JPH0345357B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a nuclear reactor refueling method applied when exchanging fuel in a core of a boiling water nuclear reactor.

Technical background of the invention and its problems The core of a boiling water reactor basically consists of a set of four fuel assemblies surrounding one control rod a having a cross-shaped cross section, as shown in Figure 1. A large number of fuel units c each having bodies b arranged therein are arranged in the planar direction of the reactor core. For example, the planar shape of the core of an 800,000 kw class nuclear reactor is as shown in FIG. The shaded area in Figure 2 is approximately 1/4 area of the reactor core, and the remaining 3/4 area is symmetrical to this 1/4 area. I will explain on behalf of the area.

The above-mentioned fuel assembly (hereinafter simply referred to as fuel) stays in the core for four cycles, or about four years, with one normal cycle being about one year, but because the burnup differs for each cycle, the output peaking may be reduced. In order to improve fuel economy, each fuel is rearranged (shuffling) when replacing fuel at the end of one cycle. Figure 3 shows a typical conventional fuel movement. New fuel loaded at position is moved to position in the second cycle, then moved to position in the third cycle, and moved to position in the fourth cycle. After the cycle is completed, it is removed from the core.

Conventionally, almost all of the fuel in the reactor core was moved at once during fuel exchange for each cycle, which resulted in a very long time for fuel exchange, leading to longer regular inspection periods. This was causing a decline in the reactor's operating rate.

Additionally, each unit of conventional fuel is loaded so that it usually contains one bottle of new fuel for the first cycle, but as shown in Figure 4, the new fuel is better than the older, more burnt-out fuel. also has a high infinite multiplication factor and high reactivity. Therefore, in the case of a fuel unit containing a mixture of new fuel and fuel from the second cycle onwards, as described above, these fuels are inserted into and pulled out for power adjustment during reactor operation around the power adjustment control rods. When the control rods are withdrawn or inserted during power operation, the core is controlled at low power to prevent sudden changes in the output of the fuel with high linear power density. There was a problem in that the operator had no choice but to operate the rod, resulting in a large loss in operating efficiency.

Purpose of the Invention The present invention was made based on the above circumstances, and its purpose is to shorten the time required for fuel exchange, improve operating efficiency, and reduce the linear power density of fuel around the control rods for power adjustment. It is an object of the present invention to provide a method for exchanging fuel in a nuclear reactor, which can improve fuel economy.

Summary of the Invention The summary of the present invention is as follows.
That is, in the reactor core, 3rd cycle fuel units consisting only of 3rd cycle fuel are placed at control rod positions for power adjustment that are operated in and out during power operation of the reactor. Two types of mixed fuel units consisting of first cycle and second cycle fuel are arranged in the vertical and horizontal directions,
In addition, a three-type mixed fuel unit consisting of fuels for the first cycle, second cycle, and third cycle is arranged diagonally with respect to the third cycle fuel unit.

And when changing fuel after each cycle,
The old first cycle fuel is left loaded without being moved. In other words, this fuel becomes the fuel for the second cycle in the next cycle.

Further, the old second cycle fuel is taken out, and new fuel, that is, first cycle fuel, is loaded into the place where it was taken out.

Furthermore, the old third cycle fuel is removed from the core or moved to the core periphery, and the old second cycle fuel, that is, the fuel that will be the next third cycle, is loaded into the removed area.

According to the above-mentioned method of the present invention, the old fuel from the first cycle becomes fuel for the second cycle on the spot without being moved, and in the next cycle, it is taken out and new fuel from the first cycle is loaded again. Become.
In other words, the positional relationship between the fuel in the first cycle and the fuel in the second cycle changes alternately every time one cycle ends, but the position of the fuel itself remains unchanged when moving from the first cycle to the second cycle.

Therefore, there is no need to transfer the old 1st cycle fuel when changing fuel, so the number of fuel transfers is significantly reduced compared to the conventional method, in which almost all fuel was transferred.
The time required for fuel exchange can be shortened.

In addition, since the output adjustment control rod can be placed in the 3rd cycle fuel unit consisting of only the 3rd cycle fuel, it is possible to reduce the linear power density of the fuel around the output adjustment control rod. It is.

Embodiment of the Invention An embodiment of the method of the present invention will be described below with reference to FIGS. 5 and 6. FIG. 5 shows the arrangement of fuel during a certain cycle. In this core, there are 10 third cycle fuel units consisting only of third cycle fuel, excluding the outer periphery of the core.
(hatched areas on the drawing) are arranged at multiple locations. These 3rd cycle fuel units are 10...
are arranged corresponding to the position where the control rods for output adjustment, that is, the control rods for output adjustment are operated in and out for output adjustment during the output operation of the reactor. A control rod for adjustment is inserted.

In addition, in the vertical and horizontal directions for these 3rd cycle fuel units 10..., there are 2 types of mixed fuel units 11 each consisting of 2 fuels for the 1st cycle and 2 fuels for the 2nd cycle.
...(the part surrounded by a circle on the drawing) is located. These two types of mixed fuel units 11... have symmetry with respect to the core center P.
In contrast, those arranged in the vertical and horizontal directions are arranged in parallel so that two fuel bodies of the same cycle are adjacent to each other,
In other two-type mixed fuel units, two fuels of the same cycle are arranged diagonally to each other.

These two types of mixed fuel units 11... are the 3rd cycle fuel units 1 whose combustion has progressed and have decreased after the reaction.
Since they are arranged adjacent to 0..., even if two units each of the highly reactive 1st cycle fuel and 2nd cycle fuel are combined as described above, the 3rd cycle fuel unit 10... Since the reaction is suppressed, there is no problem even if such highly reactive fuels are combined.

Furthermore, in the diagonal direction with respect to the third cycle fuel unit 10..., there is a three-type mixed fuel unit 12... (encircled by a dotted line frame in the drawing) consisting of the first, second, and third cycle fuels. ) are arranged. These three types of mixed fuel unit 1
2... is one in which two second-cycle fuels are arranged diagonally to each other, and one each of first-cycle and third-cycle fuels are arranged diagonally to each other.

Moreover, in the outermost peripheral part 13 of the core, which is shown surrounded by a bold frame in the drawing, fuel with a high burnup in the fourth cycle and a part of the fuel in the third cycle are mainly arranged. Note that the fuel near the outer peripheral portion 13 is arranged in the same manner as in the conventional case.

When one cycle is completed with the above fuel arrangement, fuel exchange is performed as described below. That is, as shown in FIG. 6, in the two-type mixed fuel units 11... and the three-type mixed fuel units 12..., the fuel from the previous first cycle is left loaded without being moved. Therefore, in the cycle shown in FIG. 6, the fuel used in the old first cycle becomes the fuel used in the second cycle.

Also, remove the old 2nd cycle fuel shown in Figure 5 and place it in the 6th cycle where it was taken out.
As shown in the figure, new fuel, that is, fuel for the first cycle, is loaded.

That is, due to the above operation, the positional relationship between the fuel in the first cycle and the fuel in the second cycle is reversed.

Further, the old third cycle fuel shown in FIG. 5 is removed from the core or transferred to the position of the old fourth cycle fuel in the core outer periphery 13. Then, the above-mentioned old second cycle fuel, that is, the fuel for the third cycle in the next cycle, is loaded into the location where the old third cycle fuel was taken out.

Therefore, as shown in Fig. 6, the position of the fuel in the third cycle does not appear to change, and as in the previous cycle, the position of the fuel unit 10 of the third cycle and the three-type mixed fuel unit 12 are the same. The fuel for the third cycle will be located at .

In addition, in the three-type mixed fuel units 12..., each unit contains two pieces of new fuel in the fuel arrangement shown in Fig. 6, but these fuel units do not necessarily contain the fuel in the third cycle. It is possible to combine fuels with high reactivity in this way.

Then, the reactor is operated with the above fuel arrangement, and after one cycle, ie, 1 to 1.5 years of operation, the fuel is replaced again using the above method. That is, in the next cycle, the fuel arrangement shown in FIG. 5 will be used again.

However, according to the method of the above embodiment, in the center of the core except for the vicinity of the outer periphery of the core, the old
Since the fuel for the first cycle does not need to be moved, the effort of replacing the fuel can be saved accordingly. Whereas previously, almost all fuel other than new fuel was moved,
According to the above method, the number of transfers of fuel other than new fuel is reduced to less than half, so the time required for fuel exchange is significantly shortened.

Moreover, since the fuel unit into which the output adjustment control rods are inserted is entirely composed of 3rd cycle fuel, the linear power density of the fuel is 10kW/ft even when the control rods are completely withdrawn.
This is approximately 2kW/ft lower than when new fuel was included as before. Therefore, control rod operations during reactor power adjustment can be performed at a high power of 80 to 90%, instead of operating control rods with the power reduced to 50 to 60% as in the past. The operating rate loss is 2-3% compared to the case where
This can be reduced.

Effects of the Invention As described above, the present invention eliminates the need to move the first cycle fuel in the core except for the vicinity of the outer periphery of the core during fuel exchange, thereby reducing the time required for fuel exchange. By shortening the regular inspection period, the operating rate of the reactor can be improved. It is also possible to position the output adjustment control rod in a fuel unit consisting only of fuel in the third cycle, and in this case, the linear power density of the fuel around the output adjustment control rod can be reduced. Since there is no need to operate the control rods at a significantly lower output during output adjustment as in the past, this system has great effects in improving the operating rate, such as reducing operating rate losses.

[Brief explanation of drawings]

Figure 1 is a schematic plan view of a fuel unit showing the positional relationship between fuel assemblies and control rods, Figure 2 is a schematic plan view showing the shape of the core, Figure 3 is a schematic diagram showing the movement of conventional fuel, FIG. 4 is a diagram showing changes in the infinite multiplication factor of fuel, and FIGS. 5 and 6 are fuel arrangement diagrams of a part of the reactor core, respectively, showing the states before and after fuel exchange according to the method of the present invention. a... Control rod, b... Fuel assembly (fuel), 1
0...3rd cycle fuel unit, 11...2 type mixed fuel unit, 12...3 type mixed fuel unit, 13...
The outermost part of the core.

Claims (1)

[Claims] 1. A reactor core is constructed by arranging a large number of fuel units each consisting of a set of four fuel assemblies arranged around one control rod, and this core contains third-cycle fuel consisting only of third-cycle fuel. The unit is placed at the position of the control rod for power adjustment that is operated in and out during the output operation of the reactor, and the fuel units of the 1st cycle and the 2nd cycle are arranged in the vertical and horizontal directions relative to these 3rd cycle fuel units. Arrange two types of mixed fuel units,
In addition, a three-type mixed fuel unit consisting of fuel from the first cycle, second cycle, and third cycle is arranged diagonally with respect to the third cycle fuel unit, and when replacing the fuel at the end of one cycle, the old one
The fuel for the first cycle is loaded in the same position without being moved, and the old fuel for the second cycle is taken out and new fuel is loaded in the place where it was taken out.
Furthermore, the method for replacing fuel in a nuclear reactor is characterized in that the old third cycle fuel is removed from the reactor core or moved to the periphery of the reactor core, and the old second cycle fuel is loaded into the removed area.