JPS6275378A - Fuel aggregate - 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 [Technical Field of the Invention] The present invention relates to a fuel assembly used in a light water nuclear reactor.

[Technical background of the invention and its problems] In recent years, in light water nuclear reactors, plans have been made to lengthen the operation cycle and increase fuel burnup with the aim of reducing power generation costs, and many developments and research efforts have been made in light water reactors. It is agreed.

In order to achieve a longer operating cycle and higher burnup, it is necessary to increase the initial enrichment of uranium-235 in the fuel, and in a reactor equipped with such fuel, the nuclear characteristics of the core Various effects occur.

Among these effects, the most stringent nuclear design standard for current reactor cores is ensuring sufficient subcriticality (reactor shutdown margin) during cold state. This is uranium-235 in fuel.
As the enrichment level increases, the atomic ratio of hydrogen to uranium-235 decreases, and the reactivity coefficient due to changes in moderator (light water) density increases, resulting in a large difference in the multiplication factor between operating and cold conditions. This is because the surplus reactivity in the cold state becomes large.

Conventional countermeasures have been to increase the amount of pernable poison in boiling water reactors (BWRs) and to increase the amount of pernable poison in pressurized water reactors (PWRs).
), increasing the concentration of chemical shim is considered, but this poses a problem in terms of core characteristics and fuel economy.

[Object of the Invention] The present invention has been made in response to the above-mentioned conventional circumstances, and it is possible to sufficiently shut down the reactor without deteriorating fuel economy and various core characteristics even when using highly enriched fuel. The purpose is to provide a fuel assembly that can secure a margin.

[Summary of the Invention] That is, the present invention provides a fuel assembly in which fuel rods in which cylindrical fuel pellets are enclosed are regularly bundled in a square or hexagonal lattice. A small unit is formed by bundling a small number of fuel rods with a constant distance between their centers, and a plurality of these small units are combined, and the distance between the centers of adjacent fuel rods belonging to different small units is within the small unit. Since the distance between the centers of adjacent fuel rods is greater than the distance between the centers of the adjacent fuel rods, even when using highly enriched fuel, sufficient reactor shutdown margin can be maintained without deteriorating fuel economy and various core characteristics. It was designed so that it could be secured.

[Embodiments of the Invention] Details of the present invention will be described below with reference to embodiments shown in the drawings.

FIG. 1 shows a fuel assembly according to an embodiment of the present invention. The assembly shown in FIG. 1 consists of nine small units each consisting of fuel rods 2 arranged in three rows and three columns, or a collection of fuel rods 2 and control rod guide tubes 3, arranged in a channel box 1. There is. Also, the fuel rod pitch P which spans separate small units
2 is 1.5 times the fuel rod pitch P1 within the small unit.

Figure 2 shows a system of 2 rows and 2 columns, 3 rows and 3 columns to 6 rows, similar to the fuel assembly of this embodiment, based on a conventional fuel assembly system arranged in a square lattice with a constant fuel rod pitch. This is a graph showing the change in the infinite multiplication factor of a system in which small units consisting of six rows of fuel rods are arranged in a square lattice, and the fuel rod pitch spanning the small units is larger than the fuel rod pitch within the small unit. is the infinite multiplication factor, the horizontal axis shows the number of rows and columns, the actual f51A is the infinite multiplication factor during high temperature operation (light water is 40% steam volume fraction at 286℃), and the dotted line B is the infinite multiplication factor when it is cold (light water is 20℃). (no steam). In addition, in the system having small units, the volume ratio of light water and fuel is set to be the same as in the conventional system with a constant pitch as a standard. For this reason, as the number of fuel rod rows and columns of small units increases, the spacing between the small units also increases.

As shown in the graph of Fig. 2, the fuel assembly of this embodiment, which is composed of small units arranged in 3 rows and 3 columns, has a higher infinite multiplication factor during high-temperature operation than the conventional fuel assembly. Almost no decline. On the other hand, in the cold state, the infinite multiplication factor is effectively reduced, so a sufficient margin for reactor shutdown can be ensured. This is because the π degree of light water differs greatly between high temperature operation and cold operation, so neutron absorption in the light water region between small units is large during cold operation, and this neutron absorption is small at high temperature.

Note that if the number of fuel rod matrices of small units, that is, the spacing between small units, is increased, the infinite multiplication factor during high-temperature operation will also decrease. Approximately 5 rows and 5 columns is appropriate.

FIG. 3 shows another embodiment in which the present invention is applied to a large assembly for BWR. In the fuel assembly of this example,
Fuel rods 2/)S arranged in 3 rows and 3 columns are arranged in 2 small units with an interval wider than the fuel rod pitch within the small unit.
Five fuel assemblies are arranged in the channel box 1a, and a cross-shaped control rod 5 is inserted into each fuel assembly.

Even with a large fuel assembly configured in this manner, the same effects as in the above-described embodiment can be obtained.

In FIG. 4, a spacer 6 is provided for each small unit consisting of fuel rods 2 arranged in 4 rows and 4 columns, two partition plates 7 are provided between the small units, and the fuel rods 2 are housed in a channel box 1. 2 shows a combustion assembly according to an embodiment of the present invention. In this embodiment, no steam is generated in the light water area 8 sandwiched between the partition plates 7.

Even with a fuel assembly configured in this manner, the same effects as those of the above-mentioned embodiments can be obtained.

The present invention is also applicable to current BWR and PWR aggregations. FIG. 5 shows an embodiment in which the present invention is applied to the current BW R8X8 fuel assembly. In this embodiment, fuel rods 2 are arranged in 4 rows and 4 columns in a channel box 1c. Four small units are arranged.

With the fuel assembly constructed in this manner, it is possible to obtain the same effects as in the above-mentioned embodiments.

In addition, although the fuel assemblies of square lattice are shown in these embodiments, the present invention is limited to such embodiments.
However, it is clear that b can also be applied to, for example, a hexagonal lattice fuel assembly.

[Effects of the Invention] As described above, in the fuel assembly of the present invention, a sufficient reactor shutdown margin can be ensured without deteriorating core characteristics or fuel economy.

[Brief explanation of drawings]

FIG. 1 is a top view showing a fuel assembly according to an embodiment of the present invention;
Figure 2 shows the infinite multiplication factor during high-temperature operation and in the cold state.
It is a top view which shows the fuel assembly of m example. 1...Channel box 2...
・・・Fuel rod

Claims (1)

[Claims] (1) In a fuel assembly in which fuel rods filled with cylindrical fuel pellets are regularly bundled in a square or hexagonal lattice, the distance between the centers of adjacent fuel rods is fixed. A small number of fuel rods are bundled together to form a small unit, and a plurality of these small units are combined, and the distance between the centers of the adjacent fuel rods belonging to each different small unit is such that the distance between the centers of the adjacent fuel rods in the small unit is A fuel assembly characterized in that the distance between the centers is greater than the distance between the centers.