JPS59183394A - 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 reactor.

[Background of the invention]

Conventionally, this type of fuel assembly mainly consists of an upper tie plate,
It consists of a spacer, multiple fuel rods, a lower tie plate, and a channel box that houses them.

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 clamp plates fix both ends of a plurality of fuel rods. Each tie plate is also provided with 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 grate. The void ratio (volume ratio occupied by steam in the cooling water) at the exit of the fuel assembly is around 70%, and the average in the axial direction is 40%.
The void rate is around 1. An increase in the void fraction, that is, a decrease in the body and fraction of light water as a neutron moderator, leads to a decrease in reactivity, so according to the above-mentioned distribution of the detritus fraction,
The axial power distribution becomes a downwardly distorted distribution as shown in FIG.

In boiling water reactors, from a fuel integrity point of view, the output of
is designed to be 44 kW/rn or less.

Therefore, if the power distribution in one curve direction shown in FIG. 1 can be flattened, the maximum linear power density during operation can be reduced. By increasing the difference from the above limit value of 44 kW/rr, thermal margin and fuel integrity can be increased.

[Purpose of the invention]

Therefore, an object of the present invention is to flatten the axial power distribution of the fuel assembly by relatively increasing the neutron moderating capacity 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 and increased thermal margin.

[Summary of the invention]

The feature of the fuel assembly of the present invention is that in the first invention, a moderating member extending in the axial direction is provided in the channel box, and the moderating member has a solid moderator built in only in the upper half in the axial direction. In particular, this improves the neutron moderating capacity of the upper half of the fuel-rich assembly relative to that of the lower half. Further, in the second invention, the speed reduction member has a solid moderator built-in only in its upper half in the axial direction, and its lower half is hollow and has an open bottom end, and This is due to the provision of a side hole for cooling water to flow out.
This further increases the relative improvement in the neutron moderating ability of the upper half of the fuel-rich assembly relative to that of the lower half.

[Embodiments of the invention]

FIG. 2 is a longitudinal sectional view of one embodiment of the fuel assembly according to the present invention. In FIG. 2, the fuel assembly consists of a rectangular 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 5 installed at intervals in the axial direction inside the channel box 1. , a plurality of fuel rods 6 which penetrate through this waver 5 and have both ends fixed to the upper and lower tie plates 3 and 4, and two cylindrical fuel rods which are fixed by the upper and lower tie plates 3.4. It consists of a neutron moderation member 7.

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

As shown in a longitudinal cross-sectional view in FIG. 4, the neutron moderating member 7 has a metal moderator in the axially upper half of the inside of a circular tube 9 made of Zorcaloy-2, which is the same as the cladding tube of the fuel rod 6. It contains a certain zirconium hydride (ZrH2) IO, and the metal moderator (ZrH2) 10
A plurality of holes 11 are provided in the circular tube 9 near the lower end thereof. In addition, the lower end 12 of the neutron moderating member 7 has an open tube structure, and a portion of the cooling water flows directly into the circular tube 9 without passing through the cooling water passage hole 13 provided in the lower tie plate 4.
It flows out of the tube through a hole 11 provided in the central part in the axial direction. The lower end of the metal moderator 10 has a conical structure in order to reduce flow resistance when the rising flow inside the tube flows out of the tube.

Compared to the flow path that passes through the holes 13 for the cooling water passage in the lower tie plate 4 and ascends between the fuel rods, the pi!! Passing through the neutron flow path, that is, the circular tube 9 of the neutron moderating member 7,
In the flow path that flows out from the hole 11 provided at the center in the axial direction, no pressure loss occurs in the hole 13 of the lower tie plate 4 and the cover plate 5, so the flow rate of the coolant increases and the coolant flows inside the fuel assembly. Approximately 5% of the total coolant flow can flow through this.

FIG. 5 compares the axial void fraction distribution in the fuel assembly according to the above embodiment of the present invention with that of the conventional example. In the fuel assembly according to the embodiment of the present invention, the flow rate of the coolant flowing between the fuel rods is reduced due to the above-mentioned pi-gus flow path formed by the neutron moderating member 7, and as a result, the flow rate of the coolant flowing between the fuel rods is reduced. The det rate increased by about 4%, and the hydrogen to uranium ratio (H/
U) is approximately 3% smaller than that of conventional fuel assemblies.

On the other hand, ZrH2 is inserted as a solid moderator in the upper half of the fuel assembly according to the above embodiment of the present invention, and ZrH2 is inserted as a solid moderator.
The hydrogen density contained in rH2 is higher than the hydrogen density of 7.2 x 1022 pieces per cm' (non-boiling water), which is about 5.0 x 1022 pieces/crn3. This Z in the upper half of the fuel assembly
r H2 increases the H/U ratio in the upper half by about 4% compared to the lower half in the axial direction.

Therefore, in the fuel assembly of the above embodiment according to the present invention, H/U decreases by about 3% in the lower half in the axial direction and increases by about 4 inches in the upper half compared to the conventional fuel assembly. As a result of improving the neutron moderating ability of the upper half, the axial output peaking (ratio of the maximum value to the average value of the axial output distribution) was reduced to 3.5.
%, increasing thermal margin and fuel integrity.

FIG. 6 shows the relative improvement in the neutron moderation ability in the above-mentioned axial upper half due to the void ratio dependence of the infinite multiplication factor of the fuel assembly. In the lower half of the fuel assembly of the above-mentioned example, the void ratio increases by about 4%, so in the figure,
As shown in a, the infinite multiplication factor decreases to about 0.26% Δ at the position where the tide rate is 30 degrees. On the other hand, in the upper half of the fuel assembly, I) H/U increases by about 4 inches due to Z rH2 loading, but this corresponds to a decrease of about 5 inches when converted to void ratio. Therefore, in the lower half of the fuel assembly, b
The infinite multiplication factor is approximately 05 at the position where the void rate is 65%.
Increases to 4chi△/. As a result of the above, the infinite multiplication factor decreases to an average of 0.25%Δ/K in the lower half of the fuel assembly, and increases by an average of 0.5%ΔK/K in the upper half. This is it”
,ll'i! The unidirectional power distribution is smoothed out, and the axial power peaking (the ratio of the maximum value to the average value of the axial power distribution) can be reduced by 3.5 inches, increasing thermal margin and fuel health. It becomes possible.

In addition, in the above embodiment, when viewed from the horizontal cross section of the fuel assembly, the neutron moderator is installed at the center of the fuel assembly.
Furthermore, since the cooling water flowing out from the hole 11 in the axial center section rises mainly near the center of the fuel assembly, the neutron moderation ability in the center region of the fuel assembly is improved, and the horizontal power distribution is evened out. This also has the effect of being done.

In the above embodiment, since the cooling water inside the neutron moderating member 7 flows upward from the lower end as described above, no voids are generated inside. Also, Zr
f(2 has a melting point of about 1850°C and does not melt during reactor operation.

FIG. 7 shows another embodiment of the fuel assembly according to the invention. In this embodiment, the neutron moderating member 14 having a cross-shaped cross section is installed in the assembly, and as shown in a vertical cross-sectional view in FIG. It consists of a solid moderator 16 made of ZrH2 housed in a tank and a non-boiling cooling water section 17. The holes provided at the lower end of the outer wall 15 of the neutron moderating member 14 are provided only near the center of the fuel assembly, and the solid moderator 16 is provided at the lower end of the fuel assembly as shown in the figure. It slopes towards the center. Hole 19. The cooling water flowing out accumulates on the outer wall 15 mainly near the center of the fuel collecting body and rises.

In this embodiment as well, as in the first embodiment, by increasing the neutron deceleration capacity in the upper half of the axis, the axial power distribution can be flattened. Due to the shape and arrangement of the moderating member and the distribution of the coolant flowing out from the neutron moderating member 14, the neutron moderating ability in the center region is relatively improved in the horizontal cross section of the fuel assembly, and the output distribution in the horizontal direction is also flattened. It has the effect of being

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to relatively increase the neutron moderation capacity in the axial upper half of the fuel assembly, thereby flattening the axial power distribution and Maximum linear power density can be reduced to increase thermal margin.

Furthermore, according to the present invention, since a solid moderator is added to the fuel assembly compared to conventional fuel assemblies, the moderating capacity of the entire embroider assembly is increased, and therefore the uranium fuel assembly of the same enrichment If so, the infinite multiplication factor will be high. In other words, it has the effect of reducing the amount of uranium required to make the 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 cross-sectional view of an embodiment of the fuel assembly according to the present invention, and FIG. 3 is a diagram showing the fuel assembly of FIG. 2. Figure 4 is a vertical cross-sectional view of the neutron moderating member installed in the fuel assembly, and Figure 5 shows the axial void fraction distribution in the fuel assembly and conventional fuel assembly. 6 is a diagram illustrating the relative improvement in the neutron moderating ability of the upper half in the axial direction in the above embodiment, FIG. 7 is a horizontal sectional view of another embodiment of the fuel assembly according to the present invention, and FIG. 7 is a longitudinal sectional view of the neutron moderating member used in FIG. 7. FIG. Explanation of symbols 1...Channel box, 2...Fuel bundle, 3...Upper tie plate, 4...Lower tie plate, 5...Spacer, 6
...Fuel body, 7. Rod-shaped neutron moderating member, 8. Control rod, 9. Circular tube, 10-.
・Solid moderator (ZrH2X 11...hole, 12...lower end open tube part,
13... Lower tie plate hole, 14... Square-shaped neutron moderating member, 15... Outer wall, 16... Solid moderator (ZrH2), 1
7゛...Non-boiling cooling water section, 18.19...hole. Figure 1 Figure 3 Seki Figure 2 Figure 6 0 20 710 OθO void ratio Figure 7

Claims (1)

[Claims] 1.1 A city-shaped channel box, an upper tie plate and a lower tie plate fitted into the channel box, and a structure extending in a uniaxial direction within the channel box, the upper end and the lower end of which are respectively the upper tie plate and the lower tie plate. In a fuel assembly equipped with a plurality of fixed fuel rods, a moderating member that has a solid moderator built in only in the upper half in the axial direction and extends in the axial direction is arranged at a predetermined position in the channel box. A fuel assembly characterized by: 2. Claim 1, wherein the deceleration member is rod-shaped and is disposed approximately in the center of the fuel assembly.
Fuel assemblies described in Section. 3. A cylindrical channel box, an upper tie plate and a lower tie plate fitted into the channel box, and an upper tie plate and a lower tie plate extending in the axial direction within the channel box and having their upper and lower ends fixed to the upper tie plate and the tie plate, respectively. In a fuel assembly equipped with a plurality of fuel rods, a solid moderator is contained only in the upper half in the axial direction, and the lower half is hollow and open at the lower end, and cooling water is provided below the solid moderator. 1. A fuel assembly characterized in that an axially extending deceleration member having a horizontal hole for outflow is disposed at a predetermined position within an upper channel box. 4. The fuel assembly according to claim 3, wherein the deceleration member is rod-shaped and disposed approximately at the center of the fuel assembly. 5. The fuel assembly according to claim 3, wherein the deceleration member has a cross-shaped plate shape and is disposed approximately in the center of the fuel assembly.