JPH05223972A - Loading method for fuel of nuclear reactor - Google Patents

Abstract

PURPOSE:To obtain a fuel loading method, which secures the thermal margin of the fuel so that the operation of a single-control pattern for a long period is sufficiently possible, in an unclear reactor, wherein a large amount of MOX fuel is loaded. CONSTITUTION:A boiling water reactor is operated for a long period of a half or more of one cycle under the state wherein a control rod is inserted into the same fuel cell. In the fuel loading method of the nuclear reactor such as this boiling water reactor, the entire regions comprising the fuel cells on the control rods, which are inserted for a long period, and fuel assemblies 11 neighboring the fuel cells directly or in the diagonal pattern are made to be first regions 1. Regions, which are located at the inner side of a line 2 connecting the inserted control rods for a long period located at the outermost side among the inserted control rods for a long period and are not included in the first regions, are made to be the second region. As the new fuel assembly, which is loaded into the second region, the MOX new fuel is made more, and uranium new fuel is made less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel loading method for a nuclear reactor, in particular, a boiling water reactor.

[0002]

2. Description of the Related Art In a nuclear reactor, excess reactivity and power distribution change due to combustion of fuel during operation. To control the reactivity change during such output operation, it is necessary to operate the control rod and adjust the reactor coolant flow rate. In particular, when performing control rod operation, it is desirable to take a procedure to reduce the output of the fuel adjacent to the control rod from the viewpoint of fuel integrity, which complicates the operation and lowers the plant utilization rate. There is a drawback.

Therefore, for example, as shown in Japanese Examined Patent Publication No. 62-44632, by adjusting the burnable poison contained in the new fuel, the excess reactivity change due to combustion during the output operation is made relatively small, and the output operation is performed. A method of reducing the number of control rod operations by reducing the number of control rods used for internal control is adopted. FIG. 2 shows an example of combustion change of the excess reactivity of the reactor and an operation control rod pattern in a certain cycle. The control rod pattern diagram shows a cross-sectional view of the reactor,
One square 1 represents a cell of four fuel assemblies surrounding one control rod. Cell 2 indicated by a white triangle is a control rod insertion cell during rated operation. From this, it is understood that the control rod pattern does not need to be changed because the excess reactivity is almost constant in most of the cycles. The control rod pattern is changed only once or twice as the reactivity decreases at the end of the cycle. Therefore, most of the cycle can be operated with one control rod pattern. However, the change in the output distribution cannot be controlled during this long single control rod pattern period, so it is necessary to change the control rod pattern to suppress output peaking unless a sufficient thermal margin is secured during this period. May result in reduced plant utilization.

On the other hand, in recent years, in order to improve fuel economy, the local peaking coefficient of the fuel and the radial peaking of the reactor tend to be increased, and the thermal margin of the fuel tends to be small. For example, in Japanese Patent Laid-Open No. 61-240193, a fuel is used that has a higher reactivity gain and a higher neutron utilization rate by increasing the average enrichment of the fuel rods arranged on the outer periphery of the cross section of the fuel assembly. ing. In addition, by arranging the burned fuel at the outermost periphery of the reactor, new fuel or young fuel that has burned is loaded in the central part of the reactor, reducing the leakage of neutrons from the side of the reactor and increasing the reactivity gain. It has gained. However, according to such a method, there is a tendency that the local peaking coefficient of the fuel and the power peaking in the radial direction of the reactor increase, and in JP-A-61-240193, the output in the axial direction of the reactor is increased. By improving the peaking so as not to deteriorate the output peaking seen from the whole reactor, a method to secure a thermal margin of fuel will be taken.

[0005]

However, there is a limit to the improvement of the axial output peaking, and the above-mentioned prior art will increase the burnup of the fuel in the future, prolong the operation cycle, and further the plutonium mixed oxide fuel ( There is a problem that it is insufficient to cope with new operation such as multiple loading of MOX fuel).

An object of the present invention is to provide a fuel loading method for ensuring a thermal margin of fuel capable of sufficiently performing a long-term single control rod pattern operation in a reactor loaded with a large number of MOX fuels.

[0007]

In order to achieve the above object, the MOX fuel is mainly used as the new fuel loaded in the region surrounded by the control rod insertion cells of the long-term single control rod pattern.

[0008]

In MOX fuel, thermal neutrons are absorbed by plutonium, so neutron absorption of gadolinia, which is a combustible poison, is reduced, and as a result, the time at which gadolinia burns out is delayed and the maximum infinite multiplication factor becomes small. In Figure 3, MOX
The infinite multiplication factor combustion change of the fuel is shown in comparison with the batch-loaded uranium fuel. Therefore, the radial output peaking of MOX fuel tends to be smaller than that of uranium fuel. Also,
When performing long-term single control rod pattern operation, the region where the fuel thermal margin is severe is relatively limited. Therefore, if the MOX fuel is loaded in that region, the maximum radial power peaking of the reactor can be lowered as compared with the case where the uranium fuel is loaded, and the thermal margin of the fuel can be secured.

The reason why the region where the thermal margin of the fuel is severe is relatively limited when the long-term single control rod pattern operation is performed is shown below.

FIG. 4 shows an example of the combustion change of the maximum linear power density (hereinafter referred to as MLHGR) when the long-term single control rod pattern operation is performed. The figure shows an example in which the control rod pattern is changed twice. Normally, the MLHGR generated fuel is the fuel of the second cycle of loading in the first half of the cycle, and is the new fuel of the cycle loading after the middle of the cycle. MLHGR does not occur in the new fuel at the beginning of the cycle because the infinite multiplication factor of the new fuel is kept low by the gadolinia which is a burnable poison. Therefore, as the combustion of the new fuel progresses, the infinite multiplication factor increases and the output peaking tends to increase. This trend continues until gadolinia burns out. At the end of the long-term single control rod pattern period, the excess reactivity of the entire reactor begins to decrease, which corresponds to the time when the new fuel, gadolinia, is burned out. Therefore, the output peaking of the new fuel becomes maximum and the MLHGR also becomes maximum at this time. It is possible to reduce MLHGR to some extent by setting an appropriate control rod pattern at the beginning of the cycle or after changing the control rod pattern, but for MLHGR at the end of the long-term single control rod pattern period. Since it is not assumed that the control rod pattern is changed, it is difficult to reduce the control rod pattern, and as a result, the MLHGR at this time is often the maximum throughout the cycle.

Next, the MLHGR generation region at the end of the long term single control rod pattern period will be described. FIG. 5 shows an example of the radial radial power distribution during this period. In this figure, the horizontal axis indicates the radial position from the center of the reactor, and the vertical axis indicates the radial output peaking.

As a result, the output peaking at the control rod insertion position is 20 to 30% smaller than the average value of 1.0. Therefore, conversely, the output peaking at the control rod insertion position in the central portion of the reactor is 20 to 30%. It can be seen that the 30% output peaking is increasing. Further, the power peaking in the area outside the control rod insertion position is smaller than the power peaking in the central portion of the reactor. Therefore, the maximum radial power peaking in the long-term single control rod pattern period can be limited to the intermediate region of the control rod insertion position in the central portion of the reactor.
Since the axial power peaking of the reactor varies little depending on the reactor position, the MLH at the end of the long single control rod pattern period
GR is considered to occur in fresh fuel in this region where radial output peaking is large.

From the above, the fuel loading method according to the present invention is
By placing the OX fuel at the most effective position, it is possible to secure a thermal margin of the fuel that enables long-term single control rod pattern operation.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. The figure shows a cross-sectional view of a 1100 MWe class boiling water reactor, and one square 11 represents one fuel assembly. The bold diamonds 12 indicate the positions of the control rods to be inserted for a long time during the rated operation, and there are 13 control rods in this example. The shaded portion 1 is the first region described in the claims. The thick line 2 shows the line connecting the outer long-term insertion control rods,
The area other than the diagonal line on the inside is the second area described in the claims.
Area. The black circle 21 indicates the MOX new fuel loading position, and the black triangle indicates the uranium new fuel loading position. In this example, the second
The number of new fuels included in the area is about 1/4 of the total amount of new fuels, and batch loading of MOX fuel is usually considered to be up to about 1/3. It was done. As shown in FIG. 3, the MOX fuel delays the time when the gadolinia burns out more than the uranium fuel, and the maximum value of the infinite multiplication factor becomes small. Therefore, the radial output peaking at the end of the long term single control rod pattern period occurring in the second region You can lower the maximum value of.

Next, a second embodiment is shown in FIG. The figure shows a cross-sectional view of an 820 MWe class boiling water reactor, and in this example, there are nine fuel cells into which long-term control rods are inserted during rated operation. In this example, the number of new fuel bodies included in the second region is about 1/6 of the whole new fuel, which is small.
MOX new fuel is also placed outside the area. However, since the amount of MOX fuel arranged outside the second region is smaller than that of uranium fuel, the infinite multiplication factor characteristic of the new fuel loaded outside the second region is close to that of uranium fuel. Therefore, the second
The MOX new fuel in the region tends to delay the time when the gadolinia burns out, as in the first embodiment, and the effect of arranging the MOX new fuel in the second region is effective.

[0016]

According to the present invention, it is possible to secure a thermal margin of fuel capable of sufficiently performing a long-term single control rod pattern operation in a reactor loaded with a large number of MOX fuels.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a nuclear reactor showing a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a combustion change in excess reactivity and a control rod pattern.

FIG. 3 is an explanatory diagram of an infinite multiplication factor of MOX fuel.

FIG. 4 is an explanatory diagram of a change in combustion of maximum linear power density (MLHGR).

FIG. 5 is a radial power distribution map of the reactor.

FIG. 6 is a cross-sectional view of a nuclear reactor showing a second embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... 1st area | region, 2 ... The line which connected the outer side of the long-term insertion control rod, 11 ... Fuel assembly, 12 ... Long-term control rod insertion position, 21 ... MOX new fuel loading position, 22 ... Uranium new fuel loading position.

Claims (2)

[Claims] 1. A plurality of fuel cells each including four fuel assemblies arranged adjacent to each other and a control rod insertable in a position surrounded by these fuel assemblies and having a length of more than half of one cycle. In a nuclear reactor that operates with control rods inserted in the fuel cells for a certain period, all regions including the fuel cells of the long-term insertion control rods and fuel assemblies adjacent to or diagonally adjacent to the fuel cells are first regions. When a region that is inside the reactor with respect to a line connecting the outermost long-term insertion control rods among the long-term insertion control rods and is not included in the first region is a second region, The fuel loading method for a nuclear reactor is characterized in that the new fuel assemblies loaded in the two regions are mostly new fuel assemblies containing plutonium and few new fuel assemblies for uranium fuel. 2. The fuel loading method for a nuclear reactor according to claim 1, wherein the new fuel assemblies to be loaded in areas other than the second region contain less plutonium-containing new fuel assemblies and more uranium fuel new fuel assemblies.