JPH0668550B2 - Fuel assembly - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a fuel assembly used in a boiling water nuclear reactor, and in particular, to a conventional C core loaded with a D-lattice fuel assembly. The present invention relates to a fuel assembly that can achieve the same function as a lattice fuel assembly.

[Technical background of the invention and its problems]

Generally, in a boiling water reactor, many fuel assemblies are loaded as nuclear fuel in the core.

FIG. 5 is a cross-sectional view of a D-lattice fuel assembly currently in use. In the figure, reference numeral 1 indicates a channel box having a rectangular cross section for accommodating a plurality of fuel rods 2 arranged in a lattice. Also, the number attached to each fuel rod 2 is the uranium 235
The fuel rods 2 having the same numbers represent the same enrichment. The symbol G attached to the fuel rod 2 indicates a fuel rod containing a burnable poison, which is arranged to improve the characteristic of the reactivity change of the fuel assembly due to combustion, and the symbol W indicates the water rod ( (Water rod) is shown.

A plurality of fuel assemblies having such a configuration are loaded and used in the core, but as shown in FIG. 5, they are kept at a constant distance from the control rod 3 and an adjacent fuel assembly (not shown). Will be placed. In such a D-lattice fuel rod assembly, the width 4 of the wide water gap G _W formed between the channel box 1 and the control rod 3 and the narrow water gap G formed between the adjacent fuel assemblies. The ratio G _W / G _N of _N to width 5 is about 2.

FIG. 6 shows a lower tie plate 6 provided at the lower end of such a fuel assembly.
Is set so that its axis coincides with the axis of the channel box 1, and a fuel support metal fitting insertion portion 7 to be inserted into the fuel support metal fitting is formed at the lower end thereof.

FIG. 7 is a cross-sectional view of the lower tie plate 6, wherein 100c-100d and 101c-101d are the center lines of the lower tie plate 6 and 102c is the axis of the lower tie plate 6. Inside this lower tie plate 6, an insertion hole member 8 having a fuel rod lower end plug insertion hole 8a in the shape of an 8 × 8 square lattice is arranged, and the axis center of this insertion hole member 8 is the above-mentioned axis center. It is set to be the same as 102c.

By the way, in recent years, in order to achieve a long operation cycle and improve fuel economy, a fuel assembly capable of high burnup has been required. Therefore, the average enrichment of the uranium 235 of the fuel rods arranged in the fuel assembly tends to increase. At this time, in order to keep the local output peaking (hereinafter referred to as LPF) in the fuel assembly within the design standard limit, the number of types of uranium 235 concentration in the fuel rod (hereinafter referred to as split) is increased or the flammability is increased. A fuel assembly satisfying the design criteria has been designed by devising the position of the fuel rod containing the poison.

However, in the D-lattice fuel assembly described above, the width 4 of the wide water gap G _W is equal to the narrow water gap G _W.
Since the width of _N is approximately twice the width 5, there is a problem that the number of splits must be increased, and there is a problem that it is difficult to select the arrangement position of the fuel rod containing the burnable poison. .

Therefore, in some cases, as shown in FIG. 8, the ratio of the width 4a of the wide water gap G _W to the width 5a of the narrow water gap G _N is 1, that is, both water gaps G _W , G.
Attempts have been made to use C-lattice fuel assemblies in which the _N widths 4a, 5a are the same.

This C-lattice fuel assembly has a high structural symmetry, and the fuel rod 2
2 to 3 types can be reduced compared to the D-lattice fuel assembly, and the arrangement position of the fuel rod 2 containing the burnable poison can be easily selected, and good nuclear characteristics can be easily obtained. is there.

Therefore, at the time of exchanging the fuel assembly, even in a boiling water reactor using the D-lattice fuel assembly, it is necessary to load the fuel assembly having the same water gap width as the C-lattice fuel assembly. However, in the conventional boiling water reactor, since the fuel support fittings arranged in the lower part of the core cannot be moved, there is a problem that the C-lattice fuel assembly cannot be loaded as a new fuel assembly. is there.

[Object of the Invention]

The present invention has been made in consideration of such points, and can be loaded into a conventional core using a D-lattice fuel assembly, and has the same function as that of a C-lattice fuel assembly, It is an object of the present invention to provide a fuel assembly capable of increasing the reactivity of the core, which leads to a temperature gain, and improving the nuclear characteristics, and at the same time, reducing the number of splits to reduce the molding processing cost.

[Outline of Invention]

According to the present invention, a plurality of fuel rods are arranged in a lattice in a channel box having a rectangular cross section, and an upper tie plate and a lower tie plate are formed on an upper portion and a lower portion of the channel box, respectively, and a lower tie plate is formed. A fuel rod support hole member having fuel rod support holes arranged in a grid pattern, wherein the center of the fuel rod support hole member provided in the lower tie plate is the axis of the lower tie plate. It has been designed so that it can be loaded toward the wide water gap side with respect to the core so that it can be loaded into a conventional core that uses a D-lattice fuel assembly, and that the same function as a C-lattice fuel assembly can be obtained. Characterize.

Example of Invention

An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a cross-sectional view showing an example of a lower tie plate of a fuel assembly according to the present invention, which is 100a-100 in the figure.
b, 101a-101b are the center lines of the lower tie plate 6 in the vertical and horizontal directions, and 102 is the lower tie plate 6.
Is the axis center of. In this lower tie plate 6, as shown in FIG. 1, an insertion hole member 11 having a fuel rod lower end plug insertion hole 11a having an 8 × 8 lattice shape is arranged. the wide water gap G _W and the distance 12 between the fuel rod lower end plugs insertion hole 11a facing thereto which is formed between the channel box 1 and the control rod 3, narrow gaps are formed between adjacent fuel assemblies Distance 13 between the G _N and the fuel rod lower end plug insertion hole 11a facing this
Are distributed in the wide water gap G _W direction so as to be equal to each other. For example, in order to make the distance 12 the same as that of the C-lattice fuel assembly, the insertion hole member 11 is displaced by 2.39 mm in the direction of the upper left 45 degrees in FIG.

As shown in FIG. 2, the lower end of the channel box 1 extends to the side surface of the lower tie plate 6 and surrounds the lower tie plate 6. The channel box 1 is deviated to the wide water gap G _W side by the displacement of the insertion hole member 11 to the wide water gap G _W side, and accordingly, the channel box 1 on the wide water gap side is formed. As shown in FIG. 2, a minute gap G is formed between the lower side surface and the lower tie plate 6, and the lower end thereof has a thick-walled shape provided on the side bottom of the lower tie plate 6. It is placed and fixed on the channel box pedestal 14.

On the other hand, at the lower end portion of the channel box 1 on the narrow water gap G _N side, as shown in FIG. 2, since the position of the lower tie plate 6 is the same as the conventional one, an outward bent portion 15 is provided. The bent portion 15 is in contact with the side surface of the lower tie plate 6.

As shown in FIG. 3, a pad 1 is provided on the side of the upper end of the channel box 1 on the side of the wide water gap G _W.
6 also has a pad 17 on the narrow water gap G _N side.
Are arranged respectively. The thickness of the pad 16, only the water gap adjustment width than the conventional D lattice fuel assembly, that is, thin only deviation amount to the wide water gap G _W side of the insertion hole member 11, also the pad 17, Two or more locations are arranged on one side surface, and the thickness thereof is set to be the same as the water gap adjustment width.

In FIG. 3, reference numeral 18 is an upper tie plate handle.

Next, the operation of this embodiment will be described.

In the fuel assembly configured as described above, the ratio of the width of the wide water gap G _{W to} the width of the narrow water gap G _N is adjusted by adjusting the arrangement position of the insertion hole member 11 of the lower tie plate 6. Can be changed freely. Therefore, by setting these ratios to, for example, 1: 1 and arranging the fuel rods 2 as shown in FIG. 8, the C-lattice fuel can be added to the reactor designed to use the D-lattice fuel assembly. It is possible to use a fuel assembly having the same function and nuclear characteristics as the assembly.

As a result, LP in the early stage of combustion, which has been a problem in the past,
F can be significantly reduced, the number of splits can be reduced, and it is possible to obtain a fuel assembly with a high take-out burnup.

The LPF is a Local Peaking Fa.
It is an abbreviation for ctor (local peaking coefficient), and is an index indicating which fuel rod has the highest output by relatively comparing the outputs of the fuel rods in the fuel assembly. Specifically, the average output of each fuel rod in the fuel assembly is standardized to 1.0, and the relative value of the fuel rod having the highest output is shown.

Further, the term “split” means that the type of enrichment of U-235 in the fuel rod is divided. In other words, the fuel rods located around the neutron moderator with a large amount of neutron moderator will have too much output at the same concentration, so the concentration will be reduced, and conversely the output will be low at the same concentration in the central part with a small amount of neutron moderator. Therefore, the enrichment is increased and the output of each fuel rod in the fuel assembly is averaged. As described above, the enrichment levels of U-235 of the fuel rods in the fuel assembly are not the same, and the enrichment types of U-235 are classified into several types according to the amount of neutron moderator (water) near the fuel rods. U-for each fuel rod
A number (split number) is given according to the enrichment of 235, and the split number of the fuel rod with the highest enrichment of U-235 is set to 1, and the split number becomes smaller as the enrichment becomes lower. At the lowest U-235 enrichment, the fuel rod split number is five.

In FIG. 4, the horizontal axis shows the ratio of the wide water gap 4 to the narrow water gap 5, and the vertical axis shows the LP at the initial stage of combustion.
Taking F, the change in LPF when the ratio of the wide water gap G _W and the narrow water gap G _N is changed from 1 corresponding to the C-lattice fuel assembly to 2 corresponding to the D-lattice fuel assembly using the number of splits as a parameter. Shows. Further, the straight lines a, b, c, d shown by solid lines in the drawing show the cases where the split numbers are 1, 3, 5 and 7, respectively.

As is clear from the figure, if the fuel assembly of the present invention is used as a new fuel assembly during refueling, the number of splits can be reduced by about 2 as indicated by the broken line e, and the fuel assembly manufacturing cost can be reduced. As a result, the reactivity gain can be obtained, and thus the take-out burnup can be increased and the residual amount of uranium 235 at the end of combustion can be reduced.

〔The invention's effect〕

As described above, in the present invention, the center of the fuel rod support hole member provided in the lower tie plate is deviated to the wide water gap side with respect to the axis of the lower tie plate. Can be loaded into a conventional core using an assembly, and has the same function as a C-lattice fuel assembly,
Nuclear properties are obtained. For this reason, D has been a problem in the past.
The LPF in the initial stage of combustion of the lattice fuel assembly can be significantly reduced, the number of splits can be reduced, and the take-out burnup can be increased.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a lower tie plate showing an embodiment of the present invention, FIG. 2 is a vertical cross-sectional view showing a connection state of the lower tie plate and a channel box, and FIG. 3 is a structure of an upper end of the channel box. FIG. 4 is a plan view showing the relationship between the ratio of wide water gap and narrow water gap and LPF in the early stage of combustion, and FIG. 5 is a cross section showing a conventional D-lattice fuel assembly of 8 rows and 8 columns. FIG. 6 is a vertical sectional view showing the lower tie plate, FIG. 7 is an enlarged transverse sectional view of the lower tie plate shown in FIG. 6, and FIG. 8 is a horizontal sectional view showing a C-lattice fuel assembly. . 1 ... Channel box, 2 ... Fuel rod, 3 ... Control rod, 6 ... Lower tie plate, 11 ... Insert hole member, 1
1a ...... fuel rod lower end plugs insertion holes, 16 and 17 ...... pads, 100a-100b, 101a-101b ...... centerline, 102 ...... axis, _{G W} ...... wide water gap, _{G N} ...... narrow water gap.

Claims (3)

[Claims] 1. A plurality of fuel rods are arranged in a lattice in a channel box having a quadrangular cross section, and an upper tie plate and a lower tie plate are formed at an upper portion and a lower portion of the channel box, respectively, and a lower type is formed. In a fuel assembly in which a fuel rod support hole member having fuel rod support holes arranged in a grid pattern is provided on the plate, the center of the fuel rod support hole member is wide water gap with respect to the axis of the lower tie plate. A fuel assembly characterized by being displaced to the side. 2. The fuel assembly according to claim 1, wherein the width of the channel box is widened only on the narrow water gap side at the lower end thereof. 3. The fuel assembly according to claim 1 or 2, wherein pads are arranged on the upper four sides of the channel box.