JP2791132B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a fuel assembly used for a boiling water reactor (hereinafter, referred to as BWR).

(Prior Art) Generally, in a BWR, a large number of fuel assemblies are loaded as nuclear fuel in a reactor core.

FIG. 2 is a cross-sectional view of a currently used D-lattice fuel assembly, in which reference numeral 11 denotes a channel box having a rectangular cross section for accommodating a plurality of fuel rods 12 in a lattice shape. I have.

Here, the number given to each fuel rod 12 indicates the enrichment of uranium 235, and the fuel rods 12 having the same number have the same enrichment. Reference symbol G denotes a fuel rod containing a burnable poison disposed to improve the reactivity change characteristic of the fuel assembly accompanying combustion, and reference symbol W denotes a water rod.

A plurality of such fuel assemblies are loaded and used in the core, and as shown in FIG. 2, they are arranged at a fixed distance from the control rod 13 and the adjacent fuel assemblies.
In such a D-lattice fuel assembly, of the water gap formed between the boundary line between adjacent fuel assemblies and the channel box 11, the wide water gap 14 formed on the control rod 13 side is formed. The ratio of the width of the narrow water gap 15 formed on the non-control rod side opposite to the width is about 2: 1.

In recent years, there has been a strong demand for a fuel assembly having a high burnup to achieve a prolonged cycle burnup, and this has led to an increase in the average uranium 235 enrichment of the fuel rods 12 disposed in the fuel assembly. At this time, in order to keep the local output peaking coefficient in the fuel assembly within the standard, the number of types of uranium 235 enrichment (hereinafter referred to as the split number) of the fuel rods 12 is increased, or the fuel containing burnable poisons is increased. In order to satisfy the design standard, the position of the rod 12 is devised.

However, in the above D-lattice fuel assembly, the ratio of the wide water gap 14 to the narrow water gap 15 is large, for example, 2 so that about 7 splits are required and a fuel rod containing a burnable poison is required. 12
However, there is a problem that it is difficult to select an arrangement position of the device. In addition, when the uranium 235 enrichment is increased, it is generally difficult to secure a reactor shutdown margin.

Therefore, as shown in FIG. 3, the ratio between the wide water gap 14a and the narrow water gap 15a is set to approximately 1: 1.
A C lattice fuel assembly was developed. This C lattice fuel assembly is highly symmetrical in structure, the type of the fuel rods 12 needs to be about 5 splits, and the fuel rods 12 containing burnable poisons.
There is an advantage that it is easy to select the disposition position and to easily obtain good nuclear characteristics.

Therefore, at the time of replacement of the fuel assembly, it is desired to load a fuel assembly close to the C-lattice fuel assembly also in the D-lattice BWR.
In Japanese Unexamined Patent Publication (Kokai) No. H10-27139, by forming the fuel support fitting insertion portion in the lower tie plate formed at the lower end of the fuel assembly eccentrically from the axis of the channel box, even in the D lattice BWR, the fuel close to the C lattice fuel assembly It proposes a technology that enables loading of aggregates.

(Problems to be solved by the invention) However, in BWR, generally, as shown in FIG.
Since four fuel assemblies are arranged around one control rod 13 and all four fuel assemblies are not replaced with a new fuel assembly after one operation cycle,
Loading the C-lattice fuel assembly abruptly after a certain operating cycle is actually due to output mismatch (uneven output of the four fuel assemblies) and control rod clearance (interval between control rod 13 and channel box 11). It is difficult to implement due to problems.

The present invention has been made in view of such a point, and even when loaded into a conventional BWR core using a D lattice fuel assembly,
It has a function close to that of a C-lattice fuel assembly without causing output mismatch or control rod clearance problems, and secures reactor shutdown margin even when uranium-235 enrichment increases to prolong cycle burnup. It is an object of the present invention to provide a sufficient fuel assembly.

[Constitution of the Invention] (Means for Solving the Problems) That is, the fuel assembly of the present invention has the width of the wide water gap 14 and the width of the wide water gap in the conventional D lattice fuel assembly as shown in FIG. The distance between the fuel rods is made smaller than the distance 16 between the fuel rods in the D lattice fuel assembly without changing the distance (water film width) 17 between the inner surface of the channel box 11 on the gap side and the fuel rod closest thereto. By making the width of the narrow water gap formed on the non-control rod side larger than 1/2 of the width of the wide water gap formed on the control rod side, The width of the water film formed between them is larger than the width of the water film on the control rod side, so that it has a function close to that of the C lattice fuel assembly.

(Operation) In general, the width of the wide water gap 14 in the D lattice fuel assembly shown in FIG. 2 is often larger than the width of the wide water gap 14a in the C lattice fuel assembly shown in FIG. Until 13 is replaced
While maintaining the width of the wide water gap 14 and the width of the water film 17 on the wide water gap side,
By increasing the width of the narrow water gap 15 or the width of the water film 18 on the narrow water gap side, it is possible to obtain a fuel assembly having a function close to that of the C lattice fuel assembly and having no problem in control rod clearance. Can be. In addition, if reducing the width of the wide water gap 14 by replacing the control rod 13 is no longer a problem, the width of the narrow water gap 15 can be increased while the width of the wide water gap 14 is reduced. Similarly, the width of the water film 17 on the wide water gap side may be changed.

In addition, when a new fuel assembly is loaded after a certain operation cycle, the distance between the fuel rods in the fuel assembly is not changed greatly at once, but is changed little by little in each operation cycle. The problem of output mismatch of the aggregate can be solved.

(Example) Hereinafter, an example of the present invention is described based on a drawing.

FIG. 1 shows an embodiment of the fuel assembly according to the present invention. In the conventional D-lattice fuel assembly shown in FIG.
The width of the narrow water gap 15 is increased to 15 'by reducing 16 to 16'.

FIG. 5 shows the ratio of the width of the wide water gap 14 to the width of the narrow water gap 15 '(hereinafter referred to as GW / GN).
Expressed by ) Shows the change of the infinite gain when the GW / GN is changed from the current value of about 2 to 1 corresponding to the C-lattice fuel assembly with the infinite gain on the vertical axis, and the curvature a , B, c
Shows the case of the operation at the time of cold temperature, the operation of the low void ratio output, and the operation of the high void ratio output operation, respectively. As is clear from this figure, as GW / GN is reduced, the infinite multiplication factor at the time of cold operation is smaller, while the infinite multiplication factor at the time of output operation is slightly smaller at the low void ratio, but is smaller at the high void ratio. On the contrary, it is getting bigger.

FIG. 6 shows the GW / GN on the horizontal axis and the reactivity difference between the output operation and the cold temperature on the vertical axis. Curves d and e show the GW / GN during the low void ratio and the high void ratio output operation, respectively. It shows the change in the reactivity difference between output operation and cold temperature using GN, GW / GN
, The difference in reactivity between the output operation and the cold operation becomes smaller. Therefore, by applying the fuel assembly of the present invention, the reactivity difference between the output operation and the cold temperature can be greatly reduced while maintaining or slightly increasing the reactivity during the output operation.

Further, since the number of splits can be reduced by reducing the GW / GN, the manufacturing cost of the fuel assembly can be reduced, and the reactivity difference between the output operation and the cold operation can be reduced. Therefore, it is easy to secure a margin for shutting down the furnace, and it is possible to obtain a fuel assembly that meets the demand for longer cycle burnup, which has been studied in recent years.

Fig. 7 shows relative power on the horizontal axis and core height on the vertical axis.
It is a graph showing the average axial power distribution at the time of cold shutdown. From the viewpoint of securing a reactor shutdown margin, it is important that the difference in the degree of reaction between the power operation and the cold temperature is small in the upper part of the core, which is a high void region during the power operation, as shown in FIG. In the fuel assembly, the difference in reactivity between the fuel assembly and the cold temperature during the high void ratio output operation is particularly small. Therefore, the fuel assembly of the present invention also has an excellent effect in this respect.

Above, GW without changing the width of the wide water gap 14
Although the operation and effect of the embodiment with reduced / GN have been described, the control rod 13 can be replaced if this embodiment is performed for several operation cycles, while the width of the wide water gap 14 is reduced while the GW / What reduced GN can also be applied. If the width of the wide water gap 14 can be reduced, the distance between the control rods 13 and the fuel rods 12 is reduced, and the value of the control rods can be increased, so that it is easier to secure a furnace stop margin.

As described above, the present invention uses the current D lattice BWR
It is possible to provide a useful fuel assembly when moving to the lattice BWR.

It should be noted that the fuel assembly of the present invention is not limited to the embodiment shown in FIG. 1, but may be one in which the width of the water film 18 on the narrow water gap side is increased by reducing the distance 16 between fuel rods to 16 '. Even in this case, since the nuclear characteristics of the fuel assembly are basically determined by the arrangement of the fuel rods, the same effects as those shown in FIG. 1 are obtained.

[Effects of the Invention] As is clear from the above description, according to the present invention,
It is possible to provide a fuel assembly suitable for shifting from a conventional D-lattice BWR to a C-lattice BWR, and to obtain a fuel assembly that can cope with prolonged cycle burnup, which has been studied in recent years. be able to.

[Brief description of the drawings]

1 is a cross-sectional view showing a fuel assembly according to one embodiment of the present invention, FIG. 2 is a cross-sectional view showing a conventional D-lattice fuel assembly,
FIG. 3 is a cross-sectional view showing a C lattice fuel assembly, FIG. 4 is a cross-sectional view showing an arrangement of four fuel assemblies around a control rod,
Fig. 5 is a graph showing the change of the infinite multiplication factor by GW / GN,
FIG. 6 is a graph showing the change in the reactivity difference at the time of the cold operation during the power operation by the GW / GN, and FIG. 7 is a graph showing the average axial power distribution at the time of the cold shutdown. 11 ... Channel box 12 ... Fuel rod 13 ... Control rod 14, 14a ... Wide water gap 15, 15a, 15 '... Narrow water gap 16, 16' ... Distance between fuel rods 17 ... Wide water gap Side water film 18 …… Narrow water gap side water film

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G21C 3/30-3/328 G21C 7/00-7/02

Claims (1)

(57) [Claims] 1. A fuel assembly comprising a plurality of fuel rods arranged in a grid in a channel box having a square cross section, formed between a boundary line between adjacent fuel assemblies in a reactor core and the channel box. The ratio of the non-control rod side to the control rod side of the water gap width to be
For the lattice fuel assembly, the water gap width on the control rod side of the D lattice fuel assembly and the control rod side water film width indicating the distance between the inner surface of the control rod side channel box and the fuel rod closest thereto are changed. By adjusting the arrangement interval of the fuel rods, the water gap width on the non-control rod side becomes 1 /
2. A fuel assembly wherein the width of the water film on the non-control rod side is larger than the width of the water film on the control rod side.