JP2723252B2 - Reactor fuel assemblies - Google Patents

Description

[Detailed Description of the Invention] (Object of the Invention) (Industrial application field) The present invention relates to a fuel assembly for a nuclear reactor which is designed to improve fuel economy as much as possible.

(Prior Art) In a conventional fuel assembly for a nuclear reactor, the enrichment of fissile material loaded on each fuel rod constituting the fuel assembly was uniform in the axial direction.

However, in order to save uranium resources, the problem of so-called fuel economy has been raised in which the amount of uranium fuel required to obtain a constant output is reduced as much as possible. There has been proposed a fuel assembly which can be provided with an enrichment distribution in the axial direction by lowering the fuel at the upper end and the lower end than in the central portion, thereby reducing the leakage of neutrons from the upper and lower ends (for example, improving fuel economy). JP-A-59-38684).

FIG. 3 shows a conventional example in which the concentration of fissile material is uniform (hereinafter referred to as Conventional Example 1), and a conventional example (hereinafter referred to as Conventional Example 2) in which the concentration at the upper and lower ends is lower than that at the center. As shown in FIG. Each of these prior arts is a fuel for initial loading, and in order to suppress the axial output peaking to a certain set value (1.4 in this example), the loading amount of gadolinia, which is a burnable poison, is made different between the upper and lower fuels. As can be seen from both figures, by making the upper and lower ends of the fuel rods low-enriched natural uranium, the average enrichment of the fuel assembly can be reduced.
In this example, a uranium resource saving effect of about 4% was obtained as compared with Conventional Example 1.

(Problems to be Solved by the Invention) However, it has been found that the conventional example 2 still has room for improvement in fuel economy.

To show this, the gadolinia residual amount at the end of the cycle is shown in FIG. Gadolinia is a control material that controls the reactivity of the fuel together with the control rods to keep the core at a critical level. However, if it remains until the end of the cycle, it will reduce the reactivity and shorten the operation period. As shown in FIG. 5A, the residual gadolinia in Conventional Example 1 is significant at the upper and lower ends of the fuel assembly. On the contrary,
In Conventional Example 2, gadolinia was not loaded in the upper and lower low concentration portions, so most of the gadolinia remaining in Conventional Example 1 was eliminated, and in this sense, it was expected that there would be an effect of improving economic efficiency.

However, as shown in FIG. 6 (b), in Conventional Example 2, it was found that a large amount of gadolinia remained in portions adjacent to the upper and lower low concentration portions. The reason can be understood from the burnup distribution at the end of the cycle shown in FIG. That is, in the conventional example 2, since the upper and lower ends are made to have low concentration, combustion is delayed not only in that portion but also in an adjacent portion.

The end-of-cycle burnup distribution shown in FIG. 6 indicates that there is room for further improvement. As a result of burning as shown in FIG. 6, the output distribution at the end of the cycle is as shown in FIG. That is, in the conventional example 2, the output of the upper and lower ends is reduced by lowering the upper and lower ends compared to the conventional example 1, and the output is relatively concentrated in the center of the core.
This also improves fuel economy. However, in the conventional example 2, the upper and lower ends and the portion adjacent thereto are insufficiently burned as compared with the conventional example 1, but if this is comparable to the conventional example 1, the output at the end of the cycle is more concentrated in the center. This has the potential to further improve fuel economy.

The present invention has been made in view of the above circumstances, and its purpose is to reduce the residual gadolinia at the upper and lower ends of the fuel at the end of the operation cycle as much as possible, and to further concentrate the power at the center of the core to improve fuel economy. To provide a fuel assembly for a nuclear reactor.

[Constitution of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly for a nuclear reactor, comprising a plurality of fuel rods arranged in a lattice pattern. A first region in which the fuel assembly for a reactor has both an upper end whose length is 3/24 or less of the entire length of the fuel assembly and a lower end whose length is 2/24 or less of the entire length of the fuel assembly,
Both the upper end adjacent to the upper end and having a length of 3/24 or less of the total length of the fuel assembly and the lower end adjacent to the lower end and having a length of 2/24 or less of the entire length of the fuel assembly. And a third region excluding the first region and the second region.
And the first region has a lower fissile material content than the second and third regions, and the first region contains no burnable poison or contains burnable poison. The number of fuel rods and the concentration of burnable poisons are smaller than the third region, and the second region contains no burnable poison or the number of fuel rods containing burnable poison and the concentration of burnable poison in the first region. More than the third region.

Preferably, the first region is made of natural uranium containing no burnable poison.

More preferably, the second region has a lower fissile material content than the third region.

(Operation) According to the present invention, the upper end or lower end of the fuel is made to have a low enrichment, so that the neutrons leak out of the core, and furthermore, the flammability in the upper end or the lower end and a region adjacent thereto is reduced. Promote combustion in these areas by loading or unloading toxic poisons less or more than other areas, thus reducing gadolinia residues at the end of the cycle throughout the fuel and concentrating power in the center of the core Therefore, fuel economy can be improved.

(Example) An example of the present invention will be described with reference to the drawings.

FIG. 1 is an axial enrichment / gadolinia distribution diagram of a fuel according to a first embodiment of the present invention.

In this embodiment, the central high-concentration portion in the conventional example 2 in FIG. 4 is further divided, and the number of gadolinia is reduced and the concentration is reduced in the portion adjacent to the low-concentration portion at the upper and lower ends. When the number of gadolinia is reduced, the output increases due to an increase in the infinite multiplication factor in the early stage of combustion, so that the combustion proceeds. Therefore, with such a fuel, the gadolinia burns out at the end of the cycle due to the lowered gadolinia concentration, so that the gadolinia remaining amount in the portion where the gadolinia remaining amount is large in Conventional Example 2 can be reduced. Further, by analogy with a comparison between Conventional Example 1 and Conventional Example 2, in the present embodiment, it is predicted that the residual gadolinia increases again in the central portion adjacent to the portion where the gadolinia loading is reduced. As shown in the figure, the gadolinia is burned well by being dragged by the combustion of the gadolinia-reduced portion, so that the residual gadolinia can be reduced in any part of the fuel. In addition, as compared with the conventional example 2, the upper and lower ends and the portion adjacent thereto burn more, so that the output distribution at the end of the cycle becomes more concentrated in the center as shown in FIG. As a result, it was possible to reduce the average enrichment of the fuel assembly, and it was possible to save about 3% of the uranium resource in Conventional Example 2 and about 7% in Conventional Example 1. Here, simply lowering the gadolinia concentration increases the output in this region at the end of the cycle and prevents concentration in the center of the output distribution. It is important to reduce the gadolinia density and the number of gadolinia at the same time.

The length of each region is 3/24 or less of the total fuel length at the upper end, 2/24 or less at the lower end, and the region adjacent to these is
3/24 or less, 2/24 or less at the bottom is appropriate. The lengths of the upper end and the lower end are in accordance with the description of JP-A-59-038684, which has already been cited. In the area adjacent to the upper end, as shown in FIG. 6 (b), the residual gadolinia is not more than 3/24 of the total length, especially in the area of length 2/24. It is important to keep the concentration low. On the other hand, in this area, the output increases especially in the early stage of combustion in order to reduce the number of gadolinia, but if the number of gadolinia is reduced over a length of 3/24, the output in this area becomes excessive, causing an output peak and safety. Lower it. Similarly, in the region adjacent to the lower end, the residual amount of gadolinia is 2/24 or less of the entire length, particularly 1/24.
It is important to keep the gadolinia density low in this region, which is noticeable in the 24 length region. On the other hand, in this region, since the void ratio is small, the output tends to increase as compared to the upper portion where the void ratio is large, so that 2 /
Reducing the number of gadolinia over a length of 24 or more results in excessive output and lowers safety.

FIG. 2 is an axial enrichment / gadolinia distribution diagram of a fuel according to a second embodiment of the present invention.

In the present embodiment, gadolinia is not loaded on the upper and lower ends of the fuel adjacent to the natural uranium portion. However, since the output peaking in this region exceeds the limit value in this state, the enrichment in this portion is set lower than that in the central portion.

In the first and second embodiments, in order to suppress the output peaking, the central part is divided into upper and lower parts just in the middle, and more gadolinia is loaded in the lower part. It is not limited to this position. Also, as another means, increase the concentration of the upper part,
Alternatively, a combination of the concentration difference and the gadolinia difference can be used.

Further, in each of the above embodiments, the initially loaded fuel is described. However, in the case of the replacement fuel, fuel that has already undergone combustion is loaded in the core, and the burnup of the lower portion of these fuels proceeds more. Therefore, the output peaking is generally smaller than that of the initially loaded fuel, since the same effect as increasing the enrichment in the upper portion is obtained. Therefore, it is not necessary to make the enrichment difference or the gadolinia difference in the upper and lower portions of the replacement fuel as much as the initially loaded fuel, and in some cases, the central portion may not be divided into two regions.

[Effects of the Invention] As described above, according to the present invention, the fuel economy of the fuel assembly is reduced by reducing the leakage of neutrons from the upper and lower ends of the fuel, reducing the residual amount of gadolinia, and concentrating the output on the central portion. Properties can be significantly improved.

[Brief description of the drawings]

FIG. 1 is an axial enrichment / gadolinia distribution diagram of a fuel according to a first embodiment of the present invention. FIG. 2 is an axial enrichment / gadolinia distribution diagram of a fuel according to a second embodiment of the present invention. 3 and 4 show a conventional fuel enrichment / gadolinia distribution diagram in the axial direction, FIG. 5 shows a comparison of the gadolinia residual amount in the core height direction between the present invention and the conventional example, and FIG. FIG. 7 is a diagram comparing the burnup in the core height direction between the present invention and the conventional example, and FIG. 7 is a diagram comparing the relative output in the core height direction between the present invention and the conventional example.

Claims (3)

(57) [Claims] In a fuel assembly for a nuclear reactor constituted by arranging a plurality of fuel rods in a grid pattern, the length of the fuel assembly for a nuclear reactor is 3/24 or less of the total length of the fuel assembly. A first region having both an upper end and a lower end having a length of 2/24 or less of the entire length of the fuel assembly, and an upper end adjacent to the upper end and having a length of 3/24 or less of the entire length of the fuel assembly A second region consisting of both the adjacent portion and the lower end adjacent to the lower end and having a length equal to or less than 2/24 of the total length of the fuel assembly; and a third region excluding the first and second regions. The first region has a lower fissile material content than the second region and the third region, and the first region does not contain any burnable poison or the fuel rod contains a burnable poison. The number and burnable poison concentration are smaller than the third region,
In addition, the second region does not contain any burnable poison or the number of burnable poison-containing fuel rods and the burnable poison concentration are higher than the first region and lower than the third region. Reactor fuel assemblies. 2. The fuel assembly for a nuclear reactor according to claim 1, wherein said first region is made of natural uranium containing no burnable poison. 3. The nuclear reactor fuel assembly according to claim 1, wherein the fissile material content of the second region is smaller than that of the third region.