JPH021276B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a load following operation method for a pressurized water reactor (PWR), and particularly to a load following operation method for reducing the range of change in boron concentration and the amount of discharged water.

Conventionally, the reactivity of pressurized water reactors has been controlled by inserting and withdrawing control rods and changing boron concentration. Control using control rods allows rapid reactivity changes, 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 the rods 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 1A, the change in reactivity will be due to the change in the output level. The change in reactivity as shown by the solid line 2 in Figure 1B (this is called the output deficit) and the change in xenon (Xe) as shown by the dotted line 3
It consists of two factors: change in reactivity due to change.
As shown in Figure 2, a typical example of power loss is the power loss due to the Doppler effect due to changes in fuel temperature, which is indicated by the dashed line 4 in the figure, and the power loss due to changes in the core average temperature of the primary coolant. The sum total with the output loss shown by the dotted line 5 becomes as 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 core, but the power loss due to temperature changes increases towards the end of the life, so the overall power loss is shown as a typical example by the solid line 7 in Figure 3. As shown,
It grows larger towards the end of life.

Among reactivity changes during output fluctuations, if the output loss is compensated for by inserting and withdrawing control rods, rapid output changes can be accommodated, and the Xe change is a slow change, so boron concentration changes can You just have to deal with it.

However, when changing from 100% to 50% output, the output loss is about 0.7%△ρ, while control rod bank D, which is normally used for control, is about 1.0% with full insertion.
Since there is a control rod worth of ~1.4%Δρ, if we try to compensate for the power loss with the control rods alone, we need to insert the control rods deep below the center of the reactor core. As mentioned earlier, if the control rod is inserted deeply, the axial power distribution will be greatly distorted.
After that, output distribution fluctuations due to Xe started to occur, which became 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.

Figure 4A shows the output level change during standard load following operation, Figure 4B shows the control rod insertion degree, and Figure 4C shows the boron concentration change, and when there is an output change shown by line 1 in the figure. , the control rod is line 8 in Figure 4B.
As shown in FIG. 4C, the degree of insertion changes, and as a result, the change in boron concentration shown by curve 9 in FIG. 4C becomes a complex curve in which the output loss and the Xe component are mixed. Also, in this example, the boron concentration change width is approximately
It is about 90PPm, and if such load following operation is performed every day, approximately 90PPm of boron dilution is required per day. Since boron is mixed in the entire amount of the primary coolant, in order to dilute the boron, it is necessary to drain the primary coolant water and inject pure water in its place. The boron concentration during operation is
Approximately 1000PPm at the beginning of life to approximately 100PPm at the end of life
It decreases monotonically as the core burns up to. For this reason, when the product nears the end of its life and the boron concentration decreases, it becomes necessary to discharge a large amount of waste water to the outside for treatment in order to dilute the boron. In other words, a problem arises in that the amount of discharged water and discharged radioactive substances 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 boron concentration dilution rate per unit time, so from this point as well, it is impossible to make a rapid change in boron concentration as shown by curve 9 in Figure 4C. .

In order to solve the above-mentioned problems with changes in boron concentration, it is an object of the present invention to provide an operating method that minimizes changes in boron concentration during load following operation and reduces the amount of discharged water. . In order to achieve this objective, the load following operation method according to the present invention uses control rods to compensate for reactivity changes by reducing boron concentration changes, but in order to compensate for reactivity changes with control rods, When a positive reactivity is added, the control rod is inserted into the core, and when a negative reactivity is added, the control rod is withdrawn from the core. That is, if reactivity compensation is performed primarily using control rods, the control rods will be moved more often by insertion and withdrawal in the reactor core. Furthermore, when a control rod is deeply inserted into the reactor core, the axial power distribution is distorted toward the lower part of the core, and when the control rod is withdrawn from the core, it returns to a flat axial power distribution. If the control rods move more than in the conventional method, it becomes difficult to control the axial power distribution, so we decided to adjust the position of the control rods before the output rises and gradually increase the output to full output. Therefore, it is characterized in that axial power distribution control can be performed without significant distortion.

In the conventional operating method, when attempting to reduce the boron concentration variation range, control rods are inserted and withdrawn frequently, making it difficult to control the axial power distribution. In this invention, when the output decreases and within several hours thereafter,
Even when Xe increases, all reactivity compensation is performed by automatic control rod control, so that the boron concentration does not change. The control rod is further withdrawn and the insertion degree is about 20% to 25%,
When the axial offset (A.φ.) approaches the steady value - (5% to 10%) at full output,
Dilute the boron as needed to hold the control rod in place. Here, the axial offset (A.φ.) is an index of the axial power distribution distortion of the core.
is defined as A.φ.=q _T −q _B /q _T +q _B ×100 (%), where q _T and q _B are the outputs of the upper and lower half of the core, respectively. To increase the output, first, the control rods are fully withdrawn to increase the output as much as possible, and this output level is maintained for several hours. During this time, the reactivity due to the decrease in Xe is compensated for by inserting the control rods. The control rods are fully withdrawn again to increase the output, maintain this output level for several hours, and then insert the control rods to compensate for the Xe. Repeat this procedure to reach full power. Usually about 3~
Full power can be restored in 4 hours. In this manner, the axial power distribution after returning to full power can be easily controlled without causing large vibrations.

5A to 5C illustrate the case of the operating method according to the present invention, in which the output level change is indicated by line 10 in FIG. 5A, and the control rod insertion degree is indicated by line 11 in FIG. 5B.
The change in boron concentration is shown by curve 12 in FIG. 5C with respect to time. Line 11 in Figure 5B
As shown in FIG. 5C, the control rod movement is large, and as shown by curve 12 in FIG. 5C, the boron concentration change range is greater than that shown by dotted line 9 in the conventional method (same as curve 9 in FIG. 4C). It will be understood that the size has been reduced by about half or less, and the amount of discharged water has been reduced in proportion to this.

In addition, as shown by line 10 in Figure 5A, when the output decreases, the control rod can reduce the output in steps, so the pattern of maintaining the low output for 6 hours takes longer than the conventional method to return to full output. can be made shorter. Since the output increases step by step, the health of the fuel rods will not deteriorate as the linear output increases. In addition, the operating range that can be performed only by automatic control rod operation becomes wider, and the number of boron concentration changing operations is reduced, making operation easier.

As can be seen from line 7 in Figure 3, the output loss increases at the end of the life, so the control rod inserted when the output decreases also becomes deeper, and it is necessary to pull it out to cope with the subsequent increase in Xe. becomes possible. As illustrated in Figure 6, this relationship is expressed by converting the reactivity of the control rods inserted when the output is reduced, that is, the output loss corresponding to the output change and the reactivity change due to the increase in xenon after the output decrease, into a boron concentration change. It is shown as follows. Solid lines 13 and 14 in FIG. 6 represent changes in boron concentration corresponding to changes in xenon reactivity up to 6 hours after power reduction at the beginning and end of life, respectively, and dotted lines 15 and 16 represent changes in boron concentration at the beginning and end of life, respectively. It is expressed in terms of the boron concentration change equivalent to the output loss obtained by inserting the control rod. In other words, the dotted lines 15 and 16 represent the positive reactivity obtained by withdrawing the control rod at the beginning and end of its life, so in the output change range where the solid line is below the dotted line, it This shows that changes in xenon reactivity at low power levels can be dealt with by withdrawing the control rod, regardless of changes in boron concentration. At the beginning of life, solid line 1
3 is below the dotted line 15, and in the range A where the solid line 14 is below the dotted line 16 over almost the entire output change range at the end of the life, the xenon reactivity can be compensated by control rod operation. There is.

Therefore, near the end of the life, the output decreases, and the subsequent low output level and output increase can be covered by automatic control rod operation alone, and boron concentration changes are not required. In this case, the boron concentration needs to be changed only to a small extent due to Xe overshoot after the output increases, so it is possible to continue load following operation until the end of the life.

According to the operating method of the present invention, load following operation is possible without making the boron concentration change too large and without distorting the axial output distribution at full output. As an example, at the beginning of the first cycle life and at the end of life, the output is reduced stepwise from 100% output to 50% output, maintained at 50% output for 6 hours, and then returned to full output in stages. An example of calculation using a one-dimensional diffusion code using an electronic computer is as follows.
Examples at the beginning of life are shown in Figures 7A to 7D, and examples at the end of life are shown in Figure 8.
Shown in Figure A-8D. To simplify calculations, the control rods are not moved continuously but at intervals of about 30 minutes, so lines 19 and 24 in Figures 7C and 8C
A・φ・ shown by and line 21 in Figures 7D and 8D,
Although the boron concentration change curve indicated by 26 is jagged, it actually passes through the average value of these curves. In addition, lines 17 and 22 in the figure indicate the output level, lines 18 and 23 indicate the control rod insertion degree, and lines 20 and 22 indicate the control rod insertion degree.
25 indicates the steady value at full output. A・φ・ shown by lines 19 and 24 in FIGS. 7C and 8C returns to a steady value after the full output is restored, and Xe vibration does not occur, and curves 21 and 24 in FIGS. 7D and 8D
As shown by 26, it can be seen that the boron concentration change width is reduced to about half or less compared to the conventional one (line 9 in Fig. 4C). The movement of the control rods is similar at the beginning and end of their life, and after the output decreases
It may be maintained at approximately 25% insertion for ~3 hours.

As described above, according to the load following operation method of the present invention, it is possible to eliminate the load following operation restriction at the end of life, which was a drawback of the conventional method, and the boron concentration change range is much larger than that of the conventional method. Since the amount of water discharged from the primary coolant can be significantly reduced, the amount of water discharged from the primary coolant can be reduced, the amount of wastewater treatment operations can be reduced, and the control rod automatic operation range can be expanded, greatly improving operability during load following operation. can be improved.

[Brief explanation of drawings]

Figure 1A is a diagram showing an example of changes in output level over time during load following operation, Figure 1B is a diagram showing changes in reactivity accompanying changes in output level in Figure 1A, and Figure 2 Figure 3 is a diagram showing power loss versus power level, Figure 3 is a diagram showing power loss versus core life, Figure 4A is a diagram showing power level change versus time during standard load following operation, and Figure 4B is a diagram showing power loss versus time during standard load following operation. The figure shows the degree of control rod insertion, Figure 4C shows the change in boron concentration, Figure 5A shows the change in output level in the case of the operating method according to the present invention, Figure 5B shows the degree of control rod insertion, and Figure 5C shows the change in boron concentration. Figure 6 is a diagram showing the relationship between boron concentration change and output loss, and Figures 7A and 8A are calculations of load following operation according to the present invention at the beginning and end of life, respectively. Output level diagrams showing examples, Figures 7B and 8B show the control rod insertion degree, Figures 7C and 8C show the axial offset (A・φ・), and Figures 7D and 8D show the change in boron concentration. They are line diagrams shown respectively. In the figure, 1, 10, 17, 22... Lines indicating output level, 2... Output loss, 3... Xe change,
4, 5, 6... Output loss, 7... Output loss relative to core life, 8... Solid line showing after control rod insertion, 9
...Curve showing changes in boron concentration, 11...Line showing control rod insertion degree according to the present invention, 12...Curve showing changes in boron concentration according to the present invention.

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, and when the control rods reach approximately 20-25% insertion, the axial offset reaches full power. After reaching a negative side of about 5 to 10% from the steady value, the control rod is held in that position by control based on changes in boron concentration, and when the output increases thereafter, the control rod is pulled out in a step-like manner. Increase the output, compensate for the increase in reactivity due to the decrease in xenon after the increase in output by reinserting the control rod, and after several hours have elapsed, pull out the control rod again and increase the output in steps, repeating this until the full output is reached. A load following operation method characterized by: