JPS6230993A - Method of operating boiling water type reactor - 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 1] The present invention relates to a method of operating a boiling water nuclear reactor installed at Kyoryo Power Plant 1, etc. Concerning how to operate a nuclear reactor.

[Technical backlash of the invention and its problems 1 In general, boiling water! In the v1 reactor, before the start of its operation, fissile material that serves as fuel for ants necessary for long-term operation is loaded into the reactor core, and operation begins at u11.

The fissile material used as fuel is made by burning uranium oxide enriched with uranium-235 into cylindrical pellets 1, which are sealed in a metallic sheath made of zirconium alloy or the like to form fuel rods. These fuel rods are arranged in a square lattice of 7 or 8 in length and width, and are housed in a channel box to form a fuel assembly, and these fuel Fl assemblies are arranged in a roughly circular shape in large numbers within the reactor core. Placed.

FIG. 5 shows such a fuel assembly. In the figure, reference numeral 1 indicates a channel box, and within this channel box 1, fuel rods 2 are arranged in a square lattice. There is. Fig. 6 shows a vertical cross-sectional view of Fig. [A fuel assembly in which two of the burnt fuel rods 2 are replaced with water rods 3, and one of the fuel rods 2 arranged in the channel box 1 is replaced with water rods 3] There are a replaced fuel assembly and a 31!J type fuel assembly, which is a fuel 1 assembly in which fuel rods 2 are arranged inside a channel box 1.

The fissile material such as fuel 276 is primarily uranium-235, and thermal neutrons are absorbed by the uranium-235 nuclei, causing a nuclear fission reaction. Due to this nuclear fission reaction, a plurality of new neutrons are released, and these neutrons cause a new nuclear fission reaction. In order for such a chain reaction to continue and for the reactor to reach a critical state, at least one neutron out of the multiple neutrons produced by one nuclear fission reaction must cause the next nuclear fission reaction. Must be. Generally, the probability that neutrons generated by one nuclear fission reaction will cause another nuclear fission is called the neutron multiplication factor. 1, that is, unless this neutron multiplication factor is 1 or more, the reactor will not be in a critical state.

However, as the uranium-235 contained in the fuel assembly loaded before the start of operation progresses during the operation of the reactor,
Uranium is becoming less susceptible to nuclear fission as its dark side gradually decreases.
The relative amount of uranium-238 to uranium-235 increases. Therefore, the multiple neutrons produced by the fission reaction are
This increases the probability that it will be absorbed into uranium-238 before the next fission reaction occurs. Just like that, uranium-235
As the combustion progresses, the neutron multiplication factor gradually decreases.

Therefore, in a general method of operating a boiling water reactor, fuel assemblies having different combustion rates are simultaneously loaded into the reactor core and operated.

For example, in the operating method of a boiling water reactor with a four-button core, one quarter of the total number of fuel assemblies is replaced during each fuel exchange.

In other words, if the period from one fuel exchange to the next fuel exchange is one cycle, one fuel assembly stays in the furnace for 41 cycles. , 3rd cycle, and 4th cycle are operated with four types of fuel assemblies arranged.

FIG. 7 shows an example of a fuel assembly loading pattern for such a four-batch core. In the figure, numerals 1, 2, 3, and 4 indicate the fuel assemblies of the 1, 2, 3, and 4th cycles, respectively.

In this way, the neutron multiplication factor in the reactor in a boiling water reactor operating method in which fuel assemblies with different stay periods in the reactor, that is, with different combustion stages, is mixed, is as follows:
The result is as shown in the graph of FIG. In this graph, the vertical axis shows the neutron multiplication rate and surplus reactivity, the horizontal axis shows the passage of time, the straight line a shows the relationship between the stay time of the fuel assembly in the reactor and the neutron multiplication rate, and the straight line a It shows the relationship between the total surplus reaction mass and the in-furnace residence time for four types of fuel assemblies with different in-furnace residence periods.

As can be seen from this graph, in a boiling water reactor with a 4-batch core, the neutron multiplication factor decreases, and the fuel assemblies in the 3rd and 4th cycles are 1+J cycle [l] with a large neutron multiplication factor
It is operated in a mixed state with 2-stroke fuel assemblies to improve combustion efficiency and reduce fuel 1/knee 1st stroke. Note that the graph in FIG. 8 shows the case where the reactivity control substance 1rJ is not included in the fuel rod 2. In reality, the hatched portion of excess reactivity is controlled by a reactivity control substance, and the double hatched portion is controlled by a control rod.

However, 81 of the four-button cores described above
i In the operating method of a boiling water reactor, when decommissioning a boiling water reactor, unreacted uranium-235 in the reactor core is
There is a problem in that the operation has to be stopped when a large amount of fuel is present, resulting in a fuel strike of 11 times.

In other words, if we compare the fuel cost in normal times with the fuel cost just before decommissioning, we find that in normal times, one nuclear fuel assembly stays in the reactor for four cycles, so the fuel cost per cycle is is as shown in the following equation.

1 / 4 +1 / 4 +1 / 4 +1 /4
=1 However, the fuel cost just before decommissioning is
The fuel assembly (
Well, it will only be used for one cycle.

M): The fuel assembly loaded during the previous fuel change will be used for only 2J cycles. Based on these results, the fuel consumption from the time of fuel exchange immediately before decommissioning until decommissioning ■1
The strike is as shown in the following equation.

1 + 1/2 + 1/3 + 1/4 = 25/1242.083 This means that the fuel cost will be more than twice the normal cost. Similarly, in the previous cycle, 1/2
+ 1 / 3 + 1 / 441 / 4 = 1
6 / 12 Ki 1.333 In the cycle before that, 1/3 + l/4 + 1/4 + 1/4 = 13/12B 1.0
83, and during the three cycles immediately before decommissioning, there is a problem in that the fuel cost deteriorates significantly under the normal operating method of a boiling water reactor.

[Purpose of the Invention] The present invention has been made in response to such conventional circumstances, and is a boiling water reactor that can significantly alleviate the deterioration of fuel costs even in boiling water reactors that are about to be decommissioned. It attempts to provide a method for operating a nuclear reactor.

[Summary of the Invention] That is, the present invention provides a method for operating a boiling water reactor in which a part of the loaded fuel is exchanged every fuel exchange cycle, and a method for operating a boiling water reactor in which a part of the loaded fuel is exchanged every fuel exchange cycle. Sometimes it is possible to significantly alleviate the deterioration in fuel costs even in boiling water reactors that are about to be decommissioned by loading fuel assemblies with a lower amount of uranium metal than the normally used fuel assemblies. This is a method of operating a boiling water reactor that is characterized by the following:

[Embodiment 1 of the Invention The details of the method of the present invention will be described below with reference to the drawings.

FIG. 1 shows a fuel assembly used in the method of operating a boiling water reactor according to the present invention, and in the figure, reference numeral 1 indicates a channel box.

Inside this channel box 1, fuel rods 2 and water rods 3 are arranged in a square grid. Here, based on the fuel assembly shown in FIG. 6 in which the number of water rods 3 disposed in this channel box 1 is two, in the case of the fuel assembly used in this example, The number of water sticks 3 is the first
The number of water rods 3 is 4 in the fuel assembly shown in Figure (a), and 10 in the fuel assembly shown in Figure 1 (b). It is located.

The graph shown in Figure 2 shows the changes in the neutron multiplication factor of the fuel assembly shown in Figure 1. In Figure 1, the vertical axis shows the neutron multiplication factor, and the horizontal axis shows the core residence time. , the straight line C shows the change in the neutron multiplication factor of the conventional fuel combination, and the dotted line a is the line shown in FIG. 1(a) used in this example.
shows the change in the neutron multiplication factor of the fuel ngJ merger shown in
The dotted line indicates the change in the rate of increase of 14 children in the fuel assembly shown in FIG. 1(b) used in this example. As can be seen from this graph, the fuel east combination used in this example reduces the number of fuel rods and increases the number of water rods, so the amount of cooling water, which is a neutron moderator, in the fuel assembly is reduced. Since the increase in uranium-235 is relatively large compared to that of uranium-235 stones, and neutron deceleration occurs quickly, at the beginning of combustion, the neutron multiplication factor is higher than that of a normal fuel assembly, and the uranium-235 Because there are so few particles, the time it takes to burn them out is short, so the neutron multiplication factor decreases rapidly.

In other words, in the boiling water reactor operating method of this embodiment, the fuel assembly shown in FIG. 1(b) is loaded during the final fuel exchange prior to decommissioning, and the fuel assembly shown in FIG. At the same time, the fuel assembly shown in FIG. 1(a) is loaded.

FIG. 3 is a graph showing the relationship between neutron multiplication factor and time in the boiling water reactor operating method of the present invention. In this graph, the vertical axis shows the neutron multiplication factor, and the horizontal axis shows the residence time in the reactor. The straight line C is the conventional fuel assembly, the dotted line a is the fuel assembly in Figure 1(a), and the dotted line is the neutron multiplication factor of the fuel assembly in Figure 1(b) and the Nil state in the reactor. Shows the timekeeper. As can be seen from this graph, in the boiling water reactor operating method of the present invention, there is no H?
, Despite the fact that the decomposition of uranium-235 is small, the neutron multiplication factor is higher than that of the conventional method.
- Uranium that remains inside the reactor during decommissioning -
Since the cause of 235 can be reduced, the fuel consumption is reduced by $11.
-1 The deterioration of the strike will be significantly alleviated.

That is, in this example, the fuel cost of the fuel R-east combination shown in FIG. 1(a) is 0.9% lower than that of a normal fuel assembly.
677, and 0.8710 in the fuel assembly shown in FIG. 1(b). Therefore, the fuel lost in the last fuel exchange cycle is 0.8710 +0.967
7/2 + 1/3 + 1/4 = 1.93B, which makes it possible to reduce the fuel cost by about 14.5% compared to the conventional method, which requires 1 to 2,083 B of fuel. Also, in the fuel exchange cycle before the R final fuel exchange, 0.9677 / 2-L 1/3 + 1/4
+ 1/4 * 1.317, which is 1.6% more fuel than the conventional fuel 2]st-L333.
It is possible to reduce the number of strokes.

In this way, in the boiling water reactor operation method 1) of the present invention,
The deterioration of the fuel gas during decommissioning can be significantly reduced, and the plowing of radioactive waste generated during decommissioning can also be effectively reduced.

In this example, the fuel rods arranged in the channel box are arranged in an 8 x 8 square grid, and the case where two water rods are arranged is described as normal fuel. The invention is not limited to this embodiment, and it is of course possible to arrange the fuel assemblies in any number. In addition, in this example, the method of operating a boiling water reactor with a four-batch core was explained.
The number of fuel replacement batches is also not limited to four.

FIG. 4 shows an example of another fuel assembly used in the boiling water reactor operating method of the present invention. A fuel assembly in which fuel rods 2 are replaced with water rods 3, which were used in the C1 embodiment. can be used instead of .

FIG. 4(a) shows a fuel assembly in which the diameter of the water rod 3a is made small and has the same culm as the fuel rod 2. Figure 4 (b>) shows a fuel assembly in which the uranium-235 stones have been reduced by pulling out several fuel rods 2. Figure 4 (C) shows a fuel assembly in which fuel pellets are not filled in place of the fuel rods 2. A fuel assembly in which fuel rods 2C are arranged is shown. FIG. 4(d) shows a fuel assembly in which all fuel rods 2d have a small diameter. Of course, the amount of uranium metal may be reduced in any way.

[Effects of the Invention] As described above, the boiling water reactor operating method of the present invention can significantly alleviate the deterioration of fuel costs even in the operation of the boiling water reactor prior to its decommissioning. be able to.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing a fuel assembly used in the boiling water reactor operating method of the present invention, and FIG. 2 shows the neutron multiplication factor and reactor interior of the fuel assembly used in the present invention. A graph showing the relationship between residence time; Figure 3 is a graph showing the relationship between neutron multiplication factor and residence time in the reactor according to the method of the present invention; Figure 4 shows an example of another fuel assembly used in the method of the present invention. longitudinal section,
Fig. 5 is a perspective view of a conventional fuel assembly, Fig. 6 is a vertical cross-sectional view of Fig. 5, and Fig. 7 is a diagram of the arrangement pattern of fuel l+1 assemblies in the reactor core according to the conventional operating method of a boiling water reactor. , FIG. 8 is a graph showing the relationship between the neutron multiplication factor and the residence time in the reactor according to the conventional method. 1... Fuel rod 2... Channel box 3...
...Water rod applicant Memoto Nuclear Power Co., Ltd. applicant
Toshiba Corporation Patent Attorney Jusu Yamasa - Frame 4 (d) Figure 6

Claims (2)

[Claims] (1) In the operating method of a boiling water reactor, in which a portion of the loaded fuel is exchanged every fuel exchange cycle, during the final fuel exchange immediately before decommissioning, the normally used fuel assembly is A method for operating a boiling water reactor characterized by loading a fuel assembly with a small amount of uranium metal. (2) The amount of uranium metal in the fuel assembly loaded during each refueling other than the final refueling is less than or equal to the amount of uranium metal in the fuel assembly loaded during each previous refueling. A method for operating a boiling water reactor according to scope 1.