JPH0216477B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of operating a nuclear reactor.

This will be explained using a boiling water reactor (hereinafter abbreviated as BWR) as an example. FIG. 1 shows a horizontal cross-sectional view of a nuclear reactor, and 10 is a core lattice cell (hereinafter abbreviated as cell). In this example, 13 cells 10 are arranged horizontally within the core of the nuclear reactor. 20 is a control rod, and 30 is a fuel assembly. Cell 10 is
As shown in FIG. 2, one cross-shaped control rod 20
and four fuel assemblies 30 adjacent to and surrounding the control rod 20. Fuel assembly 3
0 usually has 63 control rods 40 and 1 water rod.
It consists of rods 41 and a channel box 42 made of zirconium alloy that houses them. The control rod 40 has a zirconium alloy cladding tube containing a large number of uranium dioxide (UC ₂ ) pellets, and the upper and lower ends of the cladding tube are sealed with plugs. The control rods 20 contain a neutron absorbing material, are selectively inserted into and removed from the gap between the fuel assemblies 30, and control the reactivity and power distribution of the reactor core. In BWR, by adjusting the core flow rate,
It is also possible to control core reactivity and power distribution.

In operation of the reactor, surplus reactivity ΔK is given to the reactor core in advance, and the reactor is controlled to be critical by adjusting the control rods 20 and the core flow rate.
The process is continued until ΔK0.0%ΔK. This process is
The next cycle is about 1/4 to about 1/3 of the total fuel assemblies 30 in the core.
is started after replacing the fuel assembly with a new fuel assembly.

The control rods 20 are inserted into the reactor core to form a predetermined control rod pattern for producing a rated reactor power, and control the reactor power while maintaining the control rod pattern for an appropriate period of time. The control rod pattern is exchanged several times during the cycle, but during the same control rod pattern, the reactor output is mainly controlled by adjusting the core flow rate.

Therefore, the cells 10 include cells in which the control rods 20 are inserted during normal operation of the BWR and cells in which the control rods 20 are not inserted. Furthermore, considering the height direction of the core, depending on the insertion ratio of control rods, the fuel assembly surrounding the control rods has a part with control rods (the part below the control upper end) and a part without control rods (the part below the control rod). above the top edge).

When the control rod 10 is adjacent to the fuel assembly 30, thermal neutrons are absorbed by the neutron absorbing material, so the neutron energy distribution within the fuel assembly 30 is distorted toward the high energy side. This is called neutron spectrum hardening. As a result, Pu ²³⁹ of U ²³⁸ , which makes up the majority of the uranium dioxide that makes up the pellets,
The conversion ratio to is increased, and the infinite multiplication factor of the fuel assembly 30 is increased compared to the case where the control rods 10 are not adjacent to each other.

Moreover, the output width controlled by the control rod 10 is
Even within the same horizontal plane, the neutron flux is not uniform within the fuel assembly 30; in the control rods 40 in the area near the control rods 10, the neutron flux is small, but in the control rods 40 in the area away from the control rods 10, the neutron flux is becomes larger. For this reason, so-called "unilateral burning" occurs, and the reactivity distribution within the fuel assembly 30 is small in the region near the control rod and large in the region away from the control rod.

Therefore, in the fuel assembly 30 where the control rods inserted for a certain period of time burn while being adjacent to each other, and then the control rods are pulled out and the control rods are not adjacent to each other, the fuel rods shown in black circles as shown in FIG. The output change is maximum at . Figure 3 shows the power distribution within the fuel assembly (A
-A′ section) is shown. In the figure, the broken line represents the power distribution when the control rods are adjacent, and the solid line represents the power distribution when the control rods are adjacent to each other.
Shows the power distribution when control rods are not adjacent. As can be seen, the change in output of the control rod 40 located at the corner closest to the axis of the control rod is the largest in the fuel assembly. Its size is
The longer the period in which the control rod is present, that is, the longer the control rod is inserted, the greater the "control rod history effect."
(Control Blad History Effects).

If the change in the output of the combustion rod 40 is too large, there is a risk that the cladding tube will crack, and the radioactive substances trapped in the cladding tube may leak outside and contaminate the surrounding area. Such damage to the control rods 40 must be absolutely avoided in order to ensure the safety of the nuclear reactor.

The above-mentioned "control rod history effect" becomes the biggest problem, especially when operating a nuclear reactor using a control cell. An example of a method for operating a nuclear reactor using a control cell is Japanese Patent Application No. 134273/1983.

When a nuclear reactor is operated using a control cell, the control rods continue to be adjacent to only the fuel assemblies that make up the control cell (lower enrichment than fuel assemblies in other cells), which causes the “control rod history effect.” ” is big. For this reason, withdrawing the control rod at the end of the cycle imposes an excessive thermal load on the fuel rods adjacent to the control rod.

If the operating period of one cycle is short, the "control rod history effect" is small and fuel damage will not occur, but if the operating period is long, there is a risk of fuel damage.

On the other hand, in recent years, in order to improve the plant utilization rate, the operating period of nuclear reactors is becoming longer, and there is an urgent need to solve the above problems.

An object of the present invention is to provide a method of operating a nuclear reactor that can ensure the integrity of fuel and improve the utilization rate of a plant.

The features of the present invention include four second fuel assemblies, a plurality of non-controlled cells including second control rods inserted between the second fuel assemblies, and an infinite multiplication factor smaller than that of the second fuel assemblies. In a method of operating a nuclear reactor having a core including four first fuel assemblies and a plurality of control cells including a first control rod inserted between the first fuel assemblies, the method includes: from 1st
The reactor is operated by inserting the first control rod into the control cell and completely withdrawing the second control rod from the non-controlled cell until the first predetermined period has elapsed. The reactor is operated by completely withdrawing the first control rod from the control cell and inserting the second control rod into the non-controlled cell until the second predetermined period elapses, and then the reactor is operated until the second predetermined period elapses. Later, the first
The purpose is to reinsert the control rod into the control cell and completely pull out the second control rod from the non-controlled cell again to operate the reactor.

A preferred embodiment of the present invention will be described with reference to FIGS. 4 to 7.

In this embodiment, a nuclear reactor operation method using a control cell is improved especially for long-term operation of a nuclear reactor. The main points of this example are as shown in (1) to (3) below. In other words, (1) the reactor core is composed of controlled cells and non-controlled cells, (2) during one cycle, the control rods are inserted into the control cells and operated within the allowable range of "control rod history effect",
When the allowable limit is reached, a control rod is inserted into the non-control cell, and the control rod is pulled out from the main control cell for further operation.

(3) While the non-control cell is in use, the "control rod history effect" of the fuel assembly that makes up the control cell is sufficiently alleviated, so the control cell is used again.

FIG. 4 shows an example in which a BWR core is composed of a control cell 11 and a non-control cell 12. Figures 5 and 6
The figure shows an example of the operation of the same core.

The control cell 11 consists of fuel assemblies (4 bodies) with low infinite multiplication factors and a control rod (1 control rod) for output adjustment inserted between them in order to suppress the increase in output due to the control rod history effect as much as possible. It consists of As the fuel assembly of the control cell 11, a fuel assembly with a long combustion period in the core or a fuel assembly with a low infinite multiplication factor, such as a fuel assembly with a low enrichment degree, is used.

The uncontrolled cell 12 is composed of four fuel assemblies with a high infinite multiplication factor and one control rod for reactor shutdown inserted between them. Uncontrolled cell 12
The fuel assemblies are responsible for most of the reactor output, so they are constructed of highly enriched fuel assemblies or fuel assemblies with a short combustion period in the reactor core.

Next, the core in Figure 4 is replaced with Figure 5 A, B and C.
The control rod pattern is used to operate according to the operating output curve shown in Fig. 6. In FIG. 5, the black dots indicate the control cells 11. In FIG. M in FIGS. 5A and 5C indicates that a control rod is inserted into the control cell 11, and N in FIG. 5B indicates that a control rod is inserted into the non-control cell 12. Cells in which M and N are not written indicate that the control rods are fully withdrawn.

From the beginning of the cycle (BOC) to point A, the control rod pattern (FIG. 5A) is used in which the control rods 10 are inserted only into the control cells 11 and the control rods 10 are not inserted into the non-control cells 12. Point A is the control cell 11
This is the point at which the maximum wire density (Kw/ft) after control rod withdrawal of the fuel assembly constituting the fuel assembly does not exceed the thermal limit value, and the period during which the control rod 10 can continue to be inserted into the control cell 11. That's the limit.

At point A, reactor operational seal (PCIOMR)
Based on this, the control rod pattern is exchanged from FIG. 5A (hereinafter referred to as control rod pattern A) to FIG. 5B (hereinafter referred to as control rod pattern B). In other words, from the state where control rod pattern A is at 100% output, due to the decrease in core flow rate and the insertion of control rods into the surrounding area, the state is temporarily shifted to a low output state, and then the control rod pattern A is replaced with control rod pattern B and the The inserted control rods are pulled out and the core flow rate increases to return to 100% output.

Thereafter, the control rod pattern B is used to operate until point B. At this time, since the control rods 10 have been withdrawn from the control cell 11, the "control rod history effect" caused by the operation in control rod pattern A is alleviated. That is, by combustion on the control rod side of the fuel assembly forming the control cell 11, the power distribution of the fuel assembly is flattened.

From point B to the end of the cycle (EOC), control rod pattern C is obtained by inserting the control rod into the control cell again.
(See Figure 5C). Control rod pattern B
The procedure for changing from control rod pattern A to control rod pattern B is the same as that for changing from control rod pattern A to control rod pattern B. The purpose of using the control rod pattern in which control rods are inserted into the control rod pattern 11 again is to promote the combustion of the fuel assemblies constituting the non-controlled cells 12, which has been suppressed by the operation of the control rod pattern B.

FIG. 7 shows the maximum linear power density of the fuel rods (fuel rods with black circles in FIG. 2) adjacent to the control rods of the fuel assembly constituting the control cell 11 during the above-described operation. The solid line in the figure indicates the operating method of the example (i.e.,
The broken line indicates the maximum linear output density when operating methods (i) to (iii) are used, and the broken line indicates the maximum linear output density when the control rod is inserted only into the control cell throughout the cycle.

It can be seen that when the operation method of this example is used, the maximum linear density is lower and the safety is higher than when the operation is performed by inserting the control rod only into the control cell throughout the cycle.

Applying the concept of this embodiment, it is also possible to make some of the control cells serve as non-control cells into which control rods are inserted during the cycle. but,
To this end, the number of control cells is increased compared to that of this example, (1) to prevent flattening of the radial output distribution, and (2) to ensure surplus reactivity necessary for long-term operation. It is necessary to increase the reactivity of the fuel assembly that constitutes the control cell.

Control cells are composed of fuel assemblies with a low infinite multiplication factor (and therefore low reactivity), while non-controlled cells are composed of fuel assemblies with a high infinite multiplication factor (and therefore high reactivity). be done. Therefore, the output of non-controlled cells is high and the output of controlled cells is low, so
When viewed in the radial direction, the output distribution is uneven. Therefore, in order to flatten the radial output distribution,
The smaller the number of control cells, the better.

On the other hand, in terms of core reactivity, since the object of this embodiment is a long-term nuclear reactor operation method, it is better to have fewer control cells. When the number of control cells is large, the number of non-control cells naturally decreases, and in order to secure the surplus reactivity necessary for long-term operation, it is necessary to increase the enrichment of the fuel assemblies that make up the non-control cells. This is because countermeasures are required.

In order to configure a core with a small number of control cells, a core has been proposed in which fuel assemblies with fuel enrichment distributed in two regions in the axial direction are arranged in non-control cells.
Such a reactor core is described in detail in Japanese Patent Application Laid-Open No. 1386-1986.

From the above, this embodiment has the following effects.

(1) By using a control cell in a nuclear reactor, the integrity of the fuel can be ensured even if the reactor is operated for a long period of time.

(2) Since nuclear reactors can be operated for long periods of time using control cells, (a) plant utilization rate is improved;

(b) Improved drivability.

If the long-term operation plan is shortened, operation from BOC to point A to point B in FIG. 6 can be performed. In addition, if the reactor operation is prolonged, the operation from point A to point B to EOC, that is, operation according to control rod pattern A, control rod pattern B, and control rod pattern C, as shown in Figure 6, may be performed. You can do it sequentially. This shows that the operating method of this embodiment is highly flexible.

According to the present invention, the soundness of the fuel can be ensured even if the operating cycle period of a nuclear reactor is lengthened, and the plant utilization rate can be improved by lengthening the operating cycle period.

[Brief explanation of drawings]

Figure 1 is a horizontal cross-sectional view of the reactor core, Figure 2 is
An explanatory diagram of a core lattice cell, FIG. 3 is a relative power distribution diagram within a fuel assembly, FIG. 4 is a configuration diagram of a core to which an embodiment of the present invention is applied, and FIGS. 5 A, B, and C are operation diagrams of the present invention. An explanatory diagram showing the change in the control rod pattern when the method is applied, FIG. 6 is a characteristic diagram showing the relationship between core average burnup and reactor power when the present invention is applied, and FIG. FIG. 3 is a characteristic diagram showing a change in linear power density of a fuel rod adjacent to a control rod in a fuel assembly that constitutes the fuel assembly. 11...Control cell, 12...Non-control cell, 20
...Control rod, 30...Fuel assembly, 40...Control rod.

Claims (1)

[Claims] 1. A plurality of non-control cells including four second fuel assemblies and second control rods inserted between the second fuel assemblies, and four non-control cells each having an infinite multiplication factor smaller than that of the second fuel assemblies. In the method of operating a nuclear reactor having a core including a first fuel assembly and a plurality of control cells including a first control rod inserted between the first fuel assemblies, the method includes: Until the first predetermined period elapses, the reactor is operated by inserting the first control rod into the control cell and completely withdrawing the second control rod from the non-controlled cell; After the first predetermined period elapses and until the second predetermined period elapses, the first control rod is completely pulled out from the control cell, the second control rod is inserted into the non-control cell, and the second control rod is inserted into the non-control cell. After the reactor is operated and the second predetermined period has elapsed, the first control rod is reinserted into the control cell and the second control rod is completely pulled out from the non-controlled cell again to remove the atoms. A method of operating a nuclear reactor, characterized by operating a reactor.