JPH08233967A - Fuel assembly and initial loading reactor core - Google Patents

Abstract

PURPOSE: To contrive the flatness of axial output distribution by specifically treating an area division or the like of a degree of enrichment of a short fuel rod and another degree of enrichment of a long fuel rod. CONSTITUTION: Average degrees in each axial traverse section indicate 1.10wt.% in the lower part, 1.28wt.% in the central part and 1.35wt.% in the upper part except for upper and lower end parts. A short fuel rod P is filled with fuel pellets in the lower part and the central part thereof. A degree of enrichment of the fuel rod P indicates 0.711wt.% and is made low as compared with the average degrees of the enrichment of the traverse section of the lower part and the central part. A long fuel rod 3 has a boundary of the degree of the enrichment at a position of 8/24 of the fuel effective length, the lower part indicates 0.711wt.% and the central part and the upper part are 1.4wt.%. As the constitution, axial enrichment distribution in the enrichment part of a lower degree of the enrichment is divided into three areas and becomes high as it goes upward. Flat axial output distribution can be obtained by simpler enrichment distribution by treating such a method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly and an initially loaded core of a nuclear reactor, and more particularly to a fuel assembly and an initially loaded core suitable for use in a boiling water reactor capable of increasing burnup.

[0002]

2. Description of the Related Art Generally, in a boiling water reactor, the average enrichment of the fuel assemblies loaded in the so-called initially loaded core during the first operation is the same and of one kind. In the nuclear reactor, about 1/3 to 1/4 of the total number of fuel assemblies are taken out for each cycle and replaced with fresh fuel, but the average enrichment of the initially loaded core fuel assemblies is within 2 to 3 cycle cores. Since it is set to be combustible, the operation cycle using the initially loaded core fuel assembly (hereinafter referred to as the first cycle, the second cycle is the cycle in which fuel is partially exchanged and subsequently operated).
In the fuel exchange at the end of the cycle (third cycle, third cycle, etc.), combustion has not progressed sufficiently, that is, uranium 235
It is uneconomical to take out a large amount of residual fuel assemblies from the core.

The new fuel assembly loaded at the beginning of the operation cycle after the second cycle is called a replacement fuel assembly, and the core loaded with the replacement fuel assembly continuously for several cycles after the first cycle. Is a cycle in which the fuel components in the entire furnace have reached a substantially constant state, and the thermal characteristics are the same as those of the previous cycle and the next cycle and are stable. This is called the equilibrium cycle, and the core that has become the equilibrium cycle is called the equilibrium core.

In such a nuclear reactor, an intermediate cycle (hereinafter,
It is preferable that the thermal characteristics and the cycle incremental burn-up in the transition cycle) converge to the same degree as those of the equilibrium cycle or rapidly. However, when the average enrichment of the fuel assembly is one type as in the conventional initially loaded core, the transition to the equilibrium cycle takes a long time, and the change in the number of refueling elements in the transition cycle is large, which is not always satisfactory. It wasn't something.

For this reason, in a boiling water reactor, various types of fuel assemblies having different average enrichments are combined to form an initially loaded core, which is taken out from a fuel assembly having a low average enrichment every cycle. It has been attempted to increase the average take-out burnup of the initially loaded fuel assembly and replace it with the new fuel assembly by swiftly shifting to the next cycle. Such a technique is disclosed, for example, in JP-A-57-84.
No. 86 publication.

On the other hand, as for the replacement fuel, two kinds of fuel rods, that is, a long fuel rod having a long axial length and a short fuel rod having a short axial length filled with a fuel substance are used, and the fuel enrichment is increased. FIG. 7 of Japanese Patent Laid-Open No. 3-273189 discloses a fuel assembly for increasing burnup and improving fuel economy.

Therefore, even in the initially loaded core, Japanese Patent Laid-Open No. 3-2
Assuming the loading of replacement fuel corresponding to high burnup as shown in Fig. 7 of 73189 publication, while maintaining good core characteristics in the transition cycle, the average unloading combustion of the initially loaded fuel assembly It is desirable to have an initially loaded core that can increase the frequency.

Further, Japanese Patent Laid-Open No. 63-121789 discloses a technique for flattening an axial power distribution by devising an axial gadolinia distribution in an initially loaded core. Further, in Japanese Unexamined Patent Publication No. 4-128688, the cross sectional area of the water rod at the lower portion in the axial direction is made smaller than that at the upper portion in the axial direction, and a short fuel rod filled with natural uranium is arranged in place of it to dispose the upper and lower regions. There is disclosed a technique for reducing the difference in the moderator-to-fuel ratio (H / U ratio) to make the fuel highly reactive and flatten the output distribution during operation.

[0009]

In the prior art disclosed in Japanese Patent Laid-Open No. 63-121789, since gadolinia reduces the reactivity of fuel, when gadolinia is mixed in the lowest enrichment fuel, It is uneconomical because the burnout of the fuel after combustion in the cycle is reduced.

Further, even if the conventional technique disclosed in Japanese Patent Laid-Open No. 4-128688 is used, the H / U ratio of the upper region is smaller than that of the lower region, and the output distribution in the axial direction cannot be sufficiently flattened. In order to further flatten the axial power distribution and bring the H / U ratio of the upper region closer to the lower region, it is necessary to increase the number of short fuel rods. As the number of short fuel rods increases, the amount of uranium loaded per fuel assembly decreases, resulting in a disadvantage in fuel economy. Further, since the shape of the water rod is different between the upper part and the lower part, the time and effort required for the molding process increase.

An object of the present invention is to provide a fuel assembly which can cope with a longer operating cycle and a higher burnup of fuel without increasing the time and effort required for forming a fuel, and can flatten the axial power distribution. It is to provide a core for initial loading.

[0012]

The above object is to provide two different axial lengths, a long fuel rod having a long axial length and a short fuel rod having a short axial length filled with a fuel substance. In a fuel assembly composed of a plurality of fuel rods having a plurality of fuel rods, the enrichment of the fuel with which the short fuel rods are filled is lower than the average enrichment in the cross section of the fuel assembly, and the long fuel rods have a high enrichment. And the lower part having a lower enrichment than the upper part, and the boundary between the upper part and the lower part is located at an axial position different from the upper end of the fuel filling portion of the short fuel rod.

Further, in an initially loaded core constructed by combining a plurality of types of fuel assemblies having different average enrichments, at least one type of fuel assemblies among the plurality of types of fuel assemblies has a fuel substance inside. The fuel filled in the short fuel rod is composed of a plurality of fuel rods having two different axial lengths: a long fuel rod having a long axial length and a short fuel rod having a short axial length. Is lower than the average enrichment in the cross section of the fuel assembly, the long fuel rod is divided into an upper part having a higher enrichment and a lower part having a lower enrichment than the upper part, and the boundary between the upper part and the lower part is This is achieved by setting the axial position different from the upper end of the fuel filling portion of the short fuel rod.

[0014]

[Function] In order to make the axial power distribution flat and to transfer the thermal characteristics smoothly (stably) to the equilibrium core, the reactivity characteristics of the average fuel assembly of the equilibrium core are simulated in the initially loaded core. This is not enough, and it is necessary to simulate the reactivity distribution in the axial direction of the fuel assembly.

By shifting the axial reactivity distribution of the lowest enrichment fuel in the initially loaded core corresponding to the fuel just before unloading in the equilibrium core to the upper side, the axial power distribution is flattened and the thermal characteristics are smoothed. Can be moved to. FIG. 9 shows an example of axial distribution of fissile material at the beginning of the cycle of fuel immediately before the equilibrium core extraction. When configuring an initially loaded core, it is ideal that the axial enrichment distribution of the lowest enrichment fuel simulates the axial distribution of fissile material shown in Fig. 9, but the enrichment distribution in the fuel assembly is complicated. Becomes too much, and the time and effort for the fuel forming process increase.

On the other hand, in the fuel assembly using the short fuel rods, the average enrichment of the cross section of the fuel assembly above the effective portion of the short fuel rods is changed by lowering the enrichment of the short fuel rods. Instead, the average enrichment of the cross section of the fuel assembly below the effective portion of the short fuel rod can be reduced. That is, it is possible to add a distribution of enrichment in the axial direction of the fuel assembly by making the enrichment of pellets filled in the fuel rod uniform.
The time and effort required for fuel molding can be reduced.

Therefore, according to the present invention, in a fuel assembly using two types of long fuel rods and short fuel rods, the fission in the early cycle of the fuel just before the equilibrium core extraction is performed with a simpler concentration distribution in the fuel assembly. Since it is possible to approach the axial distribution of the organic substance, the axial power distribution can be flattened and the thermal characteristics can be smoothly transferred to the equilibrium core without increasing the labor for fuel forming. Along with this, it is possible to cope with a longer operating cycle and higher burnup of fuel.

[0018]

EXAMPLE A first example in which the present invention is applied to a boiling water reactor will be described below with reference to FIGS. 1 to 3 and 8.
3 is a quarter cross section of a boiling water reactor core according to the present invention, FIG. 8 is a partial vertical cross section of a fuel assembly used in the core of FIG. 3, and FIG. 1 is a low enrichment fuel assembly according to the present invention. FIG. 2 shows the fuel rod configuration of the first embodiment of the body, and FIG. 2 shows the cross section of the low enrichment fuel assembly of the first embodiment.

The reactor shown in FIG. 3 is a boiling water reactor of 1100 MWe class, which is composed of 764 fuel assemblies and 185 control rods. In FIG. 3, 1 is a high enrichment fuel assembly, 2 is a medium enrichment fuel assembly, 3 is a low enrichment fuel assembly, and 5 is a control rod.

In FIG. 8, the fuel assembly 16 includes a square tube channel box 12 and a channel box 12.
And a fuel bundle housed inside. The fuel bundle includes an upper tie plate 18 and a lower tie plate 19 provided on the upper and lower portions of the channel box 12.
And a plurality of fuel spacers 20 axially spaced inside the channel box 12, and a long fuel rod penetrating the fuel spacer 20 and fixed at both ends to an upper tie plate 18 and a lower tie plate 19. 15 and the water rod 13 and the fuel spacer 20 to penetrate the lower tie plate 1
9 and a short fuel rod 23 whose lower end is fixed.

A low-enrichment fuel assembly has a structure as shown in FIG.
Fuel rods 1-4 are long fuel rods,
The fuel rod P is a short fuel rod. The active fuel length of the short fuel rod (the length of the loaded fuel) is within the range of 1/24 to 15/24 from the lower end of the active fuel length of the long fuel rod. Further, as shown in FIG. 2, eight short fuel rods are arranged at a predetermined interval in the second column from the outermost periphery of the fuel grid of 9 rows and 9 columns. The fuel rod 3 is divided into an upper part and a lower part at the position of 8/24 of the active fuel length, and the enrichment of the upper part is 1.4 wt%.
It is higher than the concentration of 0.711 wt% at the bottom.
Also, the low enrichment fuel assembly does not use fuel rods containing gadolinia.

In FIG. 1, the average enrichment in each axial cross section is 1.10 wt% (bottom), 1.28 wt% (center), 1.28 wt% (bottom) from the bottom, excluding the upper and lower ends of the natural uranium region. 35 wt% (upper part). The short fuel rod P is
Fuel pellets are filled in the lower part and the central part. The enrichment of the short fuel rods P is 0.711 wt%, which is lower than the average enrichment of the lower and central cross sections. The long fuel rod 3 has a boundary of enrichment at a position of 8/24 of the active fuel length, the lower part is 0.711 wt%, and the central part and the upper part are 1.4.
It is set to wt%. With this configuration, the axial enrichment distribution in the enrichment portion of the low enrichment fuel is divided into three regions, and the enrichment becomes higher as it goes upward.

The solid line in FIG. 4 shows the axial power distribution at the beginning of the cycle when this low-enrichment fuel is loaded in the initially loaded core shown in FIG. The peaking in the axial direction is small and flat as compared with the broken line which is a comparative example in which the enrichment of the short fuel rod is not lower than the average enrichment of the cross section. FIG. 5 shows the distribution of the fuel rod enrichment of the comparative example in which the enrichment of the short fuel rod is not lower than the average enrichment of the cross section. In FIG. 5, the average enrichment in each axial cross section is 1.17 wt% from below except for the upper and lower ends of the natural uranium region.
(Bottom), 1.35 wt% (center), 1.35 wt%
The difference in enrichment between the lower part and the central part is the same as in the case of FIG. 1, but the difference in enrichment between the central part and the upper part is eliminated. Further, the complexity of the enrichment distributions in FIGS. 1 and 5 is about the same.

FIG. 6 is based on the enrichment distribution of FIG.
This is a comparative example in which an axial enrichment distribution equivalent to the above is obtained, and the enrichment is further divided into a central portion and an upper portion of the long fuel rod 3. In FIG. 6, the average enrichment in each cross section in the axial direction is 1.17 wt% (bottom), 1.35 wt% (center), 1.42 wt% (from the bottom, excluding the upper and lower ends of the natural uranium region). The difference in enrichment between the lower part and the central part and the difference in enrichment between the central part and the upper part are the same as in the case of FIG. Therefore, when the low-enrichment fuel shown in FIG. 6 is loaded into the initially loaded core shown in FIG. 3, the axial power distribution in the initial stage of the cycle is almost equal to the solid line in FIG. The degree distribution is more complicated than that in FIG. 1, and the labor required for the fuel forming process increases.

From the above, as shown in FIG. 1, the enrichment of the short fuel rod is made lower than the average enrichment in the cross section of the fuel assembly, and at the axial position below the upper end of the effective fuel portion of the short fuel rod. By using a long fuel rod in which the enrichment in the upper portion is higher than that in the lower portion, it becomes possible to obtain a flat axial power distribution with a simpler enrichment distribution.

Next, a second embodiment of the low enrichment fuel assembly according to the present invention will be described with reference to FIG. In this embodiment,
The active fuel length of the short fuel rod P is from the lower end of the active fuel length of the long fuel rod to the axial position of 1/24 to 8/24. Further, the long fuel rods 3 are divided into upper and lower enrichments at axial positions of 15/24 from the lower end, and the lower portion has 0.711 wt% and the central portion and upper portion have 1.4 wt%. The average enrichment in each axial cross section is 1.10 wt% from below, excluding the upper and lower ends of the natural uranium region.
(Bottom), 1.14 wt% (center), 1.35 wt%
(Upper part), and the axial enrichment distribution in the fuel enrichment portion becomes higher as it goes upward.

Therefore, as shown in FIG. 7, the enrichment of the short fuel rods is made lower than the average enrichment in the cross section of the fuel assembly, and at the upper axial position above the upper end of the effective portion of the short fuel rods. By using a long fuel rod having a higher enrichment than that of the lower portion, it is possible to obtain a flat axial power distribution with a simpler enrichment distribution, which can reduce the time and effort required for forming the fuel.

[0028]

As described above, according to the present invention, in a fuel assembly using two types of long fuel rods and short fuel rods, a simpler fuel assembly enrichment distribution immediately before equilibrium core extraction Since it is possible to approach the axial distribution of fissile material at the beginning of the fuel cycle, the axial power distribution is flattened without increasing the time and effort required for the fuel forming process, and the thermal characteristics are smoothly transferred to the equilibrium core. Can be made. Along with this, it is possible to cope with a longer operating cycle and higher burnup of fuel.

[Brief description of drawings]

FIG. 1 is a diagram showing a fuel rod structure of a first embodiment of a low enrichment fuel assembly according to the present invention.

FIG. 2 is a cross-sectional view of the low enrichment fuel assembly of the first embodiment.

FIG. 3 is a quarter cross-sectional view of a boiling water reactor core according to the present invention.

FIG. 4 is a diagram showing an axial power distribution at the beginning of a cycle using the low enrichment fuel assembly of the first embodiment.

FIG. 5 is a view showing a fuel rod structure of a comparative example of a low enrichment fuel assembly.

FIG. 6 is a view showing a fuel rod structure of another comparative example of the low enrichment fuel assembly.

FIG. 7 is a view showing a fuel rod structure of a second embodiment of the low enrichment fuel assembly according to the present invention.

8 is a partial vertical cross section of a fuel assembly used in the core of FIG.

FIG. 9 is a diagram showing an example of axial distribution of fissile material at the beginning of the cycle of fuel immediately before equilibrium core removal.

[Explanation of symbols]

1 ... High enrichment fuel assembly, 2 ... Medium enrichment fuel assembly, 3
... low enrichment fuel assemblies, 5 ... control rods, P ... short fuel rods.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mitsuya Nakamura 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi factory (72) Inventor Hajime Aoyama Seven, Mika-machi, Hitachi-shi, Ibaraki 2-1-1, Hitachi Energy Research Institute, Ltd. (72) Inventor Junichi Koyama, 7-2-1, Omika-cho, Hitachi City, Ibaraki Hitachi Energy Research Institute, Ltd.

Claims (6)

[Claims] 1. A plurality of fuel rods having two different axial lengths, a long fuel rod having a long axial length filled with a fuel substance and a short fuel rod having a short axial length. In the fuel assembly, the enrichment of the fuel with which the short fuel rods are filled is lower than the average enrichment in the cross section of the fuel assembly, and the long fuel rods have a higher enrichment and a lower enrichment than the upper portion. A fuel assembly, wherein the fuel assembly is divided into a lower portion, and a boundary between the upper portion and the lower portion is located at an axial position different from an upper end of a fuel filling portion of the short fuel rod. 2. The fuel assembly according to claim 1, wherein the plurality of fuel rods do not contain a burnable poison. 3. An initially loaded core comprising a combination of various types of fuel assemblies having different average enrichments, wherein at least one of the various types of fuel assemblies has a fuel substance inside. It is composed of a plurality of fuel rods having two different axial lengths, that is, a filled long fuel rod having a long axial length and a short fuel rod having a short axial length, and the fuel filled in the short fuel rod. Is lower than the average enrichment in the cross section of the fuel assembly, the long fuel rod is divided into an upper part having a higher enrichment and a lower part having a lower enrichment than the upper part, and the boundary between the upper part and the lower part is An initially loaded core, which is located at an axial position different from the upper end of the fuel filling portion of the short fuel rod. 4. The initially loaded core according to claim 3, wherein the plurality of fuel rods do not contain burnable poisons. 5. An initially loaded core comprising a combination of various types of fuel assemblies having different average enrichments, wherein at least the lowest enrichment of the various types of fuel assemblies has a fuel substance inside. And a plurality of fuel rods having two different axial lengths, that is, a long fuel rod having a long axial length and a short fuel rod having a short axial length. The enrichment of the fuel is lower than the average enrichment in the cross section of the fuel assembly, and the long fuel rod is divided into an upper part having a higher enrichment and a lower part having a lower enrichment than the upper part, and a boundary between the upper part and the lower part is An initially loaded core, which is located at an axial position different from the upper end of the fuel filling portion of the short fuel rod. 6. The initially loaded core according to claim 5, wherein the plurality of fuel rods do not contain burnable poisons.