JPS6158789B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear reactor core, and particularly to a nuclear reactor core in which fuel cells having different average infinite multiplication factors are arranged.

In boiling water reactors, changes in core surplus reactivity are adjusted using fuel assemblies containing gadolinia, a burnable poison. The excess reactivity of the reactor core, which could not be adjusted by gadolinia, was controlled by control rods. Furthermore, because the control rods must also perform power distribution control at the same time, in conventional boiling water reactors, a large number of control rods are used for power adjustment, and the power is reduced during the cycle through flow control, etc. It is necessary to replace the control rod pattern. Therefore, this was a problem in terms of simplifying core operation and improving plant utilization.

In view of these points, a nuclear reactor core has been proposed in which the range of change in core surplus reactivity is reduced to minimize operations by control rods. As an example of such a core, a small number of fuel cells (control cells) consisting of four low-enrichment fuel assemblies are placed in the central region of the core, and control rod operations are, in principle, performed within the control cells. The core configuration is being considered. FIG. 1 shows the core configuration in an equilibrium cycle. In the core 7, 8 indicates a control cell. The average neutron infinite multiplication factor of the control cell 8 is lower than the average neutron infinite multiplication factor of the fuel cells other than the control cell. 1 to 6 indicate the number of years the replacement fuel assembly stays in the core.

In a control cell with a small average neutron infinite multiplication factor, when the control rods are operated during the rated operation of the reactor, sudden changes in power peaking will not occur, and therefore the reactor core flow rate can be adjusted to reduce the power. This eliminates the need to pull out the control rod.

By arranging the control cell 8 in this manner, in addition to allowing the control rods to be operated only within the control cell 8, the arrangement has a large effect in contributing to flattening the radial power distribution of the core. Generally, in a core uniformly loaded with the same fuel cells, the output ratio is high at the center of the core, making it difficult to flatten the radial power distribution. It can be seen that the control cells 8 arranged as shown in FIG. 1 reduce the power ratio at the center of the core and contribute to flattening the radial power distribution. This situation is illustrated in Figure 2. A in the figure is a part where the control cell 8 is arranged. Figure 2 shows the radial power distribution of the core when all the control rods are withdrawn at the end of the cycle. It can be seen that since the output ratio at the position of the control cell 8 is small, the radial output distribution is flattened.

However, if the control cells 8 are gathered in the central region of the reactor core, the control cells 8 can be Power peaking tends to become large in the core region located outside the cells 8, resulting in a reduction in thermal margin.

In contrast to a core in which the control cells are gathered in the central region of the core, a core in which a large number of control cells 8 are arranged throughout the core has been conventionally considered. However, in this case, compared to the core shown in Figure 1, it is necessary to perform shuffling when replacing fuel assemblies, and the number of control rods used for power adjustment during reactor operation increases. Therefore, it was difficult to improve the plant utilization rate.

In consideration of the above points, an object of the present invention is to provide a nuclear reactor core that contributes to flattening the radial power distribution of the core in a core with a small range of change in surplus reactivity.

A feature of the present invention is that a fuel cell composed of four fuel assemblies includes a plurality of first fuel cells composed of four first fuel assemblies containing no burnable poison, and one fuel cell composed of four first fuel assemblies containing no burnable poison. a plurality of second fuel cells that are first fuel assemblies except for a third fuel assembly containing a toxic substance;
a plurality of third fuel cells each having a second fuel assembly containing no burnable poison other than one third fuel assembly, and the infinite multiplication factors of the first, second and third fuel assemblies are: The relationship is (first fuel assembly) < (second fuel assembly) < (third fuel assembly), the first and second fuel cells are arranged concentrically, and the first fuel cell is located from the center of the core. The second fuel cell is located outside the first fuel cell.

First, a method of configuring a reactor core with a small range of change in core surplus reactivity, which is a premise of the present invention, will be explained.

Figure 3 shows changes in the infinite neutron multiplication factor (k∞) of fuel assemblies with various combustion characteristics. 11 in the figure is a highly enriched fuel assembly containing gadolinia, which is a burnable poison, and the increase in k∞ at the beginning of combustion is due to the combustion of gadolinia. curve 1
It is considered that gadolinia is burned out at the point where 1 is the maximum burnup. Generally, the burnup at which gadolinia burns out is designed to be close to the cycle burnup. Straight lines 12 and 13 are k∞ curves of respective fuel assemblies that do not contain gadolinia and whose enrichment levels decrease in the order of straight lines 12 and 13. If the rate of increase in k∞ in the early stage of combustion of the fuel assembly of curve 11 is about three times the rate of decrease in k∞ of other low-enrichment fuel assemblies, then each fuel exchange unit is It consists of four fuel assemblies, one of which is a high enrichment fuel assembly containing gadolinia (having the characteristics of curve 11), and the other three are low enrichment fuel assemblies that do not contain gadolinia. If we use an aggregate (having the characteristics of straight lines 12 and 13), the average k∞ over the cycle becomes flat,
It is possible to reduce the range of changes in core surplus reactivity.

In the initial core configured in this way, the low enrichment fuel assembly is taken out at the end of the cycle, and a replacement fuel assembly that is the same as the high enrichment fuel assembly containing gadolinia is loaded in its place. ,
Even in the next cycle, the change in core surplus reactivity can be reduced. In subsequent cycles,
By sequentially replacing low-enrichment fuel assemblies with replacement fuel assemblies, it becomes extremely easy to approach an equilibrium cycle. If the low-enrichment fuel assemblies are further divided into three types and the enrichments are successively made, it will be advantageous in terms of fuel economy by replacing the one with the lowest enrichment first. .

In the method for loading fuel assemblies described above, a fuel cell consisting of four adjacent fuel assemblies is configured as a unit, and shuffling is not performed when replacing fuel assemblies in principle. In a core without shuffling, self-flattening of the power distribution as the burnup of the fuel assembly increases is effective.
It has been recognized that the plant utilization rate can be improved by shortening the fuel assembly replacement period. In the reactor core of the present invention, the first and second fuel cells, whose average neutron infinite multiplication factor is lower than the average neutron infinite multiplication factor of the core, are composed of four fuel assemblies and are arranged concentrically. , because the above-mentioned no-shear fling is assumed.

The reason why the first fuel cells and the second fuel cells are arranged alternately in the reactor core is that in a nuclear reactor in which a small number of control cells are arranged in the reactor core, the output ratio in the area away from the control cells tends to be large. , in order to suppress this.

In an equilibrium cycle core, there are four
Since there are two fuel assemblies, curve 1 in Fig. 3
By using a replacement fuel assembly having the combustion characteristics shown in No. 1, it is possible to construct a core with a small range of change in core surplus reactivity.

FIG. 4 shows an embodiment of the present invention constructed based on the above-mentioned concept. In this example, the electrical output
1100MWe, power density 50KW/, number of fuel assemblies
This was applied to 764 initial loading cores. In the core 14, 17 is a fuel assembly with the lowest enrichment, 18 is a fuel assembly with medium enrichment, and 19 is a fuel assembly with high enrichment containing gadolinia. Low enrichment and medium enrichment fuel assemblies 17 and 18 do not contain gadolinia. 15 is a first fuel cell (control cell) consisting of four low enrichment fuel assemblies 17; 16 is one high enrichment fuel assembly 19 containing gadolinia and three low enrichment fuel assemblies 1;
This is a second medium enrichment fuel cell composed of 7 fuel cells.

In the embodiment shown in FIG. 4, the third highly enriched fuel cell occupying the central region of the core includes one fuel assembly 19 and one fuel assembly 1, as shown at 20 in FIG.
8 and one fuel assembly 17. Various configurations of the third fuel cell are possible, and other embodiments are shown in FIGS. 6 and 7. 3 in Figure 6
The fuel cell 21 includes one fuel assembly 19 and 3
It consists of a fuel assembly 18 of the body. 3 in Figure 7
Each fuel cell 22 includes one fuel assembly 17, 1
Consists of 8, 19 and 25. The enrichment level of fuel assembly 25 is the same as that of fuel assembly 18 and fuel assembly 1.
It has an intermediate concentration of 9.

FIG. 8 shows another embodiment of the second fuel cell, which is composed of two fuel assemblies 17 and two fuel assemblies 18.

In the embodiment shown in FIG. 4, the second fuel cell 16
A large number of highly enriched fuel assemblies 19 are loaded in the outer region of the fuel cell. This has two purposes: to maintain the core reactivity at the end of the cycle, and to flatten the radial power distribution by increasing the power ratio in the region around the core.

FIG. 9 shows the k∞ curve of the gadolinia-containing high-enrichment fuel assembly 19 used in the embodiment of FIG. 4. In this embodiment, the fuel assemblies have enrichment levels of 3.04% by weight in the upper part of the reactor core and 2.82% by weight in the lower part of the reactor core. The purpose of providing a difference in concentration between the upper and lower portions is to compensate for the distortion in the output distribution caused by the void distribution. Note that the k∞ curve in FIG. 9 is the value when the void ratio is 40%. The number of fuel rods containing gadolinia in the fuel assembly 19 is eight, and the concentration of gadolinia is 4.0% by weight in the upper part of the core and 4.5% by weight in the lower part of the core.

The characteristic 30 in FIG. 9 indicates that the enrichment level of the fuel assembly 19 is
3.04% by weight, characteristic 31 indicates that it is
These are the characteristics when the content is 2.82% by weight.

FIG. 10 shows the k∞ curve of the fuel assembly 18 and FIG. 11 shows the k∞ curve at a void ratio of 40% for the fuel assembly 17. In the case of this embodiment, the enrichment of the fuel assembly 18 is 1.95% by weight in the upper part of the core and 1.74% by weight in the lower part of the core, with the enrichment of the fuel assembly 17 being 1.11% by weight. Gadolinia was not included in either case.

By combining fuel assemblies with the above-mentioned combustion characteristics, the average neutron infinite multiplication factor of the four bodies can be increased by 5% compared to the average neutron infinite multiplication factor of the entire core.
% or more lower first fuel cell 15 and second fuel cell 16
can be configured.

The above embodiments have been described with reference to the initial core. In an equilibrium cycle core, fuel assemblies with different lengths of stay in the reactor coexist. The replacement fuel assembly 19 containing gadolinia is shown in curve 1 in FIG.
1, so if the third fuel cell 20 is composed of four fuel assemblies having different lengths of stay in the reactor, a core with small core surplus reactivity can be created. Figure 12 shows the power density 44KW/,
An example of application to an equilibrium cycle core with 840 fuel assemblies is shown. In FIG. 12, 35 is a first fuel cell (control cell), 36 is a second fuel cell, and 37 is a third fuel cell. First fuel cell 3
The configuration of 5 consists of 4 replacement fuel assemblies with advanced combustion or 4 fuel assemblies with low enrichment of about 1.1% to 1.3% by weight, and is used once every 4 years or once every 2 years. The method will be to replace it. FIG. 13 shows the configuration of the second fuel cell 36 at the beginning of the cycle. In Fig. 13, 40 is a fuel assembly that has been in the reactor for 5 years, 41 is a fuel assembly that has been in the reactor for 4 years, 42 is a new fuel assembly, and 43 is a fuel assembly that has been in the reactor for 3 years. be. At the beginning of the next cycle, the fuel assembly 42 is moved to a specific fuel assembly position for the third year of stay in the reactor, and that fuel assembly is loaded in place of the fuel assembly 42. Further, the fuel assembly 40 is taken out of the reactor and a new fuel assembly is loaded. The average enrichment of the replacement fuel assembly is 2.82% by weight.

With the core configuration shown in this embodiment, even in an equilibrium cycle core of a large core, it is possible to suppress the increase in power peaking in areas other than the area where the first fuel cell 35 is arranged, and increase the thermal margin. can be achieved.

As described above using the embodiments, in order to flatten the radial power distribution, it is effective to arrange fuel cells with low or medium enrichment at positions where peaking is large.

According to the reactor core configuration according to the present invention, the radial power distribution of the reactor core can be easily flattened, the thermal margin is increased, and the reactor can be operated with only a small number of control rods in the control cell. It becomes possible.

[Brief explanation of the drawing]

Figure 1 is a core configuration diagram in an equilibrium cycle with a control cell, Figure 2 is a characteristic diagram showing the radial power distribution in the core of Figure 1, and Figure 3 is k∞ change due to combustion of various fuel assemblies. Figure 4 is a structural diagram showing an embodiment of the present invention in which a second fuel cell with an intermediate enrichment is placed outside the control cell; Figures 5 to 7 are Explanatory diagrams showing various embodiments of a highly enriched third fuel cell placed in the center of the reactor core, FIG. 8 is an explanatory diagram of other embodiments of a medium enriched second fuel cell, and FIGS. 9 to 11. is the characteristic diagram of k∞ of high enrichment fuel assembly, medium enrichment fuel assembly and low enrichment fuel assembly, the first
FIG. 2 is a block diagram of a balanced cycle core according to another embodiment of the present invention, and FIG. 13 is a block diagram of a second fuel cell in the balanced cycle core. 14... Core, 15... First fuel cell, 16...
...Second fuel cell, 17, 18, 19...Fuel assembly, 20...Third fuel cell.

Claims (1)

[Claims] 1. In the core of a nuclear reactor having a large number of fuel cells including four adjacently arranged fuel assemblies and control rods that can be inserted into positions surrounded by these fuel assemblies, the fuel cells are , the first fuel assemblies are the first fuel assemblies other than a plurality of first fuel cells composed of four first fuel assemblies that do not contain a burnable poison, and a third fuel assembly that contains one burnable poison. a plurality of second fuel cells, and a plurality of third fuel cells each having a second fuel assembly that does not contain a burnable poison other than one third fuel assembly; The infinite multiplication factor of the three fuel assemblies is (1st fuel assembly) < (2nd fuel assembly) < (3rd fuel assembly)
The first fuel cell and the second fuel cell are arranged concentrically, the first fuel cell is arranged within a range from the center of the core to about one-half of the core radius, and A nuclear reactor core, wherein two fuel cells are arranged outside the first fuel cell.