JPH01304387A - Fuel assembly for nuclear reactor - Google Patents

Abstract

PURPOSE:To improve fuel economy by maintaining low enrichment at the top end or bottom end of fuel, thereby decreasing the residual amt. of gadolinia at the top and bottom ends of the fuel at the end period of an operation cycle as far as possible and concentrating the output to the center of the reactor core. CONSTITUTION:This fuel assembly is constituted by further dividing the central high enrichment part in the axial direction of the fuel and decreasing the number of the gadolinia in the part adjacent to the low enrichment parts at the top and bottom ends to lower the enrichment. The residual amt. of gadolinia where the residual amt. of gadolinia is high is decreased by this fuel. Further, the central part adjacent to the parts where the loading amt. of the gadolinia is decreased burns most in follow up to the combustion of the parts where the loading amt. of the gadolinia is decreased and, therefore, the residual amt. of the gadolinia is decreased in any parts of the fuel. Since the combustion at the top and bottom ends and the parts adjacent thereto progresses most, the output distribution at the end period of the cycle is more concentrated and distributed to the center. As a result, the average enrichment of the fuel assembly is decreased and the output is concentrated more to the center of the core. The fuel economy is thus improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a fuel assembly for a nuclear reactor, which is designed to maximize fuel economy.

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

However, in order to conserve uranium resources, the problem of so-called fuel economy has been raised, in which the amount of uranium fuel required to obtain a certain output is reduced as much as possible. A fuel assembly has been proposed that can improve fuel economy by reducing neutron enrichment distribution in the axial direction by lowering the concentration at the upper and lower ends than in the center and reducing leakage of neutrons from the upper and lower ends (e.g. JP-A No. 59-38684).

Figure 4 shows a conventional example in which the degree of enrichment of fissile material is uniform (hereinafter referred to as conventional example 1), and a conventional example in which the degree of enrichment at the upper and lower ends is lower than in the center (hereinafter referred to as conventional example 2). It is shown in FIG. All of these conventional examples are fuels for initial loading, and in order to suppress the axial output peaking to a certain set value (1.4 in this example) or less, the loading amount of gadolinia, which is a burnable poison, is varied between the upper and lower portions of the fuel. There is. As can be seen from both figures, by using low-enriched natural uranium at the upper and lower ends of the fuel rods, the average enrichment of the fuel assembly can be lowered, compared to conventional example 2.
Compared to Conventional Example 1, a uranium resource saving effect of about 4% is obtained.

(Problems to be Solved by the Invention) However, it has been found that even with d3 in Conventional Example 2, there is still room for improvement in fuel economy.

To illustrate this, the amount of gadolinia remaining at the end of the cycle is not shown in FIG. Gadolinia, together with the control rods, is a control material that controls the reactivity of the fuel and maintains the reactor core at a critical level, but if it remains until the end of the cycle, it will reduce the reactivity and shorten the operating period. Become. The residual amount of gadolinia in Conventional Example 1 is significant at the upper end of the fuel assembly, as shown in FIG. 6(a). On the contrary,
In conventional example 2, gadolinia is loaded in the upper and lower end low concentration sections.

Therefore, most of the remaining gadolinia 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 the portions adjacent to the upper and lower end low concentration portions. The reason for this can be seen from the burnup distribution at the end of the cycle shown in FIG. That is,
In Conventional Example 2, since the upper and lower ends are low enriched, combustion is delayed not only in those parts but also in the adjacent parts.

The burn-up at the end of the cycle shown in FIG. 7 shows that there is still room for further improvement. As a result of combustion as shown in FIG. 7, the output distribution at the end of the cycle becomes as shown in FIG. 8. That is, in Conventional Example 2, compared to Conventional Example 1, the output at the upper and lower ends is lower due to the lower concentration at the upper and lower ends.
The power output is relatively concentrated in the center of the core, which also improves fuel economy. However, in conventional example 2, compared to conventional example 1, the upper and lower ends are
:The combustion in the adjacent part is insufficient, but if this is the same as the conventional example 1, the output at the end of the cycle will be more concentrated in the center, and there is a possibility of further improving fuel economy. be.

The present invention was made in view of the above circumstances, and its purpose is to reduce as much as possible the amount of gadolinia remaining in the upper and lower ends of the fuel at the end of the operating cycle, and to further concentrate the power in the center of the reactor core to improve fuel economy. An object of the present invention is to provide a fuel assembly for a nuclear reactor.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly in a nuclear fuel assembly configured by arranging a plurality of fuel rods in a lattice shape. The content of fissile material in the lower end or at least one of the lower ends of the fuel assembly is lower than the content of fissile material in other parts of the fuel assembly; It is characterized in that the content of the burnable poison in a portion adjacent to at least one of these portions is smaller than that in the other portions, or that the burnable poison is not loaded in these portions.

(Function) According to the present invention, the leakage of neutrons to the outside of the reactor core is reduced by making the upper end or the lower end of the fuel have a low enrichment level, and the leakage of neutrons to the outside of the reactor core is generally reduced. By reducing the loading amount of burnable poison in adjacent areas compared to other areas or not loading it, combustion in these areas is promoted, thereby reducing the amount of gadolinia remaining at the end of the cycle throughout the fuel, and at the same time reducing the amount of burnable poison in the center of the core. Since the output can be concentrated in the fuel economy, fuel economy can be improved.

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

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

In this embodiment, the central highly enriched portion in Conventional Example 2 shown in FIG. 5 is further divided, and the number of gadolinia is reduced and the concentration is lowered in the portions adjacent to the lower enriched portions at the upper and lower ends. According to such a fuel, the amount of gadolinia remaining in the portion where the amount of remaining gadolinia was large in Conventional Example 2 can be reduced. Furthermore, conventional example 1 and conventional example 2
By analogy, in this example, it is predicted that the not-linear residual amount will increase again in the central part adjacent to the part where the gadolinia loading city is reduced.
As shown in the figure, this part also burns well due to the combustion of the part with a small amount of cadrinia loaded, so the fuel)
It is possible to reduce the amount of residual cadrinia in any part of 1. In addition, compared to conventional example 2, "1 lower end d"3
As a result of the combustion progressing well in the area and adjacent areas, 1
The distribution of the horns at the end of the no-cycle becomes more concentrated in the center as shown in FIG. As a result, the average enrichment of the fuel assembly could be reduced, and uranium resources could be saved by about 3% compared to Conventional Example 2 and by about 7% compared to Conventional Example 1.

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 length of the adjacent region is 3/24 or less at the top and 2/24 or less at the bottom, or as appropriate. be.

FIG. 2 is an 11 diagram showing the axial enrichment/cadrinier fraction of fuel according to the second embodiment of the present invention.

In this example, cadrinia is not loaded in the upper and lower ends of the fuel adjacent to the natural uranium portions. However, if left as is, the peaking in this area would exceed the limit value, so the concentration level in this area is set lower than in the central area.

In the first and second embodiments below, in order to suppress output beaking, the central part is divided into upper and lower parts exactly in the middle, and more gadolinia is loaded than the lower part, or the position of the upper and lower divisions is near this (=j). However, it is not necessarily limited to this position.Also, another method is to increase the concentration at the top
Alternatively, a combination of enrichment difference and gadolinia difference can also be used.

In addition, in each of the above examples, the initial loading fuel is explained, or in the case of replacement fuel, fuel that has already experienced combustion is loaded into the reactor core, and with these fuels, the burnup in the lower part is more advanced. This has the same effect as increasing the enrichment at the top, so it generally produces more fuel than the initial load)J
Peaking is small. Therefore, in the case of replacement fuel, there is no need for the initially loaded fuel to have a difference in enrichment or gadolinia between the upper and lower sides, and in some cases, it is not necessary to divide the central portion into two regions.

FIGS. 3(a) to 3d) illustrate the axial enrichment and gadolinia content of the fuel in the third to sixth embodiments of the present invention. d3, a5 in each of these figures, the horizontal axis (j1) represents the enrichment, the vertical axis represents the core height, and the portions where the amount of quadrinia loaded is small or not loaded at all are shown with diagonal lines.

Figure 3(a) shows the upper end, and Figure 3(b) shows the lower end. In Fig. 3 (C) cf5 and Fig. 3 (d), both the lower end and the lower end have low enrichment, but the amount of cadrinia loaded is low at either end or at either end. It's the adjacent part.

In each of the above-mentioned embodiments, the effect becomes smaller when compared to the first and second embodiments, or the effect of improving fuel economy is still obtained. Further, each of the third to sixth embodiments can be provided with the same degree of enrichment and trilinear loading as the first and second embodiments.

[Effects of the Invention] As explained hereinafter, according to the present invention, by reducing the leakage of neutrons from the lower part of the fuel, reducing the residual amount of gadolinia, and concentrating the output in the central part, the fuel assembly can be improved. can significantly improve the body's fuel economy.

[Brief explanation of the drawing]

FIG. 1 is an axial enrichment/gadolinia 9411 diagram of fuel H according to the first embodiment of the present invention, and FIG. 2 is an axial enrichment/cadrinia distribution diagram of fuel H according to the second embodiment of the present invention. Figure 3 (
a) to (d) are axial enrichment/gadolinia distribution diagrams of the fuel of the third to sixth embodiments of the present invention, and Fig. 1 d3 to Fig. 5 are axial direction diagrams of the conventional fuel H. Figure 6 is a diagram comparing the not-linear residual amount in the core height direction between the present invention and the conventional example.
The figure is a comparison of the burnup in the core height direction of the present invention and the conventional example, and Figure 8 is a comparison of the burnup in the core height direction of the present invention and the conventional example)
Illustrated in a diagram comparing j. (8733) Representative Patent Attorney W. Inomata (1 idiot) 1. Figure 1. Figure 2. Figure 3. Uniform force diagram 8

Claims (1)

[Claims] In a nuclear reactor fuel assembly constructed by arranging a plurality of fuel rods in a lattice, the fissile material content of at least one of the upper end and lower end of the fuel assembly is higher than that of the other parts of the fuel assembly. The content of burnable poison is lower than the content of fissile material, and the burnable poison content of the upper end or the lower end and the parts adjacent to at least one of these parts is lower than the other parts, or these parts A fuel assembly for a nuclear reactor, characterized in that it is not loaded.