Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NUCLEAR REACTOR PLANT
Back~round of the Invention
This invention relates to a nuclear reactor plant
to serve as the heat source component of a power plant for
the production of heat or electricity. The reactor plant
includes means for cooling the reactor core in the event of
impairment of the primary heat transport circuit of the
power plant.
Under normal operating conditions, the reactor
plant serves as a major component in a heat transport
circuit which also contains a pump and one or more heat
exchangers as other major components of the circuit. Such
a circuit is referred to as the primary heat transport
circuit of the power plant.
Under a variable set of abnormal operating
conditions, including the nominal "shutdown" reactor
condition when decay heat continues to be released, the said
reactor draws on a supply of reserve coolant contained in a
tank which is an integral part of the reactor plant. The
consequent exchange of coolant between the reactor core and
the reserve coolant tank provides, in the interests of
reactor safety, auxiliary cooling to the core of the reactor
under conditions of zero or reduced primary flow, loss of
heat sink, or other form of impairment of the primary heat
transport circuit.
As this auxiliary cooling is sustained in the
absence of external power or specific operator action, the
reserve coolant tank together with the other plant
associated with this action is referred to as the passive
cooling system.
The reactor core, the coolant-carrying components
of the primary heat transport circuit, and the said passive
cooling system, together with the coolant itself, are
identified as the primary heat transport system of the said
nuclear power plant.
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1 33 3 94 1
Prior art reactors known as the PIUS* described in
Canadian Patent 1,173,570 (Hannerz); the SECURE* described
in Canadian Patent 1,070,860 (Blomstrand et al.); and the
TRIGA* Power Reactor, described in F.C. Foushee, R.W.
Schleicher, G. Schlueterand and J.S. Yampolsky, "Small Triga
Power Reactors for District Heating", Nuclear Europe, 12
(1984) pp. 33-35, and R.W. Schleicher, "Triga Power System:
A Passive Safe Co-Generation Unit for Electric Power and Low
Temperature Heat", Small Reactors for Low Temperature Heat
Applicationsj IAEA-TECDOC-463 (International Atomic Energy
Agency, Vienna, 1988) pp. 45-55, are power light water
reactors which offer capabilities for defaulting passively
to passive cooling under abnormal operating conditions which
would otherwise tend towards accident conditions. However,
the plant arrangements used are too massive and bulky and
lack the features of the present invention which permit
application in relatively confined spaces. Moreover, none
of the basic principles on which such prior art reactors
depend lends itself to dealing with the problems of ship
reorientation and motion.
~ummarY of Invention
There is provided according to the invention a
nuclear reactor plant comprising a core, means for
controlling the reactivity of said core, radiological
shielding around said core, inlet means to conduct coolant
into said core, outlet means to conduct coolant out of said
core, a reserve coolant tank; and port means in said inlet
means and in said outlet means for conducting coolant
between said reserve coolant tank and said inlet means and
outlet means by convection during impairment of normal flow
of coolant through said inlet and outlet means, and for
permitting a through-flow of coolant through said inlet
means and outlet means during normal operating conditions.
According to one embodiment, the reactor plant
comprises a light-water-cooled reactor core, bracketed by
inlet and outlet coolant plena, each coupled to one or more
ducts leading away from the core at various orientations.
* Trade Mark
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The invention further provides a nuclear reactor
plant comprising: (a) a core; (b) means for controlling the
reactivity of said core; (c) a primary heat transport circuit
for cooling said core under normal operating conditions; (d)
radiological shielding around said core; (e) inlet means in
said primary heat transport circuit to conduct coolant into
said core, said inlet means comprising an inlet header, an
inlet plenum adjacent to said core and a plurality of inlet
ducts leading from said header to said inlet plenum; (f)
outlet means in said primary heat transport circuit to conduct
coolant out of said core, said outlet means comprising an
outlet header, an outlet plenum adjacent to said core and a
plurality of outlet ducts leading from said outlet plenum to
said outlet header; (g) a reserve coolant tank; and (h) port
means in a plurality of said inlet ducts and in a plurality of
said outlet ducts. The port means and inlet and outlet ducts
are configured to provide a plurality of convective paths
maintaining hot legs and cold legs of a thermosyphon to
maintain by convection, and regardless of orientation of said
reactor plant with respect to gravity, the circulation of
coolant between said reserve coolant tank and said core
through said port means during impairment of normal flow of
coolant through said inlet and outlet means. The port means
permit a flow-through of coolant through said inlet means and
said outlet means during normal operating conditions.
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At any given time each duct carries some fraction of the
coolant flow through the core.
At its end remote from the core, each duct
terminates in a special branching device, herein referred to
as a port, designed in any one of a number of ways, such
that:
(i) under normal operating conditions, with respect to
both the inlet and the outlet ducts, virtually all
of the coolant flow in a duct originates directly
from, or passes directly to, the nearest of the
two coolant headers (designated as either the
inlet or outlet header, respectively) which
connect the said reactor plant as a series
component within the primary heat transport
circuit, and
(ii) under conditions departing from normal, branch
flow in the ports can develop such that the
combination of such flows in all of the ports
produces rates of net coolant exchange between the
core and the reserve coolant tank that are
adequate for assuring the safety of the reactor
plant.
The specific type, or principle of operation, of
the said ports applied within the reactor primary heat
transport system is not critical to the nominal functioning
of the said nuclear plant. In fact, such ports, any
associated auxiliary equipment, and the primary heat
transport system that accommodates their effective operation
may be specified as necessary to meet the particular reactor
application requirements. Such specification would
accommodate a prescribed level of safety and include the
design approach to safety, as may be required or deemed
appropriate in a particular reactor application.
The detail of a suitable type of port or its
operation is not included as the subject of the present
invention, as such may be devised by those skilled in the
art of thermal-hydraulic systems design. However, a
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preferred method of providing the port function in a manner
in keeping with a high standard of reactor safety and with
the general objectives relating to the present invention, is
through the principle and application of the specific type
of ports referred to as hydrodynamic ports. Such
hydrodynamic ports and their application in achieving a
range of objectives for nuclear reactors form the subject of
Canadian patent application Serial No. 611,551, filed
15 September, 1989, entitled Nuclear Reactor Cooling System
In the cases of incorporation of such ports as the
hydrodynamic ports, it may be said that the auxiliary
cooling system operation not only is sustained by passive
means but also is initiated by purely passive means.
The said reactor plant further comprises a reactor
vessel, gamma and neutron shields, a primary circuit delay
tank and reactivity control mechanisms, all located together
with such previously mentioned components as the reactor
core, the plena, the ducts, the ports, and the headers
within the boundary of the reserve tank and submerged in the
bulk of the reserve coolant, which is itself thermally
coupled to an ultima*e heat sink. All such components are
arranged in a highly integrated manner, relative to each
other and in accordance with measures taken to restrict (i)
space requirements, (ii) total mass of reactor plant, and
(iii) radiological dose as determined both at the reactor
boundary and in the vicinity of coolant inventory found
circulating through the various primary heat transport
components external to the reactor plant.
An objective of`certain embodiments of the present
invention is to achieve a nuclear reactor plant which (i) in
the case of marine applications, is sufficiently compact in
desiqn to permit its installation as a component of a
nuclear plant within small quarters such as are found on
board a submarine of significantly smaller dimensions and
displacement than are generally characteristic of nuclear
powered submarines; (ii) which presents manageable
operational radiological fields at its boundaries and in its
circulating coolant; and (iii) which accommodates a
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simplicity of design and operation appropriate to peculiar
characteristics required of the plant and its application.
Such characteristics include: (a) a modest plant output,
(b) a very small, non-expert, or non-existent operating
staff in attendance; accommodated by virtue of the basic
reactor concept and its amenability to computer assisted
operation, (c) a high standard of nuclear safety and plant
reliability, and (d) a limited space and weight allowance
for auxiliary equipment.
A further objective of the invention is that the
reactor plant, when installed on an unsteady platform, such
as on board a submarine, be capable of sustaining normal
operation over a sufficiently wide range of motion.
A still further objective is that the passive
cooling system remain functional when plant motion and
orientation both extend outside the normal operating range,
even when operator intervention and internal and external
power sources become unavailable.
A still further objective is that, while
accommodating these system operational objectives, the
reactor plant is also of a physical make-up that can be
embodied in a form and structure which exhibits strength and
resilience appropriate to the accelerating motion and
displacement that can be anticipated for a submarine
platform.
In general terms, these objectives are achieved in
certain embodiments of the invention by the following means:
(i) Providing a reactor layout which concentrates, to
the degree possible, all radiation sources and
shielding mass as close as practicable to the
geometric centre of the reactor plant boundary,
i.e., central to the outer boundary of the reserve
coolant tank;
(ii) Providing a highly integrated design approach in
which: (a) components perform dual roles, e.g.,
the coolant contained in the reserve coolant tank
and in the delay tank, as well as structures such
as the reactor vessel, also serve as neutron and
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gamma shields; (b) all shields, the reserve
coolant tank excluded, recapture within the
primary heat transport circuit a significant share
of the penetrating radiation energy, which
otherwise would escape from the process of
conversion to useful heat or electrical energy;
(c) design simplifications result, in certain
manifestations of the invention, from the use of
a single water quality management system. (Such
a simplification is possible in an arrangement in
which a single water inventory is made common to
all components in a set and is allowed to
circulate among them at an adequate turnover rate
without compromising their principal functions.
Examples of components sharing the water inventory
include the primary heat transport circuit, the
neutron shield tank, the reserve coolant tank, and
the reactivity mechanism coolant and lubrication
systems); and (d) the delay tank is internal to
the reserve coolant tank and, therefore, the
latter affords some shielding against radiation
emitted from the former. Further, the flow
pattern in the delay tank is arranged, by the
positioning of internal baffles and the inlet and
outlet locations, so that the segments of the tank
which contain coolant of the highest specific
radioactivity during reactor operation are at the
side of the reactor core where they least expose
operating personnel and sensitive equipment to
radiation.
(iii) Employing commonly accepted methods for safely
managing the excess reactivity required for long
fuel- burnup life and acceptable demand load-
following capability. These may include the use
of fuel which contains burnable absorber and, as
well, provides a large, prompt negative
temperature coefficient of reactivity.
In certain embodiments of the invention, provision
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is also made for reactor regulating absorber rods
and fuel-follower reactor shutoff rods, each type
of rod being manipulated automatically by a
reactivity control mechanism employing current
technology with regard to safety and reliability
in accordance with signals received from the power
plant's control computer of highly reliable
design.
In other embodiments, where it may be desirable to
avoid all direct mechanical means of controlling
reactivity, and to effect reactor shutdown
automatically without involving the in-core
complexity of control mechanisms, the present
invention accommodates such avoidance by the
addition of dissolved neutron absorber in low
concentrations to the coolant of the primary heat
transport circuit and in high concentrations to
that of the reserve coolant tank, and providing
outside the said reactor plant the chemical plant
necessary to maintain the concentration difference
as necessary. The dominance of the large mass of
the reserve coolant at the higher absorber
concentration and the incorporation of suitable
ports, will result in reactor shutdown immediately
following the development of coolant exchange flow
(through the ports) between the two regions of
differing concentrations.
(iv) Providing a cooling circuit configuration
involving the ducts and ports that, even in a
space of limited extent, assures sufficient
convective cooling at all times and at all reactor
orientations with respect to gravity, to dissipate
thermal power to the ultimate heat sink as
necessary to meet safety requirement. Further,
such configuration does not allow the basic
thermal performance of the plant to be
significantly compromised over a range of
operating circumstances including significant
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random or periodic accelerations of the platform
of installation. Moreover, the configuration
provides for the transition of the heat transport
system from the normal operating mode to the
passive decay heat dissipation mode by strictly
passive means, if safety design criteria so
require. In such an embodiment, the interplay of
the passive cooling system with the primary heat
transport circuit under nearly-normal operating
conditions can contribute in a positive manner to
overall plant performance and safety.
Some embodiments of the invention provide a self-
shielding/self-contained reactor plant for power production
that is accommodated in a small space, i.e.,
characteristically 3.7 m in diameter, 30 m3 in volume and 70
tonnes in weight. It is anticipated that such reactors may
be successfully scaled-up by as much as 10 to 30 times in
power, accompanied by an approximate doubling in linear
dimensions, while still retaining most of the design and
operating features of the basic invention.
Some embodiments also provide a power reactor,
sized to operate at the low end of power level range
normally associated with nuclear reactors dedicated to power
production, and which, on the one hand, delivers its heat
energy at a temperature as high as within a few degrees of
the saturation temperature of its coolant at normal
operating conditions, and, on the other hand, retains the
safety of the passive (convective) cooling of a swimming-
pool type of research reactor, which is characterized by a
large thermal ballast maintained as the massive highly sub-
cooled pool water. In the present reactor plant, a massive
body of highly sub-cooled reserve coolant is likewise freely
available at all times as auxiliary cooling, and
particularly during those times when operating conditions
depart from normal.
Some embodiments also provide a reactor with a
passive cooling system designed to function at orientations
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.
departing from normal and under accelerating motion (as when
a ship in which such a reactor is installed is subjected to
currents and surface waves), and in which the said cooling
system is such that it does not detract from the ability of
the reactor to operate efficiently under normal conditions.
Brief Description of the Drawing
The drawing is a vertical cross-section of an
embodiment of the reactor plant made according to the
invention and shown in normal operation, and at a normal
orientation with regard to the direction of gravitational
forces. The drawing shows, by means of arrows, a finite
amount of exchange flow of coolant between the reactor core
1 and the reserve coolant tank 30, superimposed on the
dominant flows for normal operation. The reactor plant
configuration has substantially radial symmetry about the
vertical centre line indicated.
The numerals of the drawing further identify
certain elements of the said reactor plant according to the
following list:
201 - reactor core assembly
2 - fuel elements
3 - top grid-plate
4 - bottom grid-plate
5 - regulating rod
256 - regulating rod guide tube
7 - shutoff rod
8 - shutoff rod guide tube
9 - rod drive mechanisms
10 - radial reflector
3011 - reactor vessel
12 - inlet plenum
13 - outlet plenum
14 - inlet ducts
15 - outlet ducts
3516 - radial gamma shield
17 - lower axial gamma shield
18 - upper axial gamma shield
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- 19 - shielding tank
20 - shielding tank laminates
21 - inlet ports
22 - outlet ports
23 - inlet header
24 - outlet header
25 - thermal insulating layers
26 - delay tank
27 - delay tank flow baffles
28 - main inlet conduit
29 - main outlet conduit
30 - water filled reserve coolant tank
31 - control rod mechanism deck
32 - reactor vessel cover
33 - collected gas relief spigot
34 - reserve tank access hatch
Detailed Description of the Preferred Embodiments
In the drawing, reactor core 1 is built up of a
plurality of fuel elements 2 arranged so that water
circulating among the elements serves as both moderator and
coolant. The preferred selection of fuel type depends on
the particular reactor embodiment, but the choice of
uranium-zirconium-hydride fuel, or similar fuel consisting
of a fissile-fertile nuclide mixture alloyed with metal
hydride to yield significant hydrogen moderation within the
fuel matrix, provides the advantages of (i) a strongly
enhanced prompt negative temperature coefficient of
reactivity, (ii) a favourable through-core coolant velocity
profile and magnitude for a given mass flow, and (iii) a
reduced in-core water volume and, hence, a reduced
production rate of radioactive isotopes which are subject to
transport throughout the primary heat transport circuit.
The fuel elements, or assemblies of such elements as the
case may be, are suspended between the upper and lower grid
plates 3 and 4 which, in addition to supporting the fuel,
provide openings to facilitate the passage of coolant
traversing the core component of the primary heat transport
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1 335941
circuit. The grid plates are designed also in such a way
that they hold the fuel elements or assemblies firmly in
place in the face of acceleration imposed externally on the
reactor plant, while providing for their insertion,
securing, release and removal by remote manipulation during
refuelling operations.
The core-adjacent components of the primary heat
transport circuit consist of the inlet and outlet plena 12
and 13, and the inlet and outlet ducts 14 and 15. Only four
of the ducts are shown in the sectional drawing although,
in general, a plurality of at least eight of such ducts
arranged with radial symmetry about the axis shown may be
preferred for installations on mobile platforms such as
those arising in marine applications. Each of the inlet and
outlet ducts leads outward from the reactor core and
connects with an inlet port 21 or an outlet port 22,
respectively.
The ports and their means of control are designed
in such a way that, during normal reactor operation, they
accommodate predominantly through-flow in which essentially
all of the coolant circulating through the core is matched,
by means of the said ports, to the flow circulating in the
remaining components of the primary heat transport circuit.
Such components include the inlet and outlet header networks
23 and 24, the delay tank 26, the main inlet and outlet
conduits 28 and 29, as well as the other major components of
the primary heat transport circuit located externally to the
reactor plant as currently defined. These latter components
include the primary coolant circulation pump and the primary
heat exchangers, which are parts of the primary heat
transport circuit but not parts of the subject reactor
plant. It is noted, however, that in some embodiments of
the reactor plant such components might be located within
the reserve coolant tank.
In addition to providing through-flow, the ports
and their means of control are designed so that for a range
of off-normal operating conditions, such as those arising
during an accidental impairment of operation of the
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circulation pump or the heat exchangers of the primary heat
transport circuit, the ports accommodate lateral flow. Such
flow provides for a naturally convected exchange of coolant
to be established and maintained between the reactor core 1
and the reserve coolant tank 30, in response to one or more
of a wide range of accident scenarios.
The reserve tank 30 is normally maintained full of
coolant water maintained at a temperature considerably lower
than that of the primary heat transport circuit, by virtue
of (i) layers of insulation 25 applied to all of the outer
boundaries of those components of the primary heat transport
circuit enclosed by the reserve tank, and (ii) provision of
facilities, not shown in the drawing, for the removal of
heat from the reserve coolant to external heat sinks. Such
facilities may be designed by those skilled in the art of
thermal hydraulics so that, under normal operating
conditions, the heat removal rate is sufficient to maintain
the reserve coolant temperature at a stand-by level low
enough to provide the necessary thermal ballast to meet
anticipated accident scenarios, and, under reactor shutdown
conditions, to accomplish the passive removal of decay heat
in a manner which avoids possibility of overheating the fuel
elements in the absence of further operator intervention.
Depending on the circumstances of a particular application,
the equipment used to maintain the stand-by temperature may
be in common with that for passive decay heat removal,
although this would not be a necessity from an operational
or safety point of view.
For the reactor in its normal upright position,
the natural convection circuit consists of (i) the core 1 in
combination with the several outlet ducts 15 acting in
parallel with each other to serve as the hot leg of the
circuit, (ii) the reserve coolant tank 30 in combination
with the several inlet ducts 14 acting in parallel with each
other to serve as the cold leg of the circuit, and (iii) the
ports themselves, 21 and 22, operating in lateral-flow to
complete the various branches of the circuit, thus creating
a thermosyphon. All of the said components are designed to
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~assure that, once the convective cooling paths are established
following a safety-related initiating event, adequate cooling may
be maintained for an indefinite period without the requirement
of further action on the part of human operators, automatic
safety systems, or external energy sources. The circuit, so
described, constitutes the passive cooling system of the subject
reactor operating in its most likely mode, namely, the one in
which the reactor remains in its upright orientation.
For other reactor orientations of any extreme, it will
be apparent from an examination of the drawing that, so long as
lateral flow in either direction remains possible for each port,
the multi-directional arrangement of the ducts assures a
sufficient variety of convective paths maintaining the hot legs
and cold legs of the thermosyphon to permit, regardless of
orientation, passive core cooling at levels more or less
equivalent to that at the normal upright orientation. Thus, it
may be said that a novel aspect of the present invention is that
it provides for passive cooling to be sustained even if the
reactor departs radically from its normal orientation with
respect to the direction of gravitation, as in the case of
postulated accident conditions on board a ship powered by the
reactor.
Inlet ports 21 and outlet ports 22 are designed to
perform a switching operation from through-flow to lateral-flow
mode, and vice-versa, in response to changing operating
circumstances. In some embodiments, however, the ports are
designed to allow in some circumstances the superposition of
through-flow and lateral-flow, for the further enhancement of
operational and safety performance. The choice of a port design
fulfilling prevailing safety and reliability requirements may be
undertaken by those skilled in the art of thermal hydraulic
design.
It may be recognized in the course of such a design
process, however, that, to fully meet prevailing requirements of
safety and reliability in the face of other constraints relating
to space allowances, costs, and special operational
circumstances, it may be desirable to avoid in the port design
any dependency of the passive cooling system
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on either specific operator action or the presence of so-
called actively operated components, with respect not only
to its passive cooling function, as described above, but
also to its mode of deployment. In a passive cooling system
incorporating this further step in passive design, reliance
on mechanical response to electrical sensing devices,
operator intervention, and sources of external energy would
be specifically avoided in the design of the port and any
associated control elements. Thus, a passive cooling system
which has already the property of being able to continue
indefinitely to cool the reactor without need of external
action or energy supply, once passive cooling is
established, now has, in addition, the quality of being able
to initiate such passive cooling without external action or
energy. Such a system may be described as possessing
passive cooling initiated by passive means. A cooling
system matching such a description, including specific
aspects of the port design is the subject of the copending 5
patent application referred to above.
The inlet header 23, consists of a ~
manifold or other piping arrangement which distributes, via
the inlet ports 21 operating in the through-flow mode, the ,
pumped flow from the main inlet conduit 28 in equal measure
to the several inlet ducts 14. The outlet header
24, consists of a manifold or other piping arrangement which
collects, via the outlet ports 22 operating in the through-
flow mode, pumped flow in equal measure from the several
outlet ducts 15 and directs it to the delay tank 26.
The primary purpose of the delay tank 26 is to
provide a dwelling place for the short-lived neutron
activation products, created in the primary coolant as it
flows through the core, to decay to acceptable levels before
they are carried in the coolant to primary heat transfer
components located outside the reactor plant. Such
components include heat exchangers which may have secondary
media that are particularly vulnerable to radiation damage
or which may be approached occasionally by operators. In
the present embodiment, the delay tank is annular in shape,
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co-axial with the reactor itself, and located at the
periphery of the shielding tank 19 with which it shares a
common boundary. In this configuration the delay tank is
shielded from significant secondary neutron activation, but
is itself relatively shielded as viewed from locations
outside the reactor plant. As well as performing the usual
role of minimizing the direct streaming flow of coolant from
inlet to outlet while introducing a minimum of through-tank
head loss, the judicious arrangement of baffling plates 27
within the delay tank, affords the opportunity to control
the distribution of specific activity within the tank so
that sectors of highest activity are the most shielded as
viewed from the direction of regions of greatest radio-
sensitivity lying outside the reactor plant. Such an
arrangement is a significant factor in minimizing total
shielding weight required for the plant.
The primary heat transport system of the subject
reactor, in summary, operates as follows. Under normal
operating conditions the cooling water is circulated through
the primary heat transport circuit by means of the pump
located externally the reactor plant. For such operation,
the ports, 21 and 22, remain closed to lateral flow due
either to physical boundaries or hydrodynamic effects,
depending on the type of port specifically incorporated in
the design. Under these conditions, the reactor power is
regulated to achieve the desired conditions in the circuit,
taking into account the usual plant operation considerations
such as the thermal load demanded by the external equipment.
In the event that the primary circuit becomes
impaired due to a loss of thermal load or loss of pump
operation, the ports switch from the through-flow mode to
lateral-flow mode and the reactor core is subsequently
cooled by natural convection exchange flow between the core
and the reserve coolant tank. This passive mode of cooling,
which delivers no power to the normal load, can be allowed
to continue until the reserve coolant, which was originally
maintained at a temperature considerably lower than that of
the primary circuit, approaches the boiling temperature of
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water at the maintained pressure. The total time required
for this condition to be reached is considerably lengthened
if, in the meantime, the reactor core is placed in the
shutdown state. Following reactor shutdown it is possible,
by virtue of the reserve tank cooling system, to assume that
the reactor will be adequately cooled from a safety point of
view for an indefinitely long period. For these reasons and
because at long times after shutdown, only the decay heat is
produced in the reactor core, such a reactor is said to
possess a passive decay heat removal system. It is seen
that, so long as the ports remain open to lateral flow, the
passive decay heat removal process continues to operate
regardless of the orientation of the reactor with respect to
gravity.
If, on the other hand, it is in the nature of an
accident that the reactor, operating initially at some
elevated pressure and a corresponding elevated temperature,
undergoes a sudden depressurization due to a break in the
primary circuit, a flashing to steam and an accompanying
release of energy will occur in the coolant of the primary
heat transport circuit. If, however, the ports become
immediately open to lateral flow, the major part of the
total coolant inventory of the primary heat transport
system, having been maintained as the reserve coolant at
less than the saturation temperature for atmospheric
pressure, has remained in the liquid phase during the
depressurization and becomes available for quenching the
steam and restoring core cooling to a stable long term
passive cooling process. Here too, it is assumed that the
reactor core has in the meantime reached a stable shutdown
state early in the scenario.
If in either of the above scenarios the ports were
of a design such that they switched to the lateral-flow
yielding the passive cooling mode without reliance on
operator action, automatic mechanical responses to sensors,
or external energy, the passive cooling system can be said
to have been initiated by passive means.
Further, if either of the above scenarios included
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a reactor shutdown not dependent on operator action,
automatic mechanical reaction to sensors, or external energy
sources, the reactor may be said to have a passive reactor
shutdown system. The preferred embodiment is capable of
such a feature through the addition of absorber to the
reserve coolant, as well as the necessary plant to manage
the absorber content in the primary circuit of the system as
the reactor becomes critical and assumes normal operation.
This system variant would be seen to be particularly
meritorious from a safety point of view if deployed in
conjunction with the passively initiated passive cooling
system.
For scenarios involving loss of reactor
regulation, the reactor relies for its safety on either
neutronic feedback, as provided by the prompt negative fuel
temperature coefficient, or on secondary effects caused by
changes in the coolant condition. Such changes include
phase change, or the induction of absorber dissolved in the
reserve coolant as mentioned above.
The penetrations of the reserve tank boundary to
admit the main inlet and outlet conduits are located high up
on the walls of the reserve tank. This location is chosen
to minimize the effect of a postulated breakage, in either
the conduits or the connected components occurring outside
the reserve tank, on the inventory of water in the reserve
tank. The appropriate placing of syphon breaks within the
system may be of assistance in this regard.
Returning, specifically, to a consideration of the
neutronic aspects of the reactor core 1, the beryllium
reflector 10 plays an important role keeping the basic
radial power peaking factor close to unity. To this end,
such a reflector is both necessary and particularly
effective when used in conjunction with a small hydrogen
moderated core. Also in this regard, the preferred use of
"burnable" neutron absorber interspersed in the fuel matrix,
in order to provide uniform reactivity compensation
throughout the core over the fuel burnup lifetime, avoids
the severe flux perturbations which would otherwise be
1 33394 1
caused by necessary movable mechanical absorbers.
As indicated, the presence of both the reflector
and the burnable absorber support radial uniformity of the
neutron flux, and hence uniformity of power density and
coolant conditions throughout the core. This uniformity is
a vital requirement in a low-pressure power reactor in which
acceptable efficiency of the energy conversion process
requires the average core temperature to be close to the
maximum core temperature, which, in turn, will have the
saturation temperature corresponding to the operating
pressure as its extreme upper limit. A consideration of
burnable or controllable absorber added to the coolant, as
an alternative to the above-mentioned movable or fuel-added
absorber, is disregarded in the compact reactor embodiment
depicted in the drawing, because of the need to avoid the
added plant and operational complications.
The regulating absorber rods 5, which are of
sufficient number and unit reactivity worth to cover with
some redundancy the reactor's operating reactivity margin,
are made to travel by rod drive mechanisms 9 within
associated guide tubes 5 located in a distributed manner
throughout the core. Interspersed among the regulating rods
and fuel rods of the core are shutoff rods 7, which are of
sufficient number and reactivity worth to cancel under all
conceivable operating circumstances the reactor's operating
reactivity margin, are made to travel by separate rod drive
mechanisms 9 within associated guide tubes 8. Each shutoff
rod has an attached fuel rod which, under normal operating
conditions with the shutoff rods fully withdrawn, occupies
the core lattice site reserved for the receiving of the
shutoff rod on shutdown. This arrangement is of particular
importance in the preferred embodiment, in order to further
support the favourable flux peaking factor mentioned above,
and to assure full-complement fuel loading under normal
operation in the interests of infrequent refuelling. The
rod drive mechanisms specified must exhibit high performance
capabilities with regard to reliability, miniaturization (in
the case of a particularly compact plant) and the
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1 33394 1
distinctness of operational principles between regulating
and shutoff rod functions.
In addition to the qualifying remarks already made
with regard to the regulating and shutoff rods, other
aspects of reactor control normally observed by those
skilled in the arts of reactor physics and control are
assumed to apply. It is possible to eliminate the need for
the shutoff rods and their associated mechanisms, if, for
example, the shutoff function is made to follow the onset of
passive cooling by means of neutron absorber previously
dissolved in the reserve coolant being drawn into the core.
Also, the required total reactivity worth of the regulating
rods may be reduced by adding and subtracting chemical shim
in controlled amounts with respect to the circulating
primary coolant, or by reliance on the built-in tendency of
the reactor for self regulation to a constant core
temperature by virtue of the negative temperature
coefficient of reactivity.
The reactor vessel 11 serves as a principal
support structure for many components of the reactor plant.
Its layout also accommodates the remote disassembly of
certain components, including the rod drive mechanisms and
the fuel elements or assemblies, for the purpose of
servicing and refuelling the reactor. In addition to
supporting the core 1 by means of the upper and lower grid
plates 3 and 4, the vessel supports the reflector 10, the
lower and upper gamma shield inserts 17 and 18, and the
control rod mechanism deck 31. It also provides for the
coupling of the inlet and outlet ducts 14 and 15 to the
inlet and outlet plena 12 and 13, respectively, and for the
locating of the radial component of the gamma shield 16.
The reactor vessel 11 and the borated water shielding tank,
19, together form a welded unit which is supported within
the reserve coolant tank 30 by reactor support structures
not shown in the drawing, but which may be specified by
those skilled in the art of structural design.
The space enclosed by the shielding tank 19, and
not occupied by other reactor components, is filled by
-- 19 --
1 333941
reactor shielding material possessing the ideal reactor
shielding properties of (i) hydrogen moderation, (ii)
relatively radiation-free thermal neutron capture, and (iii)
gamma radiation absorption. In the preferred embodiment,
this space is filled with a composite made up of thermal
neutron and gamma ray absorbing plates interlaminated with
water. The plates are composed of a readily available
stainless steel alloy, containing boron as a major
constituent. An advantage of this shielding design is that,
by the circulation of the water component, it facilitates
the removal of penetrating radiation heating deposited
within the shield during reactor operation. This design
also simplifies the problem of managing the liquid shielding
material, since the liquid in this case consists of
relatively pure water, rather than the more conventional
aqueous solution containing neutron absorbing chemicals.
In the preferred embodiment, the water of the
shielding tank 19 communicates with the water of the primary
heat transport circuit through relatively small openings not
shown in the drawing. Under normal operating conditions, a
small fraction of the coolant flow returning through the
main inlet conduit is diverted into the shielding tank while
an equal amount is entrained with the flow emerging from the
reactor core and eventually leaving the reactor plant
through the main outlet conduit. In the interests of
achieving the most efficient recapture of radiation heat
deposited in the shielding tank for subsequent conversion to
other energy forms in the plant's main energy conversion
unit, the magnitude of such flow through the shielding tank
is such that the merging core and shielding flows combine at
more or less the same temperature. In practice such flows
will be relatively small, typically one tenth of the mass
flow through the core, and this fact, coupled with the
fairly large volume of shield water and the reduced activity
of the incoming water due to the delay tank, prevents an
unwanted buildup of radioactivity in the shielding tank.
Nevertheless, the recapture of the radiation heating of the
shield may yield, in the case of a small reactor core, a 10
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1 33394 1
percent enhancement in the total converted energy output for
a given fission rate.
Also as a result of these shield cooling
arrangements in which different portions of the same body of
water serve various functions at different times, the shield
tank water will share not only operating pressures and
temperatures similar to those of the reactor core, but will
undergo continual chemical upgrading by virtue of the main
water chemistry plant normally connected to the primary heat
transport circuit. The advantages of this arrangement with
regard to the elimination of the added weight, volume and
complexity of a water management plant explicitly for the
maintenance of shielding solution, will be particularly
apparent in applications where compactness of plant is
essential.
Provisions similar to those described above for
coupling the shielding water system to the primary cooling
system under normal operating conditions should, in
principle, also be present when the reactor is being cooled
by the passive cooling system. Under these latter
conditions, however, the reactor will have been placed in
the shutdown state, during which transition the radiation
heating is reduced disproportionately faster than the core
power, and it is unlikely that any special cooling
provisions will be required. In any case, small passages to
facilitate adequate natural convection between the shielding
water and the main passive cooling circuit under shutdown
conditions, without compromising either of the primary
cooling functions, may be readily devised if necessary.
It may be noted that the principle of sharing a
single water chemistry plant may be extended, depending
primarily on the particular port design adopted, to include
the reserve coolant which resides in the reserve coolant
tank 30. Such inclusion is particularly appropriate in
reactor plants using the hydrodynamic ports referred to
above, or other port designs which can allow a small
constant exchange of coolant between the circulating coolant
and that of the reserve tank. In such cases, the normal
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1 333941
residual rate of coolant exchange between the reserve
coolant tank and the circulating primary coolant may
suffice. It must be borne in mind, however, that such
exchange rates are subject, under normal operating
conditions, to an imposed upper limit to prevent (i) undue
losses of energy from the primary circuit to the reserve
tank, and also (ii) the significant build-up of core
activation products in the reserve tank that accompanies the
exchange of primary and reserve coolants.
Layers of thermal insulation 25 are placed, as
shown in the drawing, on all surfaces which demarcate the
thermal boundaries between the body of reserve coolant
itself, and those components of the primary heat transport
circuit enclosed within the reserve tank. Clearly, both the
shielding tank 19 and the delay tank 26, which share a
common hydraulic boundary, may be considered for present
purposes as components of the primary circuit, as all
components located within them are allowed to rise to
general level of the normal operating temperature of the
primary circuit and need not be insulated individually. The
insulation used may be of the wettable type, but it must be
capable of physically withstanding the effects of hot water
suddenly depressurizing within the matrix of the insulation
and flashing to steam, if the insulation system is to remain
intact following a loss of coolant event involving coolant
phase change.
The materials of the gamma shielding components,
16, 17 and 18, are of a high-Z variety, typically, lead, or
tungsten. If necessary to prevent distortion due to
overheating, cooling passages may be arranged in these
components to permit the circulation of the water from the
nearby shielding tank 19. The surfaces of the gamma
shielding components, including the cooling passages, are
clad with material which is chemically compatible with the
shielding tank water. The actual dimensions of the gamma
shielding components, including the laminate structures of
the shielding tank 19, may be optimised for a particular
application by those skilled in the art of radiation
1 333941
shielding design. Also depending on the particular
application, auxiliary shielding components, not shown in
the drawings,-may be added in and around the reactor plant,
and in the vicinity of adjacent equipment. Such localized
shielding may be necessary to counteract the possible
streaming of radiation from the core through the inlet or
outlet ducts and ports. Such shielding may be arranged as
an integral part of the ducts or ports themselves.
The head space above the control rod mech~n;cm
deck 31, is likewise filled with water communicating
marginally with the main body of reactor coolant. This
water, as well as contributing to the general shielding,
also may provide cooling and lubrication to the rod drive
mechanisms, depending on their particular design. For a
reactor designed to opérate at temperatures elevated above
the operating temperature for the rod drive mechanisms, the
thermal insulation must be applied to the control rod
mechanism deck, rather than over the upper part of the
reactor vessel and its cover 32, as shown. In any case,
provision must be made, in the form of suitable passages,
not shown, for the immediate equalization of pressures
between the head space and the remainder of the reactor
vessel, to eliminate any tendency for the position of the
regulating or shutoff rods to be altered inadvertently, in
the event of pressure transients which may arise during
operation. Although provisions for the control of the
coolant pressure and gas content is made at points of the
primary heat transport circuit lying outside the reactor
plant, the collected gas relief spigot 33 appropriately
coupled to the gas control system allows the timely removal
of any gases accumulating within the reactor vessel.
The reserve coolant tank 30, and the reactor
assembly it contains, have low elevation profiles to
facilitate installation in a low-profile environment such as
on board a small or medium-sized submarine vehicle. In such
a marine application, the reserve coolant tank may stand
alone within the vehicle structure or it may be an integral
part of the submarine structure, namely, the main pressure
1 33394 1
.
hull or a secondary inner hull. In a land-based
application, the reserve tank could be in the form of an in-
ground pool, or could serve as a liner for an in-ground tank
structure which serves as a backup container for the reserve
tank, offering, at the same time, an opportunity for
monitoring for leaks in the reserve tank using methods which
are commonly applied in the industry.
The arrangement of the reactor plant, while
determined largely by thermal hydraulic and compactness
considerations, also facilitates refuelling and other
aspects of reactor servicing. The reactor core region is
accessed by first erecting auxiliary shielding, if
necessitated by a small shielding space allowance in the
particular installation. The tank is in the form of
vertical extension sealed temporarily to the reserve tank 30
and enclosing a large area around the reserve tank access
hatch 34. After filling the extension with water of reactor
purity, the hatch cover 34 is removed, followed by the
reactor vessel cover 32. At this point, the water of the
auxiliary shielding tank will have become united with the
water of the reactor systems, thus forming a single body of
liquid. After assuring that the reactivity control and
shutoff m~c-h~nisms have placed the core in its least
reactive state, the shutoff rods, 5 and 7, are decoupled
from their control mechanisms 9. The latter are then
removed, leaving the shutoff rods in the core. Following
this, the control rod mechanisms deck 31, some boron
absorbing laminates, and the upper axial gamma shield 18,
which also serves as the upper boundary of the outlet plenum
13, are removed in that order from within the reactor vessel
11. At this point defuelling operations may begin,
followed, at the last, by the removal of the control and
shutoff absorbers, including the follower fuel rods.
Fuelling operations involve the same procedural steps, but
in reverse order and with an approach-to-critical procedure
before reassembly of the shielding components and
reconnection of the reactivity mechanisms.
Embodiments which do not require that the passive
1 333941
cooling circuit be available at all physical orientations of
the plant (eg. land-based plants) may have a configuration
with as few as one inlet duct and one outlet duct,
connecting with the reactor core. These can be offset
relative to the core axis in order that ready access to the
core for servicing and refuelling is available. In the case
of a research reactor, such ready access may facilitate the
design and operation of experimental facilities installed in
or near the reactor core.
