JPS5912155B2 - Nuclear reactor fuel mounting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel installation method for installing a nuclear fuel assembly in the core of, for example, a B, W, or R type nuclear reactor.

Conventionally, the fuel mounting system for this type of boiling water reactor was
As shown in the figure, in the core after the second cycle (the second refueling cycle, one cycle is carried out once a year), one set of four fuel bodies (one unit body)
A set of three old fuel assemblies a and one new fuel assemblies are loaded into the reactor core.

The number of new fuel assemblies installed at the beginning of each fuel exchange cycle is approximately % of the total number of fuel assemblies in the second cycle.
After that, although there is a slight increase or decrease in the number of new fuel assemblies installed, when the new and old fuel assemblies a and b are replaced in order, and after the 5th or 6th cycle of the vehicle, Currently, the number of fuel bodies in the reactor core has settled down to approximately % of the total number of fuel bodies.

In this way, when new and old fuel assemblies a and b are placed and installed in the reactor core in accordance with the reactor fuel assembly replacement (replacement) planning process, the power distribution of the core is Arranging the new and old fuel assemblies so that the new and old fuel assemblies are arranged so that the cross section of the reactor core is divided into eight equal parts with respect to the axis 0 means that the new and old fuel assemblies are arranged in advance in the reactor. It is also considered desirable because the instrumentation equipment allows accurate core management.

Therefore, conventionally, the fuel mounting method for this type of nuclear reactor is
As shown in Figure 1, new fuel assemblies a are installed around the control rods (not shown) on the X and Y axes of the core, and the new and old fuel assemblies ayb for the above percentage are Of the four fuel assemblies, two new fuel assemblies will be installed.

In this case, when the control rod is withdrawn in a cold state at the time when the combustion of the reactor has progressed, the bundle average ko.

There is a risk that there will not be sufficient margin for reactor shutdown at the end of one cycle, when the power (kW) peaks.

For this reason, conventional methods were to (1) load only old fuel assemblies a around the control rods on each of the X- and Y-axes to prevent an excess of 4 in the bundle average, or (2) to reduce the core power distribution. At the cost of % targets, only 1 out of 4 units will be equipped with a new fuel tank.

However, the former method has drawbacks such as lowering the cycle burn-up, while the latter method reduces the accuracy of managing the core power distribution.

In view of the above-mentioned points, the present invention, after the second cycle, installs new fuel assemblies with high gadolinia in the core in a cross shape, and installs new fuel assemblies with low gadolinia in other parts of the core. A fuel installation method for a nuclear reactor that aims to improve the shutdown margin of a nuclear reactor and increase the burn-up of each cycle by arranging target fuel bodies. It provides:

Hereinafter, the present invention will be described with reference to an illustrated embodiment.

In FIG. 2, reference numeral 2 denotes a core within a pressure vessel 1 of a nuclear reactor, and this core 2 has an X-axis 3 and a Y-axis 4 formed in a cross shape. On the X-axis 3 and Y-axis 4, new fuel bodies 5 and old fuel bodies 6 (fuel bodies continuously loaded from the previous cycle) are loaded alternately in parallel in the second cycle and thereafter.

The new fuel assembly 5 has a gadolinia additive concentration about 3 to 6% higher than that of the old fuel assembly 6, that is, it has a highly burnable substance added thereto, and is arranged in pairs in a cross shape.

On the other hand, there are three old fuel bodies 6 and 1 in other parts of the core 2.
The new fuel assemblies 7 with low gadolinia in the body are mounted so as to form one unit, and the new fuel assemblies 7 with low gadolinia have a gadolinia additive concentration of about 2 to 10% lower than that of the old fuel assemblies 6.
It has a concentration of 3%, that is, a low combustible substance is added.

Therefore, each unit constituted by one new fuel body 7 made of low gadolinia is installed in other parts of the core 2.

By the way, in boiling water nuclear reactors, control rods containing 840 are used to control the reactivity of the reactor.

Additionally, as a design standard for the reactivity control ability of a nuclear reactor, if all control rods are inserted into the reactor core when the reactor is cold, one control rod may be completely withdrawn from the reactor core for some reason. However, it is stipulated that the reactor is subcritical.

This is called reactor shutdown margin.

Therefore, in a nuclear reactor that has just started operation, if the fuel reduction in the reactor progresses and the reactivity decreases to the extent that rated operation is no longer possible, the reactor will be shut down and some parts of the burnt area will be removed. The old fuel assembly 6 is removed and, as described above, the new fuel assembly 5 with high gadolinia and the new fuel assembly 7 with low gadolinia are loaded in this order.

At this time, the new fuel bodies 5 and 7 are required to have the number and bundle average enrichment that can ensure the cycle combustion height.

Generally, sufficient new fuel assemblies are required to compensate for the decrease in bundle average kOO due to the combustion of old fuel assemblies 6.

However, if ■ is too large in the bundle average of the new fuel assembly, there will be insufficient margin for reactor shutdown, so burnable substances such as gadolinia are added to prevent ■ from becoming too large in the bundle average. .

On the other hand, if too much gadolinia is added at a high concentration to limit the size of the bundle average kc-O from the cold state to the rated operation, the fuel body may As the bundle average kOO decreases, the reactivity margin of the core decreases, so a balance between these is necessary.

In other words, in the conventional second cycle and later cycles, the new fuel assembly used one type of gadolinia, so when considering reactor shutdown margin, two of the four fuel bodies around the control rods were used, as shown in Figure 1. It was not possible to load more than one new fuel body.

For this reason, there is a risk that the cycle burnup will be insufficient.

In the present invention, by installing each new fuel body 5.7 in the reactor core 2 as shown in FIG. 2, when the fuel bundle is in a cold state,
When the control rods are withdrawn, the bundle average (■) shows that when the fuel enrichment is kept constant and the concentration of gadolinia added is increased, the new fuel assemblies 5 and 7 have burnup curves as shown in the graph of FIG. Form.

That is, when the gadolinia concentration is increased, the peak of ■ in the bundle average becomes smaller and gentler due to the reduction of burnable poisons.

Therefore, the installation of the new fuel bodies 5 and 7 shown in FIG. 2 of the present invention skillfully utilizes the characteristics of the gadolinia concentration described above.

In other words, when conventional old fuel assemblies 6 are loaded as a set of four, as shown in FIG. lower.

On the other hand, when the new fuel bodies 5 and 7 of the present invention are installed,
As shown in FIG. 7, the four-bundle burn-up prevents the carbon dioxide from monotonically decreasing during the cycle and satisfies the core's reaction margin.

In addition, in the present invention in which the new fuel bodies 5 and 7 shown in FIG. As shown in Figure 5, the average burnup of the four bundles is based on the burnup of the old fuel at the beginning of the cycle. This will fill the reactor core's reaction margin.

Next, the embodiment shown in FIGS. 3 and 8 is another embodiment of the present invention, in which high gadolinia is formed in a cross shape on the X-axis 3 and Y-axis of the core. New fuel body 5
are installed in two rows in parallel, and the old fuel assembly 6 is installed only in the 0 section at the center of the core, and is substantially the same as the specific example described above. When a new fuel assembly 5°7 is installed as shown in FIG.
The bundle average burnup is designed to prevent a monotonous decrease in carbon dioxide during the cycle and to satisfy the reaction margin of the reactor core.

As described above, according to the present invention, in the fuel assemblies installed in the reactor core in the pressure vessel, after the second cycle, new fuel assemblies 5 with high gadolinia are installed in the core 2 in a cross shape. Since the new fuel assembly 7 with low gadolinia is installed in other parts of the reactor core 2, it is not only possible to maintain sufficient reactor shutdown margin, but also to reduce the This arrangement has excellent effects such as being able to averagely increase the burn-up of each cycle.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view for explaining a conventional nuclear reactor fuel installation method, FIG. 2 is a cross-sectional view of a nuclear reactor fuel installation method according to the present invention, and FIG. 3 is a cross-sectional view for explaining a conventional nuclear reactor fuel installation method. The figures illustrating the examples, FIGS. 4 to 8, are graphs showing bundle average burnup. : 1...Pressure vessel, 2...Reactor core,
3...X-axis, 4...Y-axis, 5...
...New fuel body, 6...Old fuel body, 7...
・New fuel body.

Claims (1)

[Claims] 1 In the fuel assemblies installed in the core in the pressure vessel, after the second cycle, new fuel assemblies with high gadolinia are installed in the core in a cross shape, and new fuel assemblies with high gadolinia are installed in other parts of the core. A nuclear reactor fuel installation method characterized by installing a new fuel body made of gadolinia.