JPS61260188A - Fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel assembly loaded into the core of a nuclear reactor.

[Technical background of the invention and its problems]

Generally, in a boiling water nuclear reactor, a long fuel assembly, in which a fuel bundle consisting of a large number of fuel rods and water rods is loaded into a channel box, is loaded into a reactor core.

It is desired to improve fuel economy in nuclear reactors, and one method for achieving this is to improve the burnup at the time of fuel removal, that is, the fuel removal burnup.

In order to improve this fuel extraction burnup, it is necessary to improve the reactivity when the fuel assembly is loaded into the reactor core.

The relationship between the rate of increase ΔEd/Ed in fuel extraction burnup and the rate of increase Δ in core reactivity can be expressed as follows.

Here, C is a constant.

Further, the reactivity K of the core can be expressed as HKeff-εpfη(1-L)-(2) according to the four-prison formula.

Here, ε: fast fission effect p: probability of escaping resonance absorption f: Thermal neutron utilization rate η: Reproduction rate L: Probability that neutrons leak from the core It is.

According to equation (2), KeH can be improved by increasing f. Furthermore, by reducing L, K can be improved.

rf Conventionally, the fuel assembly A was formed as shown in FIGS. 12 and 13.

FIG. 12 simultaneously shows the positional relationship between the control rod B, which has a cross-shaped cross section, and the fuel assembly A. The fuel assembly A is formed by arranging fuel rods in an 8×8 square lattice. Each fuel rod has its type 1, 2, 3.4, 5. It is indicated by the symbols G and W, and the specific configuration is the 13th
As shown in the figure. FIG. 13 shows the uranium enrichment level (hereinafter referred to as enrichment level) and the concentration of gadolinia (Gd203 or below, abbreviated as Gd), which is a type of burnable poison, at 24 nodes of the total length. Fuel rod type 1 in Figure 13
~5 are each filled with enriched uranium only, G is filled with enriched uranium and Gd, and W
indicates a water rod.

However, in the conventional fuel assembly A formed in this way, as shown in Fig. 14, many neutrons leak at the upper and lower ends of the core, so the thermal neutron flux is lower than that at the center. It has become noticeably smaller.

(Object of the invention) The present invention has been made with these points in mind,
It has almost the same average enrichment of the fuel assembly as conventional fuels, has a large fuel extraction burnup, has excellent safety, is easy and inexpensive to manufacture, and is also excellent in economic efficiency. The purpose is to provide fuel assemblies.

(Summary of the Invention) A fuel assembly of the present invention is a fuel assembly loaded into a core of a nuclear reactor constituted by a plurality of fuel rods, wherein the fuel assembly is provided with a natural uranium portion at at least one of the upper and lower ends. At the same time, an enriched uranium section is provided between them, and when this enriched uranium section is divided into three parts in the axial direction into an upper part, a central part, and a lower part, the cross-sectional enrichment degree of the upper part of the enriched uranium part is lower than that of the other central part and lower part. , by combining a fuel rod having two regions with different enrichment degrees and a fuel rod having two regions with different enrichment degrees, in which the cross-sectional enrichment of the lower part of the enriched uranium part is lower than the other upper part and the central part. The fuel rod is formed such that the cross-sectional average enrichment of the fuel rod is higher than the cross-sectional average enrichment of the upper and lower parts, and a part of the fuel rod contains a burnable poison having two or more regions with different concentrations in the axial direction, The burnable poison concentration is characterized in that the burnable poison concentration in the upper part of the fuel rod is lower than in other parts.

[Embodiments of the Invention] First, a fuel assembly of the present invention will be conceptually explained with reference to FIGS. 7 and 8.

In the present invention, a fuel assembly is formed as follows (1), (2), <3>, and (4).

(1) Natural uranium parts are provided at the upper and lower ends of the fuel rod.

As described above, the amount of neutron leakage is large at the upper and lower ends of the core, so the thermal neutron flux is significantly smaller than at the center.

Therefore, the upper and lower ends of the fuel rods are filled with natural uranium, increasing the enrichment in the center. However, the average enrichment of the entire fuel assembly will be approximately the same as in the past.

As a result, U-235 increases in the high-thermal neutron region of the fuel, and U-235 decreases in the low-thermal neutron region of the fuel, resulting in a higher thermal neutron utilization rate f than in the conventional fuel assembly. At the same time, U-235 at the upper and lower ends of the fuel is reduced, reducing the neutron flux and reducing the probability that neutrons will leak from the reactor or core.

Due to this phenomenon, the reactivity Keft of the core increases and the fuel extraction burnup improves.

Note that natural uranium may be provided at at least one of the upper and lower ends.

(2) When the enriched uranium part of the fuel rod excluding the natural uranium part is divided into three parts in the axial direction: an upper part, a middle part, and a lower part, the concentration of burnable poison in the axial direction is made lower in the upper part than in the other parts. .

Burnable poisons such as Gd are used to control excess reactivity in the reactor core, but they may remain in low-output areas without being burned out within the scheduled operating period. The absorption of neutrons by this residual burnable poison reduces the absorption of neutrons by fissile materials such as U-235, and reduces the thermal neutron utilization rate f.

Generally, in a boiling water reactor, the void ratio is large in the upper part V4 of the reactor core, and the energy spectrum of neutrons is hard, so it is thought that the residual amount of burnable poisons will be large.

Therefore, in the present invention, as shown in FIG. 7(b),
When the enriched uranium part excluding the natural uranium part is divided into three parts in the axial direction, the concentration of burnable poison in the upper part is lower than that in the other central part and lower part.

This makes it possible to reduce the residual amount of burnable poisons compared to conventional fuel assemblies, improve the thermal neutron utilization rate f, and reduce the core reactivity K. n can be increased.

Further, the concentration of the burnable poison may be made different in the center and lower parts, excluding the upper part where the concentration is low, to further flatten the output distribution in the axial direction (Japanese Patent Publication No. 58-23913
(see publication).

(3) The enrichment degree in the axial direction of the enriched uranium portion of the entire fuel assembly is formed such that the cross-sectional average enrichment degree of the central portion is higher than that of the other upper and lower portions.

As shown in FIG. 7(a), the enriched uranium part between the natural uranium parts at the upper and lower ends is divided into 31 regions in the axial direction, and the cross-sectional average enrichment is made to be high in the central part, Furthermore,
The upper and lower parts are formed with low concentration.

By forming the central part to have a higher concentration than the lower part, in combination with the above-described method of distributing the burnable poison, it is possible to flatten the power distribution in the axial direction of the core. Therefore, it is possible to alleviate the increase in the axial power distribution due to the installation of natural uranium at the upper and lower ends.

Furthermore, as described in (2), by reducing burnable poison near the upper ends of the fuel rods, the reactor shutdown margin is reduced. However, by forming the upper part to have a lower concentration than the central part, this can be compensated for and a sufficient margin for reactor shutdown can be ensured. In this case, by setting the boundary of the concentration of the burnable poison and the boundary of the enrichment degree of enriched uranium to the same height, it is possible to ensure a more sufficient margin for reactor shutdown.

(4) As mentioned in (3), by providing a fuel rod with a lower cross-sectional enrichment in the upper part than in the other parts, and a fuel rod with a lower cross-sectional enrichment in the lower part than in the other parts, The cross-sectional average enrichment is formed higher than that of the upper and lower parts.

In order to divide the cross-sectional average enrichment of the enriched uranium part into three parts in the axial direction as shown in Figure 7(a), fuel rods with the enrichment divided into two at different vertical positions as shown in Figure 8 are used. Combine. In other words, by combining fuel rods with low enrichment only in the upper and lower parts (No. 8, left and center), the cross-sectional average enrichment in the central part where the high enrichments overlap, as shown in the right of the figure, is increased. are doing. The fuel rods whose enrichment is divided into two in this way may be part or all of the fuel assembly.

It is easier to combine fuel rods whose enrichment is divided into two in the axial direction in this way than to manufacture fuel rods whose enrichment is divided into three in the axial direction, as shown in Figure 9, for example. , manufacturing costs are also low.

Next, specific embodiments of the present invention shown in FIGS. 1 to 6 will be described.

1 and 2 show one embodiment of the invention.

Reference numeral A1 in FIG. 1 is the fuel assembly of this embodiment adjacent to the control rod B, and inside there are fuel rods 11, 12, 13, 14, 15°G1 and water as shown in FIG. Rods W are loaded in an 8×8 square grid.

3 and 4 show other embodiments of the present invention, and FIG.
As shown in Figure 1, the fuel rod G has a Gd concentration divided into three in the axial direction.
2 is used to form the fuel assembly A2, and the other configurations are exactly the same as the embodiment shown in FIGS. 1 and 2.

The structure and operation of each fuel rod in these examples will be explained.

(1) Natural uranium section Each fuel rod 11, 12, 13, 14, 15. One node each at the upper and lower ends of G1 and G2 is filled with natural uranium. The overall cross-sectional average enrichment of the fuel assemblies A1 and A2 in this portion is 0.711% by weight (hereinafter referred to as (Wlo'')).

FIG. 5 shows the relationship between an increase in axial peaking and an increase in fuel extraction burnup, using the number of filling nodes at the upper and lower ends of natural uranium as a parameter. As can be seen from the figure, when the number of natural uranium nodes is increased, the increase in fuel extraction burnup tends to reach saturation despite the increase in axial peaking. Therefore, it is sufficient to fill one nod of natural uranium at the upper and lower ends of the fuel rod, and as shown in the figure, a sufficient effect can be obtained even if only one of the upper and lower ends is filled.

This reduces the bundle average enrichment of the fuel assembly to 2.89.
(Wlo), which is almost the same as the conventional 2.90 (Wlo), can increase the cross-sectional average enrichment in the axial center compared to the conventional example, improve the thermal neutron utilization rate f, and at the same time Reactivity K is improved by reducing leakage of neutrons from the upper and lower ends of the fuel rod.

ft (2) Axial distribution of burnable poison In fuel rod G1 shown in Figures 1 and 2, Qd is used as the burnable poison, and the upper part of the enriched uranium part excluding the natural uranium part is Gd11 degrees of 3 nodes is 2
．． -0 (Wlo), and is formed to be lower than the Gd11 degree 4.0 (Wlo) of the other 19 node portions. The reason why the upper three nodes are made low in concentration is because, as shown in Figure 60, even if the low concentration portion is increased further, the effect of increasing the fuel extraction burnup will not change much. be.

This reduces the loss of reactivity due to residual burnable poisons such as Gd, and reduces the reactivity K of the entire fuel assembly. rr can be significantly improved compared to the conventional example.

In addition, as shown in fuel rod G2 in FIGS. 3 and 4,
Divide the 19-node part into 2 more parts to obtain Gd16 degrees 3.0 (
12 node part of wlo), Gd concentration 4.0 (wlo
) may be used to flatten the axial output distribution.

(3) Distribution of the axial enrichment of the enriched uranium portion of the entire fuel assembly In this example, the cross-sectional average enrichment of the enriched uranium portion of each fuel assembly A and A2 is calculated by dividing the upper three nodes by 2. .98
(wlo), the 12 nodes in the center are 3.19 (wlo
) The lower 7 nodes are divided into 3 into 2.98 (Wlo).

By forming the upper part to have a lower enrichment than the central part, the reactor shutdown margin is improved. In addition, the center part is
By forming it in a compressed state, the axial power distribution is flattened.

As a result, the linear power density of the reactor core can be lowered, making it possible to operate the reactor with sufficient margin for the operating limit value.

The difference in enrichment between the center and bottom and the boundary position are as follows:
These values are selected to maximize the effect of flattening the axial output distribution.

(4) Distribution of axial enrichment of each fuel rod In this embodiment, the enrichment of the upper three nodes of the enriched uranium part like the fuel rod 11 is set to 2.9 (Wlo), and the enrichment of the other parts is set to 4. 0 (wlo), and the enrichment level of the lower seven nodes of the enriched uranium part like the fuel rod 12 is set to 2.9 (wlo), and the enrichment level of the other parts is set to 4.9 (wlo). O(wlo
) is formed lower than the fuel assembly A1. As mentioned above, the cross-sectional average concentration of A2 as a whole is set to 3 minutes.

Note that the fuel rods 13, 14, and 15 are each formed with a single enrichment.

On the other hand, if the cross-sectional average enrichment of the entire fuel assembly is set to 3
To separate the fuel rods 22.2 in Figures 10 and 11,
This can also be achieved by dividing the axial enrichment of each fuel rod itself into thirds, as shown in 3.24. however,
When manufacturing each fuel rod 22.23° 24, the types of enriched uranium to be filled and the types of fuel rods increase, making process control and quality control complicated.

In contrast, in this example, the axial concentration is 2 degrees.
Since it is only necessary to separate the fuel rods into parts, and there are fewer types of fuel rods to be separated, process control, quality control, etc. are easy.
The manufacturing cost of fuel can be reduced and fuel economy can be improved.

〔Effect of the invention〕

As described above, the fuel assembly of the present invention can achieve a high fuel extraction burnup while having approximately the same average enrichment as conventional fuel, and it is possible to achieve safety such as thermal margin when loading the reactor core and reactor shutdown margin. It has excellent properties, is easy to manufacture, low cost, and has excellent fuel economy.

[Brief explanation of drawings]

1 to 8 show embodiments of the fuel assembly of the present invention, FIG. 1 is a cross-sectional view of one embodiment, and FIG. 2 is a cross-sectional view of one embodiment of the fuel assembly. A block diagram showing enrichment and gadolinia distribution. Figures 3 and 4 are similar to Figures 1 and 2 of other examples. Figure 5 is an axis with the number of nodes in the natural uranium part as a parameter. Figure 6 is a characteristic diagram showing the relationship between an increase in directional peaking and an increase in fuel extraction burnup. Figures 7(a) and 8 are conceptual diagrams of the present invention, Figure 9 is a conceptual diagram when the enrichment degree of the enriched uranium portion of one fuel rod is divided into three, and Figures 10 and 11 are The figures are similar to Figures 1 and 2, which embody Figure 9, Figures 12 and 13.
The figure is similar to FIGS. 1 and 2 of the conventional example, and FIG. 14 is an axial distribution diagram of the core average thermal neutron flux in the conventional example. A1. A2... Fuel assembly, 11.12, 13°14
．． 15. G1. G2...Fuel rod. Applicant's agent Inomata Kiyoshi Concentration/I Riki 1 Hit 〃Trinian Momme → (
a) (b) No. 8 Somanbo Kokuzuchi Diagram 9

Claims (1)

[Scope of Claims] 1. In a fuel assembly to be loaded into the core of a nuclear reactor that is composed of a plurality of fuel rods, the fuel assembly is provided with a natural uranium portion at at least one of the upper and lower ends, and an enriched uranium portion is provided between the fuel rods. When a uranium part is provided and this enriched uranium part is divided into three parts in the axial direction into an upper part, a central part, and a lower part, the cross-sectional enrichment of the upper part of the enriched uranium part is lower than the other central part and lower part, and the enrichment levels are different. By combining a fuel rod having two regions and a fuel rod having two regions of different enrichment, in which the cross-sectional enrichment of the lower part of the enriched uranium part is lower than the other upper part and the central part, the cross-sectional average of the central part can be reduced. The fuel rods are formed so that the enrichment is higher than the cross-sectional average enrichment of the upper and lower parts, and some of the fuel rods contain a burnable poison having two or more regions with different concentrations in the axial direction, A fuel assembly characterized in that the concentration of the burnable poison in the upper part of the fuel rod is lower than in other parts. 2. The fuel assembly according to claim 1, wherein some of the fuel rods of the plurality of fuel rods constituting the fuel assembly are formed with a single enrichment. 3. The fuel assembly according to claim 1, wherein the axial length of the natural uranium portion is 1/24. 4. The axial length of the enriched uranium part is 11/12, and the axial lengths of the upper, middle, and lower parts are 1/8 and 1, respectively.
2. The fuel assembly according to claim 1, wherein the fuel assembly is 7/2 and 7/24.