JPS59217189A - Fuel assembly - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a fuel assembly loaded into a boiling water nuclear reactor.

[Background of the invention]

Conventionally, a fuel assembly is mainly composed of an upper tie plate spacer, a plurality of fuel rods, a lower tie plate, and a channel box that houses these. A plurality of spacers are provided at intervals in the axial direction within the channel box, and support a plurality of fuel rods in alignment. The upper and lower tie plates fix both ends of the plurality of fuel rods,
Each tie plate has a plurality of holes for cooling water passages.

In the above fuel assembly, slightly unsaturated cooling water flows between the fuel rods from the holes in the lower tie plate, and as it flows between the fuel rods from the bottom to the top, it is heated and boils, forming a two-phase flow. It flows out through the holes in the upper tie plate. The void ratio (volume ratio occupied by steam in the cooling water) at the exit of the fuel assembly is around 70 inches, and the average in the axial direction is 40%.
The void ratio before and after. An increase in the void fraction, that is, a decrease in the volume fraction of light water as a neutron moderator, leads to a decrease in the reaction, so the above-mentioned void fraction distribution causes the axial power distribution to become a downwardly distorted distribution as shown in FIG.

For boiling water reactors, from a fuel integrity perspective.

It is designed so that the linear power density (output per unit length of fuel rod) during normal operation is 44 kW/m or less. Therefore, if the axial power distribution shown in Figure 1 can be flattened, the maximum linear power density during operation will be reduced, and the difference from the above limit value of 44kw/m will increase, resulting in thermal room.

Fuel health can be increased.

[Purpose of the invention]

An object of the present invention is to flatten the axial power distribution of a fuel assembly by relatively increasing the neutron moderating ability in the axial upper half of the fuel assembly.

The object of the present invention is to provide a fuel assembly with reduced maximum linear power density during operation and increased thermal margin.

[Summary of the invention]

In order to achieve the above object, in the fuel assembly of the present invention, solid moderators are loaded in the upper and lower halves in the axial direction, so that the neutron moderating capacity in the upper half in the axial direction is increased relative to that in the lower half. A neutron moderating member increased in number is installed in the fuel assembly.

The solid moderator accommodated in the fuel assembly of the present invention is a material that does not melt during reactor operation, that is, has a melting point of 400'C.
The above materials, for example, ZrH2゜'I''Hz, Guar7
Use Phyto, Bed, Be2C, etc.

As a method to relatively increase the neutron moderating capacity in the upper axial half compared to the lower half: (1) Using the same solid moderator in the lower axial half and the lower half, the moderating capacity in the horizontal section is increased. A method of increasing timber stacking in the upper half of the shaft compared to the lower half.

(2) A method in which a solid moderator with a large neutron moderating capacity is installed in the upper half, and a solid moderator made of a different filtering material with a relatively low neutron moderating capacity is installed in the lower half.

etc. are possible.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to Examples.

In FIG. 2, the fuel assembly consists of a square channel box 1 and a fuel bundle 2 housed inside the channel box 1. The fuel bundle 2 includes an upper tie plate 3 and a lower tie plate 4 fitted to the upper and lower parts of the channel box 1, and a plurality of spacers installed at intervals along the axial direction inside the channel box 1. 5 and a plurality of fuel rods 6 passing through the spacer 5 and having both ends fixed to the upper and lower tie plates 3.4, and two cylindrical neutron moderating rods fixed to the upper and lower tie plates 3.4. It consists of member 7.

A cross-sectional view of a fuel assembly according to the invention is shown in FIG. Two neutron moderating members 7 are arranged with respect to the cross-shaped control rod 8 at positions corresponding to water rods in a conventional fuel assembly.

As shown in FIG. 4, the neutron moderating member 7 includes metal moderators in the axial upper and lower halves of a circular tube 9 made of Zircaloy-2, which is the same as the cladding tube of the fuel rod 6. Cylinders 10 and 11 made of zirconium hydride (ZrH2) are housed. The outer diameter ratio of these cylinders housed in the upper and lower halves is 2:1, and the ZrH2 amount ratio in the horizontal section is 4:1 between the upper and lower halves. A gap 12 between the circular tube 9 and the column 11 made of ZrH2 is filled with helium gas.

The hydrogen density contained in ZrHz is 7.2 x 1022 pieces/sub8, and the hydrogen density of light water (non-boiling water) at a temperature of about 280°C during nuclear reactor operation is 5.0 x 1022 pieces/17 n'
is more than. Therefore, in the fuel assembly according to the present invention,
Compared to a conventional fuel assembly, ZrHg loading increases the hydrogen-to-uranium ratio (H/U ratio) in the axial upper and lower halves by about 4 and is, respectively, and the neutrons in the upper half are relatively As a result of the improved deceleration capability, axial power peaking (the ratio of the maximum value to the average value of the axial power distribution) can be reduced by about 2%, and thermal margin and fuel health can be increased.

In addition, in this example, when viewed in the horizontal cross section of the aggregate, the neutron moderator is installed at the center of the aggregate, so the neutron moderating ability in the center area of the aggregate is improved, and the output distribution in the horizontal direction is flattened. This also has the effect of being done.

In this example, one type of solid moderator ZrHz was used in the neutron moderating member, but different types of solid moderators were used in the upper and lower parts in the axial direction. By housing the moderator in the upper half, it is also possible to reduce axial output beaking.

FIG. 5 shows another embodiment of the fuel assembly according to the present invention. In this embodiment, a neutron moderating member 13 with a cross-shaped cross section is installed within the assembly. As shown in FIG. 6, the neutron moderating member 13 has an outer wall 14 made of Zircaloy-2, a solid moderating section 15 made of Z r H2 housed in the upper half, and a Z r H2 housed in the lower half. It consists of a solid deceleration section 16 made of H2 and a space 17 filled with helium gas.

In this embodiment as well, as in the first embodiment, the axial output distribution can be flattened by increasing the neutron moderating capacity in the upper half of the axis, and the arrangement of the neutron moderating member also makes it possible to In the horizontal cross section, the neutron moderating ability in the central region is relatively improved, and the horizontal output distribution is also flattened.

〔Effect of the invention〕

As explained above, according to the present invention, by relatively increasing the neutron moderating ability in the upper axial part of the advisory assembly, the axial power distribution is flattened, and the maximum linear power density during operation is It is possible to reduce the thermal margin and increase the thermal margin.

Furthermore, in the present invention, since a solid moderator is added to the fuel assembly compared to conventional fuel assemblies, the deceleration capacity of the entire assembly is increased. Therefore, if the fuel assemblies have the same enrichment, the infinite multiplication factor will be high. In other words, it is also possible to reduce the amount of uranium required to make a nuclear reactor critical.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the axial void fraction distribution and power distribution in a conventional fuel assembly, FIG. 2 is a longitudinal sectional view of the fuel assembly according to the present invention, and FIG. 3 is a horizontal sectional view of the assembly. FIG. 4 is a longitudinal sectional view of a neutron moderating member installed in the assembly, FIG. 5 is a horizontal sectional view of another embodiment of the present invention, and FIG. 6 is a longitudinal sectional view of the neutron moderating member. . 1... Channel box, 2... Fuel bundle,
3... Upper tie plate, 4... Lower tie plate, 5... Spacer, 6... Fuel rod, 7... Neutron moderating member, 8... Control rod, 9... Yen Tube, 10...
・Solid speed reducer 'Z r H2 installed in the upper half in the axial direction
-11... Consolidated moderator Zr installed in the lower half in the axial direction
) 12, 12... Helium gas filled area, 13...
・Cross-shaped neutron moderating member, 14...outer wall, 15・
...Solid moderator ZrH2.1 installed in the upper half in the axial direction
6...Solid moderator ZrHz installed in the lower half of the axis
-17...Heritsu Kaya! Fixed ship
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Claims (1)

[Claims] 1. A cylindrical channel box, an upper tie plate and a lower tie plate that are fitted above and below the channel box, and which are installed at intervals along the axial direction within the channel box. In a fuel assembly having a plurality of spacers and a plurality of fuels with both ends fixed to the upper and lower tie plates through the spacers, a solid moderator is loaded in each of the upper and lower halves in the axial direction. A fuel assembly characterized in that a neutron moderating member is provided in which the neutron moderating ability in the upper half in the axial direction is relatively increased compared to the lower half. 2. As the neutron moderating member, a cylindrical neutron moderating member is arranged instead of the plurality of fuel rods, passes through the spacer like the fuel rods, and has both ends fixed by the upper and lower tie plates. A fuel assembly according to claim 1, characterized in that: