JPH11101888A - Fuel assembly and core of reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance a fuel economy near a C-lattice core even in a D-lattice core by disposing a number of moderating rods in an anti-channel fastener side area rather than the channel fastener side area in a fuel assembly body. SOLUTION: In the case where the inside of a fuel assembly body 90 is divided into the first area of a channel fastener side (right above) and the second area of an anti-channel fastener side (left below) by a diagonal line, all moderating rods 10 are disposed in the second area. The right above corner faces the wide gap water area of a D-lattice core, which has large moderation effect of neutrons by water as compared with a narrow gap water area. The neutron moderation effect is increased in the area and fission reaction is easily produced by collectively disposing the moderating rods 10 of the large neutron moderation effect in the narrow gap water area side of the inside of the fuel assembly 90. Thus, easy production of the fission of the inside of the fuel assembly is made substantially uniform even in the D-lattice core, a degree of freedom of fuel design is increased and fuel profitability can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water nuclear reactor, and more particularly to a fuel assembly in which control rods are inserted more than a fuel assembly in which control rods are not inserted. The present invention relates to a fuel assembly suitable for loading a wide D lattice core.

[0002]

2. Description of the Related Art In a boiling water reactor, water in the core plays a role of neutron deceleration and a role of fuel cooling. Neutron moderation plays an important role, since fissile material mainly reacts with moderated neutrons (thermal neutrons) to cause fission. In general, the neutron moderation effect increases as the width of the water region increases.

[0003] In the core of a boiling water reactor, the spacing between the C lattice cores in which the intervals between the fuel assemblies are all equal and the interval between the fuel assemblies in which the control rods are inserted are set between the fuel assemblies in which the control rods are not inserted. There is a D lattice core wider than the spacing. Therefore, in the D-lattice core, the neutron moderating effect is larger in the gap water region (hereinafter referred to as the wide gap water region) in which the distance between the fuel assemblies is large than in the narrow gap water region (hereinafter referred to as the narrow gap water region). Is big. Therefore, in the fuel assembly loaded in the D-lattice core, the fuel rods facing the wide gap water region and the fuel rods facing the narrow gap water region have different outputs.

[0004] It is better that the variation in the output of each fuel rod in the fuel assembly is small. If the variation in the output is large, the maximum output per unit length of the fuel rod, which is a safety restriction of the reactor (maximum line) ) And the limit of the output that can be taken out by one fuel assembly (limit output) become stricter, and the degree of freedom in designing the fuel assembly decreases. Due to the effect of this restriction, the D lattice core has a fuel extraction burn-up (energy that can be extracted from a unit weight of fuel) several percent smaller than the C lattice core, resulting in poor fuel economy.

[0005] As a conventional technique for solving such a problem, a nuclear fuel report, March 1992, pp11-1 is known.
5 and Proceedings of the Fifth International Topic
alMeeting On Reactor Thermal Hydraulics, Vol.1, pp
212-216 describes a fuel assembly in which water rods are arranged asymmetrically. This conventional example also describes that the average core burn-out of the fuel of the D-lattice core can be increased by 1% or more than the conventional one.

[0006]

However, even in the conventional example described above, the average core take-out burnup is smaller than that of the C lattice core.
Fuel economy is poor.

[0007] The object of the present invention is to provide a D-grating core with C
An object of the present invention is to provide a nuclear reactor core capable of achieving fuel economy close to that of a lattice core and a fuel assembly used for the nuclear reactor core.

[0008]

In order to achieve the above object, the present invention provides a fuel cell system comprising a plurality of fuel rods arranged in a square grid of 9 rows and 9 columns or more, and a water cross section larger than the fuel rods. A rod, a plurality of moderator rods filled with a solid moderator having a greater hydrogen atom number density than 70 atm of saturated water;
A lower tie plate that holds lower ends of the water rod and the reduction rod; an upper tie plate that holds at least upper ends of the fuel rod and the water rod; and a channel for fixing a channel box to the upper tie plate. A fuel assembly provided with a fastener and a first diagonal line on the channel fastener side of the fuel assembly.
When the region is divided into the region and the second region on the side opposite to the channel fastener, the speed reduction rods are arranged more in the second region than in the first region.

Further, in a nuclear reactor core having a plurality of fuel assemblies and a plurality of control rods, wherein a distance between the fuel assemblies at a position where the control rod enters is wider than that at a position where the control rod does not enter. The fuel assembly is loaded as a fuel assembly.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a D lattice core of a boiling water reactor will be described below. FIG. 1 is a cross-sectional view of a first embodiment of a fuel assembly according to the present invention, FIG. 2 is a longitudinal sectional view of the first embodiment of the fuel assembly, and FIG. 3 is a first embodiment of a nuclear reactor core according to the present invention. It is a 1/4 schematic cross section. In the present embodiment, the present invention is applied to a fuel assembly having a 9 × 9 (9 rows × 9 columns) fuel rod array.

The fuel assembly 90 includes 71 fuel rods and 2 fuel rods.
A large diameter water rod 3, three reduction rods 10, an upper tie plate 5 and a lower tie plate 6 for holding the upper end and lower end of the fuel rod, the water rod 3, and the reduction rod 10; , Water rods 3, seven fuel spacers 4 holding the reduction rods 10, and a channel box 7 surrounding the fuel spacers 4.

The fuel rods are composed of 66 long fuel rods 2 having a long active fuel length, and five short fuel rods 9 having a shorter active fuel length than the long fuel rods 2. The short fuel rods 9 are loaded at three corners of the second layer from the outermost periphery of the fuel assembly and at intermediate positions between the corners of the second layer. Water rod 3
Are loaded in the central area of the fuel assembly where the seven fuel rods can be placed. The fuel spacers 4 are arranged at substantially equal intervals in the axial direction of the fuel assembly.

The moderator rod 10 has a solid moderator 2 in a circular pipe 20.
5 is filled. As the solid moderator 25, Zr,
A hydride such as U, Ti, Mg, Li, and Ce can be used. As shown in FIG. 4, these hydrides have a hydrogen atom number density higher than that of saturated water at a high temperature and a high pressure (core conditions during operation of a boiling water reactor are about 70 atmospheres and about 285 degrees Celsius). Is high.

As shown in FIG.
The solid moderator 25 may be processed into a pellet shape, and the pellet filled in the circular tube 20 may be used. In this case, since the contact area between the circular pipe 20 and the solid moderator 25 becomes small, the chemical reaction between the solid moderator 25 and the circular pipe 20 does not cause much problem. As a material of the circular tube 20, zircaloy-2 or zircaloy-4, which are zirconium alloys having low neutron absorption, are preferable.

In this embodiment, a cruciform control rod (not shown) is inserted into the wide gap water area B of the D lattice core shown in FIG. 3, and no control rod is inserted into the narrow gap water area A.
The fuel assembly 90 is arranged such that the upper right corner of FIG. 1 faces the wide gap water region B of FIG. In other words, the lower left corner of FIG. 1 is the narrow gap water area A of FIG.
It is arranged so that it faces. As a result, the fuel assembly 90
The deceleration rods 10 are concentrated on the narrow gap water area A side (the lower left side in FIG. 1).

Although not shown, a channel fastener for fixing the channel box 7 of the fuel assembly 90 to the upper tie plate 5 is provided at the upper right corner of FIG. That is, the speed reduction rod 10 in the fuel assembly 90
Are concentrated on the lower right corner opposite to the upper right corner where the channel fastener is installed. In other words, when the inside of the fuel assembly 90 is divided into a first area on the channel fastener side (upper right side) and a second area on the opposite channel fastener side (lower left side) by a diagonal line X, all the reduction rods 10 They are arranged in two regions. In each of the following embodiments, the positional relationship of the fuel assemblies 90 in the above-described D lattice core is the same. The neutron moderating effect of water in the narrow gap water region A in FIG. 3 is smaller than that in the wide gap water region B. Therefore, the fuel assembly 90
Within, the fuel rods near the narrow gap water region A are less likely to cause a fission reaction than the fuel rods near the wide gap water region B. In the present embodiment, the deceleration rod 10 having a larger neutron deceleration effect than water is concentrated on the narrow gap water region A side in the fuel assembly 90, thereby increasing the neutron deceleration effect in this region. To facilitate a fission reaction.

Accordingly, even in the D-lattice core, the likelihood of nuclear fission in the fuel assembly is made substantially uniform, and the degree of freedom in fuel design can be increased. Accordingly, the fuel economy can be improved, so that the fuel economy close to that of the C lattice core can be achieved even with the D lattice core.

As shown in FIG. 6, the moderator rod 10 may be a solid moderator 25 in powder form and filled in a circular tube 20. In this case, the cost of processing the solid moderator 25 into a pellet can be omitted. Further, as shown in FIG. 7, the speed reduction rod 10 can be a metal rod made of a solid moderator 25. In this case, the pipe 20
Since the portion is also the solid moderator 25, the amount of the solid moderator 25 is larger than in FIGS. Furthermore, as a speed reduction rod 10,
As shown in FIG. 8, a cylinder 20 may be filled with fuel pellets 15 in a lower region and a solid moderator 25 in an upper region.

During operation of the reactor, water as a moderator flows upward from below the fuel assembly. Water does not boil below, but as it flows upward, it absorbs the heat generated by the fuel and starts boiling. When boiling is started, steam and water are mixed, and the hydrogen atom density per unit volume is lower than in the case where boiling is not performed. Typically, the volume fraction of steam in the upper region of the fuel assembly is about 70%.
%, The hydrogen atom number density at this time is about 1/3 of that in the case of not boiling. That is, the neutron moderating effect of water in the upper region of the fuel assembly is smaller than that in the lower region.

Therefore, by employing the structure shown in FIG. 8 for the moderator rod 10, the neutron moderating effect can be increased by the solid moderator 25 in the upper region of the fuel assembly where the neutron moderating effect is originally small. In this case, the uniformity of the neutron moderating effect due to the presence or absence of the moderator rod 10 is greater in the upper region of the fuel assembly than in the lower region, so the effect per fixed amount of the solid moderator 25 is greater. In addition, in the case of the speed reduction rod 10 shown in FIG. 8, the fuel can be loaded more than in FIGS. Accordingly, the average enrichment of the fuel can be reduced, so that fuel economy can be improved.

When the moderator rod 10 shown in FIG. 8 is used, it is better to set the upper region filled with the solid moderator to the region where the water in the fuel assembly boils and the proportion of steam is large. That is, it is preferable that the length of the upper region of the reduction rod 10 be about １ ／ to の of the entire length of the reduction rod 10 from the upper end of the reduction rod 10.

Although FIGS. 5 to 8 show an example in which the outer diameter of the reduction rod 10 is the same as that of the fuel rod, the outer diameter of the reduction rod 10 may be larger than that of the fuel rod. In this case, since the amount of the solid moderator 25 in the speed reduction rod 10 increases, the neutron moderating effect per speed reduction rod can be increased. Conversely, the neutron moderating effect per moderating rod may be small, and if it is desired to reduce the pressure loss of the core, the outer diameter of the moderating rod 10 may be smaller than that of the fuel rod. The decrease in the neutron moderating effect due to the reduction of the moderator rod 10 can be supplemented by increasing the number of the moderator rods 10.

The position where the speed reduction rod 10 is provided in the fuel assembly 90 is effective in a region where the neutrons are not significantly decelerated. This is because the neutron moderating effect of the solid moderator 25 is greater in the region where the neutrons are not moderated, and the neutron absorption by atoms other than hydrogen of the solid moderator 25 is greater in the region where the neutron moderation is large. Therefore, it is preferable that the installation position of the speed reduction rod 10 be a region apart from the place where water as the moderator exists. That is, as the position where the speed reduction rod 10 is provided in the fuel assembly 90,
A position adjacent to the outermost layer and the center water rod 3 of the fuel assembly is not preferable, and a position not adjacent to the water rod 3 in the second or third layer from the outermost layer is preferable.

Next, a second embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 9 is a cross-sectional view of the second embodiment. This embodiment is different from the first embodiment in that
The number and arrangement of zeros, and the other configuration is the same as that of the first embodiment. In the present embodiment, the number of reduction rods 10 is increased to five,
In the second layer from the outermost periphery of the fuel assembly 90, the fuel assemblies 90 are intensively arranged on the lower left corner side. Also in this embodiment, the upper right corner corresponds to the corner where the channel fastener is installed.

In this embodiment, the same effects as in the first embodiment can be obtained. Further, in the case of the present embodiment, the neutron deceleration effect on the narrow gap water region side (the lower left corner side in FIG. 9) is larger than that of the first embodiment because the number of the speed reduction rods 10 is increased. Although the number of the long fuel rods 2 is reduced by increasing the number of the speed reduction rods 10, the neutron deceleration effect of the neutrons is increased.
Since the enrichment of the uranium can be reduced, the uranium enrichment cost can be reduced.

Next, a third embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 10 is a cross-sectional view of the third embodiment. This embodiment is different from the first embodiment in the number and arrangement of the reduction rods 10, and the other configurations are the same as those in the first embodiment. In this embodiment, the number of reduction rods 10 is increased to four, three near the lower left corner of the second layer from the outermost periphery of the fuel assembly 90, and one at the lower left corner of the third layer from the outermost periphery. Have been placed. Also in this embodiment, the upper right corner corresponds to the corner where the channel fastener is installed. In this embodiment, the same effects as in the second embodiment can be obtained.

Next, a fourth embodiment of the fuel assembly according to the present invention will be described. This embodiment has the same cross-sectional configuration of the fuel assembly as that of FIG. 1 except that the length of the speed reduction rod 10 is shorter than that of the long fuel rod 2. Other configurations are the same as those of the first embodiment. In this embodiment, the same effects as in the first embodiment can be obtained. Furthermore, in the present embodiment, since the reduction rod 10 is shortened, the flow path cross-sectional area of the water in the upper region of the fuel assembly 90 increases, so that the pressure loss in the fuel assembly 90 can be reduced. Accordingly, the effect of reducing the discharge pressure of the pump for supplying water to the core can be obtained.

The effect of reducing the pressure loss by shortening the reduction rod 10 is great in the upper region of the fuel assembly 90 where the proportion of steam is high. Therefore, the length of the speed reduction rod 10 is preferably about half or more of the length of the long fuel rod 2. Further, also in this embodiment, by arranging the reduction rod 10 as shown in FIG. 9 or FIG. 10, the same effect as in the second embodiment or the third embodiment can be obtained.

Next, a fifth embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 11 is a cross-sectional view of the fifth embodiment. In this embodiment, 10 × 10 (10 rows and 10 rows)
The present invention is applied to a fuel assembly having an array of fuel rods. The fuel assembly 90 of this embodiment includes 87 fuel rods, two large-diameter water rods 3, and five reduction rods 10. The fuel rods are all long fuel rods 2. Other configurations are the same as those of the first embodiment of FIG.

The water rod 3 is loaded in the central area of the fuel assembly where eight fuel rods can be arranged. Reduction rod 10
Are concentrated on the lower left corner side of the second layer from the outermost periphery of the fuel assembly. Also in this embodiment, the upper right corner corresponds to the corner where the channel fastener is installed. Even if the arrangement of the fuel rods changes as in the present embodiment, the same effects as in the first embodiment can be obtained.

Next, a description will be given of a sixth embodiment of the fuel assembly according to the present invention. This embodiment has the same cross-sectional configuration as that of the fuel assembly shown in FIG.
Is shorter than the long fuel rod 2. Other configurations are the same as those of the fifth embodiment. In this embodiment, the same effects as in the fifth embodiment can be obtained. Furthermore, in the present embodiment, since the reduction rod 10 is shortened, the flow path cross-sectional area of the water in the upper region of the fuel assembly 90 increases, so that the pressure loss in the fuel assembly 90 can be reduced. Accordingly, the effect of reducing the discharge pressure of the pump for supplying water to the core can be obtained. The length of the speed reduction rod 10 is preferably at least half the length of the long fuel rod 2 as in the fourth embodiment.

Next, a seventh embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 12 is a cross-sectional view of the seventh embodiment. This embodiment is different from the fifth embodiment in the number and arrangement of the reduction rods 10, and the other configuration is the same as the fifth embodiment. In this embodiment, the number of the speed reduction rods 10 is increased to seven, and the fuel rods 90 are intensively arranged on the lower left corner side of the second layer from the outermost periphery.

In this embodiment, the same effects as in the fifth embodiment can be obtained. Further, in the case of the present embodiment, the neutron deceleration effect on the narrow gap water region side (the lower left corner side in FIG. 12) is larger than that of the fifth embodiment because of the increase in the number of speed reduction rods 10. In addition, although the number of fuel rods is reduced by increasing the number of the speed reduction rods 10, the neutron deceleration effect is increased. Thus, the cost of enriching uranium can be reduced.

Next, an eighth embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 13 is a cross-sectional view of the eighth embodiment. This embodiment is different from the fifth embodiment in the number and arrangement of the reduction rods 10, and the other configuration is the same as the fifth embodiment. In the present embodiment, the number of the speed reduction rods 10 is increased to six, and five are provided on the lower left corner of the second layer from the outermost periphery of the fuel assembly 90 and one is provided on the lower left corner of the third layer from the outermost periphery.
Books are arranged. In this embodiment, the same effects as in the seventh embodiment can be obtained.

Next, a ninth embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 14 is a cross-sectional view of the ninth embodiment. This embodiment differs from the first embodiment in the shape of the water rod 3 and the type of fuel rod. In this fuel assembly, all the fuel rods are long fuel rods 2, and one square water rod 3 is installed in the central area of the fuel assembly where nine fuel rods can be arranged. . Other configurations are first
This is the same as the embodiment. As in the present embodiment, even if the shape of the water rod 3 changes, the same effect as in the first embodiment can be obtained.

Next, a fuel assembly according to a tenth embodiment of the present invention will be described with reference to FIG. FIG. 15 is a cross-sectional view of the tenth embodiment. In the present embodiment, the present invention is applied to a fuel assembly having a fuel rod array of 10 × 10. The fuel assembly 90 of the present embodiment includes 86 fuel rods, one water rod 3, and five reduction rods 10. The fuel rods are all long fuel rods 2. The other configuration is the same as that of the ninth embodiment in FIG.

The water rod 3 is loaded in a central area of the fuel assembly where nine fuel rods can be arranged. Reduction rod 10
Are concentrated on the lower left corner side of the second layer from the outermost periphery of the fuel assembly. Also in this embodiment, the upper right corner corresponds to the corner where the channel fastener is installed. Even if the arrangement of the fuel rods changes as in the present embodiment, the same effects as in the ninth embodiment can be obtained.

The arrangement of the reduction rod 10 in the ninth embodiment may be changed as shown in FIGS. 9 and 10, or the arrangement of the reduction rod 10 in the tenth embodiment may be changed as shown in FIGS. 12 and 13. . In this way, by changing the arrangement of the reduction rod 10, the same effect as in the seventh and eighth embodiments can be obtained.

In the above embodiment, the example in which the reduction rod 10 is arranged only in the second area on the side opposite to the channel fastener (lower left side) has been described. However, the reduction rod 10 is disposed in the first area on the channel fastener side (upper right side). 10 may be arranged. However,
Also in this case, the number of the speed reduction rods 10 in the first area needs to be larger than that in the second area. Furthermore, burnable poisons can be mixed into the solid moderator, in which case the fuel design flexibility is further increased and the fuel economy may be improved.

[0040]

According to the present invention, the non-uniformity of the neutron moderating effect in the horizontal section (cross section) of the core of the boiling water reactor can be reduced even in the case of the D lattice core of the boiling water reactor. Economics can be achieved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a first embodiment of a fuel assembly according to the present invention.

FIG. 2 is a longitudinal sectional view of a first embodiment of the fuel assembly according to the present invention.

FIG. 3 is a quarter of the first embodiment of the reactor core according to the present invention;
FIG.

FIG. 4 is a diagram showing a hydrogen atom number density of a solid moderator.

FIG. 5 is a partial vertical sectional view showing an example of a speed reduction rod.

FIG. 6 is a partial longitudinal sectional view showing an example of a speed reduction rod.

FIG. 7 is a partial vertical sectional view showing an example of a speed reduction rod.

FIG. 8 is a partial vertical sectional view showing an example of a speed reduction rod.

FIG. 9 is a cross-sectional view of a second embodiment of the fuel assembly according to the present invention.

FIG. 10 is a cross-sectional view of a third embodiment of the fuel assembly according to the present invention.

FIG. 11 is a cross-sectional view of a fifth embodiment of the fuel assembly according to the present invention.

FIG. 12 is a cross-sectional view of a seventh embodiment of the fuel assembly according to the present invention.

FIG. 13 is a cross-sectional view of an eighth embodiment of the fuel assembly according to the present invention.

FIG. 14 is a cross-sectional view of a ninth embodiment of a fuel assembly according to the present invention.

FIG. 15 is a cross-sectional view of a tenth embodiment of a fuel assembly according to the present invention.

[Explanation of symbols]

2 ... Long fuel rod, 3 ... Water rod, 4 ... Fuel spacer, 5
... upper tie plate, 6 ... lower tie plate, 7 ... channel box, 9 ... short fuel rod, 10 ... reduction rod, 15
... Fuel pellet, 20 ... Circular tube, 25 ... Solid moderator, 90
... a fuel assembly.

Claims (9)

[Claims] 1. A plurality of fuel rods arranged in a square lattice of 9 rows and 9 columns or more, a water rod having a larger cross-sectional area than the fuel rods, and a hydrogen atom number density higher than 70 atm of saturated water. A plurality of speed reducers filled with a solid moderator, a lower tie plate holding lower ends of the fuel rods, the water rods and the speed reducers, and an upper type holding at least upper ends of the fuel rods and the water rods; And a channel fastener for securing a channel box to the upper tie plate, wherein the fuel assembly is diagonally divided into a first region on the channel fastener side and a second region on the anti-channel fastener side. When divided into regions, the fuel rods are arranged more in the second region than in the first region. 2. A fuel assembly loaded in a reactor core wherein a distance between fuel assemblies at positions where control rods enter is wider than that at a position where control rods do not enter. A rod, a plurality of moderator rods filled with a solid moderator having a greater hydrogen atom number density than 70 atm saturated water, an upper tie plate holding at least the upper ends of the fuel rods and the water rods, and a channel box. A fuel assembly comprising a channel fastener for fixing to the upper tie plate, wherein the fuel assembly is divided into a first region on the channel fastener side and a second region on the opposite channel fastener side by a diagonal line. The fuel assembly according to claim 1, wherein the speed reduction rods are arranged more in the second area than in the first area. 3. The fuel assembly according to claim 1, wherein the plurality of speed reducers include a nuclear fuel pellet in a lower region and a speed reducer filled with the solid moderator in an upper region. 4. The fuel assembly according to claim 1, wherein the plurality of speed reducers includes a speed reducer having a shorter overall length than the entire length of the fuel rod. 5. The fuel assembly according to claim 1, wherein the plurality of speed reduction rods are arranged inside the outermost periphery of the fuel assembly. 6. A vehicle according to claim 5, wherein said plurality of speed reduction rods are:
A fuel assembly, wherein the fuel assembly is arranged at a position not adjacent to the water rod. 7. The fuel assembly according to claim 1, wherein the plurality of speed reduction rods are disposed only in the second region. 8. The fuel assembly according to claim 1, wherein the plurality of speed reduction rods are arranged in a second or third layer from the outermost periphery of the fuel assembly. 9. A reactor core comprising a plurality of fuel assemblies and a plurality of control rods, wherein a distance between the fuel assemblies at a position where the control rod enters is wider than that at a position where the control rod does not enter.
A reactor core, wherein the fuel assemblies according to claim 1 are loaded as the plurality of fuel assemblies.