JPH0241714B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of operating a boiling water nuclear reactor (hereinafter referred to as BWR) that performs load following control operation.

In BWR, from the viewpoint of preventing fuel rod damage due to mechanical interaction between fuel pellets and cladding material,
Control rod withdrawal is limited to when the linear power density of the fuel rod is below a certain threshold or below the linear power density (hereinafter referred to as envelope) that the fuel rod has already experienced. Furthermore,
Power increases above a threshold or envelope are performed by core flow rate control, and the rate of increase is also limited.

For BWRs subject to the above operating restriction conditions, one possible method of operation that requires large output changes, such as load following control operation, is to control the output distribution by operating control rods during operation, but this method requires control rod operations at the right time and in the right amount, requires highly accurate core condition monitoring, and complex operational operations.

An object of the present invention is to provide a method for operating a boiling water reactor that can perform load following operation that can simplify the operation and reduce the rate at which the reactor output exceeds the envelope.

A feature of the present invention is that before performing load following operation, all of the control rods constituting the steady state control rods for which the first envelope was created are withdrawn at both the core center and the core periphery or at the core center. Load following is achieved by inserting control rods in the current state to create a second envelope that reduces the output at the bottom of the core compared to that of the first envelope, and controlling the core flow rate using the control rod pattern created in the second envelope. It's about driving.

The present invention is based on the conventional method shown in FIG. 1 by the inventors.
This was done based on new knowledge obtained by examining the characteristics of load following operation in BWRs. The results of this study will be explained below. Here, we will explain using an example of a BWR with an electrical output of 784 MW, a power density of 51 kW/ft, and a control rod withdrawal threshold of 8 kW/ft. FIG. 1 shows temporal changes in core thermal output 10, core flow rate 11, and core average xenon concentration 12 during load following operation by core flow rate control, and the control rod pattern (1/4 core). In this example, the high output level is 100%, the low output level is 65%, the low output holding time is 8 hours, and the output change time is 1 hour. In the control rod pattern in the same figure,
The number shown at the control rod position indicates the control rod insertion rate (number of notches); 24 indicates the control rod is fully inserted, and 0 or a blank area indicates the control rod is fully withdrawn. The envelope in terms of operation limit conditions is 100% of the rated thermal output, which is shown at point A in the diagram.
This is the linear power density distribution during steady operation at 100% rated core flow rate. Generally speaking, when operating a BWR under load following control, the most severe operating restriction conditions occur several hours after returning to the high output level, and in this example, at point B, which is 6 hours after returning to the high output level. be. At this point, the xenon concentration is at its lowest value and core flow must be reduced by approximately 6.5% to compensate for the positive reactivity due to the reduced xenon concentration and maintain 100% thermal power. FIG. 2 shows the axial linear power density distribution 13 for a typical fuel bundle at this point in time compared to the corresponding envelope 14. As shown in the figure, the linear power density at this point increases in the lower part of the core, and the peak position in the lower part of the core shifts downward. As a result, the linear power density exceeds the envelope by a maximum of 1.2 kW/ft at the peak point in the lower core. This is due to a decrease in the xenon concentration in the lower part of the core,
This is due to the downward shift of the void generation point as the core flow rate decreases.

The inventors have developed the present invention having the above-mentioned characteristics as a result of the new knowledge obtained in the above-mentioned study, that is, various studies aimed at suppressing the excess of the envelope of the lower core power peak during load-following operation. We arrived at the result of obtaining a driving method. This operation method creates Envelope 1 during steady operation, then inserts control rods to create Envelope 2 in which the output at the bottom of the core is reduced compared to Envelope 1, and the control rod pattern that created Envelope 2. Load following operation is performed using core flow rate control in In order to create Envelope 2, the control rods that make up the control rod pattern of Envelope 1 are completely withdrawn at both the core center and the core periphery, or at the core center. A certain control rod is inserted.

FIG. 3 is a diagram showing an embodiment of the present invention using the same core as the above example, with a thermal output of 15 and a core flow rate of 16.
Time change and control rod pattern (1/4 core)
a and b are shown. That is, it is assumed that steady operation was being performed with control rod pattern a until time C in FIG. 3 with thermal output of 100% and core flow rate of 100%.
Before the start of load following control operation, control rod pattern b is realized by inserting a total of 24 control rods, each with two notches, in the center of the reactor core at positions obliquely intersecting with the control rods inserted during steady operation. At this time, by inserting the fully withdrawn control rods two notches at a time, the linear power density at the top of the core increases to a maximum of 0.6kW/
It rises by about ft. Therefore, in order to comply with the above-mentioned operation restriction conditions, the following procedure is required. That is, at time C, the thermal output is reduced to about 95% by reducing the core flow rate to about 91%. Next, insert the control rod and realize pattern b. After that, increase the core flow rate to 100% thermal output and 100% core flow rate.
Realize. Next, an envelope is created by operating the control rod in pattern b for a certain period of time (12 hours). As a result, the envelope in the subsequent load following control operation is a composite of the linear power density distributions at time C and time D, and is shown in FIG. 4 for a typical fuel bundle. In Figure 4,
Curves 17 and 18 are linear power density distributions (envelopes) at time points C and D in FIG. 3, and curve 19 is a composite envelope of these. In this example, the reactivity reduction due to control rod insertion was approximately
Δk is 0.0019%, and the increase in core flow rate to compensate for this is approximately 0.05% and can be ignored. As a result, at time D, a state of 100% thermal output and 100% core flow rate can be achieved, similar to time C. Here, Δk is the neutron multiplication factor in nuclear fission.

In this state, load follow control operation is performed with a high output level of 100% and a low output level of 65%. The most severe operation restriction condition is at time E, which is about 6 hours after returning to high output. In FIG. 4, curve 20 is the linear power density distribution in a typical fuel bundle at time E, and the amount exceeding envelope 19 is the maximum.
This is approximately 0.7kW/ft, which is approximately 40% less than before.

As described above, by inserting the fully withdrawn control rods in the center of the core shallower than the inserted control rods before the start of load following control operation, the amount of envelope excess during load following control operation can be reduced by approximately 40%. % can be reduced. Therefore, the risk of fuel rod breakage can be reduced. In particular, since only the control rod insertion operation is performed to form the envelope before changing the output due to load following operation, the operation of the nuclear reactor becomes simple. Especially when changing the output of load following operation,
Since this is done through core flow rate control, the operation at that time is significantly simplified. Furthermore, the economic efficiency and safety of BWR can be improved.

The insertion depth and number of control rods in the center of the core are sufficient to reduce the output in the lower part of the core by the range of output change during load following control operation with respect to the envelope during steady operation, and at the same time It is necessary that the envelope obtained by combining the envelope obtained by control rod operation and the envelope during steady operation does not have dips in the lower part of the core. As a result, the electrical output
784MW, 24 BWRs with a power density of 51kW/
Each is about 1 to 3 notches.

Next, FIG. 5 is a diagram for explaining another embodiment of the present invention using the same core as the embodiment described above, and shows temporal changes 21 and 22 of thermal output and core flow rate, and a control rod pattern (1/4 Core) c, d, and e are shown. In FIG. 5, up to point F, the control rod pattern c
Assume that steady operation was performed with 100% thermal output and 95% core flow rate. Prior to the start of load-following control operation, the control rods in the center of the core that are diagonally intersecting with the control rods inserted during steady operation are each made two notches, for a total of 12 control rods, and the control rods in the periphery of the core are Control rod pattern e is realized by inserting a total of 12 control rods, including 4 control rods inserted with 10 notches and 8 control rods inserted with 15 notches. This control rod operation maximizes the linear power density at the top of the core.
It increases by about 1.1kW/ft. Therefore, in order to carry out the control rod operation described above within the range of operation restriction conditions, the following operating procedure is required.

First, at point F, the core flow rate is reduced to about 85%, thereby reducing the thermal output to about 95%. In this state, control rods at the core periphery are inserted to realize pattern d. Next, by gradually increasing the core flow rate, the thermal output is returned to 100% at a rate of increase in output that is less than the rate of increase determined by the operation limit conditions. Since the control rod operation at the core periphery causes a decrease in reactivity of approximately 0.18Δk, the core flow rate will be approximately 100% when the thermal output returns to 100%, which is approximately 5% lower than the core flow rate during steady operation. %It has increased. Then, at point G, the core flow rate is reduced again to about 91%, thereby lowering the thermal output to about 95%. Note that the time from reaching 100% heat output to point G may be arbitrary, but
In this embodiment, it is set to 2 hours. In this state, the control rod pattern e is realized by inserting the control rods in the center of the core. After that, the core flow rate was gradually increased while observing the above-mentioned operating limit conditions, and the thermal output was increased to 100%.
Return to %. The reactivity reduction due to this control rod operation is approximately 0.0012% Δk, and the core flow rate when the thermal power returns to 100% is approximately 100%, similar to the previous time. Finally, an envelope is created by operating the control rod in pattern e for a certain period of time (12 hours).

Through the operations described above, the envelope during load follow control operation is a composite of the linear power density distributions at time F and time H, and is shown in FIG. 6 for a typical fuel bundle. In Figure 6, curve 23,
24 is the linear power density distribution (envelope) at times F and H in FIG. 5, and 25 is the envelope obtained by combining these.

Next, in the control rod pattern e, a high output level is set.
Perform load following control operation at 100% and low output level 65%. As a result, the most severe operating restriction conditions are at point I, approximately 6 hours after returning to high output. 6th
Curve 26 in the figure is the linear power density distribution of a typical fuel bundle at point I, and the amount exceeding the envelope 25 is about 0.6 kW/ft at maximum. This excess amount has been reduced by approximately 50% compared to before.

As described above, before the start of load following control operation, the fully withdrawn control rods in the center of the core are inserted shallowly compared to the inserted control rods, and the control rods in the periphery of the core are inserted in the axial position. By inserting it to the center, the envelope excess amount during load following operation can be reduced by approximately 50%.
% can be reduced. Significant suppression of this envelope excess can reduce the risk of fuel rod failure. Furthermore, in this embodiment as well, like the previous embodiments, the driving operation is simplified and the economy and safety of the BWR can be improved.

In this example, the control rods at the core periphery are inserted first, and then the control rods at the center of the core are inserted. It is obvious that a method of inserting one control rod or a method of inserting both at the same time would produce the same effect as the present embodiment.

In addition, in this example, the insertion depth of the control rods at the core periphery is appropriate to be inserted to the center in the axial direction in order to reduce the output in the axial lower part of the center of the core, and the number of control rods inserted is as follows. It is determined from the difference between the core flow rate during steady operation and the maximum core flow rate allowed for operation.

In the above two embodiments, the control rod pattern during steady operation is an example that does not include control rods that are adjacent to each other. The present invention can be similarly applied to cases where there is.

According to the present invention, it is possible to simplify the operating operation during load following operation of a boiling water reactor, and it is also possible to significantly suppress the degree to which the reactor output exceeds the envelope during the operation.

[Brief explanation of drawings]

Figure 1 is a diagram showing changes in core characteristics during load-following control operation of a conventional boiling water reactor; Figure 2 is a linear power density distribution characteristic diagram in the axial direction of a typical fuel bundle in Figure 1; 3 and 4 show an embodiment of the present invention, and FIG. 3 is a diagram showing the relationship between changes in control rod patterns and changes in core characteristics;
Fig. 4 is a linear power density distribution characteristic diagram in the axial direction of a typical fuel bundle in Fig. 3, Figs. 5 and 6 show other embodiments of the present invention, and Fig. 5 shows a change in the control rod pattern. FIG. 6 is a graph showing the linear power density distribution characteristic in the axial direction of a typical fuel bundle.

Claims (1)

[Claims] 1. Operate in the steady state in which the first envelope is created, and before performing load following operation, among the control rods constituting the control rod pattern in the steady state, both the core center portion and the core peripheral portion or the core center A second envelope is created in which the control rods in a fully withdrawn state are inserted in the section to reduce the power at the lower part of the core than that of the first envelope, and the second envelope is
A method of operating a boiling water reactor characterized by performing load following operation by controlling the core flow rate using a control rod pattern that creates an envelope.