JPH027436B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a load following operation method for a pressurized water nuclear reactor.

Recently, load following ability has been desired in nuclear power generation. In other words, in the past, the proportion of nuclear power generation in total power generation was small, so nuclear power generation was not expected to have the ability to follow the load, but in recent years, as the proportion of nuclear power generation in total power generation has increased. As a result, load following capability has come to be desired in nuclear power generation as well, and along with this, it has become necessary to follow the reactor core output, and it has also become necessary to follow the reactor core reactivity control. Ivy. Changes in reactivity when the core output fluctuates, such as during load-following operation, are mainly caused by changes in reactivity due to changes in output level (this is called power loss), as shown in a typical example in Figure 1. It consists of two factors: a change in reactivity due to a change in quinocene (Xe). As a typical example is shown in FIG. 2, the power loss is the sum of the power loss due to the Doppler effect due to the fuel temperature change and the power loss due to the core average temperature change of the primary coolant. The Doppler power loss does not change much during the life of the core, but the power loss due to temperature changes increases towards the end of the core life. The overall output loss becomes large at the end of the life, as shown in a typical example in FIG.

Reactivity control devices for pressurized water reactors have traditionally included control rods and boron concentration change systems.
The control rods enable fast reactivity changes, and the boron concentration change system increases or decreases the boron concentration in the primary coolant to enable slow reactivity changes, so most of the reactivity changes during output fluctuations are , insert a control rod to compensate for the output loss,
If you compensate by pulling out, you can respond to quick output changes.
Since the Xe change is a slow change, it is thought that it can be dealt with by changing the boron concentration.

However, when changing from 100% to 50% output, the output loss is about 0.7% Δρ, while control rod bank D, which is normally used for control, has a control rod worth of about 1.0 to 1.4% Δρ when fully inserted. In order to compensate for the power loss with only the control rods, it is necessary to insert the control rods deep below the center of the reactor core. If the control rod is inserted deeply, the axial power distribution will be greatly distorted, and power distribution vibration due to Xe will also occur, which becomes a problem. In order to avoid such axial power distribution distortion, it has recently become a standard operating method to vary the power without inserting the control rods too deeply. In this case, since the control rod alone cannot compensate for the power loss, the remaining reactivity will depend on changes in boron concentration, and rapid power fluctuations will not be possible. Furthermore, the instantaneous return ability to immediately return to the original full output level from a reduced output level will naturally be insufficient because the control rods are not inserted as deep as the output loss. Figure 4 shows changes in output level, degree of control rod insertion, and boron concentration changes during conventional standard load following operation.As is clear from Figure 4, the control rods are not inserted very deeply into the core. Therefore, there is a lack of quick output fluctuation ability and immediate recovery ability, and a solution to these problems is desired. In addition, some techniques (for example, Japanese Patent Publication No. 51-47837, Japanese Unexamined Patent Publication No. 51-47837,
54-17498) has been proposed, but no satisfactory results have been obtained.

This invention has been made in view of the above circumstances, and is a pressurized water type atom that can 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 purpose is to provide a method for operating a furnace.

In response to this purpose, the load following operation method for a pressurized water reactor of the present invention provides a control rod bank having a control rod value approximately equivalent to a change in reactivity due to a change in output during load following operation of a pressurized water reactor. The reactor is inserted into the core with no bank overlap or with a small bank overlap, and the power is rapidly reduced.The subsequent decrease in reactivity due to the increase in xenon is compensated for by changes in boron concentration, and the subsequent increase in power is compensated for by changes in boron concentration. In the future, in an operating method in which the change in reactivity due to the change in output is compensated for by withdrawing the control rod, and the increase in reactivity due to the decrease in xenon after the increase in output is compensated for by the change in boron concentration, the change in axial offset due to the insertion of the control rod is It is characterized in that the control rod operation range is set near the bottom of the core, which is the range where the absolute value of the axial offset decreases when the control rod is further inserted after the curve exceeds the maximum value on the negative side.

The details of this invention will be explained below with reference to one embodiment.

First, an important point is that in this invention, when inserting control rods into the reactor core, no bank overlap is provided, and the control rods are inserted in such a way that there is no bank overlap or there is very little bank overlap. . In other words, the axial offset (AO) is an index of the axial power distribution distortion of the core.
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. In conventional control rod banks, in order to improve the continuity of reactivity between each bank, each bank is inserted with a certain degree of height, which is called bank overlap, as shown in Figure 5A. There is. That is, in a typical example, the first bank has 128 steps (total steps are 228 steps).
When this happens, the next bank will begin to be inserted at the same time. Therefore, as the control rod is inserted, the axial offset increases more and more toward the negative side as shown by line A in FIG. 6, and does not return to its original state. Therefore, in the conventional method, when the control rod is inserted a certain depth, the AO inevitably becomes large on the negative side, making it difficult to control the power distribution. However, as in the present invention, as shown in FIG. 5B, if the overlap of the control rod banks is eliminated and each bank is inserted independently, when the control rods are nearly fully inserted, the AO will be as shown in FIG. 6. As shown by line B, it returns to the original state again. Therefore, in the conventional method, A.
The control rod was inserted shallowly so as not to make O. too negative, but with the method of this invention the same A.
Even if it is O., it can be inserted almost to the bottom of the reactor core.
Note that there is no problem in the existence of control rod bank overlap as long as it is small. In other words, the 6th
As shown in the figure, in the conventional method, among the AO change curves (line A in the figure) due to control rod insertion,
Whereas the range in which the negative absolute value of AO increases when a control rod is inserted in the upper part of the core, the method of this invention uses the AO change curve when inserting a control bank without bank overlap. It is characterized by utilizing the range (line B in the figure) in which the negative absolute value of AO decreases when a control rod is inserted near the bottom of the core. This makes it possible to insert control rods deeply without causing distortion in the axial power distribution, which was impossible with conventional methods.
It is possible to have quick load change capability and instant full output return capability.

When power decreases, AO changes to the positive side due to changes in core average temperature. Since the range of change in AO changes depending on the temperature coefficient of the moderator, it changes depending on the core life as shown in FIG. 7, a typical example. The control rod position is determined by moving the control rods near the bottom of the core so that the AO shifts to the positive side due to this power change is offset and the AO hardly changes from the full power state.

It is desirable that the control rod worth be equivalent to the output loss of the desired output change. For example, in the case of a change in output from 100% to 50% due to the output loss shown in FIG. 3, the required control rod worth is approximately 0.7 to 1.0% Δρ. In this example, the control rod worth is set at approximately 0.8% Δρ, and when the control rod worth becomes insufficient near the end of its life, another control bank is inserted from the top of the core to compensate, and the AO is distorted to the negative side. The control rod bank located at the lower part of the core may be moved further down the core to compensate for this. Further, a slight increase or decrease in reactivity may be compensated for by changing the boron concentration. In this way, the standard control rod bank can cope with power losses during power changes throughout its life.

In addition, when the output change range changes, such as when changing from 100% to 30% output, a control bank with a control rod worth corresponding to the output loss in that case is determined as a standard, and each output change These control rod banks can be selected depending on the width. Control rod bankworth can be used to increase or decrease the number of control rods in each bank (from 9 to 5, for example), or to increase or decrease the number of absorption rods in one control rod (from 20 to 5, for example).
(reduced to 12).

〔Example〕

As an example of this, the control rod changes in the case of load following, in which the output decreases stepwise from 100% output to 50% output, maintains it at 50% output for 6 hours, and then increases the output stepwise again to 100% output, AO Examples of calculations of changes and boron concentration changes using a one-dimensional diffusion calculation code are shown in FIGS. 8 and 9.

FIG. 8 is an example of calculation at the beginning of the first cycle life, and FIG. 9 is an example of calculation at the end of the first cycle life. Each control rod worth is approximately 0.7,
This is an example of 0.8%Δρ. It can be seen that AO, which is an index of axial power distribution distortion, is maintained close to a steady value regardless of output fluctuations. Also, since the control rods are inserted quite deeply, they have the ability to return immediately if they are pulled out. It can also be seen that the boron concentration forms a curve that corresponds to changes in xenon. Also, from these figures, there is still quite a bit of room for the control rods to be fully inserted, so the other control banks should be further inserted from the top of the core, and for the transition to the negative side of AO, the previous control bank should be further inserted below the core. It can be seen that there is enough room to absorb some increase or decrease in the output deficit if they are offset.

As is clear from the above description, according to the present invention, control rods having a force approximately equivalent to the required power loss are inserted near the bottom of the core without control rod bank overlap or with small bank overlap. By keeping the axial offset almost the same as before insertion, rapid load changes can be made without distorting the axial power distribution, and load following operation with immediate recovery capability is possible. A follow-up driving method can be obtained.

[Brief explanation of drawings]

Figure 1 is a graph showing an example of the breakdown of reactivity changes during output fluctuations, Figure 2 is a graph showing an example of changes in output loss due to output level, Figure 3 is a graph showing an example of changes in output loss due to lifespan, Figure 4 is a graph showing an example of changes in control rod insertion degree and boron concentration during output fluctuations in a conventional operation method, Figure 5 is a graph showing an example of control rod bank insertion, and Figure 6 is a graph showing an example of control rod insertion degree and boron concentration changes during output fluctuations in a conventional operation method. Graph showing examples of changes in shear offset, No. 7
The figure is a graph showing an example of axial offset change due to output change, Figure 8 is a graph showing an example of calculation of specifications during load following operation at the beginning of the first cycle life, and Figure 9 is a graph at the end of the first cycle life. 3 is a graph showing an example of calculation of specifications during load following operation.

Claims (1)

[Claims] 1. A control rod bank with a control rod value approximately equivalent to the change in reactivity due to a change in output during load following operation of a pressurized water reactor is inserted into the core with no bank overlap or with small bank overlap. , the output is rapidly decreased, and the subsequent decrease in reactivity due to the increase in xenon is compensated for by the change in boron concentration, and when the output is subsequently increased, the change in reactivity due to the output change is compensated for by withdrawing the control rod, and the output is increased. In an operating method in which the increase in reactivity due to the subsequent decrease in xenon is compensated for by changes in boron concentration, if the curve of axial offset change due to control rod insertion exceeds the maximum value on the negative side, and further control rods are inserted, the axial offset will increase. A method of load following operation of a pressurized water reactor, characterized in that the operating range of control rods is set near the lower part of the reactor core, which is the range in which the absolute value of decreases.