JPS6228437B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a core structure of an initially loaded core of a nuclear reactor.

Conventionally, in boiling water nuclear reactors, two methods have been used for the configuration of the core portion of the initially loaded reactor core.
The first method is to use one type of fuel assembly in order to simplify the core configuration. However, with this method, gadolinium oxide (G
^d ₂ O ₃ ) (hereinafter referred to as gadolinia) burns quickly and in order to obtain stable surplus reactivity characteristics throughout the cycle, it is necessary to use a product containing a high concentration of gadolinia, which has complex combustion characteristics. That was the difficult point. The second method is to obtain appropriate surplus reactivity characteristics by using two or more types of fuel assemblies with different average enrichments and approximately realizing a core configuration during an equilibrium cycle. However, this method
Not only did the structure of the reactor core become complicated, but the design and manufacture of the fuel assembly were also complicated.

The core structure of the initial loading core of the nuclear reactor of the present invention is as follows:
The purpose is to reduce the change in the surplus reactivity necessary for combustion in the reactor core due to combustion, and to realize a nuclear reactor that can adjust the output level by controlling the flow rate without operating the control rods at any burnup. and consisting of a first fuel rod containing nuclear fuel and a burnable poison and a second fuel rod containing nuclear fuel but not containing a burnable poison, at least two types having different ratios of the number of first fuel rods and second fuel rods. Four fuel assemblies with the same average enrichment are considered as one unit of the reactor core configuration, and these four fuel assemblies contain at least one type of fuel assembly whose infinite neutron multiplication factor increases with combustion. It is characterized by including at least one type of fuel assembly whose infinite neutron multiplication factor decreases.

As mentioned above, the method using one type of fuel assembly requires a high concentration of gadolinia, which has complex combustion characteristics, while the method using several types of fuel assemblies with different average enrichment Since the configuration and fuel assembly design are complicated, the present invention uses fuel assemblies with one type of average enrichment and utilizes the combustion characteristics of gadolinia to approximately achieve an equilibrium cycle core configuration. This is what I did. The equilibrium cycle core configuration is a method of configuring the core by using several fuel assemblies with different core residence years as one unit. In the first year of stay in the core, the infinite neutron multiplication factor increases with combustion, and thereafter the infinite neutron multiplication factor decreases, and by combining these increasing and decreasing characteristics, a surplus It is designed to reduce the range of change in reactivity.

That is, in the present invention, 4
The fuel assembly of the core is considered as one unit of the core configuration, and these four
The fuel assembly includes at least one type of fuel assembly whose infinite neutron multiplication factor increases and at least one type of fuel assembly whose infinite neutron multiplication factor decreases as it burns, thereby combining the characteristics of increase and decrease of the infinite neutron multiplication factor. This reduces the range of change in surplus reactivity.

In order to obtain such an increase/decrease characteristic of the infinite neutron multiplication factor, we use the burnable poison gadolinia. Increasing the concentration of gadolinia delays the combustion of gadolinia,
Increasing the number of fuel rods containing gadolinia in the fuel assembly reduces the infinite neutron multiplication factor at the start of the cycle. Therefore, the gadolinia concentration in the fuel assembly and the number and position of gadolinia-containing fuel rods are adjusted to obtain combustion characteristics with an appropriate infinite neutron multiplication factor. Therefore, when the gadolinia concentration is the same, a fuel assembly in which the infinite neutron multiplication factor increases due to combustion has a greater number of fuel rods containing gadolinia than a fuel assembly in which the infinite neutron multiplication factor decreases.

In order to reduce the change in surplus reactivity, the characteristic of increasing the infinite neutron multiplication factor in particular needs to be continued throughout the cycle, and this can be achieved without using a compound containing gadolinia at high concentrations with complex combustion characteristics. In order to achieve this, it is necessary to delay the combustion of materials containing gadolinia at relatively low concentrations. In the present invention, by utilizing the fact that the thermal neutron flux distribution in the fuel assembly decreases from the outer periphery to the center, the gadolinia-containing fuel rods are concentrated in the center of the fuel assembly. It has realized the characteristics of

Examples will be described below.

The example used here is a thermal output of 3300MW,
Operation period 15 months, cycle burnup 9000MW ^d /st
It is a boiling water reactor with 764 fuel assemblies loaded in the core, the core height is 146 inches, and the fuel assembly spacing is 6 inches. First, the configuration of the fuel assembly used in the example will be explained.

FIG. 1 shows the configuration of a fuel assembly in which the infinite neutron multiplication factor increases with combustion, and the number of gadolinia-containing fuel rods is eight. In this diagram,
1 is a fuel rod, 2 is a fuel assembly, and the numbers written inside each fuel rod indicate the enrichment of uranium and the concentration of gadolinia. The numbers in the box indicate the type of fuel rod, G ₁ and G ₂ indicate fuel rods containing gadolinia, and the numbers in parentheses below them indicate the number of each fuel rod 1 per fuel assembly 2. The average concentration is 2.25% by weight. In addition,
The letter W written in the fuel assembly 2 indicates a water rod.

In this case, the reason why the concentration of gadolinia is set to 5.0% by weight is to prevent thermal conductivity from decreasing and combustion characteristics to become complicated if the concentration of gadolinia exceeds 6.0% by weight.

Curve 5 in FIG. 3 shows the change in the infinite neutron multiplication factor due to combustion of the fuel assembly shown in FIG. 1 when the guide rate is 40%. The horizontal and vertical axes of this figure represent the burnup (GW ^d /st) and the vertical axis, respectively.
It has an infinite neutron multiplication factor. Although the infinite neutron multiplication factor increases at the beginning of the cycle, the cycle burnup shown in line 6 is 8.0GW ^d /st before the cycle burnup of 9.0GW ^d /st.
It has been decreasing since. This is gadolinia
This is because they are burned out before 9.0GW ^d /st.
In this way, when the combustion characteristics of the infinite neutron multiplication factor change from increasing to decreasing before the cycle burnup,
At the end of the cycle, the infinite neutron multiplication factor of all fuel assemblies decreases, and it cannot be expected to reduce the change in surplus reactivity. Therefore, in order to obtain a fuel assembly in which the infinite neutron multiplication factor increases throughout the cycle, it is necessary to delay the combustion of gadolinia at a concentration of 5.0% by weight.

Thermal neutron flux in the fuel assembly has a distribution that decreases from the outer periphery to the center, and mutual interference increases when gadolinia-containing fuel rods are placed next to each other. If the fuel rods containing gadolinia are arranged as shown in the cross section of the body, it can be expected that the combustion of gadolinia will be delayed. In Figure 2, the black circle 3 indicates the fuel rod containing gadolinia, and the black circle 4 indicates the fuel rod containing gadolinia.
White circles indicate fuel rods that do not contain gadolinia. The fuel enrichment arrangement of the fuel assembly shown in FIG. 2 is the same as that of the fuel assembly shown in FIG. The change in the infinite neutron multiplication factor due to combustion of the fuel assembly shown in FIG. 2 is shown in curve 7 in FIG. 3 for the case of a void ratio of 40%. As can be seen from curve 7, due to the delay in the combustion of gadolinia, the infinite neutron multiplication rate shows an increasing characteristic up to a cycle burnup of 9.0 GW ^d /st and beyond. Hereinafter, a fuel assembly including eight gadolinia-containing fuel rods and in which gadolinia combustion is delayed will be referred to as a fuel assembly A.

First, in one embodiment, the four fuel assemblies that constitute the reactor core include fuel assembly A, which does not contain gadolinia, and whose fuel enrichment arrangement is the same as that in Figure 1, so that infinite neutrons increase throughout the cycle. A nuclear reactor was constructed using two fuel assemblies of decreasing magnification (hereinafter referred to as fuel assemblies B).

Curve 8 in FIG. 4 shows the change in the infinite neutron multiplication factor due to combustion of fuel assembly B in the case of a void ratio of 40%. The horizontal and vertical axes of this figure show burnup (GW ^d /st) and infinite neutron multiplication factor, respectively.For comparison, fuel assembly A
The change in the infinite neutron multiplication factor is shown as curve 7. Comparing curves 7 and 8, it can be seen that in fuel assemblies A and B, the enrichment and arrangement of the fuel rods that make up these are the same, so the infinite neutron multiplication factors after gadolinia burns are almost the same. Recognize.

Figure 5 shows a cross section of the core of a boiling water reactor to which this embodiment is applied, where one box represents one fuel assembly, and 12 shows four fuel assemblies. It is one unit of the core configuration consisting of. Symbols 8 and 0 in the boxes represent fuel assemblies A and B, respectively, and numbers 8 and 0 represent the number of gadolinia-containing fuel rods in fuel assemblies A and B, respectively. Four fuel assemblies in one cell, indicated by 12, are arranged so as to surround a cross-shaped control rod inserted into the cell.

Curve 15 in Figure 10 shows the change in surplus reactivity accompanying combustion of the core, and the horizontal axis of this figure
The vertical axis shows burnup (GW ^d /st) and core surplus reactivity (%Δk/k), respectively. curve 1
5, the excess reactivity of the core in this example is small and remains almost constant until the middle of the cycle. The reason for the change at the end of the cycle is that the burnup in the relatively high-output part at the center of the core is higher than the average burnup in the core, and the burning of gadolinia is progressing in this region. Curve 15
The range of change in surplus reactivity in the range of 0.2 GW ^d /st to 9.0 ^{GW d} /st is 1.45% Δk/k.

Next, in another embodiment, four fuel assemblies, which are units of the core configuration, include one fuel assembly A and a fuel assembly containing one fuel rod containing gadolinia so that the infinite neutron multiplication factor is reduced by combustion. body (hereinafter referred to as
A reactor core was constructed using one fuel assembly (hereinafter referred to as fuel assembly C) and two fuel assemblies containing two fuel rods containing gadolinia (hereinafter referred to as fuel assembly D).

For both fuel assemblies C and D, the enrichment and arrangement of the fuel rods that constitute them are the same as in Figure 1, and the positions of the gadolinia-containing fuel rods are as shown by 3 in Figures 6 and 7, respectively. It is. Note that the concentration of gadolinia is 5.0% by weight in both cases. Curves 9 and 10 in FIG. 4 show changes in the infinite neutron multiplication factor due to combustion of fuel assemblies C and D for a void ratio of 40%.

FIG. 8 shows a cross section of this embodiment. Similarly to FIG. 5, one cell represents one fuel assembly, and 12 represents a core composed of four fuel assemblies. It is one unit of composition. Symbols 8, 1, and 2 in the boxes indicate fuel assemblies A, C, and C, respectively.
D, and the numbers 8, 1, and 2 indicate the number of gadolinia-containing fuel rods in each fuel assembly. Curve 16 in FIG. 10 shows the change in surplus reactivity accompanying combustion of the core in this example. Curve 16 0.2GW ^d /st~
The range of change in surplus reactivity in the range of 9.0GW ^d /st is 1.8%Δk/k.

The range of change in the surplus reactivity of the core in these examples is 1.45%Δk/k for the former and 1.8%Δk/k for the latter.
k, and the number of control rods required to suppress this change is approximately 15 for the former and approximately 18 for the latter.
Also, as can be seen from curves 15 and 16 in Figure 10,
In both cores of the two examples, the change in excess reactivity was small from the early cycle to the middle of the cycle.
This shows that it is possible to keep control rod operations to the minimum necessary.

Here, for comparison, the surplus reactivity of a core constructed from one type of fuel assembly that has been conventionally used was determined. The enrichment and arrangement of the fuel rods in the fuel assembly used were the same as in the case of FIG. 1, and the number of gadolinia-containing fuel rods was three. A cross section of the fuel assembly is shown in FIG. The gadolinia concentrations in the fuel rods 13 and 14 are 4.0 and 6.0% by weight, respectively.
It is. The change in the infinite neutron multiplication factor accompanying the combustion of this fuel assembly is shown in the fourth graph for the case of a void ratio of 40%.
This is shown in curve 11 of the figure. As can be seen from curve 11 in Figure 4, if the number of fuel rods containing gadolinia in the fuel assembly is less than this, the infinite neutron multiplication factor will increase, so the surplus reactivity will also exceed the necessary value, and the number of fuel rods containing gadolinia will increase. When the number is increased, the change in the infinite neutron multiplication factor becomes large, and the range of change in the surplus reactivity also becomes large.

The first factor is the change in surplus reactivity associated with the combustion of the core.
This is shown in curve 17 in Figure 0. The change in the surplus reactivity is very similar to the change in the infinite neutron multiplication factor shown in curve 11 in FIG. 4, but the point at which the surplus reactivity reaches its peak is earlier than that of the infinite neutron multiplication factor. This is also due to the fact that combustion progresses quickly in the central part of the reactor core, as mentioned above. ^0.2GWd /st～ ^9.0GWd /st
The range of change in surplus reactivity within the range is 2.55%Δk/k
Looking at the changes, it is clear that the amount of control rods inserted must be increased because the surplus reaction increases until the middle of the cycle, and then the control rods must be withdrawn repeatedly as the surplus reactivity decreases.

On the other hand, the range of change in the surplus reactivity of the reactor reaction in the two embodiments of the present invention is 1.45%Δk/k,
1.88% Δk/k, and the number of control rods can be reduced by about 11 and about 8, respectively, compared to the conventional method. Furthermore, due to the difference in the change in surplus reactivity, the present invention can simplify operation compared to the conventional method, and these results show that the present invention is extremely effective.

As described above, the core structure of the initially loaded core of the nuclear reactor of the present invention minimizes the change in surplus reactivity necessary for combustion in the reactor core due to combustion, and allows control rods to be operated almost at any burnup. This makes it possible to realize a nuclear reactor in which the output level can be adjusted by controlling the flow rate without any problems, and has great industrial effects.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a fuel assembly of eight fuel rods containing gadolinia used in an embodiment of the core structure of a nuclear reactor of the present invention, and FIG. Fig. 3 is a diagram showing the structure of a fuel assembly in which the , a characteristic diagram showing the change in infinite neutron multiplication factor due to combustion of various fuel assemblies used in the example, FIG. 5 is a cross-sectional view of the reactor core of one example, and FIGS. 6 and 7 are the fuel containing gadolinia. A configuration diagram of a fuel assembly containing one and two rods, respectively, FIG. 8 is a cross-sectional view of the core of another embodiment, and FIG. 9 is a configuration diagram of a fuel assembly containing three fuel rods containing gadolinia. 10 are characteristic diagrams showing the relationship between the excess reactivity and burnup of the core in the embodiments of FIGS. 5 and 8 in comparison with the conventional case. 1... Fuel rod, 2... Fuel assembly, 3... Fuel rod containing gadolinia, 4... Fuel rod not containing gadolinia,
13, 14...Fuel rod containing gadolinia.

Claims (1)

[Scope of Claims] 1 Consisting of a first fuel rod containing nuclear fuel and a burnable poison, and a second fuel rod containing the nuclear fuel but not containing the burnable poison, the first fuel rod and the second fuel rod Four fuel assemblies of at least two types with different ratios of numbers and the same average enrichment are considered as one unit of the core configuration, and these four fuel assemblies are fuels whose infinite neutron multiplication factor increases as they burn. A core structure of an initially loaded core of a nuclear reactor, comprising at least one type of fuel assembly and at least one type of fuel assembly whose infinite neutron multiplication factor decreases as it burns. 2. The four fuel assemblies include a first fuel rod containing uranium oxide and gadolinium oxide, a second fuel rod containing uranium oxide but not gadolinium oxide, and the first fuel rod. The core structure of an initially loaded core of a nuclear reactor according to claim 1, comprising a second fuel assembly having a smaller number of fuel assemblies than in the first fuel assembly.