JPS6071987A - Method of operating boiling-water 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] The present invention relates to a method of operating a boiling water nuclear reactor that improves fuel economy by improving the method of loading fuel assemblies.

[Technical Background of the Invention] In general, a boiling water nuclear reactor is constructed by loading four fuel assemblies 2 around a control rod 1 having a cruciform cross section as shown in FIG. . . , and as shown in FIG. 2, a plurality of these unit lattices 3 . . . are arranged in a lattice shape to constitute a reactor core. Incidentally, in such boiling water reactors, the fuel assembly is replaced approximately every year, or every cycle of operation, but this fuel assembly is designed to be burned for 1 to 4 cycles. , 173 to 1/4 of the total fuel assemblies are replaced during refueling.

Therefore, when operating with fuel assemblies replaced every third, the fuel assembly with the most advanced combustion after the 3rd cycle,
The core is loaded with 1/3 of fuel assemblies in which combustion has progressed to a moderate degree, fuel assemblies in which combustion has not progressed, that is, fuel assemblies with infinite multiplication coefficients of the lowest, medium, and highest.
Thereafter, every time the fuel is replaced, the fuel assembly with the lowest infinite multiplication factor is taken out and a new fuel assembly is loaded in its place, so that the core remains in the same state from then on, and such a core is called an equilibrium core. However, the fuel assemblies are loaded for the first time after constructing a nuclear reactor; in the reactor core, that is, the first loaded reactor core, it is either all fuel assemblies or new fuel assemblies, so during the first 2 and 3 cycles, the fuel assemblies are completely loaded. This was uneconomical as unburned fuel assemblies had to be removed from the reactor core during refueling.

In order to improve this problem, the fuel assemblies in the initial loading core have three types of enrichment: high enrichment, medium enrichment, and low enrichment, i.e., those with the highest infinite multiplication factor, medium enrichment, and At least three types of fuel assemblies are prepared, and three 17'3 fuel assemblies of these three types are loaded to form the initial loading core. Since such an initially loaded core is in the same state as the above-mentioned equilibrium core, fuel assemblies with a low infinite multiplication factor from the time of the first fuel exchange, that is, fuel assemblies in the same state even if combustion has progressed sufficiently, Since it is taken out from the core, the economy of fuel H is improved.

[Problems with the background technology] By the way, there is a demand for further improvement in fuel economy in commercial nuclear reactors, but it is difficult to further improve fuel economy with the methods described above. there were. In other words, since the method described above puts the initially loaded reactor core in the same state as the equilibrium core, the number of fuel assemblies to be removed from the reactor core during refueling and the number of new fuel assemblies to be newly loaded are limited. The number of bodies is fixed at 1/3 of the total combustion 111 assembly in each case, and there is no room to change these. In addition, in a boiling water reactor, only some control rods are inserted into the reactor core as adjustment rods during reactor operation, and the remaining control rods are completely withdrawn. Adjustments are made. Therefore, the unit grid, that is, the control cell containing this adjustment rod, is accompanied by fluctuations in output during operation, and the fuel assembly loaded in this control cell is subject to severe thermal conditions.

For this reason, a fuel assembly with low output, that is, a fuel assembly with the lowest infinite multiplication factor, is loaded in this control cell, and the thermal conditions of all fuel assemblies are averaged to increase the output of the entire core and fuel It is necessary to aim for efficient combustion. In addition, since there is a large amount of neutrons leaking at the outermost periphery of the core, it is necessary to load the fuel assembly with the lowest infinite multiplication coefficient also in this outermost periphery to reduce the number of neutron troughs. Therefore, there is little room to change the loading pattern of fuel assemblies due to these conditions.Therefore, it is not possible to freely change the number of fuel assemblies to be replaced during fuel exchange or the loading pattern of fuel assemblies. Therefore, there is almost no room for improving fuel economy ↑η-.

(Object of the Invention) The present invention has been made based on the above circumstances, and its purpose is to provide a method of operating a boiling water reactor that can further improve fuel economy. .

"Summary of the Invention" That is, the present invention provides a method in which, in the initial loading core, the fuel assemblies are of a plurality of types with different enrichment degrees, and the fuel assemblies with a maximum degree of 1cm are loaded at the outermost position of the core. In addition, the fuel assembly with the lowest enrichment is loaded into the unit cell containing the control rods used as adjustment rods, and after 15-cycle operation, the fuel assembly with the lowest infinite multiplication factor is placed at the outermost position of the core. It is loaded into the unit grid that includes the control rods used as adjustment rods.Therefore, in the initially loaded reactor core, the highest enrichment fuel with a high infinite multiplication factor is placed at the outermost periphery of the core, where the output decreases due to neutron leakage. Since the fuel assemblies are loaded, the output of the entire core increases in the first cycle of combustion, and the fuel assemblies can be burned efficiently.In addition, in this initially loaded core, the highest enriched fuel is placed on the outermost periphery. This is because the fuel assemblies with the lowest enrichment are loaded, so the number of loaded fuel assemblies with the lowest enrichment is reduced. Therefore, in the first refueling, the fuel assemblies with the lowest enrichment are not fully burned. It was taken out of the reactor core in a state of
Moreover, since the number of fuel assemblies with the lowest enrichment is small, the fuel economy in this first cycle is greatly improved. Note that although the number of fuel assemblies with the highest enrichment in the initially loaded core will increase compared to before, these fuel assemblies with the highest enrichment will eventually be removed from the reactor in a state where combustion has progressed during the transition to the equilibrium core. Therefore, over a period of -0- several cycles, this increase in the number of fuel assemblies loaded with the highest enrichment does not cause a decrease in fuel economy.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 3 to 6. Figure 3 shows the loading pattern of the fuel assemblies in the initial loading core. These Figures 3 to 6 schematically show the '1/4 reactor core, where A... is the control rod, first B...
. indicates a fuel assembly, and C-... indicates a unit grid, ie, a control cell, containing a regulating rod and a control rod used in No. 1. In addition, the numbers placed within squares B... indicating fuel assemblies indicate the type of fuel assemblies, and the loading pattern of these fuel assemblies is based on the center line X-X, Y-Y for the entire core.
All are symmetrical. In this Figure 3, the number 1 is the fuel assembly with the highest enrichment, for example, 3.00% by weight, and the number 72 is the medium enrichment, which means the enrichment is 2°07Φ product.
In the fuel assembly, the number 3 is the lowest enrichment, and if λ is the enrichment 1.
99% by weight fuel assembly is shown.

Therefore, in this initially loaded core, fuel assembly B with the highest enrichment
... is loaded onto the outermost periphery of the core, and the fuel assembly with the lowest enrichment is loaded into the control cell C-. In addition, fuel assemblies B having the highest enrichment, intermediate enrichment, and lowest enrichment are loaded in other regions so as to be evenly distributed. Therefore, the total number of fuel assemblies loaded in this core is 560, and 1/3 of this number is about 187, but fuel assemblies B with the highest enrichment... The number of fuel assemblies loaded is 252, which is the largest number, and the lowest enrichment fuel assembly B... is 116, which is the same number as the number loaded.
There are 192 medium enrichment fuel assemblies B, which corresponds to about 1/'3.

Then, the core is burned in this initial loading core pattern for about one year, or one cycle. Combustion in this cycle involves a large number of fuel assemblies with the highest enrichment, and since these fuel assemblies with the highest enrichment are loaded around the core, the output of the entire core is higher and more averaged. and efficient combustion. Therefore, even the fuel assembly B with the lowest enrichment level is efficiently combusted. In addition, fuel assemblies B with the highest enrichment loaded in the outermost area of the reactor core are affected by leakage of neutrons to the outside of the core, and fuel assemblies B with the highest enrichment loaded in other areas...・Combustion does not proceed as fast as it does.

Then, after the first cut/full operation is completed, the first fuel change is performed. This fuel exchange sucks! Ii! At the same time, all 116 fuel assemblies B... were removed from the reactor core, and 116 new fuel assemblies were loaded in their place. The loading pattern of the fuel assembly in this case is shown in FIG. The numbers written in this Figure 4 are for fuel assembly B...
The number 1 indicates the highest infinite multiplication factor, that is, the new fuel assembly, and the number 2 indicates the one with the intermediate infinite multiplication factor, that is, the highest enrichment in the initial loading core. The number 3 is the one with the lowest infinite multiplication factor, that is, the one with medium enrichment in the initially loaded core. The loading pattern of the fuel assemblies in this second cycle is as shown in Fig. 4, in which the fuel assemblies with the lowest infinite multiplication coefficients are loaded into the outermost periphery of the core and the control cell C'... has an infinite multiplier of 9 - the highest, middle, and lowest numbers have an average variance of 1
Load with no.

Then, a second cycle operation is performed with this loading pattern. The loading pattern after the second cycle is basically the same as the balanced core, and is not intended to particularly improve fuel economy, but to smoothly transition to the balanced core.

Next, when this second cycle operation is completed, a second fuel exchange is performed. In this fuel exchange, 180 of the 192 medium-enriched fuel assemblies B in the initially loaded core are removed from the core, the remaining 12 are left, and new fuel assemblies 180 are loaded in their place. Therefore, in this fuel exchange, there are four types of fuel assemblies with different infinite multiplication coefficients,
The loading pattern is shown in FIG. The number 1 in FIG. 5 indicates the one with the highest infinite multiplication coefficient, that is, the new fuel assembly, and the number 4 indicates the one with the lowest infinite multiplication coefficient, that is, the 12 units mentioned above. The loading pattern of this third cycle is also such that fuel assemblies B with a low infinite multiplication coefficient 10 are loaded on the outermost periphery of the core and on the control cells C.

Then, when the third cycle of operation is completed, a third fuel exchange is performed. In this third refueling, fuel assembly B...2527, which had the highest enrichment in the initially loaded core,
+ has the lowest infinite multiplication coefficient. Therefore, in this fuel exchange, 12 fuel assemblies with the lowest infinite multiplication coefficient in the third cycle and 752 of these 252 fuel assemblies c15 are removed from the core, and new fuel assemblies 1 are replaced in their place.
(loaded with 64 bodies). The 100 bodies left in the back of fuel assembly B, which had the highest enrichment level in the first loaded core, are those that were loaded at the outermost periphery of the core in the first loaded core, in other words, the fuel assemblies that had not progressed to much combustion. Choose the one that isn't there. Then, in this way, the core finally shifts to an equilibrium core.

(Effect of the invention) As mentioned above, the present invention (J)
1) The fuel assemblies with the highest enrichment are loaded at the outermost position of the core, and the fuel assemblies with the highest degree of compression are used as adjustment rods. The unit grid containing the control rods is loaded, and after one cycle of operation, the fuel assembly with the lowest infinite multiplication coefficient is loaded at the outermost periphery of the core and the unit grid containing the control rods used as adjustment rods. Therefore, in the first-loaded reactor core, the highest enrichment fuel assembly with a high infinite multiplication factor is loaded at the outermost part of the core where the output decreases due to neutron leakage. The overall output increases and the fuel assemblies can be burned efficiently.Also, in this initially loaded core, the fuel assemblies with the highest degree of illumination are loaded around the outer periphery of the spark, so the minimum enrichment The number of loaded fuel assemblies is reduced. Therefore,
In the first fuel exchange, the fuel assembly with the lowest enrichment is taken out from the core in a state where combustion has progressed sufficiently, and since the number of fuel assemblies with the lowest enrichment is small, the fuel assemblies in this first cycle are Economic efficiency is greatly improved. In addition, although the number of fuel assemblies with the highest enrichment loaded into the initially loaded core will increase compared to before, these fuel assemblies with the highest enrichment will eventually reach a state where combustion has progressed during the transition to an equilibrium core. Since the fuel is taken out from the core at the same time, the increase in the number of fuels loaded with the highest enrichment fuel set 11: during the period 1]I: of the late cycle will not cause a decrease in the economic efficiency of the fuel. The effect is huge.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view of a unit cell, and FIG. 2 is a schematic plan view of a part of the core. 3 to 6 are schematic diagrams showing loading patterns of fuel assemblies explaining one embodiment of the present invention. A...Control rod, B...Fuel assembly, C
...Unit cell, C-...=ln1~roll cell. Patent attorney on behalf of the applicant Takehiko Suzue 13-

Claims (1)

[Claims] In a method of operating a boiling water reactor in which four fuel assemblies are loaded around a control rod to form a unit cell, and a core is formed by arranging a plurality of these unit cells in a lattice pattern, the initial loading is In the reactor core, multiple types of fuel assemblies with different enrichments are used, and the fuel assemblies with the highest enrichment are loaded at the outermost position of the core, and the fuel assemblies with low R enrichment are used as adjustment rods. Load the unit grid containing the control rods, and after one cycle of operation, load the fuel assembly with the lowest infinite multiplication coefficient to the outermost position of the core and the unit grid containing the control rods used as adjustment rods. Characteristic operating method of boiling water reactor.