Note: Descriptions are shown in the official language in which they were submitted.
CA 0202~713 1998-02-11
.~
PRESSURE-TUBE TYPE HEAVY-WATER MODERATED NUCLEAR REACTOR
The present invention relates to a pressure
tube type nuclear reactor which uses heavy water as the
neutron moderator, and, more particularly, to an
improvement in the reactivity coefficient which is a
critical factor when controlling the operation of a
nuclear reactor.
The structure of a conventional tube type
nuclear reactor which uses heavy water as the neutron
moderator and uses light water as the coolant will be
described.
A plurality of calandria tubes are inserted
into a calandria tank which contains heavy water. A
pressure tube is inserted into each of the calandria
tubes, the pressure tube accommodating sealed nuclear
fuel. As a result, nuclear reaction heat is supplied
from the nuclear fuel to the light water which passes
through the pressure tube and serves as the coolant.
Thus, heated light water is taken out as hot and high
pressure steam. The pressure tubes for the pressure tube
type nuclear reactor are, for example as disclosed in
Japanese Patent Laid-Open No. 55-94190, disposed so as to
form a square lattice configuration. In order to improve
the control performance of the core of the nuclear
reactor of the type described above, a variety of
structures have been disclosed as follows:
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(1) A structure disclosed in Japanese Patent
Laid-Open No. 58-34386 and arranged in such a manner that
the pressure tubes are, as the basic configuration,
arranged so as to form a square lattice configuration.
The intervals between the lattices of the pressure tubes,
in which both a control rod and instrumentation guide
tubes are not provided, are narrowed.
(2) A structure disclosed in Japanese Patent
Laid-Open No. 49-103091 and arranged in such a manner
that a carbon rod is disposed in the heavy water
moderator in the calandria tube.
(3) A structure disclosed in Japanese Patent
Laid-Open No. 61-26891 and arranged in such a manner that
the pressure tubes and the heavy water tubes are
alternately disposed.
(4) A structure disclosed in Japanese Patent
Laid-Open No. 52-61697 and arranged in such a manner that
a water sealing rod is inserted into the pressure tube.
(5) A structure disclosed in Japanese Patent
Laid-Open No. 52-100073 and arranged in such a manner
that the outer diameter of the calandria tube at the
central portion of the core is arranged to be larger than
that at other portions.
In the conventional pressure tube type nuclear
reactor in which the pressure tubes are arranged so as to
form a square lattice configuration, the pressure tubes
and the calandria tubes are inserted into the calandria
tank. Therefore, if the interval between the formed
CA 0202~713 1998-02-11
lattices of the pressure tubes is narrowed, the pitch of
through holes formed in the surface of the calandria tank
is also narrowed, causing the strength of the calandria
tank to be deteriorated. Therefore, it has been
difficult for the interval between the disposed lattices
of the pressure tubes to be narrowed. If the interval
between the disposed lattices of the pressure tubes is
widened, the ratio of heavy water to nuclear fuel is
raised. The pressure tube type nuclear reactor of the
type described above tends to show a large positive void
reactivity, causing the self-control performance of the
nuclear reactor to become unsatisfactory.
In order to overcome the above-described
problem, the above-described structures (1) to (5) have
been disclosed. However, according to the structure (1),
the pitch of the positions of the pressure tubes become
non-uniform and complicated. Therefore, the evaluation
of the performance of the core and operational control
become unsatisfactory. ~urthermore, since the pitch of
the positions of the pressure tubes is partially
narrowed, the strength of the calandria tank at the
position at which the pressure tube passes through may be
partially deteriorated. In order to overcome the above-
described problem, the size of the calandria tank must be
enlarged. According to the structure (2), heavy water is
replaced by carbon rods, in part. However, since the
carbon rod is a relatively large neutron-absorber, the
neutron economy becomes deteriorated. In addition, since
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the carbon rods are disposed simply in parallel and among
the pressure tubes, spaces in which the carbon rods are
provided and spaces around the carbon rods must be
provided, causing the overall size of the calandria tank
to be enlarged. According to the structure (3), since
the heavy water tubes are disposed simply in parallel to
the pressure tubes, similar to the structure (2), the
size of the calandria tank becomes too large. According
to the structure (4), the fuel pins in the pressure tubes
are in part replaced by the water sealing rods in the
pressure tubes. Therefore, the quantity of the fuel is
reduced. In order to compensate for the reduced space
for the fuel, the size of the pressure tube for
accommodating the fuel and the calandria tank for
accommodating the pressure tubes are enlarged
excessively. According to the structure (5), the
diameter of the calandria tube at its intermediate
portion is arranged to be larger than that at other
portions. Therefore, the diameter of the hole formed in
the calandria tank for the purpose of inserting the
calandria tube is necessarily enlarged. As a result, the
interval of the holes must be enlarged for the purpose of
securing the strength of the calandria tank. Therefore,
the size of the calandria tank becomes too large.
When the size of the calandria tank becomes too
large, the quantity of heavy water to be contained
therein becomes too large or a satisfactory effect of
reducing the quantity of heavy water cannot be obtained.
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.
Therefore, the improvement in the self-control
performance of the nuclear reactor by reducing the volume
ratio of heavy water to fuel cannot be achieved, and what
is even worse, the size of the calandria tank becomes too
large.
An object of the present invention is to
improve the self-control performance of a pressure-tube
type nuclear reactor without enlarging the size of the
calandria tank.
In order to achieve the above-described object,
a first aspect of the present invention lies in a
pressure-tube type heavy-water moderated nuclear reactor
arranged in such a manner that pressure tubes each of
which accommodates nuclear fuel and through each of which
a coolant passes are accommodated in respective calandria
tubes disposed in a calandria tank which contains heavy
water, the pressure-tube type nuclear reactor comprising:
a structure arranged in such a manner that the pressure
tu~es are arranged so as to form an equilateral
triangular lattice configuration. As a result, the
volume ratio of heavy water to fuel can be reduced
without shortening the distance between pressure tubes.
A second aspect of the present invention
lies in a pressure-tube type heavy-water moderated
nuclear reactor arranged in such a manner that pressure
tubes each of which accommodates nuclear fuel and through
each of which a coolant passes are accommodated in
respective calandria tubes disposed in a calandria tank
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'_
which contains heavy water, the pressure-tube type
- nuclear reactor comprising: a structure arranged in such
a manner that a member which replaces the heavy water
positioned near the outer surface of the calandria tube
is positioned near the outer surface of the same. Since
the member is disposed around the calandria tube so as to
replace heavy water, the quantity of heavy water around
- the calandria tube can be reduced. Therefore, the volume
ratio of heavy water to fuel can be reduced. Since the
member to replace heavy water is disposed around the
pressure tube, the size of the calandria tank can be
reduced in comparison to a structure arranged in such a
manner that the member is disposed between the calandria
tubes after it has been separated from the pressure tube.
A third aspect of the present invention lies in
a method of assembling a pressure-tube type nuclear
reactor in which spaces for containing heavy water are
formed by hermetically fastening calandria tubes which
are inserted into a calandria tank to the component part
of the calandria tank, the method of assembling a
pressure-tube type nuclear reactor comprising the steps
of: securing the calandria tubes to the component part of
the calandria tank; introducing cylinders each of which
is made of a material which does not excessively absorb
neutrons and which is sectioned into a plurality of
sections into portions around the calandria tubes; and
assembling the cylinders sectioned into the plurality of
sections so as to cover the outer surfaces of the
CA 02025713 1998-02-11
calandria tubes. Since a member to replace a portion of
- heavy water is divided into sections so as to be
introduced into a portion around the calandria tube
before it is fastened to the portion around the calandria
tube, the working space for fitting the member to replace
heavy water and the interval between the pressure tubes
can be reduced. As a result, the volume ratio of heavy
- water to fuel can be improved without enlarging the
calandria tank.
A fourth aspect of the present invention lies
in a method of assembling a pressure-tube type nuclear
reactor in which spaces for containing heavy water are
formed by hermetically fastening calandria tubes which
are inserted into a calandria tank to the component part
of the calandria tank, the method of assembling a
pressure-tube type nuclear reactor comprising the steps
of: introducing cylinder members each of which is made of
a material which does not excessively absorb neutrons
into the spaces for containing heavy water; inserting the
calandria tubes into the cylindrical members thus
introduced when the calandria tubes are inserted into the
component part of the calandria tank; and fastening the
calandria tubes to the component part of the calandria
tank. When the calandria tube is inserted into the
calandria tank, the cylinder member is previously
introduced into the calandria tank. Then, the calandria
tube is inserted into the cylinder member simultaneously
with the insertion of the calandria tube into the
",~
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calandria tank. Therefore, the cylinder member serving
-- as the member to replace heavy water can be disposed
around the calandria tube without the necessity of
preparing a wide working space in the calandria tank.
A fifth aspect of the present invention lies in
a member to replace a heavy water moderator of a
pressure-tube type nuclear reactor to be disposed around
- a calandria tube, the member to replace a heavy water
moderator comprising: an assembled structure in the form
of a cylinder into which the calandria tube can be
introduced. Since a cylindrical member to replace heavy
water for surrounding the calandria tube can be obtained,
it can be used so as to surround the calandria tube. As
a result, heavy water around the calandria tube can be
replaced by the member so that the volume ratio of heavy
water to fuel can be improved.
A sixth aspect of the present invention lies in
a pressure-tube type nuclear reactor according to first
aspect, wherein the volume ratio of heavy water to fuel
is arranged to be 9.0 or less. In addition to the effect
obtainable from the first aspect, the void reactivity
coefficient of the coolant in the pressure tube can be
assuredly made closer to zero or negative values since
the pressure tubes are disposed in the equilateral
lattice configuration in such a manner that the density
of the configuration is determined so as to make the
volume ratio of heavy water to fuel 9.0 or less.
, . .
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Therefore, the self-control performance of the core of
the nuclear reactor can be further improved.
A seventh aspect of the present invention lies
in a pressure-tube type nuclear reactor according to the
second aspect, wherein the volume ratio of heavy water to
fuel is arranged to be 9.0 or less. In addition to the
effect obtainable from the second aspect, the void
reactivity coefficient of the coolant in the pressure
tube can be assuredly made closer to zero or negative
values since the members to replace heavy water are
disposed around the pressure tubes in such a manner that
the density of the configuration is determined so as to
make the volume ratio of heavy water to fuel 9.0 or less.
Therefore, the self-control performance of the core of
the nuclear reactor can be further improved.
An eighth aspect of the present invention lies
in a pressure-tube type nuclear reactor according to the
first aspect, wherein some of the pressure tubes are
replaced by control rod guide tubes, neutron
instrumentation guide tubes or poison tubes for the
pressure-tube type nuclear reactor at the position at
which the pressure tubes are posltioned and the volume
ratio of heavy water to fuel is arranged to be 9.0 or
less. In addition to the effect obtainable from the
first aspect, all of the tubes such as the pressure
tubes, control rod guide tubes, neutron instrumentation
guide tubes or poison tubes for the nuclear reactor can
be arranged in the triangular lattice configuration with
, ~ , .
CA 0202~713 1998-02-11
which the quantity of heavy water can be reduced.
Therefore, the quantity of heavy water can be easily
reduced.
A ninth aspect of the present invention lies in
a pressure-tube type nuclear reactor according to the
first aspect, wherein a control rod with a fuel follower
is positioned in a coolant in some of the pressure tubes.
In addition to the effect obtainable from the first
aspect, an effect can be obtained in that the control
rods with a fuel follower inserted into the coolant in
some of the pressure tubes can be cooled by the coolant
which passes through the pressure tube.
A tenth aspect of the present invention lies in
a pressure-tube type nuclear reactor according to the
first aspect, wherein a cylinder surrounding the outer
surface of the calandria tube is disposed so as to
replace heavy water. In addition to the effect
obtainable from the first aspect, the neutron economy can
be maintained at a satisfactory level since the member to
replace heavy water surrounding the calandria tube does
not excessively absorb neutrons. In addition, since the
calandria tube is surrounded by a cylinder, heavy water
can be equally replaced by the cylinder from the outer
surface of the calandria tube. Therefore, the intensity
of the neutrons effecting the fuel can be distributed
equally around the calandria tubes.
An eleventh aspect of the present invention
lies in a pressure-tube type nuclear reactor according to
-- 10
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the second aspect, wherein the member for replacing heavy
water is disposed in such a manner that the member forms
a cylinder which surrounds the calandria tube after
assembled. In addition to the effect obtainable from the
second aspect, heavy water can be equally replaced by the
cylinder from the outer surface of the calandria tube
since the calandria tube is surrounded by the cylinder.
Therefore, the intensity of the neutrons effecting the
fuel can be distributed equally around the calandria
tubes.
A twelfth aspect of the present invention lies
in a pressure-tube type nuclear reactor according to the
first aspect, wherein a cylinder surrounding the outer
surface of the calandria tube is disposed so as to
replace heavy water and the cylinder has its wall
arranged to be a hollow shape. In addition to the effect
obtainable from the first aspect, the neutron economy can
be further improved since the cylinder has a hollow wall.
In addition, since heavy water for the volume for the
hollow portion can be replaced, a large quantity of heavy
water can be replaced by a light weight structure.
A thirteenth aspect of the present invention
lies in a pressure-tube type nuclear reactor according to
the second aspect, wherein the member for replacing heavy
water is a cylinder which surrounds the calandria tube
and the cylinder has its wall arranged to be a hollow
shape. In addition to the effect obtainable from the
second aspect, the neutron economy can be further
CA 0202~713 1998-02-11
~~ improved since the wall of the cylinder is in the hollow
shape. Furthermore, since heavy water can be replaced by
the volume for the hollow portion, a large quantity of
heavy water can be replaced by a light weight structure.
A fourteenth aspect of the present invention
lies in a pressure-tube type nuclear reactor according to
the second aspect, wherein the member which replaces
heavy water is made of a zirconium alloy, aluminum or an
aluminum alloy. In addition to the effect obtainable
from the second aspect, an improved neutron economy can
be obtained since the member to replace heavy water is
made of a material such as a zirconium alloy, aluminum or
aluminum alloy which do not easily absorb neutrons.
A fifteenth aspect of the present invention
lies in a pressure-tube type nuclear reactor according to
the second aspect, wherein the calandria tubes are
arranged to be in an equilateral triangular lattice
configuration. In addition to the effect obtainable from
the second aspect, the volume ratio of heavy water to
fuel can be improved by the arrangement of the
equilateral triangular lattice configuration of the
calandria tubes and the substitution of heavy water by
the member. Therefore, if the volume ratio of heavy
water to fuel cannot be easily adjusted by either of the
above-described means, both of the above-described means
are employed so that it can be easily adjusted.
A sixteenth aspect of the present invention
lies in a pressure-tube type nuclear reactor according to
- 12 -
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the second aspect, wherein the member which replaces
- heavy water is disposed in a region which shows a
relatively high output density in the calandria tank. In
addition to the effect obtainable from the second aspect,
the distribution of the outputs can be uniformed by
arranging the balance of the outputs from a region in
which the output density is high and that from a region
in which the output density is low since the above-
described cylinder is positioned in the region in which
the output denslty is relatively high.
A seventeenth aspect of the present invention
lies in a pressure-tube type nuclear reactor according to
the second aspect, wherein the member which replaces
heavy water is arranged in such a manner that its
thickness is enlarged in proportion to the output
density. In addition to the effect obtainable from the
second aspect, the quantity of heavy water to be replaced
can be enlarged in proportion to the output density in
the region in which the output density is high since the
above-described cylinder is arranged to have an increased
thickness in proportion to the output density.
Therefore, the volume ratio of heavy water to fuel can
be, in proportion to the output density, adjusted in the
region in which the output density is high.
Other and further objects, features and
advantages of the invention will appear more fully from
the following description provided in conjunction with
the accompanying drawings, in which:
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-
Fig. 1 illustrates a first embodiment of the
present invention and is a plan view which illustrates
the plan configuration of calandria tubes of a pressure-
tube type nuclear reactor;
Fig. 2 illustrates the first embodiment of the
present invention and is a vertical cross sectional view
which illustrates the core of the pressure-tube type
nuclear reactor;
Fig. 3 illustrates the first embodiment of the
present invention and is an enlarged view which
illustrates the configuration of pressure tubes each of
which serves as a unit of a portion including control rod
guide tubes or neutron instrumentation tubes shown in
Fig. 1;
Fig. 4 is a graph which illustrates the
relationship between void reactivity coefficient and
volume ratio of heavy water to fuel of the pressure-tube
type nuclear reactor;
Fig. 5 illustrates a second embodiment of the
present invention and is a plan cross sectional view
which illustrates the relationship among the calandria
tubes, cylinder disposed outside the calandria tubes and
control rod guide tubes or neutron instrumentation tubes
disposed in the calandria tank of the pressure-tube type
nuclear reactor;
Fig. 6 illustrates a second embodiment of the
present invention and is a vertical cross sectional view
which illustrates the calandria tubes and cylindrical
- 14 -
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portions disposed outside the calandria tubes according
to the embodiment shown in Fig. 5;
Fig. 7 illustrates the second embodiment of the
present invention and is a plan view which illustrates
the configuration of the tubes and the cylinders when
viewed from a portion above an upper tube sheet according
to the embodiment shown in Fig. 6;
Fig. 8 illustrates the second embodiment of the
present invention and is a plan view which illustrates
the overall configuration of the tubes in the calandria
tank of the pressure-tube type nuclear reactor;
Fig. 9 illustrates the second embodiment of the
present invention and is a perspective view which
illustrates the cylinder;
Fig. 10 illustrates the second embodiment of
the present invention and is a plan cross sectional view
which illustrates a fastener portion for the cylinder;
Fig. 11 illustrates the first embodiment of the
present invention and is a plan view which schematically
illustrates the equilateral triangular lattice
configuration of the tubes;
Fig. 12 illustrates a conventional structure
and is a plan configuration view which schematically
illustrates square lattice configuration of the tubes;
Fig. 13 illustrates a third embodiment of the
present invention and is a plan configuration view which
partially illustrates a hollow cylinder;
. . .
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-
~- Fig. 14 illustrates a conventional structure
and is a plan configuration view which illustrates the
overall plan configuration of the calandria tubes of the
pressure-tube type nuclear reactor;
Fig. 15 illustrates a conventional structure
and is a plan configuration view which illustrates a
portion of the configuration of the pressure tubes shown
in Fig. 14 in an enlarged manner;
Fig. 16 is a graph which illustrates the
relationship between the neutron multiplication factor
and the volume ratio of heavy water to fuel of the
pressure-tube type nuclear reactor;
Fig. 17 illustrates a fourth embodiment of the
present invention and is a plan configuration view which
illustrates the plan configuration of the tubes in a
quarter portion of the core of the pressure-tube type
nuclear reactor;
Fig. 18 illustrates the fourth embodiment of
the present invention and is a vertical cross sectional
view which illustrates a half portion of the core of the
pressure-tube type nuclear reactor;
Fig. 19 illustrates the fourth embodiment of
the present invention and is a plan cross sectional view
which illustrates a quarter portion of the calandria
tube; and
Fig. 20 is a graph which illustrates the output
peaking coefficient in the axial direction of the core
according to the present invention.
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The quantity of heavy water for one pressure
tube can be expressed by hatched area Slin the cross
sectional view of Fig. 11, the area S1 being obtained from
the following equation:
S1 = 2~ Q12 ~ ~R12 ~-- (1)
where ~1: interval between lattices
R1: outer radius of calandria tube
On the other hand, the quantity of heavy water
for one pressure tube arranged in a square lattice
configuration can be expressed by hatched area S2 in the
cross sectional view of Fig. 12, the area S2 being
obtained from the following equation:
S = (e)2 _ ~(R)2 ...................... (2)
where ~2: interval between lattices
R2: outer radius of calandria tube
Assuming that the triangular lattice and the
square lattice have the same interval between lattices
and the diameter of the calandria tube and therefore
el = ~2 = ~ and Rl= R2= R, heavy water volume reduction
ratio ~M for one pressure tube, obtained from a
comparison made between the equilateral triangular
lattice configuration and the square lattice
configuration, can be expressed by the following equation:
~M = (S2 - Sl)/S2
= (1 - ~3/2) x {1 - ~(R/Q)2}-l (3)
Specifically, it is assumed that ~ = 24 cm and R = 8 cm
so as to be substituted into Equation (3), a reduction
~0 ratio ~M of about 0.21 is obtained.
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Reduction ratio ~L of the diameter of the core
portion which is a critical factor to determine the size
of the calandria tank can be obtained from the following
equation:
~L = {ez ~ (2~3 )oSel}/e2
= 1 - (~3)os
. 0.07 (4)
That is, assuming that the lattice intervals
are the same, the quantity of heavy water can be
significantly reduced (about 21~ according to the above-
described example) and the diameter of the core can be
reduced by 7~.
Therefore, according to the equilateral
triangular lattice configuration, the volume of heavy
water can be reduced without shortening the interval
between the pressure tubes and the volume ratio of heavy
water to fuel can be reduced. Therefore, the size of the
calandria tank can be reduced, causing a satisfactory
operational performance and an economical advantage to be
obtained.
With respect to the second aspect of the present
invention, the volume ratio of heavy water to fuel (D/F)
can be obtained from the following equation in the case
where the member having a thickness t is successively
placed around the pressure tube (the member having a
cylindrical shape):
D/F = {e - ~(R + t)2}/n~r2
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- where ~ = interval between lattices
R = outer radius of calandria tube
t = thickness of cylinder
n = number of fuel pins per fuel assembly
r = radius of fuel pellet
The improved degree QD/F of the volume ratio
D/F of heavy water to fuel can be obtained from Equation
(5) as follows:
~D/F = -{7r(R + t) 2 _ 7rR2}/n~r2 ... (6)
A first embodiment of the present invention
will now be described.
A plurality of calandria tubes 1, control rod
guide tubes 4 and neutron instrumentation tubes 5 are
inserted into a calandria tank 2 in which heavy water 10
serving as a neutron moderator is enclosed.
A pressure tube 3 is inserted into the
calandria tube 1, the lower end of the pressure tube 3
being connected to a pipe 31 through which light water
passes through. The other end of the pressure tube 3 is
connected to another pipe 32.
The pressure tube 3 accommodates a fuel
assembly constituted by binding a plurality of fuel pins
each of which contains nuclear fuel.
Light water is introduced into the pressure
tube 3 through the pipe 31 by a pump (not shown). As a
result, light water in the pressure tube 3 is heated by
the fuel assembly so that it boils. As a result, light
water becomes a two phase ~low consisting o~ steam and
- 19 -
CA 0202~713 1998-02-11
' _
- liquid at high temperature and high pressure so that the
two phase flow passes through the pipe 32. It is then
taken outside the core so as to act as energy to rotate
the turbine of a generator.
As shown in Fig. 1, according to this
embodiment, the calandria tubes 1 are arranged to be in
the form of equilateral triangular lattices each of which
includes a control rod guide tube 4 or a neutron
instrumentation tube 5.
The above-described calandria tubes 1, the
control rod guide tubes 4 and the neutron instrumentation
tubes 5 are hermetically connected to an upper tube sheet
6 and a lower tube sheet 7, as shown in Fig. 2, in order
to enable the calandria tank 2 to contain heavy water.
According to the above-described equilateral
triangular lattice arrangement of the calandria tubes 1,
an effect can be obtained in that the necessary volume of
heavy water can be reduced in comparison to a
conventional square lattice arrangement shown in Figs. 14
and 15.
The manner in which to dispose the control rod
guide tube 4 will be described.
As shown in Fig. 3, when the control and
instrumentation pipes such as the control rod guide tube
4, the neutron instrumentation tube 5, a poison tube and
the like are disposed in the central portion of the
triangular lattice, predetermined mechanical strength
must be secured in a joint (usually a rolled joint)
- 20 -
CA 0202~713 1998-02-11
'~_
~ between the above-described pipes and the calandria pipe
1 and the upper tube sheet 6. Therefore, a minimum gap 8
must be secured between the pipe having the largest outer
diameter (usually, the control rod guide pipe 4) and the
calandria tube 1. As a result, the maximum diameter of
the above-described pipes are determined.
Since the equilateral angular lattice enables
the above-described maximum diameter to be reduced in
comparison to the square lattice, the control rod guide
pipe 4 must be disposed to the same position at which the
pressure tube 1 is disposed in the case where a large-
diameter control rod guide tube 4 must be employed. In
this case, since the necessity of a space in which the
control rod guide tube 4 is disposed in the equilateral
triangular lattice can be eliminated, the gap between the
pressure tubes 1 can be narrowed. Therefore, a
preferable volume ratio of heavy water to fuel can be
realized even if the pressure tube 1 is replaced by the
control rod guide tube 4.
Furthermore, in some instances, fuel control
rods are inserted into some pressure tubes 1 in the
equilateral triangular lattices so as to replace the
control rod guide tubes 4. In order to prevent a
deterioration in the volume ratio of heavy water to fuel
due to the above-described provision of the control rods,
the gap between the pressure tubes 1 must be reduced by
the degree which corresponds to the eliminated space in
which the control rod guide tube 4 is disposed in the
CA 02025713 1998-02-11
equilateral triangular lattice area. In this case, since
light water is, as the coolant, passed through the
pressure tubes 1, the control rod in the pressure tube 1
can be satisfactorily cooled. Therefore, a control rod
with a fuel follower can be employed as the control rod.
In a structure in which the control rod with a fuel
follower is employed, the fuel can be introduced by the
degree of the gradual removal of the control rod from the
core at the end stage of the combustion. Therefore, the
deterioration in the output at the final stage of the
combustion in the core can be prevented.
A specific example of the equilateral
triangular lattice will be described. When the control
rod guide tube 4 is positioned at the position at which
the pressure tube 1 has been disposed, essential
dimensions of a large-size reactor arranged for the
purpose of realizing a desired volume ratio of 8.0
between heavy water and fuel are as follows, where the
size of the lattice is 25 cm and the number of the fuel
pins in the fuel assembly is 54:
Distance between lattices ~ : 25.0 cm
Inner diameter of pressure tube: 12 cm
Configuration: Equilateral triangular lattice
configuration
Number of fuel pins in fuel assembly: 54
Diameter of fuel pellet: 1.0 cm
Outer diameter of calandria tube: 16 cm
- 22 -
,
CA 0202~713 1998-02-11
As a result, the above-described volume ratio
- of 8.0 between heavy water and fuel can be achieved.
A second embodiment of the present invention
will be described.
Referring to Figs. 5 and 6, a second embodiment
of the present invention, structured in such a manner
that a cylinder is disposed around each of the pressure
tubes arranged to be in a square lattice configuration,
will be described. Fig. 5 illustrates a unit of the
lattices in the calandria tank 2 according to the present
invention, the unit being composed of four calandria
tubes 1 arranged to be square lattice configuration. A
pad 30 is held between the calandria tube 1 and a
cylinder 9 in order to form a proper gap around the
calandria tube 1. The cylinder 9 is then placed on a
lower tube sheet. The cylinder 9 is made of a material
such as metal of a type which does not excessively absorb
neutrons and which has satisfactory strength, the metal
being exemplified by Zircalloy-2, zirconium-niobium alloy
and aluminum alloy. The cylinder 9 can be sectioned into
two or three sections in its circumferential direction so
as to be fastened easily as described above. A fastener
11 is slid in a direction designated by an arrow 12 as
shown in Fig. 9, so as to realize the state of the
fastening as shown in Fig. 10. If necessary, the
cylinder 9 may be shortened in its axial direction (by 50
cm to 200 cm), so that it can be introduced into the
calandria tank 2 and easily fastened there as shown in
CA 0202~713 1998-02-11
Fig. 6 after the calandria tube 1 has been connected to
the upper tube sheet 6 and the lower tube sheet 7 of the
calandria tank 2. After all of the cylinders 9 have been
fastened as described above, heavy water serving as the
moderator is introduced into the calandria tank 2.
As shown in Fig. 7, the portion at which the
control rod guide tube 4 passes through the upper tube
sheet 6 of the calandria tank 2 must secure predetermined
mechanical strength at the joint between the calandria
tube 1 and the control rod guide tube 4 and the upper
tube sheet 6. Therefore, the minimum gap 8 between the
calandria tube 1 and the control rod guide tube 4 must
have the necessary size.
Usually, it is difficult to enlarge the
diameter of the calandria tube 1 at the upper tube sheet
6. Therefore, the diameter of the calandria tube 1 in the
core in the calandria tank 2 is made larger than that in
its upper portion.
However, the enlargement of the diameter of the
calandria tube or the arrangement of the drawing of the
shape of it encounters a certain limit in terms of the
manufacturing difficulty and the mechanical structure.
Therefore, it is difficult to considerably enlarge the
diameter of the calandria tube in the calandria tank.
Therefore, according to the present invention,
the cylinder 9 is disposed around the calandria tube 1 so
as to overcome the above-described difficulty.
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CA 0202~713 1998-02-11
The control rod guide tubes 4 and the neutron
instrumentation tubes 5 of a number necessary to control
and to measure the output from a nuclear reactor are, as
shown in Fig. 7, disposed properly in the gaps between
the calandria tubes 1.
A specific example which is able to
quantitatively realize the effect of the cylinders 9 will
be described.
In this case, the configuration is arranged to
be the square lattice configuration as shown in Fig. 8.
Lattice interval ~: 24.5 cm
Calandria tube outer radius R: 8 cm
Number n of fuel pins of fuel assembly: 54
Fuel pellet radius r: 0.5 cm
Cylinder thickness: 1.0 cm
- As a result of the effect of the cylinder
provided, the volume ratio of heavy water to fuel can be
improved from 9.4 to 8.1.
A third embodiment of the present invention,
provided with a cylinder having a different structure
from that of the cylinder 9 according to the second
embodiment, will be described. According to the third
embodiment, an economical effect can be obtained. The
difference in the structure of the cylinder 9 will be
described.
As shown in Fig. 13, the wall portion of the
cylinder 9 is arranged to be a hollow structure. An
outer plate 13 and an inner plate 14 of the hollow
- 25 -
CA 0202~713 1998-02-11
cylinder is made of Zircalloy-2 or an aluminum alloy
- (1100). If necessary, a spacer 16 may be disposed so as
to maintain the gap in the hollow portion and separated
portions 15 are joined by the fastener 11 or by welding.
The above-described separated portions 15 are connected
to each other by the fastener 11, the portions 15 in
which the circumferential end portions of the outer plate
13 and those of the inner plate 14 are hermetically
welded by a welded portion 18. Furthermore, the axial
end portions of the hollow cylinder are hermetically
closed so that a hollow portion 17 is formed. The
effective thickness of the hollow cylinder is defined
from the outer surface of the outer plate 13 to the inner
surface of the inner plate 14.
The volume ratio of heavy water to fuel can be
improved from 10 to 8.2 when a hollow cylinder having the
effective thickness t = 1.2 cm is used in a square
lattice core arranged as follows:
Lattice interval e 24 cm
Calandria tube outer radius R: 8 cm
Number n of fuel pins of fuel assembly: 48
Fuel pellet radius r: 0.5 cm
An example of a structure arranged in such a
manner that the control rod guide tube 4 is disposed at
the central portion of the triangular lattice and the
cylinder is disposed around the calandria tube for the
purpose of sufficiently improving the volume ratio of
- 26 -
CA 0202~713 1998-02-11
heavy water to fuel will be described. In this case, the
- arrangements and the effect are as follows:
Inner diameter of pressure tube: 12 cm
Configuration: Equilateral triangular lattice
configuration
Lattice pitch: 25.5 cm
Number n of fuel pins of fuel assembly: 54
- Fuel pellet diameter: 1.0 cm
Outer diameter of calandria tube in the core
portion: 16 cm
Outer diameter of calandria tube at the upper
tube sheet: 15 cm
Outer diameter of control rod guide tube: 5 cm
Minimum gap at the upper tube sheet: 4.7 cm
lS Effective thickness of the cylindrical wall:
0.5 cm
Volume ratio of heavy water to fuel: 7.9
According to the above-described examples, the
volume ratio of heavy water to fuel can be properly
designed.
The void coefficient changes depending upon the
type of fuel (oxide fuel containing plutonium or uranium
dioxide fuel), the composition, the enrichment and the
like. However, since the volume ratio of heavy water to
fuel can be arranged to be 7.9 to 8.2, according to the
present invention, the void coefficient can be made a
value in the vicinity of zero, in the order of 10-5 ~k/k/~
- 27 -
CA 0202~713 1998-02-11
void. Therefore, an excellent operating efficiency can
- be obtained.
Furthermore, since the diameter of the core and
the quantity of heavy water can be reduced, an economical
S effect can be obtained.
An example of a method of fastening the
cylinder 9 to the portion around the calandria tube 1
will be described.
The manufactured cylinder 9 is placed on the
lower tube sheet 7 prior to the fastening of the
calandria tube 1 to the calandria tank 2. The calandria
tube 1 is then inserted into the lower tube sheet 7. At
this time, the calandria tube 1 is also inserted into the
cylinder 9. Thus, the structure in which the cylinder 9
as an alternative to heavy water is disposed around the
calandria tube 1 can be manufactured.
According to the above-described manufacturing
method, the necessity of the work for assembling the
cylinder 9 around the calandria tube 1 performed in a
narrow space can be eliminated. Furthermore, the
necessity of providing a wide working space around the
calandria tube 1 can be eliminated. It is preferable
that the cylinder 9 be disposed in such a manner that it
is held between the upper and the lower tube sheets 7 and
8 via a spacer.
Accordingly, the structure is arranged in such
a manner that the calandria tubes 1 are disposed in the
core in the form of a triangular lattice, the volume
- 28 -
CA 0202~713 1998-02-11
ratio of heavy water to fuel can be properly determined
- and the coolant void reactivity coefficient can be made
closer to negative values as much as possible.
Therefore, the self-control performance peculiar to a
nuclear reactor can be improved. For example, since the
quantity of heavy water can be reduced by about 20~ and
the diameter of the core can be reduced by about 7~ in
the same lattice interval as that of the conventional
structure, an economical effect can be obtained.
Furthermore, when a cylinder is, as an
alternative to heavy water, fastened around the calandria
tube 1, the coolant void reactivity coefficient can be
made a value further closer to a negative value.
Therefore, the self-control performance can be improved.
In addition, when both the above-described
triangular lattice configuration and the cylinder are
simultaneously employed, the design can be significantly
freely made for the purpose of improving the operation
efficiency and the economical effect.
A fourth embodiment of the present invention
further established for the purpose of making the
distribution of outputs from the pressure type nuclear
reactor uniform will be described.
As shown in Fig. 4, in a design range of a
nuclear reactor which uses heavy water as the moderator,
when the volume ratio of heavy water to fuel is reduced,
the coolant void reactivity coefficient is usually
reduced (the self-control performance is improved). As
- 29 -
CA 0202~713 1998-02-11
shown in Fig. 16, the neutron multiplication factor is
reduced when the ratio of the volume of a heavy water
moderator to the volume of fuel has become reduced.
Usually, the more the neutron multiplication
factor is, the more the burn-up of the same fuel can be
enlarged (the fuel economy can be improved). Therefore,
a proper volume ratio of heavy water to fuel can be
selected in such a manner that both the self-control
performance and the fuel economy are maintained at a
satisfactory degree.
Since a larger quantity of neutrons leak in the
peripheral portion in the core in comparison to that in
the central portion of the core, the neutron
multiplication factor becomes reduced. Therefore, the
output density becomes lowered and a uniform output
distribution cannot be obtained.
Accordingly, according to the fourth
embodiment, an improvement of the self-control
performance and a uniform output distribution can be
simultaneously achieved.
Fig. 17 illustrates a quarter portion of the
arrangement of calandria tubes 1, neutron instrumentation
tubes and control rod guide tubes in the calandria tank
2.
As shown in Fig. 19, the calandria tube 1
includes the pressure tube 3 through which a coolant 22
passes. The pressure tube 3 also includes fuel pins 21
which contain pellet nuclear fuel. In the calandria tank
- 30 -
CA 0202~713 1998-02-11
2 which contains heavy water 10, the calandria tubes 1
are disposed in a square lattice configuration by the
number necessary to obtain the rated output from the
pressure tube type nuclear reactor. Furthermore, the
control rod guide tubes 4 and the neutron instrumentation
tubes 5, which are necessary to control the core or to
perform the instrumentation, are properly disposed among
the above-described lattices. If necessary, poison tubes
for introducing poison for controlling the output of the
core are provided. The above-described tubes are
inserted into the calandria tank 2 and are hermetically
joined to the upper tube sheet 6 and the lower tube sheet
7 of the calandria tank 2 so that the calandria tank 2
contains heavy water.
As shown in Figs. 17, 18 and 19, the cylinder 9
is disposed around the calandria tube 1 disposed at the
central portion of the core in the calandria tank 2.
The cylinder 9 may be fastened in such a manner
that it is supported by the calandria tube 1 or by the
lower tube sheet 7 via a spacer.
The cylinder 9 is made of a material, which
does not excessively absorb neutrons, such as a zirconium
alloy (Zircalloy-2, Zircalloy-4, zirconium-niobium alloy
or the like), aluminum or an aluminum alloy.
In order to enlarge the volume which can be
replaced by heavy water, the wall of the cylinder 9 is
arranged so as to form the hollow portion 17 which
contains a chemically stable gas such as helium or argon.
CA 0202~713 1998-02-11
It is preferable that the hollow portion 17 be filled
with a chemically stable gas such as helium or argon in
the structure according to the above-described
embodiments in the case where the cylinder 9 is arranged
to be the hollow portion. In order to seal the thus
enclosed gas, the hollow portion 17 is hermetically
sealed similar to the above-described embodiments. The
cylinder 9 may be sectioned into sections similar to the
above-described embodiments. In this case, the end
portions thus sectioned are structured so as to be joined
to each other by a fastener similar to the above-
described embodiments.
Sixty fuel pins are collected so as to form a
fuel assembly which is included in the pressure tube. An
example, arranged in such a manner that the cylinder 9
having hollow wall portion is disposed around the
calandria tube 1 as an alternative to heavy water, will
be described in terms of the quantitative effect.
According to this example, the present
invention is applied to a 1,000 MW electric output
pressure tube type nuclear reactor in which heavy water
is used as the moderator and light water is used as the
coolant. A specific example of the structure is as
follows:
Power output: 1000 MW Number of pressure tubes:
688
Number n of fuel pins of fuel assembly: 60
Effective height of the core portion: 3.7 m
CA 0202S713 1998-02-11
Radius of core portion: 7.8 m
~ Fuel pellet radius r: 0.475 cm
Inner diameter of pressure tube: 12.3 cm
Pressure tube lattice interval Q: 24.5 cm
Outer radius R of calandria tube: 8.25 cm
Outer diameter of the control rod guide tube:
9.5 cm
Effective thickness of hollow cylinder: 1.8 cm
Number of calandria tubes 1 in which hollow
cylinders are provided: 344
Axial length in which the hollow cylinder is
disposed: 2.22 m
Minimum interval in the upper tube sheet
(between the calandria tube 1 and the control
rod guide tube 4): 4.3 cm
According to this example, the volume ratio of
heavy water to fuel in the portion to which no cylinder
is fastened is 9.1, while that in the portion (the
central portion of the core) to which the cylinder is
fastened is 6.7. As described above, when the volume
ratio of heavy water to fuel in the central portion of
the core is lowered with respect to that in the
peripheral portion, the neutron multiplication factor in
the central portion of the core becomes reduced in
comparison to that in the peripheral portion. Therefore,
the number of neutrons generated in the central portion
is reduced, causing a curve showing the power density
distribution to be changed from the dotted line to a
CA 0202~713 1998-02-11
solid line as shown in Fig. 20 as well due to the
- deterioration in the power distribution in the peripheral
portion caused from the neutron leakage in the
peripheral portion. As a result, the power distribution
in the axial direction (vertical direction) of the core
can be made more uniform. Since a similar effect can be
obtained in the radial direction of the core, the power
distribution in both the axial direction and the radial
direction of the core can be made more uniform.
10As a result, the total (effective) volume ratio
of heavy water to fuel in the core can be made a value in
the vicinity of 8. Therefore, as shown in Fig. 4, the
void coefficient can be made a value in the vicinity of
zero in an order of 10-5 ~k/k/% void. As a result, a
satisfactory operating performance can be obtained and
the power distribution can be improved by about 20%
without deterioration in the neutron economy, causing an
economical effect to be obtained. As described above,
according to the present invention, the volume ratio of
heavy water to fuel can be determined to a desired value
so that the coolant void coefficient can be made a value
further closer to the negative side. As a result, the
self-control performance peculiar to the neutron reactor
can be improved. Furthermore, since the cylinders 9 can
be partially disposed in the portions showing a high
power density so as to make the power distribution in the
core to be uniform, the number of the control rods
necessary to uniform the power distribution can be
''. ,' '
CA 0202~713 1998-02-11
reduced. In addition, the performance of the core can be
easily controlled, the operation performance can be
improved and an economical effect can be obtained.
A structure arranged in such a manner that the
thickness of each of the cylinders 9 is enlarged in
inverse proportion to the distance from the radial center
of the core will further enable the power distribution in
~ the radial direction to be uniformed. In addition, a
structure arranged in such a manner that the thickness of
each of the cylinders 9 is enlarged in inverse proportion
to the distance from the axial center will enable the
power distribution in the axial direction to be
uniformed.
Furthermore, if the calandria tubes are
disposed in a triangular lattice configuration, the
volume ratio of heavy water to fuel can be adjusted
without the necessity of enlarging the calandria tank.
Therefore, the adjustment and design can be further
freely performed.
In any of the above-described embodiments, it
is preferable that a certain gap be provided between the
calandria tube and the cylinder for the purpose of
passing heavy water.
