Note: Descriptions are shown in the official language in which they were submitted.
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The present invention concerns a control system for nuclear
reactors and is particularly well suited for light-water reactors used for
ship propulsion power, involving high load change rates.
The control of nuclear reactors is generally accomplished by
control rods, also called adsorber rods, which contain a neutr~n-absorbing
material and are run in or out of the core in some controlled manner. In
addition to the absolutely necessary requirement that the reactor can be
shut off by these control rods quickly and safely under any operating ~ `
condition, it is expected that upon normally operating the control rods, no ~ -
excessively high values of the neutron flux or of the core temperature occur,
and that the reactor core achieves in all areas a burnup of the nuclear fuel,
which is as uniform as possible. Temperature and neutron flux peaks must be
avoided, because the latter limit the total power output of the reactor core
in view of the permissible mechanical material stresses. Uniform burnup is
desirable for economic reasons, so that it is not necessary to replace
individual, burned-up fuel elements while others are still far from having -~
reached their possible burnup. ,'r, '
In the German Auslegeschrift 1,244,307 issued to Siemens AG on ~
July 13, 1967, a method for influencing the neutron multiplication factor of ;
a nuclear reactor core with a multiplicity of control rod clusters~ i.e. a -
fixed assembly of some control rod is described, in which the rods are moved ~ ;
in a defined sequence individually or sequentially in groups, as a function
of a control signal It is proposed there to move the rod clusters, which
are of uniform mechanical design, in dependence on the magnitude of the
control signal with different frequency and individually or in groups of
different size, stepwise, in cyclic sequence. Among other things, a method `
is given, by which it is proposed that a constant advance of the reactivity -
is achieved when the control rods are inserted.
The German Patent 2,007,564 issued to Siemens AG on October 18,
1973 has as its object to make the flux
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distribution as uniform as possible also, in the travel direction of the
control rods. It is assumed here that the control of power does not require
large means and that the flux distribution is not distorted excessively in ;
the radial direction (over the cross section of the core). The method
proposed there for controlling the power of a pressurized-water reactor by ` ~
means of several adjustable control rod clusters, which can be moved -
differently in two groups, is distinguished by the feature that the one group ~ -
is moved with the other group only in the event of load changes, but has
otherwise a depth of immersion of between about 8 and 20% of the core
height, while the other group is adjusted between this depth of immersion
and 100% of the core height, in order to compensate the fuel temperature
feedback effect of the reactivity, which occurs in the event of load changes.
~ oth of the above proposed solutions re~uire a relatirely large
amount of control means.
It is an object of the present invention to propose a control for
nuclear reactors with high load change rates, which can be realized with a `
small amount of control means without thereby making the maximum power form
factor larger and the minimum DNBR (ratio between DNB heat flux and local
heat flux) smaller, than with a control with a small load change rate and/or
a large amount of control means, and wherein at the same time the high load ~ -
change rates are made possibly by a nearly linear control characteristic.
It is here proposed to solve the problems stated above, namely, -~
limiting the power form factors and an advance of the reactivity as
constant as possible in both cases when the control rods are inserted, by -
an action, designed according to the invention as a combination, of the
movable control rod system and a system of fixed non-moving burnable
poisons in the core. `
In a basic embodiment of the invention, there is placed in an
unpoisoned core a fixed system of burnable poisons, the length of which, as
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compared to the active length of the fission zone, is shortened on one side,
e.g., at the top in the case of control rods inserted from the top, and
therefore does not absorb in the upper region of the active core.
The movable control rods are inserted into this core from the top.
With the control rods partially inserted, e.g., half-way or two-thirds, one
can subdivide such a core in the axial direction roughly into three regions:
an upper region, in which only the absorption of the control rods is effective;
a lower region, in which only the absorption of the burnable poisons is
effective; and a middle region, in which the absorbing effect of both systems
is superimposed. It should be noted here that control rods or burnable
poisons influence the neutron flux not only immediately at the point of their
geometrical location, but that they also have a certain spatial influence
range, from which they absorb neutrons and in which they therefore lower the
neutron flux. The superimposed absorption action of the two systems
proposed here falls, with the control rods inserted partially, just into
the ~'middle~ region~ which experience has shown to have hi~h axial power
form factors, so that there, a reduction, as intended by the invention, of `;~
the maximum power form factors is brought about, with an increase of the
relatively low power form factors in the lower and upper regions with
reduced neutron absorption.
With this method, one tolerates for the core without control rods
and for the core with fully inserted control rods, to the extent that they
occur in power operation~ maximum axial power form factors, which are
higher than they would be without the method according to the invention. ~ -
With appropriate optimization, the present invention brings about, through
the combined actlon of control rods and burnable poisons, that approximately
constant power form factors are obtained for all control rod positions that
occur in power operation. These power form factors are then lower than
the maximum values that would be Qbtained without the proposed procedure
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under otherwise the same boundary conditions, so that a system, designed
according to the invention in this manner, can be run with a higher total
power output. In addition, the power form factors, depending on the control
rod position and the corresponding axial position of the power maximum, can
be matched to the axially variable coolant conditions as far as film boiling
is concerned, so that a constant film boiling margin is obtained,
independently of the control rod position. This film boiling margin is the
ratio of the critical heating surface loading to the actual heating surface
loading of the fuel rod cladding tubes, the critical heating surface loading
being determined by a sudden drop of the heat transfer coefficient, which is
an indication of the incipient film boiling. Because of the poorer heat
transfer, film boiling can lead to a burn out of the cladding tubes.
By shortening the rods with the burnable poisons at the top as ~ -
compared to the active length of the fission zone, as described above, one
obtains an approximately constant advance in reactivity when the control
rods are inserted from the top, i.e., one obtains a linearized control
characteristic if the reactivity of the core is plotted versus the position
of the control rods. With a layout according to the state of the art~ -
without the poison shortening as defined in accordance with the invention,
relatively small reactivity advances are obtained, when the control rods `~
are inserted into the uppermost region of the core. With the proposal
presented here~ one obtains in the upper, poison-free core region a zone
with high local reactivity and relatively great flux weight, so that the
reactivity advance is increased in this region if the control rods are
inserted there, and overall, an approximately constant reactivity change-is
obtained. With this method, smaller reactivity advances are obtained when `~
the control rods are moved in the lowest region of the core. However, this ~-
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operating range of the control rods can be excluded from practical use.
The control system proposed here offers the following advantages: ~-
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1. As described above, one obtains in power operation
approximately constant power form factors with variable rod position within
a practical operating range. This option is particularly important if one
wants to compensate the relatively large reactivity variations, which are due
to variable xenon concentration, by means of the control rod system; in some
situations, control rod positions with deeply inserted control rods result
here. The method proposed here makes it possible to limit the maximum
axial form factors to relatively low values also for these control rod
positions.
2. For technical control reasons (e.g., load change rate)~ large
reactivity advances are frequently of interest in the entire range of
adjustment of a group of control rod clusters. However, the maximum
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reactiYity advance is limited upward for safety reasons. With the method
proposed here, which leads to a linearization of the control curve, i.e.,
approximately constant advance of the reactivity in the entire operating
range, the minimum reactivity advance relevant from a control point of view -
is largely matched to the maximum reactivity advance relevant from a safety
point of view, so that overall, an improvement results due to the then ~
higher minimum reactivity advance from the control point of view. -
3. In addition, an approximately constant reactivity a~vance -
often leads to a simplified control technique, for self-evident reasons. -
4. The control proposed here permits to combine two or three
control rod cluster groups in one simultaneously moved control rod cluster ~- -
group or bank. If operating with such a combined control rod bank, one `
obtains normally higher axial power form factors than with operating a group
or subgroup. ~hese higher for~ factors are reduced again by the control
proposed here. The combination of control rod cluster groups into a bank `
offers the following advantages: -
- Simpler control rod cluster motion programs with simplified control
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means can be used. --
- If the mechanical control rod travel velocity is limited upward, -
larger reactivity advances versus time can be made available by a bank
combining control rod cluster groups.
- Even if the linearization of the control curves, as proposed here,
has been accomplished, different control rod cluster groups frequently still
have different reactivity advances. This difficulty is avoided by the -
combination of such control rod cluster groups in an overall bank and a -
further matching between a minimum and a maximum reactivity advance with the
advantages described under 2. is brought about.
In the German Offenlegungsschrift No. 1,909,109 issued to General
Electric on September 18, 1969 and German Patent No. 1,279,230 issued to
Gesellschaft fur Kernenergieverwertung in Schiffbau und Schiffahrt on
May 22, 1969 and in the German Journal Atomkernenergie 12 (1967), pages - -
285 and 286, the spatially variable design of burnable poisons is used for
the purpose of influencing the long-term behavior of a core:
- with respect to an op*imum reactivity behavior (e.g., excess
reactivity of the core, constant with time),
- for achieving a power distribution which is stationary over one ~-
burnup cycle, e.g., is constant with time, for instance, for one year,
- for obtaining uniform burnup of the poisons. .~ -
In the mentioned publications, the control rod system, particularly
with partially inserted control rods, is not taken into consideration. The
methods proposed in the present application comprise a spatially variable ` ;
design of the fixed, burnable poisons for the purpose of obtaining
approximately constant maximum power form factors through cooperation with
the control rod system for all control rod positions that occur in power ~ ~
operation. The control rod positions vary here in time in the second, ~ ;
minute or hour scale with greatly varying, i.e., non-stationary, power ~
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distributions individually; as mentioned, only the maximum values of the
form factors remain approximately constant here.
In accordance with this invention there is provided a controllable
light-water nuclear reactor comprising a reactor core composed of fuel
assemblies and a fixed system of burnable neutron poisons in non-uniform
distribution throughout the core and with an absorber system movable in an
axial direction of the core and comprising adjustable control rods which can
be inserted in said direction into the reactor core from one side of the
core; wherein the improvement comprises the neutron poisons being concentrated
in the axial direction of the reactor core on the side of the reactor core
which is facing away from the side where the control rods are inserted, said
poisons and control rods being interrelated so that for all control rod
positions which occur in power operation, the maximum power form factor for
all these control rod positions viewed together is lower than it would be
without the use of the burnable neutron poisons, due to the combined action
of the movable control rods and the fixed system of burnable neutron poisons,
said control rods being in the form of control rod clusters and all control -
rod clusters serving for the control of the core power being connected to- :
gether for joint actuation. .
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In summary, the interaction of burnable poisons, of control rods
and of a variable rod position is characteristic for the proposals given
here.
In the German Patents 2,007,564 and 1,244~307, detailed subdivisions
of the control rod clusters into control rod cluster groups and detailed
control rod motion programs are given with the objective to limit the axial
power form factors when the control rods are inserted, and to achieve
constant reactivity advance. The present invention solves these problems
by the combined deployment of control rods and burnable poisons, so that
one can get along with a simple control rod cluster motion scheme.
In the following, the principles of this invention are
exemplified by the application of the invention to the core layout for a
220-MWth pressurized-water reactor for a ship's propulsion system as will
be explained below;
Having reference to the accompanying drawings:
Figure 1 schematically shows a cross section of the reactorls
core layout to show the arrangement of the fuel assemblies; -
Figure 2 is like Figure 1 but shows the arrangement of the neutron
poison;
...
Figure 3 graphically shows the core's axial power distribution ~ ~
when a group of core reactivity control rod clusters is fully withdrawn -
from the core;
Figure 4 is like Figure 3 but shows the distribution when the ;;
group is half-inserted;
Figure 5 is like Figure 3 but with the group one-quarter inserted;
Figure 6 is the same but with the group fully inserted; ~
Figure 7 graphically shows the invention's nearly linear -
reactivity change as the control rod cluster group is inserted from beginning
to end of the insertion travel; and
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Figure 8 is a schematic view of a vertical section of the core,
taken on the line I-I in Figure 2.
In Figure 1 the reactor core is assumed to have twenty-four
square fuel assemblies 1, each fuel assembly having 21 x 21 positions which
can be used for fuel rods, poison rods and control rod guide tubes, in which
latter axially movable control rods are guided. The control rods are to be
inserted into the fuel assembly from above, and are mechanically
interconnected to form clusters of control rods., i.e., jointly movable
control rod cluster groups each having a common drive. The combination of
the control rod clusters into jointly moving control rod cluster groups each
with a common drive, is designated by the letters A to D3. For controlling
the power and xenon reactivities, the equivalent control rod cluster groups -
B or C are employed, as desired, which each cause a reactivity change of
about 7% over their insertion travel. The control rod cluster groups A,
Dl, D2 and D3 are shutdown rod clusters which in normal reactor operation
are maintained outside the core. As shutdown rod clusters serve also the ;
control rod clusters of that control rod cluster group B or C which are not
used for power control at the time in question. No control rods are
inserted mto fuel assemblies without letter designations in Figure 1. In
the following~ only the control rods of the cluster group B will be
considered. All other control rods are assumed to be fully withdrawn,
Gadolinium in the form of ~d203 is used as the neutron poison. ~-
The total reactivity of these neutron poisons, which are arranged according `
to Figure 2 in the fuel assemblies in the form of special rods at positions
not occupied by the control rod guide tubes, or fuel rods is about 11% at
the start of the reactor operation. It is uniformly distributed, for instance,
over the lower part of the reactor core and decreases to zero in steps
steadily toward the core's top. The core has a poison length distribution
such as is seen in Figure 2. These poison rods, which are shortened at the
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top by at least one-seventh as compared to the core's active fission
material zone of 1750 mm, have a length distribution chosen so that the
following effects are obtained in cooperation with the movable control
rod cluster system of the group B.
- The core with Group B fully withdrawn shows the axial power
distribution according to Figure 3. (Curve A), which has a higher local
hump in comparison with the analogous power distribution without the ~-
described shortening (Curve B).
- The core with Group B half-inserted shows in Figure 4, with the
mentioned shortening of the poisons, an axial power distribution with a ~
distinctly lower local maximum (Curve C~ than without the mentioned shorten- : -
ing of the poisons (Curve D). ~.
- The core with Group B inserted in the upper quarter of the fission
material zone shows, with the mentioned shortening of the poisons, an about
cosine-shaped course of the axial power distribution (Figure 5~ Curve E)~
as is shown by the core without control rods (Figure 3, Curve A) without
using the mentioned shortening of the poisons. The analogous power
distribution without the mentioned shortening of the poisons is shown by
Curve F in Figure 5, which has a locally higher axial power maximum. Since,
however, the absorbers of the Group B have, in the core given as the example,
with full load and Xe-equilbrium, approximately the last-mentioned depth of ~
insertion~ about the same axial power distribution (Figure 5, Curve E) is ~ .
obtained in this case with the combined system of the mentioned shortening
of the poisons and the movable control rods of the Group B as the core
without control rods has in ordinary pressurized-water reactor layouts
(Figure 3, Curve A).
- The core with Group B fully inserted shows in Figure 6 with the ~ ~
mentioned shortening of the poisons about the same axial power behavior . .
(Curve G) as the core without control rods with this layout (Figure 3,
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Curve A) and, without the mentioned shortening of the poisons, a flat ~ ~
axial power distribution (Curve H), as could be seen already in Figure 3, ~ -
(curve B). -
- At the same time, an almost linear reactivity change per travel
distance is achieved by the system of poisons and absorbers combined in the
manner described, from the start of the insertion on~ when the absorbers of
Group B are inserted from the top (Figure 7, Curve I), this shape remains
almost to the end of the insertion travel. This control characteristic is
quite contrary to the behavior which the core has without the mentioned -
combination of poisons and movable absorbers (Figure 7, Curve K), here, the
control characteristic exhibits twice a typically S-shaped course (Curve K,
Part Bl and B2) with large differences between the maximum and the minimum
reactivity advance per unit distance of insertion with the above-mentioned
disadvantages. The doubly S-shaped course was caused by the division,
necessary inthe last-mentioned case, of Group B into two subgroups Bl and B2
(Figure 1, core cross section), in order not to let the local power `~
increases become undesirably large. The system of poisons and movable
absorbers, combined in the manner mentioned, makes possible the combination
of the subgroups Bl and B2 in Bank B as well as the linear shape of the
control characteristic shown (Figure 7, Curve I). Thus, it is possible to ~ ~-
use one bank with constant travel velocity for the control of the power. ~ -
The bank B has then, with the same travel velocity of the control rod
cluster groups as with the subgroups Bl and B2, a larger reactivity change
per unit insertion distance in the control range of interest than the
subgroups Bl or B2, which further differ greatly with respect to their
reactivity deviation per total travel distance (Figure 7, Curve K, Part B
and B2, respectively). Thus~ a considerably higher load change rate is
achievable with the same travel velocity of the control rod clusters by
the control characteristic of the systems combined in the manner described ;~
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than without the combined systems.
Figure 8 shows in a simplified presentation a vertical long-
itudinal cross section through the reactor core corresponding to Section
I-I in Figure 2. As already explained in connection with the description
of Figure 1, the reactor core consists of 24 square fuel assemblies 81,
four of which are shown in the longitudinal cross section. Each of these
fuel assemblies contains in a square arrangement 21 x 21 positions for fuel
rods, poison rods and control rod guide tubes in which the control rods
move. In the interest of a better presentation, only some of the poison
rods 82 are shown in each case, which extend over the entire height of the
fuel assembly and contain reactor poison in the zone shown in black in
Figure 8. For the sake of clarity, the fuel rods are not shown. In all of
these fuel assemblies, an upper region 83 is completely free of burnable
poison, while in the region below it the poison rods 82 in part of the
reactor contain reactor poison over their remaining length and in the other
part of the reactor about every second poison rod, has a shorter length
of poison. In the two central fuel assemblies of Figure 8, it is shown
that the poison rods in the vicinity of the reactor center have a greater
active poison length than the poison rods arranged in the same fuel assembly
further away from the center. Above the reactor core, numerous control rods
84 are shown in the fully withdrawn state, which are moved as a cluster for
each fuel assembly by means of a common spider 85 from which the rods
depend, and by a rod drive 86, jointly in the axial direction. The rod
drives 86 of different clusters can be combined as groups or banks and
moved jointly, intercdupled by electrical means (not shown~.
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