JPH0527075B2 - - Google Patents

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, as shown in Fig. 1, has four fuel assemblies 2 loaded around a control rod 1 having a cross-shaped cross section, and a unit grid 3. Further, as shown in FIG. 2, a plurality of these unit lattices 3... are arranged in a lattice form to constitute the 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. When replacing fuel, 1/3 to 1/4 of the total fuel assembly is replaced. Therefore, when operating by replacing fuel assemblies by 1/3, after the 3rd cycle, there will be fuel assemblies with very advanced combustion, fuel assemblies with moderate combustion, and fuel assemblies with no progress in combustion. In other words, the core is loaded with 1/3 of the fuel assemblies with the lowest, middle, and highest infinite multiplication coefficients, and after each fuel change, the fuel assembly with the lowest infinite multiplication coefficient is taken out.
If a new fuel assembly is loaded instead, the core will remain in the same state from then on, and such a core is called an equilibrium core. However, in the reactor core loaded with fuel assemblies for the first time after constructing the reactor, that is, in the first loaded reactor core, all fuel assemblies are new fuel assemblies, so
During the first two to three cycles, fuel assemblies that have not been completely burned must be removed from the core during fuel exchange, which is uneconomical.

In order to improve this problem, the fuel assemblies in the initial loading core have three types of enrichment: high enrichment, low enrichment, and low enrichment. Three types of fuel assemblies are prepared, and 1/3 of each of these three types of fuel assemblies is loaded to form the initial loading core. Since such an initially loaded core is in the same state as the equilibrium core, a fuel assembly with a low infinite multiplication coefficient from the time of the first fuel exchange, that is, a fuel assembly in the same state as when the fuel has sufficiently advanced, is Since it is removed from the core, fuel economy is improved.

[Problems with the Background Art] Incidentally, 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. Ta. 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 are fixed to 1/3 of the total fuel assembly,
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. Adjustment of reactivity is made. As a result, the unit grid, ie, the control cell that includes this adjustment rod, undergoes 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. Furthermore, since a large amount of neutrons leak out of the core at the outermost periphery of the core, it is necessary to load fuel assemblies with the lowest infinite multiplication coefficients also in this outermost periphery to reduce the amount of neutrons leaking. Therefore, there is no room for changing the loading pattern of the fuel assembly under these conditions. Therefore, since it is not possible to freely change the number of fuel assemblies to be replaced or the loading pattern of fuel assemblies during fuel exchange, there is little room for improving fuel economy.

[Purpose 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, in the present invention, in the initially loaded reactor core, a plurality of 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 the lowest enrichment are loaded at the outermost position of the core. is loaded into a unit grid containing control rods used as adjustment rods, and after one cycle of operation, the fuel assembly with the lowest infinite multiplication factor is placed at the outermost periphery of the core and the unit containing control rods used as adjustment rods. It is loaded onto a grid. Therefore, in the first-loaded core, the highest enrichment fuel assembly with a high infinite multiplication factor is loaded at the outermost periphery of the core, where output decreases due to neutron leakage. The overall power of the core increases, and the fuel assemblies can burn more efficiently. Furthermore, in this initially loaded core, the fuel assemblies with the highest enrichment are loaded on the outermost periphery, so the number of fuel assemblies with the lowest enrichment is reduced accordingly. Therefore, in the first refueling, the fuel assemblies with the lowest enrichment are taken out from the core with sufficient fuel, and since the number of fuel assemblies with the lowest enrichment is small, this first Fuel economy in the cycle is significantly 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 several cycles, this increase in the number of fuel assemblies with the highest enrichment degree does not result in 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 combustion assembly in the initially loaded core. These Figures 3 to 6 schematically show the 1/4 core, where A... is the control rod, B... is the fuel assembly, and C'... is the control rod used as the adjustment rod. A unit cell or control cell containing bars is shown. In addition, the numbers written inside square B, which indicates fuel assemblies, indicate the type of fuel assemblies, and the loading pattern of these fuel assemblies is relative to the center lines X-X and Y-Y for the entire core. All are symmetrical. In this Figure 3, number 1 is a fuel assembly with a maximum enrichment, for example, 3.00% by weight, and number 2 is a fuel assembly with a medium enrichment, for example, 2.07% enrichment.
Weight % fuel assembly, Math 3 indicates a fuel assembly with the lowest enrichment, for example 1.99 weight %. In this initial loading core, the fuel assemblies B with the highest enrichment are loaded onto the outermost periphery of the core, and the fuel assemblies with the lowest enrichment are loaded into the control cell C'. Further, fuel assemblies B having the highest enrichment, middle enrichment, lowest enrichment, etc. are loaded in other regions so as to be evenly distributed. Therefore, the total number of fuel assemblies loaded into the core is 5,600, and 1/3 of this is approximately 187, but fuel assembly B with the highest enrichment... The number of fuel assemblies is 252, which is more than the amount loaded in the fuel assembly B, and the fuel assembly B with the lowest enrichment is 116, and the fuel assembly B with the middle enrichment is 192, which is equivalent to about 1/3. It is the body.

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. This results in more efficient combustion. Therefore, the fuel assembly B having the lowest enrichment level is also efficiently combusted. Note that fuel assembly B with the highest enrichment loaded in the outermost area of the core is more likely to burn than fuel assembly B with the highest enrichment loaded in another area due to neutron leakage outside the core. is not progressing.

After the first cycle of operation is completed, the first fuel exchange is performed. In this fuel exchange, all 116 fuel assemblies B, which had the lowest enrichment level, were removed from the core, and new fuel assemblies 116 were replaced in their place.
Load the body. The loading pattern of the fuel assembly in this case is shown in FIG. The numbers written in Fig. 4 indicate the degree of infinite multiplication coefficient of fuel assembly B..., number 1 indicates the one with the highest infinite multiplication coefficient, that is, the new fuel assembly, and number 2 indicates the infinite multiplication coefficient. The number 3 indicates a medium multiplication factor, that is, the highest enrichment in the initially loaded core, and the number 3 indicates the lowest infinite multiplication factor, that is, the medium enrichment in the initially loaded core. The loading pattern of the fuel assemblies in this second cycle is as shown in Figure 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'... is loaded by distributing the highest, middle, and lowest infinite multiplication factors into the average.

Then, a second cycle operation is performed with this loading pattern. The loading pattern after this second cycle is basically the same as the equilibrium core,
It does not particularly improve fuel economy;
This is to ensure a smooth transition to an equilibrium core.

Next, when this second cycle operation is completed, a second fuel exchange is performed. In this refueling, 180 out of 192 fuel assemblies B with intermediate enrichment in the initially loaded core were removed from the core, leaving the remaining 12.
Instead, a new fuel assembly 180 is loaded. Therefore, in this fuel exchange, there are four types of fuel assemblies with different infinite multiplication coefficients, and their loading patterns are 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 having a low infinite multiplication coefficient 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...252, which had the highest enrichment in the initially loaded core, had a lower infinite multiplication factor. In this fuel exchange, 152 of these 252 fuel assemblies and the 12 fuel assemblies with the lowest infinite multiplication coefficient in the third cycle are removed from the core, and 164 new fuel assemblies are loaded in their place. In addition, fuel assembly B, which had the highest enrichment level in the initially loaded core...
The 100 bodies to be left in the reactor core will be those that were loaded at the outermost periphery of the core during the initial loading, meaning those that have not yet undergone much combustion. Then, in this way, the core finally shifts to an equilibrium core.

〔Effect of the invention〕

As described above, the present invention provides a plurality of types of combustion assemblies with different enrichments in the initially loaded reactor core, and loads the fuel assemblies with the highest enrichment at the outermost position of the core, and loads the fuel assemblies with the highest enrichment at the outermost position of the core. The fuel assembly is loaded into a unit cell containing control rods used as adjustment rods, and after one cycle of operation, the fuel assembly with the lowest infinite multiplication factor is placed at the outermost periphery of the core and the control rods used as adjustment rods are loaded. This is to load the unit cell containing the Therefore, in the first-loaded core, the highest enrichment fuel assembly with a high infinite multiplication factor is loaded at the outermost periphery of the core, where output decreases due to neutron leakage. The output of the entire reactor core increases, and the fuel assemblies can be burned efficiently. In addition, in this initially loaded core, the fuel assemblies with the highest enrichment are placed on the outermost periphery, so the number of fuel assemblies with the lowest enrichment is reduced accordingly. Therefore, in the first refueling, 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 combustion assemblies with the lowest enrichment is small, this first Fuel economy in the cycle is significantly improved. In addition, 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 core in a state where combustion has progressed during the transition to an equilibrium core. Therefore, over a period of several cycles, an increase in the number of loaded fuel assemblies with the highest enrichment degree has a great effect, such as not causing a decrease in fuel economy.

[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'...Control cell.

Claims (1)

[Claims] 1 This is the first method for 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 unit cells in a lattice pattern. In the loaded core, the fuel assemblies are of multiple types with different enrichment levels,
The fuel assembly with the highest enrichment is loaded at the outermost position of the core, and the fuel assembly with the lowest enrichment is loaded into a unit grid containing control rods used as adjustment rods.
A method for operating a boiling water reactor, characterized in that after cycle operation, the fuel assembly with the lowest infinite multiplication factor is loaded at the outermost periphery of the core and into a unit cell containing control rods used as adjustment rods. .