30 (Some applications, which may stand to gain the most from
passive cooling, may also impose overriding requirements
excluding a more complex form of passive safety.)
E. The fundamental principles are universally
adaptable to a sufficient degree that the basic reactor
35 design, including the approach to incorporating passively
initiated passive cooling, can be applied to advantage in
a range of circumstances, including the following, taken
individually or in combination: (a) a range of power
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1~26~6
output ratings; (b) a range of power reactor purposes,
including those for space and process heating and those
producing shaft power or electricity; (c) pool-type
research reactors where the thermal-hydraulic configuration
of the invention permits: (i) enhanced cooling, and thus
higher sustained levels of power and, hence, neutron flux,
for a given fixed reactivity margin, and (ii) effective
localization of activation products within the primary
transport circuit; (d) limited head space above the reactor
; 10 as in submarine applications of reactors; (e) varying
orientation of reactor plant (which translates as
variability of the gravitational g-vector), as in the case
of a submarine-borne reactor during either normal operation
` at sea, or when capsized; (f) varying magnitude of the
acceleration due to gravity during reactor operation, as
for a submarine-borne submarine riding surface waves
(bearing in mind that the acceleration due to gravity is a
key parameter in all passive cooling designs); (g) a
requirement for assured reactivity insertion accompanying
initiation of shutdown cooling through high density neutron
absorber dissolved in the pool water; and (h) a low margin
`~ of auxiliary power available to operate the primary
circulation pump.
In a reactor cooling system of the kind referred
to in the present invention, the primary heat transport
circuit which is otherwise of a more-or-less standard
closed circuit design (i.e., with the coolant being
circulated through the reactor loop core and the heat
exchanging components by means of a pump) is fitted with
branching devices called hydrodynamic ports in such a way
; that passive cooling is continuously available by process-
inherent means to assure that the reactor is always
adequately cooled, even following a range of events which
would ordinarily lead to accident scenarios. It is a
feature of the hydrodynamic ports and fundamental in the
manner of their deployment that they permit reactors
embracing a broad range of types, configurations and
purposes to be accommodated in respect of the stated
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132691G
objectives of the patent, without significantly restricting
or complicating plant design or operation.
Brief De~cription of the Drawings
In drawings which illustrate embodiments of the
invention,
: Figure 1 is a sectional elevation of one
embodiment of the invention;
Figure 2a is an axial view of a hydrodynamic port;
,;. Figure 2b is a cross-section along the line AA of
Figure 2a;
Figure 3a is an axial view of a hydrodynamic port
having bypass ducts;
` Figure 3b is a sectional view along the line AA of
:, Figure 3a;
Figure 3c is a sectional view along the line BB of
, FIgure 3a;
Figure 4a is an axial view of a hydrodynamic port
with an anti-convective shroud fitted thereon;
Figure 4b is a sectional view along the line CC of
Figure 4a;
Figure 5 is a sectional elevation of a second
embodiment of the invention; and
Figure 6 is a sectional elevation of a third
embodiment of the invention.
.~ 25 The numerals of the drawings identify various
elements and components pertinent to the invention
according to the following list:
1 - reactor core assembly
2 - inlet plenum
3 - outlet plenum
4 - inlet duct
. 5 - outlet duct
6 - diffuser
. 7 - inlet hydrodynamic port
8 - outlet hydrodynamic port
9 - thermal insulating layers
3 lo - outlet conduit
- 8 -
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1326~1 ~
11 - inlet conduit
: 12 - pressurizer
. 13 - liquid-vapour interface
:. 14 - reserve coolant tank
15 - reserve tank access hatch
16 - radial gamma shield
17 - axial gamma shield
18 - fuel elements
` 19 - radial reflector
J 10 20 - circumferential exchange flow slot
: 21 - flow area defining duct
: 22 - flow area defining duct (complement)
23 - flange
24 - slot-defining plate
25 - spacer
. 26 - bypass passage
27 - bypass tube
28 - bypass slot
29 - ringed flange
30 - anti-convective shroud
. 31 - stratification grid vanes
32 - isothermal plane
33 - qua`si-stagnant zone
34 - circulated coolant
35 - reserve coolant
36 - inlet manifold
37 - outlet manifold
- 38 - "borated" water shielding tank
39 - rod drive mechanism housings
40 - reactor core access tubes
41 - baffle and shield
42 - regulating rod guide tube
43 - shutoff rod guide tube
44 - regulating rod
;~. 35 45 - shutoff rod
; 46 - reactor vessel
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132~916
Detailed Description of the Preferred Embo~iments
In the reactor plant shown in Figure 1, the primary
heat transport circuit comprises the equipment components
normally found in a typical reactor cooling system of the
5 closed circuit variety. Included are the reactor core 1
and the inlet and outlet plena, 2 and 3, respectively, all
located inside the reactor vessel 46. Also included are
the inlet and outlet ducts, 4 and 5, respectively, leading
by means of inlet and outlet conduits 11 and 10,
10 respectively, through penetrations in the walls of the
reserve coolant tank 14 to connect with external
components, principally the main circulating pump and
s combinations of heat exchanging components, such as
~ preheaters, evaporators, steam generators, or simple heat
15 exchangers. The part of the primary circuit that embraces
these latter components could be located within the reserve
coolant tank along with the reactor vessel and connecting
ducts and conduits, if required by the safety and space
restrictions of a particular design. However, the form,
20 specification, and location of the pump and heat exchanging
components, as a matter of particular note, are not
, critical to the functioning of the subject invention, and
' have not been included in the drawings.
over the range of anticipated operating and
25 shutdown conditions of the reactor of Figure 1, the coolant
~, circulating within the primary circuit will be at
temperatures considerably higher (typically by 80 - 250 C)
than the reserve coolant maintained in the reserve coolant
tank 14. Thermally insulating layers 9 of a design
30 suitable for such applications are attached to most of the
primary circuit boundaries lying within the reserve coolant
tank. Such insulation enhances the energy delivery
efficiency of the reactor and facilitates the maintaining
of the reserve coolant at a standby temperature
35 sufficiently low to support safety objectives. The role of
~3 the insulation in enhancing the actual operation of the
! passive cooling circuit is not necessarily of importance,
however, in meeting most specific reactor safety
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1326916
requirements.
The reserve coolant is maintained at suitable
standby temperatures with the aid of reserve tank coolers
(not shown in the drawings) which transport, to the
external environment, residual heat transferred from the
primary circuit to the reserve tank during normal
operations, and also the decay heat transferred to the
reserve tank by the passive cooling circuit following
emergency shutdown. The reserve tank coolers may operate
on either active or passive principles but, for consistency
with the passive safety principles of the present
invention, the capacity of the passive cooling component of
the reserve tank coolers should be adequate for safe decay
heat removal over an indefinitely long period following
reactor shutdown. The specific design of the reserve tank
coolers may be carried out by persons skilled in the art of
heat transfer as required to suit the particular reactor
and the circumstances of its application.
The elements of the primary heat transport circuit
not ordinarily found in a reactor cooling system, but which
are a key feature of the present invention, are the inlet
and outlet hydrodynamic ports, 7 and 8, located in the
circuit just upstream from the inlet duct 4 and downstream
from the outlet duct 5, respectively. The hydrodynamic
ports, exemplified by the basic version shown in Figure 2,
exhibit specific hydrodynamic propsrties when correctly
incorporated in the design of a reactor primary heat
transport circuit. The most important of these properties
are the ports' capabilities of: (i) supporting to a high
degree the continuity of primary circuit flow between the
~ducts 21 and 22 in normal reactor operating conditions,
:!(ii) offering large resistance against a tendency toward
both combining and dividing branah flows through the
exchange flow slots 20, relative to port axial flows, when
~!35 the system temperatures, primary flow or reactor
orientation deviate inadvertently from normal, and (iii)
offering relatively small resistance to large branch flows
of either the combining or dividing kind in severely
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132~9~6
abnormal or accident conditions, such as when the main
circulating pump has stopped.
- Under these latter conditions, core cooling relies
solely on naturally convective coolant circulation,
established and maintained by inherent processes. The
operative convective circuit in such conditions is between
the core 1 and the reserve coolant tank 14 and is referred
to herein as the passive cooling circuit. A principal flow
pattern for natural convection is depicted by the broken
arrows in Figure 1. The solid arrows show the flow pattern
in the primary cooling circuit under normal operating
conditions with the circulating pump on.
The above-mentioned properties of the hydrodynamic
ports are manifest in the first embodiment of the present
invention as it applies to the reactor plant of Figure 1.
j A detailed description of the operation of the cooling
- system of this plant follows.
As implied above, under normal operating conditions
a steady flow is maintained through the primary circuit by
the continuous operation of the circulating pump. The
pumping head equals the algebraic sum of the pressure
changes across individual circuit components, including (i)
-9 the heat exchanger components located externally to the
, reserve cooling tank as referred to above, and (ii) the
components forming that part of the circuit delineated by
the solid arrows and shown residing inside the reserve
coolant tank 14, namely (in flow sequence), the inlet
conduit 11, the inlet hydrodynamic port 7, the diffuser 6,
the inlet leg 4, the inlet plenum 2, the reactor core 1,
;~ 30 the outlet plenum 3, the outlet duct 5, the outlet
hydrodynamic port 8, and the outlet conduit 10.
~ For purposes of providing the constant availability'1l of passive cooling by process-inherent means during normal
`, rector operation, the hydrodynamic ports, 7 and 8, are
~ 35 designed (as clarified in Figure 2) to present no physical
; barrier against the flow of coolant between the primary
cooling circuit and the reserve coolant tank 14. However,
to avoid the undesirable transport of either thermal energy
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1326~
or radioisotopes from the primary circuit into the reserve
tank during normal operation, it is important to reduce to
acceptable levels the tendency for such exchange flow, and
also the magnitude of such flow if it should occur. (In
some reactor variations, in which safety shutdown is
induced by the onset of passive cooling, the limiting of
exchange flow in normal operations is essential also to
avoid the premature transport of dissolved neutron
absorbing nuclides from the reserve tank into the primary
circuit and the resulting reactor shutdown.)
The tendency for such exchange is reduced to
acceptable proportions, in the first instance, by the
incorporation of the port concept in the design of the
primary circuit in accordance with a specific design
requirement applying only to that part lying within the
reserve coolant tank 14 of Figure 1. The requirement is
stated as follows: At normal operating conditions, the net
pressure from all effects accumulative around the passive
cooling circuit delineated by the broken arrows in Figure
1, and tending to support net flow around this circuit,
must be substantially zero.
Basic parameters of the primary circuit that must
be taken into account in designing for zero net
accumulative pressure are the primary circuit mass flow
rate, the coolant temperature profile in the primary
circuit, the bulk temperature of the coolant of the reserve
coolant tank, and the resistive pressure drop due to the
primary circuit mass flow through the core and other
1 components in the flow path from port 7 to port 8. All
`~ 30 such parameters will have been determined as the normal
~ operating design values selected to yield optimal plant
;, energy production, efficiency and safety, as appropriate to
~! the intended application.
More specifically, the criterion for meeting the
~ 35 requirement of zero net accumulative pressure may be
;~ expressed in terms of the values of Al and A2 which are
~l shown in Figure 1 as the axial flow areas associated with
hydrodynamic ports 7 and 8, respectively. These flow areas
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~ 132691~
can be chosen by those skilled in the science of
hydrodynamic design so that the change in dynamic pressure
(velocity head change) associated with a constant mass flow
passing from the vicinity of the inlet port 7, of flow area
Al, to the vicinity of the outlet port 8, of flow area A2,
is equal to the deficit by which the net elevation
pressure, assessed accumulatively around the passive
cooling circuit, fails to match the resistive pressure
losses associated with the same primary coolant mass flow
through the core. The correct assessment of the net
elevation pressure is based on the vertical port separation
L as indicated in Figure 1, and the between-port coolant
temperature profiles (and hence the density profiles) along
paths through both the reactor vessel and the reserve tank.
The resistive losses identified above are due to the
hydrodynamic resistance offered by the core itself, and
other components between the inlet port 7 and the outlet
port 8.
Within the requirement of zero net accumulative
pressure, as spelled out in the stated criterion for
meeting the requirement, it is clear that a great deal of
- flexibility is possible in the thermal-hydraulic design of
the reactor depicted in Figure 1. For example, if the
temperatures of the profile extending through the core and
between the two ports are considerably higher than those
~il that are characteristic of the bulk of the reserve coolant,
as will certainly be the case when a relatively high
~ thermodynamic efficiency is needed in the conversion of
:~ reactor thermal power output to mechanical or electric
forms, and when the temperature of the reserve coolant is
; kept relatively low in the interests of safety, it is
possible to meet the requirement of zero net accumulative
pressure requirement without prescribing any change in flow
area in going from the vicinity of one port to that of the
other, i.e., with Al equal to A2. As the net change in
dynamic pressure from the vicinity of port 7 to port 8 is
~i zero in this case, the design criterion is simply that the
net accumulative elevation pressure be equal to the
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~ 1 326916
resistive pressure losses along the flow path through the
core and between the ports. This criterion is met by the
adequate protraction of the vertical separation L between
the ports, and seeing that the segment of the primary
circuit including the core and lying between the ports,
itself now considerably lengthened as a consequence of the
~-increased port separation, is of a design consistent with
good hydrodynamic practice.
In a different example, space limitations are
assumed to impose a restriction on the reactor plant and
~-hence on the vertical separation L between the ports in the
reactor of Figure 1. For a prescribed disposition of
normal operating temperatures throughout the system, this
reduction can be permitted, while preserving the zero net
accumulative pressure requirement, by making A2sufficiently
larger than A1 to create a net dynamic pressure decrement in
the coolant passing from port 7 to port ~. This decrement
constitutes additional driving pressure to compensate for
the reduction in the net accumulative elevation pressure
associated with the shortening of both the hot and the cold
hydrostatic columns operative around the passive cooling
circuit. As in the previous case good thermal-hydraulic
design applied to that part of the primary cooling circuit
lying between the ports and including the core is essential
i25 and may be carried out by those skilled in the science of
hydrodynamic design. The specification of such components
as the diffuser 6 and its location within the circuit must
,jibe in accordance with established design practice for such
components.
In a third example relating to the reactor depicted
,.7 in Figure 1, sufficient vertical space is postulated so
that, for the prescribed disposition of normal operating
temperatures throughout the system, the distance L between
the two ports can be made such that the net static head
becomes more than sufficient to compensate the resistive
pressure losses associated with the passage of the primary
coolant at design mass flow rate through the core. In this
case, the zero net accumulative pressure requirement may be
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132691~
met by making A2 smaller than Al so that the dynamic
pressure increment cancels the exces~ net elevation
pressure over the resistive pressure loss across the core.
Also, the excess net elevation pressure contributes, along
with the circulating pump if one is present, to overcoming
the resistive losses in those primary circuit components
~ implied as lying outside the reserve coolant tank. Again,
- the careful application of hydraulic design practice is
required.
10A principal objective of the invention is to
-~provide in a reactor cooling system, of otherwise ordinary
but efficient design, a passive cooling system capability
which is constantly available for deployment by process-
inherent means during normal reactor operation, and which
15 is capable of mitigating satisfactorily a set of reactor
accident initiating events. In the cooling system of the
reactor depicted in Figure 1, as an example, the
hydrodynamic ports, 7 and 8, are designed to present no
physical barrier against the flow of coolant between the
20 primary cooling circuit and the reserve coolant tank 14.
-~ The principal means of avoiding the potentially deleterious
effects of such an arrangement on the normal reactor
operation is an important element of this invention. The
operation of the cooling system in mitigating accidents
25 effects is now described.
In the event that pumped flow through the primary
$J circuit of the reactor cooling system shown in Figure 1
suddenly ceases during the course of normal reactor
~^ operation (due to a loss of pumping power, for example),
30 the resulting drop in axial coolant flow through the
hydrodynamic ports leads to a loss of the dynamic pressure
change that is maintained during normal operation. The net
accumulative pressure around the passive cooling circuit
becomes non-zero and dominated by elevation pressures
35 differences which are enhanced momentarily by the rising
temperature of coolant within the core. In these
circumstances, the reactor core becomes a component of the
now operative passive cooling circuit and, following some
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1326~16
reactivity-related transient oscillations, caused in part
by the sudden engagement through the lower port of the
relatively cool reserve coolant with the passive circuit,
a relatively steady flow supported entirely by natural
5 convection becomes established. The coolant column of
approximate height ~ and temperature equal to the reactor
outlet temperature serves as the hot leg, and the column of
height L partially cancelled by the column of height L1,
both columns being at the reserve coolant temperature,
10 serves as the cold leg of the operative thermosyphon.
In the event of loss of capacity of the heat sink
of the primary circuit, the resultant return of the
circulating primary coolant at significantly elevated
temperatures raises the temperature of the thermosyphon hot
15 leg (now of length L) to above-normal values, forcing the
passive circuit to operate in parallel with the pumped
circuit, thereby ameliorating the core coolant condition
due to increased core flow and, more importantly, the
combining of cool reserve coolant with primary circuit flow
20 at the inlet port 7. A similar chain of events results, in
the near term, following a loss of regulation of reactor
power event in which for a time the reactor undergoes a
~j significant inadvertent increase in reactor power.
~g
Before describing the behaviour of the reactor
~ 25 cooling system in the event of the breakage of a component,$. which has a role in defining the pressure boundary of the
primary heat transport system, it is important to describe
~i further the disposition of some of the system components,
as the nature of the consequences depends greatly on the
30 location of the break within the system, and on the
i specific arrangements whereby the system is maintained at
;3 atmospheric or elevated pressures during normal operation.
i The primary heat transport system pressure boundary
includes the reserve coolant tank and those components of
35 the primary circuit indicated with respect to Figure 1 to
3 lie outside the boundary of the reserve coolant tank,
unless of course they too are located inside the reserve
tank in the alternative arrangement mentioned earlier. The
,
~j - 17 -
.
... . . .
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1326~1 ~
reactor of Figure 1 is depicted as being maintained at an
; elevated operating pressure by means of the pressurizer 12
connected to the reserve coolant tank. A liquid-vapour
interface 13 is shown to be maintained in the pressurizer.
If the reactor is to be operated nominally at atmospheric
pressure, either the pressurizer, or the reserve tank
itself, is left open to the atmosphere in the design, or is
in some manner vented to atmosphere. The so-called (open)
pool-type reactors fall into this latter category.
Because substantially zero pressure differences
must exist across the barrier-free interfaces that occur
- between the reserve tank and the primary circuit under
normal operating conditions, and the operating pressures of
these two parts of the system are therefore intimately
related, the degree of pressurization of the primary system
; is largely unimportant in a description of the basic normal
functioning of cooling system according to the invention.
By similar reasoning, the actual point at which the
pressurizer is connected to the system is relatively
unimportant to the objectives of the present invention.
Although the pressurizer of the reactor cooling system of
Figure 1 is shown connected to the reserve coolant tank, it
may be preferable in certain circumstances to connect the
pressurizer to the primary circuit instead. In some cases,
it may be advantageous to connect a single pressurizer to
the reserve tank and the primary circuit in parallel.
These observations apply so long as the degree of
pressurization and other key operating parameters do not
stray too far from design values, in either the normal
operating or the passive cooling modes, as described
previously.
The principal objective of pressurizing the reactor
coolant in any reactor plant is to permit the transport of
heat at elevated temperatures, in the interest of enhancing
the thermodynamic efficiency of the plant, while retaining
the advantages of the coolant in its liquid state. In the
reactor plant of Figure 1, only the coolant circulating in
the primary circuit has a requirement for elevated
:,
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13~6~
temperatures in consideration of plant thermodynamics.
Therefore, the reserve coolant may be maintained at L,
relatively low temperatures; safety considerations require
that it must be maintained by means of the tank coolers
(not shown) at temperatures much lower than the saturation
temperature corresponding to atmospheric pressure. This
requirement is entirely consistent with the separate safety
requirement that the reserve coolant temperature remain low
in consideration of enhancing the passive cooling system's
potential in mitigating accidents conditions in the manner
already described.
Because of the way in which the pressures of the
primary circuit and the reserve coolant tank are linked in
the maintenance of the zero net accumulative pressure
requirement, a requirement for pressurization of the
primary circuit results also in the pressurization of the
reserve coolant tank. Thus, in a pressurized system, the
nominal operating pressure of the entire heat transport
system is required to be conservatively in excess of that
required to prevent boiling at the point of the hottest
,Jcoolant within the primary circuit. Conversely, in the
special case of the reactor not being pressurized to higher
than the ambient pressure, the reactor is constrained to
operate with core temperatures not exceeding the saturation
temperature at atmospheric pressure. The reactor shown in
Figure 1 is representative of either the pressuxized or
non-pressurized case.
The arrangement exemplified by the reactor in
Figure 1, in which the primary circuit and the reserve
~!30 coolant tank are required to operate at identically
regulated pressures, enhances the safety performance of the
reactor in the event of a primary circuit pipe break. This
follows since only the relatively small fraction of the
.total primary inventory which lies within the primary
circuit itself is at a temperature exceeding the saturation
temperature at the ambient pressure (normally one
atmosphere) and is therefore subject to rapid vaporization
and accompanying energy release at the instant of the
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132691~
break. ~he bulk of the reserve coolant inventory remains
as cool liquid immediately following the break and serves
to continue the core cooling function after entering the
core region through the ports and eventually establishing
a steady passive cooling flow.
- By suitable design, in which, for example, the
inlet and outlet conduits penetrate the wall of the reserve
coolant tank at levels higher than the locations of the
upper hydrodynamic port (as shown in Figure 1), the passive
- 10 cooling system will continue to serve the reactor core
cooling function for a considerable time following the pipe
break. The design may also call for a syphon break
arrangement (not shown) between the two conduits in order
to limit the extent of removal of reserve coolant by
syphoning action. The potential for loss of coolant
: through rupture of the reserve coolant tank itself is
minimized by care and redundancy in design.
~- In the special case of the reactor which is non
'~ pressurized relative to atmospheric pressure, and the
maximum coolant temperature does not exceed the saturation
i temperature at this pressure, a pipe break is followed by
a very orderly progression of events (due to the absence of
vaporization of liquid) culminating in the steady operation
of the passive cooling circuit in the dissipation of
residual reactor power production.
It was previously emphasized that current practice
in the art of thermal-hydraulic design would be required in
designing the conventional aspects of the primary heat
transport circuit which support normal operation. Similar
; 30 practice is to be applied in achieving a satisfactory
passive cooling design in which the core resistance for a
~ broad range of thermal-hydraulic conditions, the lengths
;J and cross sections of the inlet and outlet legs, the
standby temperature of the reserve coolant, and the reserve
tank cooler capacity must be specified. It is also a
requirement of the design that the resistances of the
hydrodynamic ports to large scale branch flows do not
adversely impede the natural convection of the passive
.
,
... . .
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132691~
cooling circuit in any depicted accident scenario,
including ones in which reactor shutdown is not effected
immediately. Further consideration of this aspect of the
detailed design of the hydrodynamic ports is given in the
paragraphs that follow.
In the selected illustrations of the response to
accident conditions of a reactor cooling system made in
accordance with the invention, no particular reference was
made to either reactivity reductions or reactor shutdown
action instigated early in the events. It is clear in all
examples given, however, that the provision for early
reactivity reduction will a~sist in arriving at a detailed
thermal-hydraulic design which will maintain the safety of
the reactor following all credible initiating events.
Furthermore, the provision of an early shutdown will
clearly extend the period of time that may be allowed to
pass before human intervention is required to assure long
term safety, following an accident. Such reactivity-
~31 limiting mechanisms are not prescribed as a part of the
present invention, but it is clear that if the reactivity
reduction or shutdown action is inherent in the processes
,j~ which are intimately related to the accident condition,
then reactivity-limiting mechanisms can be seen to be
consistent with inherent safety aspects of the cooling
system.
Two reactivity-limiting mechanisms, both of which
are generally understood by persons skilled in the art of
reactor design, are mentioned here briefly, however. One
is based on negative reactivity power coefficients
associated with either the expansion of the coolant or the
effect of fuel heating on either resonance absorption or
neutron thermalization. The other is less direct and is
~' initiated by the onset of passive cooling which, in turn,
is triggered in response to the initiation and development
of the accident scenario. The first of these mechanisms
may be adopted in the detailed design of a reactor such as
the one in Figure 1, in the selection of the fuel type and
associated core design.
~ ?~ -- 21
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~ 1326916
The second mechanism may be implemented in the
detailed design of a reactor such as that of Figure 1 by
arranging for the permanent addition of heavily
concentrated neutron absorber in solution with the reserve
coolant, and placing neutron absorber only in mild solution
with the primary coolant. The reactor, in this case, is
regulated by actively varying the concentration of absorber
- in the primary circuit. Under normal operating conditions
the two levels of solution are kept from mixing with one
another as a byproduct of the arrangement described earlier
as keeping the same two categories of coolant from mixing
~- for reasons of both thermal efficiency and prudent
radiological management. The additional requirement of
maintaining the two levels of solution in fairly strict
isolation from each other may in turn require an
elaboration of operating protocol or, preferably, the
- adoption of a somewhat more sophisticated arrangement of
the hydrodynamic ports such as the ones discussed below in
connection with the Figures 2, 3, and 4, and with the
reactors depicted in Figures 5 and 6.
In terms of strictly thermal-hydraulic
considerations, the reactor cooling system of Figure 1 does
not place any formidable restrictions on the operating
protocol of the reactor plant. In a typical start up
scenario, the primary circuit pump may first be turned on,
in which case there will be a significant bypass exchange
flow in which some circulating primary coolant enters the
reserve tank through the inlet port 7, while an equivalent
flow of reserve coolant passes into the primary circuit
~, 30 through the outlet port 8. As the reactor power is
increased after being brought to criticality, the primary
circuit warms more quickly than does the reserve coolant
and, by the time the primary circuit reaches the normal
operating temperature, it will have ceased altogether to
exchange with the reserve coolant. A less likely
alternative start up procedure is to first bring the
` reactor to an operating power level while relying on the
! convective flow of the passive system for core cooling. As
.
s - 22 -
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.
'
1~2691~
the pumped cooling circuit is brought on line, the initial
ingress flow of the passive cooling is gradually reduced to
zero and the normal operating conditions are reached.
During the course of extended normal operations of
the reactor depicted in Figure 1, a finite amount of
exchange flow may occur inadvertently due to fluctuations
in the operating paxameters which lead to momentary
departures from the zero net accumulative pressure
requirement. It is an advantage of the cooling system
- 10 constructed according to the invention that there is a
continuity in the behaviour of the exchange flow deviation
from the null point as the values of the operating
parameters vary about the precise set of values that
provide null exchange. Such advantage is manifest in the
amenability of such a functionally "well-behaved" system to
various remedial measures of confining exchange flows to
arbitrarily small values by either active control of the
operating parameters or by arrangements for passive
compensation.
Exchange flows, including those which occur during
either the start up scenarios described or inadvertent
lapses in meeting the zero net accumulative pressure
requirement, contribute accumulatively to the loss of some
-~ useful energy, to the loading of the reserve tank coolers,
and to the build up of what may be radiologically
significant quantities of radioisotopes in the reserve
coolant. In many applications, significant levels of these
effects may be acceptable, depending on the circumstances
of the particular application. If, however, such effects
must be strictly controlled, even to the point of being
eliminated entirely over a fairly broad range of operating
parameters, remedial measures in the form of either the
precise control of the operating parameters during
operation, or as preferred, through special designs of the
ports and the cooling system as a whole, may be implemented
. .,
- in such a way that the system is self-compensating by
inherent processes over a meaningful range of parameter
variations. Such special designs are embodied in the
'.
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:", -. ~ , . ..
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,
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.,
1 3 2 6 9 ~ ~
different versions of the hydrodynamic ports discussed
below with reference to Figures 2, 3, and 4, and with the
reactors depicted in Figures 5 and 6. Such special designs
would be necessary, for example, to maintain an adequate
operational separation between the reserve and the primary
coolants if reactor shutdown relies on two distinct levels
- of dissolved neutron absorber. Also, a somewhat more
elaborate start-up protocol than those just described would
be necessary in reactors involving dissolved absorber in
this way. Such protocols may be readily devised by those
skilled in the art of reactor dynamics and control.
Beyond the imposition of the zero net accumulative
pressure criterion on the primary circuit design through
the proper selection of the flow areas A1 and A2 as
described previously, additional factors which limit the
magnitude of exchange flow between the primary circuit and
the reserve tank during normal operations are vested in the
design of the hydrodynamic ports themselves. A detailed
representation of a basic version of the hydrodynamic port,
of a kind suitable for implementation in the cooling system
of the reactor of Figure 1, is shown in Figure 2. This
port constitutes an approximation of a continuous pipe made
up of two flow area defining ducts 21 and 22 with attached
flanges 23, separated axially by the branch opening
composed of a series of several slot-defining plates 24
, alternating with spacers 25 which give rise to the
circumferential exchange flow slots 20.
During normal operation, when the requirement of
zero net accumulative pressure is nominally satisfied,
essentially all flow through the port is axial and the flow
magnitude is substantially due to primary pump operation.
Under these conditions, branch flow tends to be discouraged
, on the basis that the particles of coolant involved in any
such branch flow would experience high momentum changes,
whether combining with or dividing from the main axial
flow, by passage through the slots. The purpose of the
slot and plate structure of the branch openings is to
require the largest possible momentum change for the
.,
~ - 24 -
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.
132691~
"typical" particle of coolant involved in branch flow.
Thus, the port structure shown in Figure 2 is seen to be
supportive of the first two of the previously mentioned
important properties of the hydrodynamic ports, namely, (i)
; 5 the support of the continuity of primary circuit flow
between the flow area defining ducts 21 and 22 in normal
reactor operating conditions, and (ii~ the offering of
large resistances to both combining and dividing branch
flows, when the tendency for such is created due to the
`` 10 inadvertent deviation of principal operating parameters,
such as coolant temperatures and primary flow rates, from
nominal values.
The third of the previously mentioned important
properties of the hydrodynamic ports, namely, the offering
of relatively small resistances to large branch flows, of
.either kind, in severely abnormal or accident conditions,
is characteristic of the hydrodynamic port shown in Figure
2 when properly incorporated in the reactor cooling system,
;for the following reason. In abnormal circumstances, such
`i20 as during an impairment of the primary pump, the required
,~mass flow through the core for adequate cooling is
considerably reduced due to (i) the inlet temperature being
now determined by the reserve coolant temperature which is
maintained at values greatly depressed relative to
operating core inlet temperature, and (ii) the expected
reduction in reactor power due to a suitable provision for
reactor shutdown in accident conditions. The relatively
small mass flow requirement, and the resulting
acceptability of relatively low passive cooling flows under
shutdown conditions, makes tolerable the residual
resistance which branch flows experience in negotiating the
slot-defining plates of the ports.
The detailed design of the hydrodynamic ports based
on the principles of operation just outlined may be carried
out by persons skilled in the art of hydrodynamic design.
Thus, the variables such as the axial flow area, the number
and thickness of the slots, and the sizes of the plates and
spacers may be selected to suit the capacity and the
- 25 -
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, . , , - .
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~3~91~
performance specifications for a reactor designed for a
particular application. It is an advantage of the port
concept and the manner of its implementation according to
the invention that the ports are simple of structure and,
therefore, afford great flexibility in adaptation to a
variety of reactor configurations. Nevertheless, the
~, important properties of the ports discussed above can be
enhanced through any number of refinements which may be
~ implemented by those skilled in the art of hydraulic
-~. 10 design. Such refinements may include adaptations in the
: form of the shaping of the edges of the slot-defining
plates, the minor progressing of their inside diameters,
'".! the introduction of an axial rod of cylindrical, conical or
other shape, or the "dishing" of the plates themselves.
Two specific enhancements of the important
properties of the hydrodynamic ports as refining elements
of the invention will be described in some detail. The
first enhancement is represented in Figure 3 as a
modification of the basic hydrodynamic port depicted in
Figure 2. The objective of this modification is to
enhance, relative to the port depicted in Figure 2, the
previously-mentioned second important property, namely, the
offering of large resistances to both combining and
dividing branch flows when the normal operating parameters
inadvertently deviate somewhat from nominal values, while
leaving essentially unaltered the branch flow area
available to the shutdown passive cooling mode. The second
enhancement is represented in Figure 4 as an anti-
convective shroud to be attached to each hydrodynamic port
to not only contain local exchange flow in the port, but
also to provide for the self-acting hydrostatic
compensation of pressure imbalances tending to produce
exchange flow, as discussed below.
The hydrodynamic port depicted in Figure 3 may be
described as the basic structure of the hydrodynamic port
depicted in Figure 2 modified in the following ways.
First, the port of Figure 3 has two extra slot-defining
plates 24 in the series which, with the flanges 29 which
- 26 -
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.: ,.,: : . ... ...
.... : - :-::
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1326916
are now ringed, form blind bypass slots 28, one at each end
of the series of regular slots 20. Second, the bypass
slots so-formed at either end of the series of regular
slots are connected by means of several longitudinal bypass
tubes 27 which are located symmetrically about the
`principal axis, providing a plurality of bypass passages 26
between the bypass slots and, thereby, providing passages
between the two flow area defining ducts 21 and 22.
The role of the bypass passages so defined may be
;10 stated as follows: Under ideal normal reactor operation
conditions, the ports carry only axial flow. Under such
conditions the flow experiences very little resistance in
passing through, and between, the regions of the area
defining ducts 21 and 22. In the absence of significant
levels of such resistance, a negligible pressure difference
exists between the entrances to the two bypass slots, and
negligible bypass flow is present. Therefore, under the
ideal operating conditions characterized by axial flow
only, the modified port of Figure 3 behaves within the
circuit essentially as does the basic port of Figure 2.
In the event of the reactor operation departing
from normal, the zero net accumulative pressure requirement
ceases to be satisfied and there arises in either the basic
or the modified port the tendency for branch flow, i.e.,
combining branch flow in one port and the corresponding
dividing branch flow in the other. In the case of the
modified port, however, even small branch flows of either
kind create sufficient longitudinal pressure differences
between the entrances to the bypass slots ~due to the
momentum changes inflicted on the axial flows by the branch
flows) to cause significant bypass flows through the bypass
passages.
In the case of combining branch flow, the pressure
drop and the induced bypass flow are in the general
direction of the main flow. Therefore, the bypass tubes
may be seen as assuming a share of the port's normal axial
mass flow, thereby reducing the axial flow velocity within
the port. The corresponding increase in the internal
.
` A 27
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~: .. . . . .
. : , ~ .... .
, : .. ~ .... . .
. : . :: . .
132
.~ `
static pressure in the vicinity of the port opposes the
incoming branch flow and therefore tends to correct the
initial cause, namely, the departure from zero net
accumulative pressure in the passive cooling circuit.
In the case of the dividing branch flow
simultaneously in the other port for the same causative
departure from zero net accumulative pressure, the pressure
drop and the induced bypass flow are in the general reverse
-~ direction to the main flow. Therefore, the bypass tubes
f lo may be seen as recirculating a portion of the port~s normal
axial mass flow, thereby increasing the axial flow velocity
within this port. The corresponding decrease in the
~ internal static pressure in the vicinity of the port
- restrains the outgoing branch flow and therefore tends, as
s 15 for the port experiencing combining branch, to correct the
initiating departure of the system from the required zero
net accumulative pressure in the passive cooling circuit.
Since the basic port of Figure 2 does not have the
benefit of the bypass channels and, hence, the associated
20 counteracting actions described above, a greater rate of
exchange flow may be expected in a system incorporating
such ports, in comparison with a system incorporating the
ports of Figure 3 which are the equivalent except for the
; bypass channels. In fact, it has been demonstrated
25 experimentally for branch flows in the region of greatest
s interest for present purposes (i.e., for branch flows of
less than 5 percent of the axial flow), that the branching
loss coefficients in general increase linearly with the
magnitude of the branch flow for both the ba~ic and the
30 modified ports, and that the addition of the bypass channel
can increase the linearity coefficient from zero to about
2, in the case of dividing flow, and from 4 to about 8, in
j the case of combining flow.
As discussed above with respect to the basic port,
35 the details of the structures for the modified hydrodynamic
ports can be specified according to the invention by
persons skilled in the art of hydraulic design. Included
~j in this work is the selection of the capacities and the
- 28 -
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. ~. .~ . .
132~gl6
configurations of the bypass channels so that they provide
significant enhancement of the second important property as
discussed, while not impairing significantly the third
important property, namely, the offering of relatively
small resistances to large branch flows in severely
abnormal or accident conditions.
It is an important feature of the modified
` hydrodynamic port depicted in Figure 3, that the effects at
work in opposing the departures from the zero net
accumulative pressure requirement (giving minimal exchange
flow between the primary circuit and the reserve coolant
tank) are present immediately, i.e., in the order of the
through-port transit time. Therefore, the time required
for the corrective processes within the hydrodynamic ports
furnished with bypass tubes does not limit the speed of the
response to the inadvertent variations of normal operating
conditions. The immediacy of response is of particular
importance in counteracting the influences of periodic
inertial forces that are commonly imposed on mobile
reactors, in which case the effective acceleration due to
gravity, which determines the elevation pressures operating
within the system, may be subject to severe time variation.
The discussion to this point has focused on two
principal features of the invention which provide for the
restraint of exchange flow between the primary circuit and
the reserve tank in the absence of a physical barrier
! separating the two. These features depend on (i) thecorrect specification of the hydrodynamic ports within the
system to which they are applied~ in order to fulfil the
zero net accumulative pressure requirement in the virtual
elimination of exchange flow driving pressures when system
parameters are at their normal operating values, and (ii)
the appropriate detailed design of the hydrodynamic ports
which, in either the basic or modified version, tend to
counteract exchange flow arising when the system parameters
deviate from normal. According to the invention, these
features may be embodied in a variety of reactors, of which
one example has been described with reference to Figure 1.
29 -
132`69~
- For many applications of such reactors, these two features
alone may be sufficient It is recognized, however, that
without further measures to limit exchange flow, residual
levels of such flow may persist at levels unacceptable in
some applications.
The principal reasons for the persistence of
exchange flows, in spite of the two features mentioned, are
two-fold. The first is port related and results from the
fact that a finite amount of local exchange flow will
; 10 always be present in ports defined in the manner of either
Figure 2 or Figure 3. The local exchange flow is the
result of small internal pressure differentials caused by
both the local dynamic and elevation pressure effects.
Such differentials may be experienced in progressing
longitudinally from one slot to the next, or in sampling
pressures while progressing vertically across the port
particularly when the axis is oriented more or less in line
with the horizontal. ~hese effects give rise to local
circulation of coolant into and out of the slots, even when
` 20 the net branch flow in the port may have been cancelled due
to the above-described design measures. The effects are
, aggravated by the inevitable hydrodynamic roughness near
the slots and plates as viewed from the port interior, and
the severe temperature differential in the vicinity of the
~, 25 inescapable thermal interface between the circulating
coolant and the reserve coolant.
The second reason for persistent exchange flow is
system related. Even when the earlier-described design
measures succeed in either minimizing the pressure
differentials of the system that drive exchange flows, in
the first instance, or counteracting such pressure
;; differentials as they may arise, in the second, such
measures cannot in principle eliminate the residual
;~ exchange flows absolutely. Therefore, in reactor
applications which require that exchange flows be
restricted to very small values, or indeed virtually
eliminated (as required when shutdown absorber is present
in the reserve coolant), a supplementary mechanism to
- 30 -
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` 132691~
complete the suppression of exchange flows within the
` reactor system is necessary. An active mechanism, that
regulates one or more of the main system parameters to
- eliminate instrumentally detected or anticipated residual
; 5 exchange flow within a cooling system constructed in
accordance with the invention as described to this point,
may be devised by those skilled in the art of thermal-
hydraulic control. For reasons of reliability,
.! effectiveness, universal applicability, and harmony with
the basic objectives of the invention, however, a passive
mechanism is preferred to the active one.
According to the invention, the phenomena of local
exchange flow associated with each hydrodynamic port of the
system, and the exchange flows residual in the reactor
cooling system as a whole, are addressed through the
introduction of a single attachment applied to each
hydrodynamic port of the system. The attachment, an
example of which is shown in Figure 4, is referred to
; herein as the anti-convective shroud. The anti-convective
shroud is a passive device with the capacity to suppress
the local exchange flows in the port to which it is
attached, and to accumulate incipient residual exchange
.~ flow within the system flow in such a manner that the
pressure differentials tending to drive the residual flows
25 are completely compensated.
Referring to Figure 4, the anti-convective shroud
30 is exemplified as an attachment to the hydrodynamic port
made up of the components 21, 22, 23, 24, and 25. These
components were previously identified in the description of
30 an identical example of a hydrodynamic port, namely, the
basic version shown in Figure 2, but without an anti-
convective shroud. Such an attachment may be fastened,
with similar expected advantage, to any one of a variety of
versions of hydrodynamic port, including, for example, the
l 35 modified version incorporating bypass elements as shown in
3 Figure 3, also described above.
;~ The anti-convective shroud 30 is essentially a hood
which opens downward and encloses the entire structure of
.~
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` .
,,.;,', ` ' . - , ' ,
.:., , .. . . : .. .. . .. ....
132~
:`
the hydrodynamic port to which it is attached. Apart from
the downward opening, and possible vent hole (not shown)
through the uppermost point in the shroud surface to
eliminate any gases collected, the enclosing surface of the
shroud is complete and terminates in sealed joints at the
flow defining ducts 21 and 22. The anti-convective shroud
has sufficient internal clearance from the port to avoid
; directly interfering with the flow of coolant through the
exchange flow slots 20.
10Under normal operating conditions, the shroud
provides for a largely isothermal, quasi-stagnant zone of
coolant generally enveloping the port. This zone of
coolant continually adopts the approximate temperature of
the coolant flowing axially within the port, by virtue of
the stratification qrid made up of vanes 31 placed
vertically in the downward opening and the thermally
insulating material (not shown) that may be placed on the
remaining surfaces of the shroud. Since the temperature of
the circulating primary coolant, and hence the temperature
of the quasi-stagnant zone, is considerably hotter than
that of the reserve coolant, a large thermal gradient must
exist in the vicinity of the downward opening. The purpose
of the stratification grid is to inhibit the formation of
local convective circuits in the vicinity of the opening,
thereby facilitating the orderly formation and maintenance
of thermally stratified layers of coolant ranging from the
nearly primary circuit temperatures at the top of the grid,
to typically reserve coolant temperatures at the bottom of
the grid. Regardless of the orientation of the port axis
i 30 in a particular reactor application, the specific
configuration of the anti-convective shroud must be
designed so that the alignment of the stratification grid
is nominally horizontal, in order to best support the
thermal stratification of which one isothermal plane is
identified by the numeral 32 in Figure 4.
It was implied above that, in the absence of
special measures such as the anti-convective shrouds, local
exchange flow through the slots of a given hydrodynamic
- 32 -
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port would exist as the result of small internal pressure
differentials caused by both the dynamic and the elevation
pressure effects. In the presence of the quasi-static zone
of coolant around the port, however, significant elevation
pressure differentials cannot exist, since the circulating
primary coolant inside the port and the coolant of the
quasi-static zone outside and along the port are now
essentially of the same temperature. Moreover, while the
dynamical pressure differences occurring within the port
may result in some local exchange flow through the slots of
the port, such exchange generally involves coolant of the
single temperature and is contained within the shroud.
Only if the local exchange is of such vigour as to induce
disruptive secondary exchanges through the stratification
grid will a net exchange of primary coolant with the
reserve tank occur. Such contingencies may be guarded
against in the proper detailed design of the anti-
convective shroud, carried out by persons skilled in the
art of hydraulic design. Thus, the anti-convective shroud
can be viewed as a passive device with the capacity to
suppress the local exchange flows caused by dynamic and
elevation pressure differentials in the port to which it is
attached.
It was also implied above that, in the absence of
special measures such as active primary parameter controls,
or the passive compensating effects of the anti-convective
shrouds, some residual exchange flows would persist in the
reactor cooling system as a whole. Key points of behaviour
of the anti-convective shroud which enable it to play a key
role in limiting the extent of such residual exchange flow
are described in the paragraphs immediately following.
JjConsider the vertical temperature profile of the
coolant located in the stratification grid 31 of the anti-
convective shroud exemplified in Figure 4. A transition in
i~35 coolant temperature will be observed in passing from the
top of the grid where the temperature is that of the local
primary coolant, to the bottom of the grid where the
temperature is nearly that of the reserve coolant. In a
33
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13269~
- somewhat idealized example in which a reactor cooling
system incorporating such a port is operating with
absolutely zero net exchange flow passing through the port,
the temperature profile in the grid may be characterized by
the location of the median temperature plane located, say,
midway between the upper and lower extremes of the grid and
corresponding to the isothermal plane 32 identified in
Figure 4. If, however, the sy~tem should experience
changes in operating parameters such that a small branching
flow of primary coolant enters the anti-convective shroud,
then a net downward displacement of the median temperature
' plane in effect occurs. This shift in the elevation of the
median temperature plane amounts to a lengthening of the
column of hot coolant extending below the port. As this
` 15 column constitutes a component of the passive cooling
circuit, the lengthening of this column contributes to the
change in the net elevation pressure evaluated around the
passive cooling circuit. Thus, the branch flow initiated
by a disturbance in the operating parameters contributes to
the creation of a component of driving pressure in the
passive cooling circuit.
The remarkable feature of the anti-convective
shroud in the configuration indicated is that, when such a
shroud is added to each of hydrodynamic ports used in the
cooling system arrangement exemplified in Figure 1, the
induced change in the net elevation pressure induced in the
, manner described above is always in the sense which tends
ij to cancel the initiated branch flow, regardless of its
cause. This observation applies regardless of whether it
is the inlet port or the outlet port that is involved, or
whether the initiating disturbance gives rise to an inflow
from the primary circuit into the shroud, as in the above
, example, or to an outflow from within the shroud into the
primary circuit, in which case the median temperature plane
shifts up rather than down in effecting the correct
compensation.
It will be obvious that the maximum disturbance in
the primary system that can be fully compensated by the
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i326916
interaction of coolant with the anti-convective shroud will
depend, to a large extent, on the depth of the
stratification grids for both the inlet and outlet ports,
(although the performance of the system is not necessarily
limited by the depth of the shallower grid of the system).
It may be clear that, if the median temperature plane in
responding to primary disturbances should be moved
vertically within the grid to a level where, for practical
purposes, it approaches the upper or lower limit of a given
stratification grid, further compensation may not be
forthcoming from that port assembly. However, the partial
compensation already achieved at that point will be
sustained as long as the temperatures of the coolant
residing over the full extent of the grids are sustained,
as they surely will be due to the small, but persisting
residual exchange flows passing through the quasi-static
; zones and the grids. For a given magnitude of initiating
i disturbance, the persisting exchange flows will be much
.!~ smaller in magnitude than would exist in the absence of the
compensating behaviour of the described anti-convective
shrouds and incorporated stratification grids.
~ In summary, moderate disturbances in the primary
; system operating parameters cause the anti-convective
shrouds to accumulate residual exchange flows within their
quasi-stagnant zones in such a way that the resulting
elevation pressure differentials cancel the original
tendency for the exchange flow. Thus, for a wide range of
normal operating scenarios, residual exchange flows may be
virtually eliminated. For more extreme disturbances, the
induced exchange flows may persist, but at flow rates that
have been reduced, by the presence of the anti-convective
shrouds, to persisting values that may be acceptable in
many reactor applications.
The particular design of the anti-convective shroud
enabling the cooling system to meet the requirements of a
given reactor application may be specified by those skilled
in the art of thermal-hydraulic design. In addition to
' considering the adequacy of the anti-convective shrouds in
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1326916
limiting both local and systemic residual exchange flows,
the design process must take account of the impact of the
presence of shrouds on the performance of the passive
cooling system. In each case, both the anti-convective
shroud and the stratification grid must be dtasigned to
neither significantly impede the operation of the passive
cooling system operating in the shutdown mode, nor
adversely affect the interaction of the passive circuit
with the cooling system as a whole, in accident scenarios.
A second embodiment of the invention is shown in
',J the sectional elevation of the reactor plant of Figure 5.
;~ The cooling system of this plant has all of the essential
attributes, and performs all of the basic functions,
ascribed to the reactor plant of Figure 1. In addition,
the embodiment shown in Figure 5 includes two anti-
;~ convective shrouds 30 attached to the inlet and outlet
hydrodynamic ports, 7 and 8, respectively. These shrouds,
and their associated stratification grids 31, enhance the
basic performance of hydrodynamic ports in their prescribed
'20 roles according to the invention, in the manner described
;~in some detail with reference to Figure 4. Therefore,
iduring normal operations, the reactor plant depicted in
Figure 5 may be expected, even in the presence of
significant operating disturbances and in the absence of
actively compensating control equipment, to exhibit verylittle exchange flow between the primary circuit, as
previously defined, and the reserve coolant tank 14.
Building upon the basic functions of the anti-
convective shroud and the stratification grid, as described
with reference to Figure 4, the compensating actions of
these attachments on a reactor system as a whole, in
response to inadvertent variations in the normal operating
parameters, may now be readily understood in terms of the
example provided in Figure 5.
Suppose the reactor plant of Figure 5 is operating
initially under a set of nominal operating parameters which
satisfy the zero net accumulative pressure requirement for
zero exchange flow. The criterion for meeting this
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13269~
requirement is that the change in the dynamic pressure,
associated with a constant mass flow passing from port 7 to
port 8, must equal the deficit by which the net elevation
- pressure, assessed accumulatively around the passive
cooling circuit, fails to match the resistive pressure
losses associated with the primary mass flow through the
core 1. The important point to be noted here is that, in
assessing the net elevation pressure in terms of
essentially the weight difference between a predominantly
hot and a predominantly cold column, the height of both
such columns is now effectively the vertical distance Lg
between the median temperature planes falling at
approximately the mid-elevation points in the
stratification grids of the two ports, 7 and 8. ~g is now
the operative hydrostatic height in satisfying the zero
accumulative pressure requirement, as opposed to the height
L which, in the absence of shrouds and grids as for Figure
;, 1, was taken as the operative height. A further important
point is that the magnitude of Lg can vary in response to
the receipt of small exchange flows within the shrouds, and
such variation is the basis of the mechanism for
compensating inadvertent departures of the cooling system
from the zero net accumulative pressure requirement, which
give rise to these flows initially.
Now suppose that a tendency develops within the
operating system to produce ingress exchange flow, in which
case an incipient combining branch flow occurs in port 7
and a corresponding dividing branch flow occurs in port 8.
In the embodiment of the invention depicted in Figure 5,
30 such a tendency may result, for example, from an
: inadvertent reduction in pump speed, an increase in
circulating primary coolant temperature, or an effective
:~ (inertial) increase in the effective gravitational constant
' due to upward acceleration of the reactor. Aq the slight
ingress exchange flow progresses, the median temperature
plane in the grid of the inlet port rises and the median
~. temperature plane in the grid of the outlet port falls, in
i; the manner described with reference to Figure 4. The
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1326~1~
consequent contraction in ~9 corresponds to the reduction in
the elevation head that is necessary to cancel the cause of
the initiating tendency for ingress exchange flow.
Suppose, on the other hand, that a tendency
develops within the operating system to produce bypass
exchange flow, in which case an incipient dividing branch
; flow occurs in port 7 and a corresponding combining branch
flow occurs in port 8. In the reactor plant depicted in
Figure 5, such a tendency may result from inadvertent
10 parameter variations, such as an increase in pump speed, an
increase in reserve coolant temperature, or an effective
decrease in the vertical component of Lg, if the reactor
takes on a slightly inclined orientation. As the slight
bypass exchange flow progresses, the median temperature
15 plane in the grid of the inlet port falls and the median
temperature plane in the grid of the outlet port rises,
again in the manner described with reference to Figure 4.
~ The consequent protraction of Lg corresponds to the increase
3 in the elevation head that is necessary to cancel the cause
20 of the initiating tendency for bypass exchange flow.
The foregoing operational scenario shows, by
example, how the cooling system of the reactor depicted in
Figure 5 provides, by virtue of the arrangement of the
hydrodynamic ports, anti-convective shrouds and
25 stratification grids in accordance with the invention, the
process-inherent means for regulating the cooling system
during normal reactor operations in such a way that the
potentially undesirable side-effects of the passive
shutdown system on normal operations are automatically and
30 continuously curtailed. This situation contrasts with that
:
for other reactors having passive shutdown cooling systems
~ which, as in the present invention, are deployed by
;~ process-inherent means. Such other systems depend on
sophisticated sensors and active control systems to avoid
` 35 undesirable side-effects.
In accordance with the invention, the specification
^ of details relating to the capacities of the various
component parts of the cooling system shown in Figure 5,
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.... .. . .
~` 13269~6
when adapted to a particular reactor design and
application, may be determined by those skilled in the art
and science of thermal hydraulics. Since no coolant
` pressurizer is shown as part of the cooling system in
Figure 5, it may be assumed that pressure and inventory
control equipment, in this particular embodiment, are a
part of the primary cooling circuit lying outside the
reserve coolant tank and, hence, not shown. The location
and design of such equipment is in the nature of elements
to be specified by a skilled designer to suit a particular
application.
The embodiment shown in Figure 5 also exemplifies
the adaptation of the invention to applications which place
severe restrictions on the size of the plant, while
~; 15 requiring passive cooling to become available as needed
through inherent processes. In addressing such
requirements, the vertical inter-port distance L (shown in
Figure 5) has been made shorter than that which would be
required to support the normal through-core flow rate by
natural convection alone. Consequently, the zero net
accumulative pressure requirement, as stated with reference
to the passive cooling circuit, is met by making the axial
flow area A2 f the outlet port 8 somewhat larger than the
area Al of the inlet port 7. Moreover, in the interest of
lessening the height and width requirements of the plant,
the inlet port and the inlet duct are arranged
horizontally, while the outlet port is oriented vertically
- with its anti-convective shroud and stratification grid
-. arranged symmetrically about the port axis. It should be
noted that the individual components depicted in Figure 5
are not drawn to scale and no relative size or
configurational relationships among the various components
should be inferred literally from the drawing. The
detailed design of such components, including the sizing
for a particular reactor application, may be carried out by
those skilled in the art of thermal-hydraulic design. The
drawing is intended to convey, however, representative
examples of how the basic principles of the ports and their
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132691~
attached shrouds offer flexibility in the design of
reactors for which there is a stipulated requirement of
passive shutdown cooling that becomes available, as needed,
by process-inherent means.
5The passive cooling circuit of the reactor of
Figure 5 is readily seen to include the anti-convective
shrouds 30 and the stratification grids 31 among its
' components. Accordingly, the thermosyphon hot leg,
operative during convective core coolant exchange with the
lo reserve coolant, consists of the upper plenum 3 and the
outlet duct 5 leading to the outlet port 8, plus the upper
shroud 30 and grid 31. The effective vertical height of
the hot leg is therefore approximately equal to ~, less the
vertical distance from the middle of the upper port down to
the median temperature plane of the upper grid.
Correspondingly, the effective vertical height of the cold
leg is approximately equal to Lg, less the su~ of Ll and the
vertical distance from the ~iddle of the lower port down to
the median temperature plane of the lower grid. The layers
of insulation 9 shown attached to the shrouds, as well as
to the other components of the primary circuit, are
consistent with the roles of the shrouds and grids as
~components of the passive cooling circuit. The
idetermination of the optimal dimensions to be adopted in
achieving adequate thermosyphon head, while conforming to
imposed space limitations in a particular reactor design
and application, may be performed by those skilled in the
art of thermal-hydraulic design.
,In addition to accounting for both the potentially
adverse effects of the inadvertent departures of operating
parameters from their nominal value~, and the limited space
available for the reactor plant in some applications, as
~ust described, the embodiment of the invention shown in
Figure 5 also exemplifies applications in which vertical
access to the reactor core must remain available for
jpurposes of shielding placement, control and shutoff
mechanism deployment, experimentation (as in the case of a
research reactor), and refuelling and general maintenance.
- 40 -
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132691~
Access to the top of the reactor core is retained in the
-design of the subject reactor primarily by offsetting the
outlet duct 5 immediately as it leads away from the outlet
plenum 3. This arrangement is readily accommodated by the
- 5 flexibility of design yielded by the reactor cooling system
in accordance with the invention, and leaves almost the
entire space above the core open for the placement of
equipment, instrumentation, or experimental apparatus, at
the discretion of the designer.
10The reactor plant as depicted in Figure 5 shows
typical utilization of the reactor access thus made
available. For example, the reactor access tubes 40 may be
used as control or shutdown rod guide tubes, or for in-core
flux monitoring devices, in specific reactors requiring
such facilities. The access tubes may also be used in
research reactors for the insertion of irradiation target
samples into the reactor core, or as beam tubes for the
formation and extraction of neutron or gamma-ray beams.
The reactor vessel 46 of the plant in Figure 5 is shown in
an abbreviated form. In this example, the component 41
-serves as a demountable, thermally insulated baffle which
is essential during normal operation to prevent the primary
coolant from mixing with the reserve coolant, but is
obviously not exposed to very large pressure differentials.
In some applications, combining the baffle with reactor
core shielding may be advantageous.
i~Access for servicing such facilities, as well as
for performing refuelling operations, is obtained through
the hatch 15 in the case of a closed or pressurized
reactor, or simply through the open surface of the coolant
in a pool-type reactor. For either type of reactor, it may
be advantageous in the design to abandon component 41 and
to extend the rim of the reactor vessel 46 to a much higher
level than shown, or even to the level where it joins with
the hatch (in a closed reactor), or breaks the coolant
surface (in a pool-type reactor). Such arrangements may
provide for continual access to the core while the reactor
is operating, without interfering with the operation of the
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6 9 1 ~
cooling system.
A third embodiment of the invention is shown in the
sectional elevation of the reactor plant of Figure 6. The
cooling system of this plant has all of the essential
attributes, and performs all of the basic functions, which
were generally ascribed to the reactor plant of Figure 5.
In addition, the embodiment shown in Figure 6 includes a
multiplicity of inlet and outlet hydrodynamic ports, 7 and
8, each coupled hydraulically to a corresponding inlet or
outlet plenum, 2 or 3, by means of a corresponding inlet or
;~outlet duct, 4 or 5. Leading in directions generally away
from the reactor core, the inlet and outlet ports couple
also to their respective inlet and outlet manifolds, 36 and
37. The manifolds, in turn, couple to the inlet and outlet
conduits, 11 and 10, which connect with the remaining
components of the primary circuit which, as in the first
and second embodiments, lie outside the reserve coolant
tank 14.
The arrangement of components in the third
embodiment exhibits general s y etry about the central axis
shown in Figure 6. When the reactor is at rest in its
normal physical orientation, this axis is aligned with the
vertical. Thus, the several inlet ports are normally found
at a common elevation, as are the inlet ports. The arrows
show the flow pattern of the primary coolant during normal
operation, in which case the total upstream flow to the
core is shared, more or less equally, among the several
` inlet ducts, and the downstream flow, correspondingly, by
the several outlet ducts.
30Each of the hydrodynamic ports shown in Figure 6
is fitted with an anti-convective shroud 30 and a
stratification grid 31, which are of designs similar to,
, but not necessarily identical to, those of the second
embodiment. Also shown in Figure 6 are such reactor
components as fuel elements 18, a neutron reflector 19,
gamma shielding, 16 and 17, a neutron shielding tank 38,
and various components, 42, 43, 44, 45 and 39, relating to
reactivity control. These components do not have a direct
- 42 -
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role in the functioning of the reactor cooling system
` shown, but are included in the embodiment to demonstrate
how various reactor components and a reactor cooling system
made according to the invention may co-exist in an actual
reactor.
The plant shown in Figure 6 typifies applications
in which the reactor cooling system, as well as performing
in the manner described with reference to the plants of
Figures 1 and 5, also meets the re~uirements of (i) passive
shutdown cooling being always available, regardless of the
orientation of the reactor with respect to gravity, and
(ii) the normal operation of the reactor being tolerant of
various types of dynamic motion and net displacement, both
rotational and translational, imposed within specified
limits on the reactor plant as a whole. The basic
operational details whereby these requirements may be met,
according to the invention, are now given with reference to
the reactor plant depicted in Figure 6 and defined in the
preceding paragraphs. In proceeding, an understanding of
the operation of the first and second embodiments of the
invention is presumed.
As already indicated, the primary coolant of the
; reactor in Figure 6 is circulated during normal operation
according to the pattern depicted by the arrows. The flow
to the core 1 is delivered in more or less equal shareæ by
the several inlet ducts 4, and each share is received by a
duct as an axial flow transmitted by the corresponding
inlet port 7. Similarly, the flow out of the core is
received in more or less equal shares by the several outlet
ducts 5, and each share is transmitted by a duct to become
an axial flow in the corresponding outlet port 8.
Normal operating conditions, in the application
environments for which the embodiment shown in Figure 6 is
eminently suitable, include an alignment of the central
3S axis of the reactor to coincide more or less with the
, vertical, and an absence of motion of the platform on which
,3 the reactor is mounted. These stipulations for normal
~ operations are in addition to those relating to the key
.,
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13269~ ~
design parameters, including the mass flow rate, the
circulating coolant temperature profiles, and the reserve
coolant temperature, which are chosen, as for the first two
embodiments, in the optimization of the plant's basic
thermal efficiency.
The operation of the reactor of Figure 6, under
normal conditions as defined above, may be described in
terms essentially the same as those used previously in
describing the reactors of Figures 1 and 5. For example,
any tendency towards exchange flow between the circulating
primary coolant and the reserve coolant is minimized for
all three reactors because the axial flow areas of the
hydrodynamic ports are chosen so that the zero net
- accumulative pressure requirement is fulfilled. This
requirement may be restated more aptly, however, for the
specific case of the multiple port configuration of Figure
6: At normal operating conditions, the net pressure from
all effects accumulative around each of the many
identifiable passive cooling circuits, and tending to
support net flow around any such circuit, must be
substantially zero. A passive circuit, in this context,
can be defined as any closed path in the system which
`includes the reserve coolant tank, and the two ducts
connecting with any arbitrarily chosen pair of hydrodynamic
ports, including pairs of inlet ports, pairs of outlet
ports, and pairs formed of any combination of one of each.
As in the case of the second embodiment shown in Figure 5,
the sizing of the port areas for the third embodiment shown
in Figure 6 is based on Lg, the operative hydrostatic height
now defined as the vertical distance between the inlet and
'!
outlet groups of median temperature planes 32, as well as
the nominal values of the key operating parameters.
Either of the two versions of hydrodynamic port
lpreviously described in reference to Figures 2 and 3, or
;~35 indeed a further version, may be chosen for application in
the reactor of Figure 6, depending on the degree of
resistance needed against the tendency toward exchange
flow. In the meanwhile, the anti-convective shrouds 30
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13269~
eliminate or isolate the effects of local exchange flows in
the multiplicity of ports, and the self-adjusting
capability of the median temperature planes 32, occurring
within the various stratification grids 31, serve to
automatically eliminate, or limit, incipient exchange
flows. As already described in considerable detail with
reference to the reactor of ~igure 5, such exchange flows
are brought about by inadvertent departures of the normal
operating parameters from normal. Additional sources of
tendency toward exchange flow arise in the configuration of
the passive cooling system of Figure 6, however. These may
be due to the presence of minor non-symmetries in the
manufacture of the various hydrodynamic components relating
to the multiplicity of ports and associated flow paths.
The disposition of this source of exchange flow tendency
will be addressed later, along with the effects of various
types of dynamic motion and net displacement imposed on the
reactor plant by its operating environment.
The multiplicity of passive cooling circuits
addressed in connection with the maintenance of ideal
operating conditions under normal (upright) conditions, are
also the key to the main feature of the third embodiment of
the invention, namely, the continuous availability of the
passive cooling function, regardless of the orientation
which the reactor may assume with respect to gravity. The
operation of the provisions for passive cooling may be
readily understood with reference to the plant depicted in
Figure 6. It is assumed for the present discussion that
the axial flow areas of the ports either remain the same or
increase, in passing from the inlet to the outlet port.
Consider the operation of the passive cooling
system, following the failure of the circulating pump and
j the general stoppage of flow in the part of the primary
circuit external to the reserve coolant tank, while the
reactor, for the time being, remains upright. Under these
conditions, the mass flow through the core reduces to the
level which can be supported by natural convection in the
passive cooling circuits. In this, more or less equal
- 45 -
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1326~
flows from the reserve coolant tank enter the inlet ports
through the shrouds of the inlet ports, pass through the
core, and emerge into the reserve tank as equal flows
through the shrouds of the outlet ports. Apart from the
multiplicity of flow paths, the general behaviour of the
system under these conditions is essentially the same as
for the reactors of only two ports as in the second
embodiment of Figure 5. As for that reactor, the rate of
core cooling remains adequate, in spite of the reduced
flow, because the core inlet temperature is now determined
largely by the relatively low reserve coolant temperature.
If, in the meantime, the reactor becomes nominally
shutdown, the demands on the passive cooling system are
further reduced and the reactor remains adequately cooled
for an indefinitely long period.
Consider now the case in which the reactor's
physical orientation departs to an arbitrary degree from
the upright position. It may be inferred from Figure 6
that, regardless of any such orientation, a variety of
viable passive cooling circuits is always available for
natural convection. Such circuits were defined, in the
~ discussion on the requirements for zero exchange flow under
i normal operating conditions, as including the reserve tank,
' and two of the ducts connecting with any pair out of the
multiplicity of hydrodynamic ports. Depending on the
degree of disorientation from the normal, the various
passive cooling paths may operate in concert with each
other and carry different components of the total flow
3~ through the core. Certain aspects of the detailed design
of the cooling system require special attention in respect
of passive cooling being available at all reactor
orientations. These include the fact that the majority of
the passive coolant flow in the core will be, for a range
of reactor orientations, in directions other than parallel
to the general orientation of the fuel elements, assuming
j a standard fuel arrangement. Another aspect requiring
j attention is the fact that the anti-convective shrouds,
which are designed in the first instance to optimize the
- 46 -
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;~ resistance of the cooling system to disturbances occurring
` during normal operation, give rise to non-symmetries among
the various passive cooling circuits. It may be noted,
however, that the elongation of the shrouds and the
associated grids, a desirable measure for enhancing normal
operation, does not reduce the convective driving head for
~any of the passive cooling circuits more severely than it
;does in the (worst) case of the reactor in the upright
position. These and similar aspects of design may be dealt
with appropriately by persons skilled in the art of
thermal-hydraulic design.
As just described with reference to Figure 6, the
main feature of the reactor plant, showing the third
embodiment of the invention, is the availability of
effective passive cooling at all times and in all
circumstances, including all physical orientations of the
reactor plant. Such facility for passive cooling would be
of limited value, however, if the presence of such facility
were to degrade siqnificantly the efficiency of normal
operations, or if the measures required to avoid such
degradation were to compromise the operability of the
plant. Moreover, the operating environment that requires
a passive cooling capability at all physical orientations
is likely to be a dynamic environment ~as on board ship)
for which the effects on normal operation have to be
.accommodated, in addition to the effects of inadvertent
variations in the normal operating parameters as already
discussed for all three embodiments. The manner in which
the normal operation of the reactor of Figure 6 is tolerant
of the various types of dynamic motions and net
displacements, is now described.
Consider first an incremental rotational
displacement of the reactor in Figure 6, operating
'jinitially with an upright orientation under ideal normal
l35 conditions of no exchange flow. Assuming that the rotation
,.is clockwise about an axis corresponding to a normal to the
plane of the paper of Figure 6, the distortion of the
original flow symmetries due to the changes in elevation
;~
47 -
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132691~
heads results in a dividing branch flow at the left-most
outlet port, and a combining branch flow at the right-most
inlet port. The resulting movements of the median
temperature planes in the grids of the two ports, due to
the accumulation within the shrouds of minor amounts of
exchange flow, causes the value of Lg, as it pertains to the
two ports in question, to tend to shorten toward its
original value, even though an increased vertical
separation has occurred between these two ports.
;10 Similarly, the occurrence of combining branch flow in the
right-most outlet port, and dividing branch flow in the
left-most inlet port, tends to lengthen the value of Lg, as
,it pertains to these two ports, to its original value, even
thouqh the two ports now physically have less vertical
separation. Thus it may be seen for this simple example of
rotational displacement that, by the interaction of
incipient residual exchange flows with the anti-convective
shrouds and stratification grids, the parameter Lg self-
:adjusts within the incrementally rotated system to satisfy
the hydraulic requirements for zero exchange flow with
respect to all four ports simultaneously. On extending
'this line of reasoning to more general circumstances, it
may be concluded that the parameter Lg will automatically
adjust to the value satisfying the zero flow requirement
with respect to all hydrodynamic ports in the systemsimultaneously, including those not visible in Figure 6,
even when the plant is subjected to arbitrarily complex
rotational displacements.
At certain limits of reactor inclination, it is
apparent that the range of useful self-adjustment of Lg in
the inhibition of exchange flow becomes exhausted. Such a
limit occurs when the plant inclination becomes so great
that the stratification grids of two diametrically opposite
ports can on longer both intersect a single horizontal
plane. It is apparent, also, that the said limit of
inclination in radians is approximately equal to the ratio
of the (vertical) grid length to the diametrical separation
of the said ports. ~herefore, for a given basic reactor
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confiquration, the range of inclination within which the
reactor plant can remain relatively free of exchange flow
is proportional to the length of the stratification grid in
the anti-convective shroud of each hydrodynamic port.
While lengthening the grids in the design of a reactor for
a given mobile application would increase the reactor's
immunity to exchange flow, the indiscriminate use of such
measures could compromise the effectiveness of the
reactor's passive cooling system. The reduced
effectiveness, caused by the effective shortening of the
thermosyphon hot leg, would be the most severe for a nearly
upright reactor, as suggested earlier. The task of
optimising the reactor design for a particular application
- environment, in which an optimal balance between immunity
to exchange flow and the effectiveness of the passive
cooling is achieved, may be performed by those skilled in
the art of hydrodynamic design.
It was stated earlier that any tendency towards
exchange flow between the circulating primary coolant and
the reserve coolant may be minimized for all three
embodiments, because the axial flow areas of the
hydrodynamic ports are chosen so that the zero net
, accumulative pressure requirement is fulfilled. This
~, statement implies that, in a most likely configuration of
`1 25 the third embodiment of the invention, i.e., one in which
3 the multiplicity of ducts and ports are arranged
i symmetrically about the principal axis, a certain quality
of manufacture is achievable. The quality of manufacture
, would have to be such that the total mass flow would be
30 shared identically by all ducts, and the specified axial
flow areas of the hydrodynamic ports would precisely
~A materialize in the manufacturing process, to the extent
that the zero net accumulative pressure requirement would
be truly satisfied simultaneously for all passive cooling
35 paths to be found in the system. Since it may be
impractical, however, to manufacture the hydraulic
,~ equipment to such precision, it may appear that some form
of post-assembly adjustment might be in order. Mechanisms
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for such adjustment might be suggested by those skilled in
both thermal-hydraulic and mechanical design. However, it
should be pointed out that, through the line of reasoning
used previously to show that the anti-convective shrouds
and stratification grids automatically adjust system
elevation pressures to inhibit exchange flow in the face of
disturbances in the primary parameters and changes in
reactor orientation, it may be shown that tendencies toward
exchange flow arising from the practical limitations in
;10 component manufacturing are likewise compensated, within
reasonable limits, by the self-adjusting properties of the
shroud-equipped hydrodynamic ports. It follows, as a
corollary, that any minor changes in the geometry of the
hydraulic equipment, due to corrosion, deposition, or
deformation associated with aging, are likewise compensated
automatically.
In consideration of the effects of dynamic motions
on the integrity of normal reactor operation, two types of
motion are important in the operating environment
~20 anticipated for the reactor of Figure 6, depicting the
-~jthird embodiment of the invention. The types of motion are
(i) rotational oscillations of limited amplitude about
horizontal axes, and (ii) vertical translational
oscillations of limited magnitude.
Consider first the rotational oscillations. Here
we consider only rotations about horizontal axes located
approximately at the level of the reactor core. (The
1 effects of oscillations about axes at other levels may be
3 considered approximately in terms of a superposition of a
translational component of motion on a rotational component
about a horizontal axis through the core.) Such rotational
: oscillations, typified as a rocking motion about an axis
;, through the core and normal to the paper in the
representation in Figure 6, can be considered to
potentially create exchange flow in two ways, i.e., through
inertial (accelerative) effects and through the
displacement effects, which will be dealt with in turn.
Since the rotations are defined to be about the
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core, the forces of centrifugal acceleration on the coolant
in the various ducts, 4 and 5, will tend to cancel.
However, the scooping effect, as the opening of an anti-
convective shroud 30 accelerates into the reserve coolant
35, tends to produce an exchange flow of the "inlet port
to inlet port" variety, and similarly for the outlet portæ.
While such an inertial effect persists, an incipient
exchange flow actually occurs and results in the
displacements of the median temperature planes 32 in
directions that tend to cancel the said inertial effect.
Since the inertial effect reverses direction for the second
half of the oscillation period, the displacement of the
median temperature planes 32 will be reversed. If this
reversal takes place before the ranges of travel in the
stratification grids 31 become exhausted, as will be the
case ideally, no net exchange flow will be experienced as
a result of the scooping effect. The design factors which
tend to minimize the probability of exchange flow due to
the scooping effect, as just analyzed, are (i) high
resistance of the ports to branch flow at nominal operating
conditions, by method such as those that have been
described earlier in accordance with the invention, and
~(ii) a reasonable capacity of the anti-convective shrouds
-~to accumulate incipient exchange flow~ Such factors may be
-'25 appropriately addressed in the detailed design process by
,those skilled in the art of thermal-hydraulic analysis.
The strictly displacement effects of rotational
oscillations may be analyzed using a similar line of
reasoning to the one used previously in addressing the
effects of net (static) displacements. In the case of very
slow oscillations, the very same criterion for inhibiting
exchange flow applies as for static displacements, namely,
the maximum rotational displacement from the upright
orientation of the reactor in radians should not exceed the
static limit, namely, the ratio of the length of the
stratification to the diametrical separation of the outlet
or the inlet ports. As the frequency of the oscillations
;~ speeds up for a specific reactor configuration, however,
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there will be an increase in the maximum amplitude of
oscillation that can be allowed before a net exchange flow
occurs, provided that the mid-position of the oscillations
coincides with the reactor upright position. The
i 5 permissible amplitude increases because the incipient
exchange flow rate is limited by the resistance of the
ports to branch flow at the nominal operating conditions,
and the reverse swing in each oscillation causes a reversal
of the incipient exchange flow, before the available range
travel of the median temperature plane in each port becomes
exhausted. It follows, therefore, that the period of
oscillation at which dynamic amplitudes may be allowed to
exceed the maximum permissible static displacement
corresponds approximately to twice the time required for
the incipient exchange flow to displace the median
temperature plane by half the length of the associated
grid, when the inclination of the reactor is at the static
~'~ limit. For shorter periods of oscillation, the permissible
amplitude increases more or less inversely with the period
;3 20 in a manner that depends on the resistance of the ports to
branching flows at nominal operating conditions and on the
capacity of the anti-convective shrouds.
Finally, the effects of vertical translational
oscillations on the performance of the third embodiment of
the invention are considered. With reference to the
reactor of Figure 6, suppose the reactor operating normally
is subjected to vertical oscillation as may arise if the
$. reactor is mounted on board a ship being subjected to
surface wave motion. The effect of such motion on reactor
` 30 operation may be expressed in terms of a periodic
modulation~of the net elevation pressure evaluated around
;~ any one of the passive cooling circuits which were defined
` previously. It may be clear, therefore, that, for that
`Jl part of the oscillatory cycle corresponding to the ship
being on the crest of the wave, there will be a tendency
for bypass exchange flow in the reactor cooling system. On
~ the other hand, for that part of the cycle corresponding to
,.''~J a depression, there will be a tendency for ingress exchange
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flow in the system. Whether these tendencies lead to
actual periodic exchange flows between the primary circuit
and the reserve coolant depends on a number of factors in
common with those that were considered in connection with
the rotational oscillations. The resistance of the
hydrodynamic ports, 7 and 8, to branch flow, at close to
nominal operating conditions, tends to limit the maximum
rate of accumulation of incipient exchange flow in the
anti-convective shrouds 30 during each cycle. The capacity
of each anti-convective shroud 30 determines the fraction
of the range of the stratification grid 31 that is
traversed by the advancing median temperature plane 32
during the first half of each cycle, before it recedes in
the second half. If the ports, shrouds and grids are
` 15 specified so that, for the entire spectrum of anticipated
oscillation frequencies, wave amplitudes do not exceed the
values which push the median temperature planes beyond the
-~ range provided by the grids in a single cycle, then such
motions need not result in any actual exchange of primary
coolant with the reserve tank. The specification of the
components to meet these conditions may be accomplished by
persons skilled in the art of hydraulic design.
It is in the nature of the hydraulic processes at
j work, in both the cause and the compensation of the various
influences that tend towards exchange flow, that the
corrections generated in opposition to the various causes,
`! according to the invention, will be superimposed
appropriately within the system, and the many normal
~ operating points of concern are thus diminished by the
`;i 30 described passive corrective actions operating in parallel.
The identification of the limiting sources of perturbation
on the normal operating system that dominate the tendency
i for exchange flow, and to make the necessary design choices
to adequately control the effects in a particular
application, may be carried out by those skilled in the art
of hydrodynamic design.
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