JPH0222916B2 - - 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 and a fuel assembly. [Technical Background of the Invention] The conventional approach to nuclear reactor operation has been to improve fuel integrity by increasing thermal margins such as linear power density or critical power ratio. The increase in thermal margin also makes it easier to operate the control rods and improves the operating rate of the reactor. The method of operating a nuclear reactor that provides the maximum thermal margin is one in which the power distribution is constant throughout the operating cycle, and such a power distribution is well known as the Hering distribution. An example of a Hering distribution in a boiling water reactor is shown in FIG. In boiling water reactors, void fraction distribution occurs in the axial direction, so neutrons heat up more rapidly in the lower part of the core, where the void fraction is small, than in the upper core, where the void fraction is large, and as a result, the position of the power peak is lower in the core. There has been a slight shift to FIG. 1 also shows the axial void fraction distribution b for the Hering power distribution. However, the Hering distribution is virtual, and it is not easy to achieve the Hering distribution in an actual nuclear reactor, especially in a boiling water reactor that has a distorted power distribution in the axial direction. For this reason,
Improvements were made to fuel assemblies for boiling water reactors,
By dividing the fuel assembly into two regions in the axial direction and increasing the content of fissile material in the upper part of the core than in the lower part, or by making the content of burnable poisons in the upper part of the core lower than in the lower part. It is now possible to achieve a power distribution close to the Hering distribution throughout the driving cycle. Now that the health of the fuel itself has improved due to improvements in hardware, improved fuel economy is required for reactor operating methods and fuel assemblies. I'm getting used to it. In boiling water reactors, an operating method that increases the output in the lower core than the upper core from the beginning to the middle of the operating cycle is already known as one of the operating methods that improves fuel economy. It is something that As mentioned above, in a boiling water reactor, a void fraction distribution occurs in the axial direction, and the higher the void fraction is in the upper part of the core, the more ²³⁸ U resonance absorption occurs, so more fissile material ²³⁹ Pu is produced. Therefore, from the beginning to the middle of the operating cycle, if the power in the lower part of the core is made larger than the power in the upper part of the core, the void fraction in the upper part of the core will become larger, especially if ²³⁹ Pu is A lot is generated. During this time, fuel combustion progresses toward the lower part of the core, and the reactivity decreases. Therefore, at the end of the operation cycle when such an operation is carried out, the reactivity in the lower part of the core is lower than in the upper part of the core, so the power peak occurs in the upper part of the core, and the core average void fraction decreases. becomes smaller. Therefore, the reactivity of the entire reactor core near the end of the operating cycle increases, the reactor can be operated for a longer period of time, and fuel economy improves. However, such an operation can no longer be carried out in the next operation cycle after an operation in which the output of the lower core is made larger than the output of the upper core from the beginning to the middle of the operation cycle. For example, in the case of a fuel exchange in which approximately one-third of the total fuel is replaced with new fuel, at the beginning of the next operation cycle, approximately two-thirds of the total fuel will have undergone the above operation and will be removed from the lower part of the core. It is impossible to make the power in the lower part of the core greater than that in the upper part of the core because the reactivity of the reactor is reduced. This is because in a boiling water reactor, control rods are inserted from the bottom of the core, so it is not possible to control the reactivity only in the top of the core. Therefore, in the operation cycle in which the above operation was carried out, the progress of fuel combustion due to the longer operation of the reactor will appear in the next operation cycle, and in the next operation cycle, the The length of the driving cycle becomes shorter than in the case of driving. [Object of the Invention] In view of the above points, the present invention provides a boiling water reactor that performs operation in which the output in the lower part of the core is greater than the output in the upper part of the core from the beginning to the middle of the operation cycle. The above-mentioned operation can be performed again in the next operation cycle after the operation cycle in which the operation was performed, thereby improving fuel economy. [Summary of the Invention] That is, the present invention provides, in the operation cycle following the operation cycle in which the above-mentioned operation was carried out, a fuel that is divided into two regions in the axial direction and whose reactivity in the upper region is lower than that in the lower region. By loading the fuel, the above-mentioned operation is made possible again and fuel economy is improved. [Examples of the Invention] The present invention will be described below based on specific examples. Figure 2 shows the initial operation cycle of a boiling water reactor in which the output peak occurs in the lower part of the core from the beginning to the middle of the operation cycle in a certain operation cycle (this is referred to as the Nth cycle). and axial power distribution c at the end stage,
Fig. 3 shows e and the axial void fraction distributions d and f. However, up to the (N-1) cycle, the operation is based on the Heringian distribution, the fuel has an average enrichment of 3.0% by weight, and there are 6 fuel rods containing gadolinia.
Uniform in the axial direction. Note that FIGS. 1a and 1b show the axial power distribution and axial void ratio distribution, respectively, when the Hering operation is performed in this Nth cycle, and these do not change throughout the operation cycle. Figure 3 shows the results of the next (N+
1) Axial power distributions g, i and axial void fraction distributions h, j at the beginning and end of the cycle. The new fuel loaded in the (N+1)th cycle is the above-described fuel that is uniform in the axial direction, and about one-third of the total amount is replaced by the new fuel. As already explained, at the beginning of the (N+1)th cycle, about two-thirds of the entire fuel has experienced the Nth cycle operation, so the reactivity of the lower part of the fuel decreases significantly. , No matter what control rod plan is used, it is not possible to generate a power peak in the lower part of the core as at the beginning of the Nth cycle. Perform Haring operation in the Nth cycle,
When the Hering operation is performed in the (N+1)th cycle as well, the axial power distribution and void ratio distribution in the (N+1)th cycle are almost the same as those in FIGS. 1a and 1b. FIG. 4 shows an example of the present invention. In the Nth cycle, when an operation is performed in which the output peak occurs in the lower part of the core from the beginning to the middle of the operation cycle, in the next (N+1) cycle,
2 in the axial direction as fuel loaded as new fuel.
Axial power distribution k, m at the beginning and end of the (N+1)th cycle when the fuel is divided into regions and the reactivity in the upper part is smaller than the reactivity in the lower part.
and void fraction distributions l and n. No. (N+
1) The fuel loaded as new fuel in the cycle is:
The fuel assembly is divided into two areas, upper and lower, at the central height position, and the upper part contains 6 fuel rods containing gadolinia.
There are four books at the bottom. The concentration is uniform, 3.0% by weight in both the upper and lower parts. By loading the fuel, which is divided into two regions in the axial direction and whose reactivity in the upper part is lower than that in the lower part, as new fuel for the (N+1)th cycle, it is possible to
Even in the +1) cycle, it was possible to achieve an axial power distribution comparable to that of the Nth cycle. Table 1 shows the length of the Nth and (N+1) cycles in burnup (MWd/t) when each operating method is implemented. When operating a nuclear reactor at rated output, burnup is proportional to the number of days the reactor is in operation. Therefore, it can be said that the higher the burn-up, the more efficiently the fuel was used, and the higher the fuel economy.

[Table] Here, case A is a case where Haring operation is performed in the Nth and (N+1) cycles. Cases B and C are both cases where operation is performed in which the output peak occurs in the lower part of the core from the beginning to the middle of the operation cycle in the Nth cycle.
The new fuel loaded into the cycle is different. In case B, this new fuel is a uniform fuel in the axial direction, but in case C, it is a fuel based on the present invention that is divided into two regions in the axial direction and the reactivity of the upper part is smaller than that of the lower part. . In the Nth cycle, as shown in FIG.
As a result of the operation in which the power peak occurs in the lower core from the beginning to the middle of the operation cycle, the burnup is approximately 6.1% higher than in the Hering operation. However, when uniform fuel is loaded in the axial direction as new fuel in the (N+1) cycle, as shown in Figure 3, it is not possible to generate a power peak in the lower part of the core at the beginning of the operation cycle. In the (N+1)th cycle, the burnup is approximately 5.2% smaller than in the Hering operation. Adding these together, the gain over Hering operation is less than 1%. On the other hand, based on the present invention, the (N+
1) Load fuel as new fuel in the cycle, which is divided into two areas in the axial direction and the reactivity of the upper part is smaller than the reactivity of the lower part, and as shown in Figure 3, from the beginning to the middle of the operation cycle. If the operation is performed such that the power peak occurs in the lower part of the core, a burnup equivalent to that of the Hering operation can be obtained in the (N+1)th cycle. The burnup is not larger than that in Hering operation, but the total burnup is about 3
Because half of the fuel has already experienced operation in the Nth cycle where the output peak occurs at the bottom of the core from the beginning to the middle of the operation cycle,
This is because combustion progresses more than in the case of Hering operation, and the reactivity of the fuel itself decreases. Summarizing these two operating cycles, by operating according to the present invention and using the fuel assembly,
There was a burnup gain of about 3% compared to Hering operation, and fuel economy improved accordingly. [Effects of the Invention] As described above, the present invention implements an operation in which the output of the lower part of the core is larger than the output of the upper part of the core from the beginning to the middle of the operation cycle in a boiling water reactor, and In the next operation cycle after the operation cycle, a fuel assembly that is divided into two areas in the axial direction and whose reactivity in the upper part is smaller than that in the lower part is loaded as a new replacement fuel.
By carrying out the above operation again, it has the effect of improving fuel economy. In the fuel assembly cited as an example of the present invention, the reactivity in the upper part is lower than that in the lower part due to the difference in the number of fuel rods containing gadolinia, but the reactivity in the upper part is lower than that in the lower part due to the difference in the enrichment level. A similar effect can be obtained.

[Brief explanation of drawings]

FIG. 1 shows an axial power distribution a and an axial void fraction distribution b when a Hering operation is performed in a boiling water reactor. Figure 2 shows the axial power distribution c at the beginning of the operation cycle, the void fraction distribution d, and the operation cycle when an operation in which the output peak occurs in the lower core from the beginning to the middle of the operation cycle is performed in the Nth cycle. These are the axial power distribution e and the void fraction distribution f at the final stage. The fuel is all axially uniform. Figure 3 shows the axial power distribution g and void ratio distribution h at the beginning of the operation cycle when uniform fuel is loaded in the axial direction as new fuel in the next (N+1) cycle, and the axial direction output distribution g and void ratio distribution h at the end of the operation cycle. The directional output distribution is i, and the void ratio distribution is j. FIG. 4 shows an embodiment of the present invention, in which a fuel assembly is loaded as new fuel which is divided into two areas in the axial direction and the reactivity of the upper part is smaller than the reactivity of the lower part. These are the axial power distribution k and void ratio distribution l at the beginning of the driving cycle, and the axial power distribution m and void ratio distribution n at the end of the driving cycle.

Claims (1)

[Claims] 1. From the beginning to the middle of the operation cycle, an operation is carried out in which the output of the lower part of the core is made larger than the output of the upper part of the core, and in the next operation cycle after the operation cycle in which the above operation was carried out, 2 A boiling water nuclear reactor characterized in that the fuel assembly is divided into regions and the reactivity of the upper part is lower than that of the lower part, and the above-mentioned operation is carried out again.