JPH047839B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a nuclear reactor, and more particularly to a load following operation method for a pressurized water nuclear reactor.

[Prior art] For example, JP-A-57-119297 and JP-A-57-
As stated in Publication No. 133395, etc., conventionally,
Reactivity control in pressurized water reactors is performed by inserting and withdrawing control rods and changing boron concentration.
Control using control rods can quickly change the reactivity, but since the control rods are inserted from the top of the core, there is a problem that the axial power distribution becomes distorted as they are inserted. On the other hand, the boron concentration variable system increases or decreases the boron concentration in the primary coolant, and there is no problem with the axial output distribution, but the problem is that the reactivity changes slowly because a large amount of water must be handled. There is.

Looking at the change in reactivity when there is a fluctuation in the core output, such as during load following operation, if there is a change in the output level as shown by line 1 in Figure 6A, the change in reactivity due to this change in output level is , consists of a reactivity change as shown by the solid line 2 in FIG. 6B (this is called an output deficit) and a reactivity change due to a change in xenon (Xe) as shown by the dotted line 3. As shown in a typical example in Fig. 7, the output loss is the output loss due to the Doppler effect due to fuel temperature change, that is, the output loss shown by the dashed-dotted line 4 in the figure.
Dotted line 5 in the figure due to core average temperature change of primary coolant
This is the total of the output loss shown by , and is shown by the solid line 6 in the figure.

The power loss due to the Doppler effect does not change much during the life of the reactor core, but the power loss due to temperature changes increases as it approaches the end of the life, so the overall power loss is shown as a typical example by solid line 7 in Figure 8. As shown, it becomes larger at the end of life.

Therefore, among the reactivity changes during output fluctuations, if the output loss is compensated for by inserting and withdrawing control rods, fast output changes can be accommodated, and the Xe changes are slow changes, so the boron concentration We need to respond with change.

However, when changing from 100% output to 50% output,
The power loss is about 0.7% Δρ, while the control rod bank or grouped specific control rod group normally used for control has a control rod value of about 1.0-1.4% Δρ with total insertion. If an attempt is made to compensate for this power loss with only the control rods, it is necessary to insert the control rods deeper into the core than in the axial direction. As mentioned above, if the control rod is inserted deeply, the axial power distribution will be greatly distorted, and then the power distribution will oscillate due to Xe, which becomes a problem. In order to avoid such distortions in the axial power distribution, it has recently become a standard operating method to perform power fluctuations without inserting the control rods too deeply.

During standard load following operation, if there is a change in the output level as shown by line 1 in Figure 6A, the control rod will change its insertion degree as shown by line 8 in Figure 9A.
As a result, the boron concentration undergoes a complex change in which the output loss and the Xe component coexist, as shown by curve 9 in FIG. 9B. In FIG. 9C, the axial offset change, which is an index of the axial output distribution distortion in the above case, is shown by a line 24, and it can be seen that it is controlled to a substantially constant value with the steady value as a reference. Furthermore, in this example, the boron concentration variation range is about 90 ppm, and if such load following operation is performed every day, boron dilution of about 90 ppm per day is required. Since boron is mixed in the primary coolant, boron dilution requires draining the primary coolant and injecting pure water in its place.
The boron concentration during operation decreases monotonically as the reactor core burns, from about 1000 ppm at the beginning of life to about 100 ppm at the end of life. For this reason, near the end of its lifespan,
As the boron concentration decreases, a large amount of waste water must be discharged and treated to dilute the boron. That is, a problem arises in that the amount of discharged water and discharged radioactive materials increases. Therefore, in actual load following operation, operation near the end of life is restricted due to the processing capacity of this waste water.
In addition, near the end of life, there are naturally restrictions on the dilution rate of boron concentration per unit time, so from this point of view as well, it is important to avoid sudden changes in boron concentration as shown by curve 9 in Figure 9B. Become.

As mentioned above, in order to avoid distortion of the axial power distribution, it has recently become a standard operating method to perform power fluctuations without inserting the control rods too deeply. In this case, since the control rod alone cannot compensate for the power loss, the remaining reactivity depends on changes in boron concentration, and rapid power fluctuations are not possible. Furthermore, as can be seen from Figure 9A, the immediate recovery ability to immediately return to the original full output level from a reduced output level is naturally insufficient because the control rods are not inserted as deep as the output loss. Become.

[Problems to be Solved by the Invention] Therefore, as can be understood from the above description, the conventional technology has the following problems.

Because the control rods cannot be inserted deeply into the core to avoid distortion of the axial power distribution, the ability to quickly return to full power from low power levels is lacking.

Since a significant change in the critical boron concentration is required, the critical concentration dilution ability is restricted during the period when the critical boron concentration is low near the end of the life, and the load following operation ability is restricted.

As mentioned above, since the critical boron concentration changes greatly, a large amount of primary coolant discharge water is discharged throughout the life, and water treatment requires high cost.

Due to the problems in items 1 to 1 above, it is difficult to perform fast load following operation such as step-like operation.

The load following operation method described in Japanese Unexamined Patent Publication No. 57-133395 lacks the ability to immediately return to full output. Further, as shown by line 10 in FIG. 10A, when returning from a low output level to the full output level, the output cannot be returned to the full output at once, but returns to the full output after 2 to 3 hours through several partial output levels. Furthermore, the 10th
As shown by line 12 in Figure C, although the change in critical boron concentration is smaller than that shown by line 9 in Figure 9B, it cannot be completely reduced to zero, and the ability to dilute boron concentration still decreases at the end of life. Run short. These matters will be easily understood from the typical examples shown in FIGS. 10A to 10C.

Therefore, an object of the present invention is to solve these problems and easily realize rapid power change and immediate return capability of the reactor core without causing large distortions in the axial power distribution of the reactor core. The present invention provides a load following operation method for a pressurized water reactor that can reduce the amount of water discharged for diluting boron as the element concentration changes.

[Means and effects for solving the problem] For this purpose, the load following operation method of a pressurized water reactor according to the present invention rapidly reduces the output by inserting control rods into the reactor core, and then reduces the reaction due to the subsequent increase in xenon. The reduction in power is compensated for by withdrawing the control rod, and after the control rod has been inserted about 20 to 25% and the axial deviation has reached about 5 to 10% negative than the steady value of the full output, the control rod It is held in position and controlled by pulling out the weak absorption control rod bank, and after that, when increasing the output, the output is increased to full output in steps by pulling out the control rod and the weak absorption control rod bank sequentially, and the output is increased. This is characterized by compensating for the subsequent increase in reactivity due to the decrease in xenon by reinserting the weakly absorbing control rod bank.

Load-following operation of a pressurized water reactor in this way makes it possible to easily achieve rapid power changes and immediate recovery capabilities of the core without causing large distortions in the axial power distribution of the core, and at the same time, it is possible to easily realize rapid power changes and immediate recovery capabilities of the core without causing large distortions in the axial power distribution of the core. The amount of water discharged for boron dilution can be reduced.

[Embodiments] Next, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which the same reference numerals indicate the same or corresponding parts.

In addition to the load following operation method described in JP-A-57-133395, the present invention uses control rods made of a weak neutron absorber to achieve low output levels throughout the entire period from the beginning of life to the end of life. This provides an ideal load-following operation method that has the ability to immediately return to full output from 100 to 100 kW, and enables a significant reduction in water discharge.

In the method described in JP-A-57-133395,
The power reduction occurs immediately by inserting the control rod, but in order to compensate for the change in xenon concentration after the power reduction by withdrawing the control rod, the control rod is gradually withdrawn.
Even if the control rods were fully withdrawn in an attempt to return to full power, the reactivity would be insufficient, and there would be no ability to immediately return to full power.

To solve this problem, special control rods with reactivity effects equivalent to the power loss at full power level are inserted into the core in advance, and when returning to full power level, these control rods are inserted into the reactor core. One possible method is to pull out the control rod. However, since the control rods are fully inserted into the core at full power level, control rods with extremely high reactivity values are difficult to employ because the radial and axial power distributions will be greatly distorted as they are inserted. Therefore, as a control rod used for such a purpose, a control rod cluster assembly composed of rods of a weak neutron absorber (for example, stainless steel, Inconel, etc.) is desired.

In other words, the weakly absorbing control rod cluster that can be used is the first one.
As shown in the figure, and as shown in FIG. 2, a cross section taken along the line A-A in FIG. A solid steel rod 13a can be used. In addition, as shown in FIG. 2A, a stainless steel solid rod 13a and Ag with strong neutron absorption are used.
A weak absorption control rod cluster 13 in which -In-Cd rods 13b are mixed may also be used. Furthermore, instead of the solid stainless steel rod 13a shown in FIGS. 2 and 2A, a hollow stainless steel tube 13d filled with a weak neutron absorber 13c such as alumina or zirconia inside is used as shown in FIG. 2B. You can also use In addition, the stainless steel rod 13a in FIG.
An Inconel rod may be used instead of , or an Hf rod may be used instead of the Ag-In-Cd rod 13b shown in Fig. 2A.
It is also possible to use a B ₄ C rod or the like.

FIG. 3 is a core plan view showing an example of the arrangement of weak absorption control rods. In the figure, for example, 20 weak absorption control rods 13 are arranged at positions indicated by the symbol G. A specific group of control rods is generally called a bank, but in the arrangement example shown in Figure 3, each bank is divided into five banks, _G1 to _G5 , each consisting of four weakly absorbing control rod clusters. ing. Furthermore, A, BC and D
is the control bank, and Sa, Sb, Se and Sd are the stop banks.

If the number of weak absorption control rod clusters 13 made of stainless steel or the like shown in Fig. 1 is fully inserted by diluting the critical boron concentration at all output levels, the number corresponds to the expected output deficit due to output fluctuation. In order to immediately return to full power from a low power level, it is possible to respond immediately without changing the critical boron concentration by fully withdrawing these weakly absorbing control rod clusters. The reactivity for the necessary output loss is approximately 0.7% Δρ for a change in output from 100% to 50%, as shown by line 6 in Figure 7 and line 7 in Figure 8.
However, in the method disclosed in Japanese Patent Application Laid-Open No. 57-133395, the control rod cannot be completely withdrawn at a low output level, and at least about 20 to 50%
% is inserted into the reactor core, and considering that the power increase due to control rod withdrawal can be utilized, the reactivity of the weakly absorbing control rods is approximately 0.5% Δρ ~ 1.0%
It follows that Δρ is desirable. The reason is 0.5%Δρ
If the reactivity value is smaller, there will be a problem of insufficient power loss to compensate, and if the reactivity value is larger than 1.0%Δρ, a large number of weak absorption control rod banks will be required, and their drive equipment will also be required. This is because it would be disadvantageous in terms of construction costs. If weak absorption control rods with such a reactivity effect are inserted all at once, the output distribution will be distorted, so normally 4~
We decided to divide it into 5 banks and dilute the reactivity effect for each bank. Furthermore, dividing into a large number of banks is advantageous for use in finely adjusting the critical boron concentration.

After returning from a low power level to full power by withdrawing the control rod bank and the weak absorption control rod bank, the reactivity due to the decrease in xenon concentration is compensated for by sequentially inserting the weak absorption control rod banks. be able to. The reactivity of xenon concentration change after returning from 50% output level to 100% output level is
Since it is about 0.8%Δρ, it can be compensated by fully inserting all weak absorption control rods. In this way, most of the total reaction changes during load following operation can be covered by the control rod bank and the weak absorption control rod bank, so various load following operations can be performed with little need for changes in critical boron concentration. .

FIG. 4 is a diagram showing an example of a change in the axial deviation of the power distribution due to control rod insertion, in which line 14 represents all five banks of weak absorption control rods, line 15 represents one bank of weak absorption control rods, and line 23 represents a bank of weak absorption control rods. An example of conventional full-length control rod insertion with overlap in between is shown. The weakly absorbing control rod banks are inserted into the core individually without overlap between each bank, so even if all banks are inserted at once, the axial power distribution will be distorted after all insertions, as shown by line 14. do not have. The axial deviation AO, which is an index of strain, is defined as AO = qT - qB / qT + qB x 100%, where qT and qB are the outputs of the upper and lower half of the core, respectively.

This axial deviation AO hardly changes after the weak absorption control rod is inserted, and in reality, it is inserted one bank at a time, so the deviation AO during insertion is as shown by line 15.
Since the AO change is also small, fluctuations in the power distribution due to the insertion of weak absorption control rods are hardly a problem. Therefore,
The reactivity can be finely adjusted by inserting and withdrawing the weak absorption control rod bank as needed without worrying about power distribution distortion.

In the load following operation method when the weak absorption control rods are arranged as shown in Fig. 3, Figs. 5A to 5D
As illustrated in the figure, load tracking is achieved with very fast load changes (5% output/min) as shown by line 16 in Fig. 5A and with almost zero critical boron concentration changes as shown by line 17 in Fig. 5C. Able to drive. Fifth
In Figure B, the dotted line 18 indicates the degree of insertion of the weakly absorbing control rod bank, and the solid line 19 indicates the degree of insertion of the control rod bank. It can also be seen that by withdrawal of the control rod bank and the weak absorbing control rod bank, there is complete ability to immediately return to full power from the low power level shown in section 20 of FIG. 5B. Furthermore, by slightly inserting a control rod bank under full power conditions, it is possible to completely reduce the critical boron concentration change shown by line 17 in FIG. 5C to zero. In addition, the axial offset change shown by line 25 in Figure 5D is about 5 to 10% lower than the steady value due to control rod insertion and subsequent withdrawal during power reduction.
It can be seen that the axial offset is maintained on the negative side, and after the output increases thereafter, the axial offset is controlled to a substantially constant value.

[Effects of the Invention] As described above, according to the present invention, since a weak absorption control rod bank is used, compared to the method described in JP-A-57-133395, it is possible to achieve the ideal ideal It is possible to perform load following operation.

It can handle various load change patterns, from fast load changes of about 5% output/minute to slow load changes.

Capable of load following operation with complete instantaneous full output return capability until almost the end of life.

Since it is possible to perform load-following operation in which the change in critical boron concentration is almost zero until almost the end of its life, the amount of discharged water is reduced so drastically that its treatment becomes almost no problem.

Stable load following operation with little output distribution distortion is possible.

[Brief explanation of drawings]

Fig. 1 is a side view showing a structural example of a weak absorption control rod cluster that can be used in the load following operation method of the present invention, Fig. 2 is a sectional view taken along line A-A in Fig. 1,
The figure is a cross-sectional view corresponding to FIG. 2 of another weak-absorbing control rod cluster, FIG. 2B is a cross-sectional view showing a modification of the weak-absorbing control rod, and FIG. Fig. 4 is a plan view showing an example of arrangement; Fig. 4 is a drawing showing a modification of axial deviation due to control rod insertion;
Figure A is a diagram showing changes in the output level in an example of load following operation of the present invention, Figure 5B is a diagram showing changes in control rod insertion degree in an example of load following operation of the present invention, and Figure 5C is a diagram showing changes in the degree of control rod insertion in an example of load following operation of the present invention. FIG. 5D is a diagram showing changes in boron concentration in an example of operation, FIG. 5D is a diagram showing changes in axial offset in an example of load following operation of the present invention, and FIG. 6A is an example of change in output level over time in conventional load following operation. Figure 6B is a diagram showing the change in reactivity due to the change in power level in Figure 6A, Figure 7 is a diagram showing the relationship of power loss to power level, and Figure 8 is a diagram showing the relationship of power loss to core life. Figure 9A is a diagram showing the relationship between time and control rod insertion degree during conventional load following operation, Figure 9B
9C is a diagram showing the relationship between time and axial offset change during conventional load following operation. FIG. 9C is a diagram showing the relationship between time and axial offset change during conventional load following operation. Figures 10B and 10C are diagrams showing the output level, control rod insertion degree, and boron concentration changes, respectively, when using the conventional load following operation method. 13... Weak absorption control rod cluster, 13a, 13b
and 13c...weak absorption control rod.

Claims (1)

[Claims] 1. The power is rapidly reduced by inserting the control rods into the reactor core, and the subsequent decrease in reactivity due to the increase in xenon is compensated for by withdrawing the control rods.
After the control rod reaches the insertion level and the axial deviation reaches about 5 to 10% negative than the steady value of the full output, the control rod is held in that position and control is performed by withdrawing the weak absorption control rod bank, and the subsequent output is During the ascent, the output is increased to full output in steps by withdrawing the control rods and the weak absorption control rod bank in sequence, and the increase in reactivity due to the decrease in xenon after the increase in output is compensated for by reinserting the weak absorption control rod bank. A load following operation method for a pressurized water reactor, characterized by: 2. The load following operation method for a pressurized water reactor according to claim 1, wherein the reactivity value of all banks of the weak absorption control rod cluster is 0.5% Δρ to 1.0% Δρ.