JPH0350997B2 - - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for changing control rod patterns in boiling water nuclear reactors.

[Technical background of the invention]

As shown in Fig. 1, in a boiling water reactor, four fuel assemblies 2 are loaded around a control rod 1 having a cross-shaped cross section to form a unit lattice 3 . are arranged in a grid to form a planar core 4 as shown in FIG. In addition, one square in FIG. 2 indicates one unit cell 3 .

By the way, in such a core 4 , as the burnup of the fuel progresses, the reactivity decreases.
The surplus reactivity corresponding to this decrease in reactivity is given in advance. However, since a nuclear reactor is operated continuously for about one year, if surplus reactivity is given to correspond to the decrease in reactivity during this period of continuous operation, the initial surplus reactivity becomes excessive and the margin for shutdown of the reactor core decreases. For this reason, burnable poisons such as gadolinia are mixed into the fuel to suppress excessive surplus reactivity at the initial stage of combustion. Therefore, as shown in Figure 3, the change in surplus reactivity during one cycle is kept relatively low by the effect of the burnable poison in the early stages of combustion, and then the surplus reactivity is reduced by the combustion of the burnable poison. is increasing. The surplus reactivity reaches its maximum in the middle stage of combustion when the burnable poison is completely burned, and thereafter decreases as the combustion progresses. In order to compensate for this change in surplus reactivity, boiling water reactors control the core flow rate,
During operation, the number and insertion amount of control rods (so-called adjustment rods) inserted into the reactor core, ie, the control rod pattern, are changed. The above reactivity adjustment is first performed by adjusting the core flow rate, but the fourth step is
As shown in the figure, the adjustment range A based on the core flow rate is small, so if the reactivity cannot be adjusted completely by adjusting the core flow rate, the control rod pattern is changed as shown in FIGS. 5 to 8. In addition, the fifth
Square or unit cell 3 in the figure to figure 8...
The number written inside is the control rod 1 in that unit cell...
Indicates the number of insertion notches, and the number of notches in the fully withdrawn state
24. In the fully inserted state, the number of notches is 0, and in the unit grid 3 where no number is written, the control rod 1... is shown to be in the fully withdrawn state. In the early stage of combustion, each of the nine control rods 1... is operated as shown in Figure 5.
The control rod pattern is operated with 12 notches inserted, and as the surplus reactivity increases, these control rods 1 are inserted two notches at a time, changing to the control rod pattern as shown in FIGS. 6 and 7. Furthermore, when the surplus reactivity decreases in the late stage of combustion, five of the nine control rods 1 are brought into a fully withdrawn state, and the control rod pattern is changed to that shown in FIG. By the way, control rod 1...
When a control rod is inserted, the output of the fuel in the lower part of the unit cell 3 containing the control rod decreases. Therefore, if the output of the core 4 is maintained constant in this state, the output at the lower part of the fuel in the other unit grids 3 will increase accordingly. Therefore, when the control rod pattern is changed in this way, the power distribution in the axial direction of the fuel changes as shown from B to C in FIG. 9, and the linear power density in the lower part increases. As a result, the rate of increase in linear power density in the lower part of the fuel becomes excessive, resulting in mechanical mutual interference between the fuel pellets and the fuel cladding, so-called PCI, resulting in a problem in which the integrity of the fuel cladding deteriorates. For this reason, conventionally, when changing the control rod pattern, the output of the core 4 is lowered once as shown in FIG.
The control rod pattern was changed in this low power state.

[Problems with background technology]

In the conventional method described above, the output of the reactor must be reduced when changing the control rod pattern,
There was a problem that reduced the operating rate of the nuclear reactor. In addition, when the fuel burns, xenon is produced as a fission product. Since this xenon has a large absorption cross section for thermal neutrons, the presence of this xenon can cause three-dimensional oscillations in the power distribution within the reactor core. The amount of xenon produced is small in the area where the control rod is inserted, and the amount produced is large in other areas. Therefore, in the conventional method, the control rod operation amount when changing the control rod pattern is large, and changing the control rod pattern causes a large change in the distribution of xenon in the reactor core, which is favorable for the stability of the reactor core. It wasn't something.

[Purpose of the invention]

The present invention was made based on the above circumstances, and its purpose is to be able to change the control rod pattern under rated operating conditions without significantly changing the output of the reactor, and to change the control rod pattern. It is an object of the present invention to provide a method for changing a control rod pattern of a boiling water reactor, which can improve the stability of the reactor core by reducing fluctuations in the distribution of xenon when changing the control rod pattern.

[Summary of the invention]

The present invention changes the control rod pattern by alternately repeating control rod operation and core flow rate adjustment multiple times in which the reactivity applied to the reactor core is less than or equal to the effective multiplication factor of 0.04%. By suppressing the applied reactivity of this one control rod operation to below 0.04% effective multiplication factor,
It is possible to suppress core power fluctuations during control rod operations to within the range of ±0.5%, which is the tolerance for rated operating conditions and is generally used for power supply purposes. Therefore, if the control rod pattern is changed by repeating such control rod operation and core flow rate adjustment, the control rod pattern can be changed under rated operating conditions without reducing the reactor output, and the reactor availability rate can be improved. It is possible to significantly improve the linear power density during control rod operation, and the rate of increase in linear power density during control rod operation is extremely small, so there is no possibility of PCI occurring. In addition, fluctuations in the xenon distribution during control rod operation are also reduced, which improves the stability of the reactor core.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 11 to 23. Figures 11 to 20 are diagrams showing the procedure for operating the control rods, and show schematic plan views of the reactor core. As mentioned above, one square represents one unit cell 3 , and each The numbers written inside unit cell 3 indicate the number of insertion notches for control rods in that unit cell, and unit cell 3 with no numbers written on it means that the control rods in that unit cell are fully withdrawn. Show that. FIG. 11 shows a control rod pattern before control rod operation, and this embodiment is for the case where this control rod pattern is changed to the control rod pattern as shown in FIG. 6 described above. Further, this embodiment is for a boiling water nuclear reactor equipped with a hydraulic control rod drive mechanism.

First, as shown in Fig. 12, the control rod 1 at the center position is
Manipulate the book and insert only one notch. The reactivity applied to the core when one control rod is operated by one notch corresponds to a change in the effective multiplication factor of 0.016% to 0.024%, which is less than the effective multiplication factor of 0.04%. Next, as shown in Figure 13, one control rod at the left position is
Insert only the notch. Next, as shown in FIG. 14, one control rod at the right position, which is symmetrical to the control rod at the left position, is inserted by one notch. Furthermore, the control rods are inserted one notch at a time in the order of upper position, lower position, lower left position, upper right position, upper left position, and lower right position as shown in Figures 15 to 20, and 9 control rods are inserted as shown in Figure 20. Insert the control rod one notch at a time. Next, insert the control rod in the center position one notch, then insert the control rods one notch at a time in the same order as above, and then
Change the control rod pattern to the one shown in the figure. After each control rod is operated, the reactor core flow rate is adjusted to maintain the reactor output at the rated operating state. The reactivity applied to the reactor core by operating each notch of each control rod corresponds to an effective multiplication factor of 0.016 to 0.024%, and the variation in reactor output due to changes in this effective multiplication factor is 0.2 of the rated output. corresponds to ~0.3%. Therefore, by repeating such control rod operations and core flow rate adjustments, the fluctuation range of the reactor output when changing the control rod pattern can be made extremely small as shown in D in FIG. 21. and,
Tolerance of output fluctuation in rated operation is ±0.5%
On the other hand, since the output fluctuation due to the control rod operation is 0.2 to 0.3%, the rated operating state can be maintained. Note that E in FIG. 21 shows the state of output reduction when changing the control rod pattern according to the conventional method. In addition, in the method of this embodiment, since the reactivity applied to the core by one control rod operation is small, the power distribution after control rod operation is changed from the power distribution state shown at the bottom of Fig. 22. As shown in G in Fig. 22, the change is extremely small, so even if the control rods are operated under rated operating conditions, PCI will not occur. Also,
The linear power density distribution after performing the above control rod operation from the operating state of the linear power density distribution as shown in F' in Fig. 23 becomes as shown in G' in Fig. 23, and the maximum increase in linear power density I for this example is approximately
It is 0.1KW/ft. In addition, the operating standard for preventing PCI, ie, PCIOMR, stipulates that the output increase rate is 0.1 KW/ft or less per hour, so in the case of this example, the time interval between control rod operations is more than 1 hour. shall be. However, the aforementioned xenon is produced by β-decay of iodine produced by nuclear fission, and its half-life is relatively short at 6.7 hours.
Therefore, if the time interval between control rod operations is too long, the amount of xenon produced will change due to power fluctuations during control rod operation, which is undesirable from the standpoint of core stability. Good.

Note that the present invention is not limited to the above embodiment.

For example, the present invention can also be applied to boiling water reactors using so-called fine control rod drive mechanisms (FMCRDs), which allow control rods to be operated in finer drive units. In this case, this fine control rod drive mechanism can drive control rods in finer drive units than one notch, which is the minimum control rod drive unit of conventional hydraulic control rod drive mechanisms, so that during one control rod operation, Multiple control rods can be operated simultaneously and the applied reactivity can be kept below the effective multiplication factor of 0.04%. Therefore, it is possible to simultaneously operate a plurality of control rods located at mutually symmetrical positions to further reduce fluctuations in the power distribution of the reactor core.

〔Effect of the invention〕

As mentioned above, in the present invention, the reactivity applied to the core is 0.04.
The control rod pattern is changed by alternately repeating control rod operations below the % effective multiplication factor and core flow rate adjustment multiple times. By suppressing the applied reactivity of this single control rod operation to below 0.04% effective multiplication factor, the core power fluctuation during control rod operation is kept within ±0.5%, which is the allowable error of the rated operating condition. can be suppressed to Therefore, by repeating such control rod operations and changing the control rod pattern, it is possible to change the control rod pattern under rated operating conditions without reducing the reactor's output, greatly improving the reactor's availability. Moreover, since the rate of increase in linear power density during control rod operation is extremely small, there is no possibility of PCI occurring. In addition, fluctuations in the xenon distribution during control rod operation are also reduced, so the stability of the reactor core is improved, which has great effects.

[Brief explanation of drawings]

Figures 1 to 10 show conventional examples, Figure 1 is a plan view of a part of the core, Figure 2 is a schematic plan view of the core, and Figure 3 is a line showing the change characteristics of surplus reactivity. 4 is a diagram showing the relationship between core flow rate and power, FIGS. 5 to 8 are schematic plan views of the core showing control rod patterns, and FIG. 9 is a diagram showing power distribution. FIG. 10 is a diagram showing output changes when changing control rod patterns. FIGS. 11 to 23 show an embodiment of the present invention, and FIGS. 11 to 2
Figure 0 is a schematic plan view of the reactor core showing the procedure for changing the control rod pattern, Figure 21 is a line diagram showing power fluctuations when changing the control rod pattern, Figure 22 is a line diagram showing power distribution, and Figure 23 is a diagram showing the power distribution. The figure is a diagram showing the linear power density distribution. 1...Control rod, 2...Fuel assembly, 3 ...Unit cell,
4...Reactor core.

Claims (1)

[Claims] 1. A control rod for a boiling water reactor characterized in that the control rod pattern is changed by alternately repeating control rod operation and core flow rate adjustment multiple times in which the reactivity applied to the reactor core is 0.04% effective doubling factor or less. How to change the pattern.