JPH0249678B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pressurized water nuclear reactor characterized by a power control device that can respond to rapid power changes and can control power distribution.

Conventionally, a control rod made of neutron absorbing material has been used as a means of power control in a nuclear reactor, and the neutron absorbing material of the control rod is usually set evenly in the axial direction from the upper end to the lower end. .

In addition, when controlling the output of a nuclear reactor using the control rods, the power distribution within the reactor changes due to the vertical movement of the control rods, and the reactor-specific reactions associated with changes in output change. Power distribution changes due to temperature feedback, and on the other hand, in order to operate a nuclear reactor safely and efficiently, the power distribution must be limited within a certain range. (For example, if the power in the reactor becomes excessive, the fuel in that area will be damaged.) In other words, the power of the reactor is controlled by controlling the neutron flux, that is, the reactivity, and the power distribution. For example, if there is a need,
As in the publication, a control rod in which elements with different neutron absorption capacities are arranged in a track-like manner in descending order of neutron absorption capacity has been proposed, but the specific distribution of neutron absorption capacity has not been clarified. Further, the track-shaped control rod cannot be applied to current pressurized water reactors.

Furthermore, the method currently employed as a power distribution control means is a constant axial neutron flux operation method, and to further explain this operation method, for example,
When the power is reduced from the rated power operating state, the axial power distribution of the core tends to shift toward the upper part of the core. This is because the moderator temperature decreases, which adds positive reactivity to the upper part of the core. Control rods are inserted into the reactor core from above in order to correct the distortion in the power distribution and perform reactivity control necessary to reduce the power. However, control rod insertion at this time is mainly for power distribution control, and inserting control rods to the point where sufficient reactivity is compensated for the output drop is not allowed from the perspective of power distribution control (abnormal distribution occurs).

Therefore, another reaction control method is used, namely control of the neutron absorber concentration in the primary coolant. In this control method, since the neutron absorbing material is uniformly distributed within the reactor, changes in the power distribution that pose a problem in power distribution control do not occur. Furthermore, when returning from a low output state to the rated output, the above-mentioned operation is generally reversed. However, this method has a weakness in that it also uses reactivity control by adjusting the neutron absorber concentration in the coolant. Further, boric acid is usually used as a neutron absorbing material in the coolant, and the two methods described below are currently used alone or in combination to adjust its concentration. One method is to dilute the boric acid concentration by extracting the primary coolant and simultaneously injecting an equal amount of pure water, and to concentrate it,
Highly concentrated boric acid water is injected instead of pure water during the dilution to adjust the boric acid concentration in the entire primary system, but since the amount of injection is limited compared to the amount of primary system coolant. However, there is a problem with time delays.
Another method uses an ion exchange resin and takes advantage of the difference in adsorption power of boron depending on the temperature of the resin. In this case as well, the entire primary coolant is brought into contact with the resin and the resin temperature is controlled. Due to this, we are unable to avoid delays due to
They cannot respond quickly like control rods and cannot deal with rapid output changes. That is, the rate of output change is limited by the ability to control the neutron absorber concentration in the primary coolant.

Furthermore, as an operating method to reduce the degree of insertion of control rods into the reactor core, which causes large changes in power distribution, as in Japanese Patent Publication No. 51-47837, a large number of control assemblies are arranged with the same structure and the same usage (mission). ), that is, a punctured output control bank, a Doppler control bank, and a xenon control bank, have been developed to perform output fluctuations. It controls only the Doppler effect reactivity, and at low output it is fully inserted,
At the rated output, it is used as a full drawer. However, in this method, all reactivity control necessary for output control is performed by the control rods, so it is thought that it can cope with rapid output changes, but it ignores reactivity changes due to moderator temperature changes, and output No consideration was given to distribution control.

The present invention was developed in view of the above-mentioned actual situation, and it is possible to use a plurality of control rod assemblies inserted into a reactor core formed by a large number of nuclear fuel assemblies as power control control rods. The control rods of the power control rod assemblies can be inserted and removed over the entire height of the reactor core, and the control rods of the power distribution control rod assemblies can be freely inserted and removed over the entire height of the reactor core. The absorption capacity is made uniform, while the control rods of the power distribution control control rod assembly group are fully inserted into the core only during partial power operation, and are fully withdrawn outside of the same partial power operation, and their neutron absorption capacity is It is characterized by dividing the distribution into an upper half that is smaller than the control rods of the control rod assembly group for output control and a lower half that is even lower than the upper half. The object of the present invention is to provide a pressurized water nuclear reactor that is controllable, has a quick reactivity control function with little distortion in power distribution, and eliminates the above-mentioned drawbacks.

The present invention has the above-mentioned configuration, and includes a group of control rods that can be freely moved in the vertical direction, in which the neutron absorption capacity of the control rods is set equally in the vertical axis direction, and a group of control rods that can be freely moved in the vertical direction, and the neutron absorption capacity of the control rods is set such that the neutron absorption capacity of the control rods is set to be equal in the upper and lower directions. The control rod assembly group is provided with a control rod assembly group for power distribution control that is set to a large value and is operable to move vertically in the vertical direction and is placed outside the reactor core during rated power operation. It is possible to control the low power of a pressurized water reactor by fully inserting the control rod assembly group for power distribution control, and it is possible to perform low power control of a pressurized water reactor by fully inserting the control rod assembly group for power distribution control.In addition to controlling the power of only the control rods, the neutron absorption capacity is set to be small at the bottom and large at the top. By fully inserting all control rods, it is possible to respond to rapid output changes and control the output distribution, and it has a safe, highly efficient, and quick reactivity control function with little distortion of the output distribution. Its output control performance and reliability are significantly improved.

Embodiments of the present invention will be described below with reference to the drawings.

To explain a pressurized water reactor with a thermal output class of 2.7 million KW and a standard 3-loop system, as shown in Figure 4, its core is composed of 157 fuel assemblies, as well as class-type control rod assemblies. A power control device is attached to the core, in which 48 control rod assemblies are arranged and 13 additional control rod assemblies can be attached.

The above-mentioned 48 control rod assemblies are arranged so that they can be freely moved in the vertical direction with equal neutron absorption capacity in the vertical axis direction, and are divided into six groups of control rod assemblies that are used in the same manner, that is, have the same mission. In other words, there are three banks, stop banks S _A , S _B and control banks A, B, C, D.
The banks are all control rod assembly groups for output control, and the control bank D is used during normal operation.

Furthermore, in this embodiment, in addition to the control bank, there is provided a control rod assembly group for power distribution control, ie, an output control bank P, consisting of five control rod assemblies. Each control rod assembly of the power control bank P has a configuration in which its neutron absorption capacity is set to be small at the bottom and large at the top, and the power control bank P provides negative reactivity necessary for power reduction. At the same time, the configuration described above has been adjusted to correct the distortion in the power distribution due to a decrease in power.At low power, the control rods are fully inserted across the entire length of the top and bottom of the reactor core, and at rated power, they are inserted outside the core. It is used as a full drawer that is pulled out and placed, has an output distribution control function, and also has a normal output control function, and is configured to be able to be moved vertically.

In addition, some reactivity and output distribution corrections are
It is also possible to supplement with the control bank D in the conventional manner.

The embodiment of the present invention has the above-described configuration, and includes the control rod assembly group, that is, the control bank A,
Effectiveness, practicality, and large electronics when controlling the output of a pressurized water reactor using a power control device consisting of the control rods B, C, and D, and the power distribution control control rod assembly group, that is, the power control bank P. To explain using computer simulation results, at a certain time T, the output is rapidly reduced to 50% output at a rate of 5%/min from the rated output state, and after maintaining that 50% output state until the 6th hour. Next, Figures 1 and 2 show the main parameters when a daily load following cycle operation was carried out in which the output was increased to the rated output at a rate of 5%/min and maintained at that state until the 24th hour. Figure 1 shows that only part of the control rod assembly group A, B, C, and D is used (stop banks S _A , S _Curve 1 is the relative output of the reactor, and curve 2 is the control bank A (or B, C).
Curve 3 is the relative position of control bank D in the core, where 1.0 and 0.0 indicate the fully withdrawn and fully inserted positions. In this case, the control rod assembly group for power distribution control, that is, the power control bank P, and the control bank D are used, and the curve 2' shows the power control bank P, and the first and second Curves 4 and 4' in the figure represent changes in the axial neutron velocity deviation Δ when performing operations to realize output changes, and the deviation Δ
is based on the following equation.

Δ=P _T −P _B /P _T +P _B ×P P _T : Power of the upper half of the core P _B : Power of the lower half of the core P : Relative power of the core (%) In addition, the dotted line 5 in the same figure indicates the axial direction. This indicates a certain tolerance range, that is, a tolerance range, to give flexibility to the operation in neutron east constant value operation.In the case of Fig. 1, control bank A (or B , C) are fully inserted, and the lack of reactivity is supplemented by the control bank D, and the control bank D is also inserted to keep the axial neutron flux deviation Δ within the permissible range. However, if this state continues, changes in the concentration of xenon (which absorbs neutrons but has a short half-life) among the fission products will cause the power distribution to shift in the direction where the deviation Δ is negative, that is, the power in the lower half of the core will decrease. On the other hand, the power output in the upper half of the reactor core decreases, and the imbalance between the upper and lower power outputs of the core increases. Therefore, in order to correct this, the control bank D is slightly withdrawn, and the deviation Δ is controlled within the permissible range (within the dotted line 5).

However, when the control bank is pulled out to return to the rated output, the output distribution goes out of the tolerance range on the negative side of the deviation Δ, and when the control rods are operated, the deviation Δ
In other words, even if a control rod is inserted in this state, the power distribution will further increase the power in the lower half of the core, decrease the power in the upper half, and the deviation Δ will become even more negative. Transition. Operation with constant axial neutron flux becomes impossible. In order for the above operation to be possible, all operations must be possible within the allowable range of the deviation Δ.

Further, in FIG. 2, since the output bank P is used, the change in the output distribution at low output is significantly reduced, and the deviation Δ can be controlled within the allowable range even after the rated output is restored. Said time T
A control bank is inserted a little to prevent the deviation Δ from deviating to the positive side at the 17th hour, and
After 2 hours, it is pulled out to the original position to prevent the deviation Δ from deviating to the negative side, but the deviation Δ
is well controlled.

Note that in FIGS. 1 and 2, the control bank D is not fully withdrawn in accordance with a request determined from operational necessity.

Further, FIG. 3 shows the axial neutron absorption capacity of the output control bank P, where the curve 6 is based on the control bank D and expresses its absorption capacity as 1.0, and the curve 7 is the axial neutron absorption capacity of the output control bank P. This is for the output control bank P used in the simulation of FIG. That is, as is clear from the figure, by using only the control rods and fully inserting the output control bank P, it is possible to respond to rapid output changes and control the output distribution, and the output distribution can be controlled safely and efficiently. It has a quick reactivity control function with little distortion and significantly improves the power control performance and reliability of pressurized water reactors. In this way, according to the present invention, a plurality of control rod assemblies inserted into a reactor core formed by a large number of nuclear fuel assemblies are divided into a control rod assembly group for power control and a control rod assembly group for power distribution control. The control rods of the control rod assembly group for power control can be freely inserted and removed over the entire height of the reactor core, and the neutron absorption capacity is uniform over the entire length, while the control rods for power distribution control The control rods of the control rod assembly group are fully inserted into the reactor core only during partial power operation, and are fully withdrawn outside of the same partial power operation, and the distribution of their neutron absorption capacity is determined from the control rods of the control rod assembly group for power control. Since it is divided into a small upper half and a lower half that is even lower than the upper half, the control rod assembly group for power distribution control can be quickly enclosed and used together with the power control rod to enable partial power operation. At the same time, it is possible to prevent bias in the power distribution due to the temperature drop of light water (occurring in the upper part of the core) during partial power operation, and furthermore, it is possible to reduce fluctuations in the power distribution when this is withdrawn and return to full power (rated) operation. It has the following effects.

Although the present invention has been described above with reference to embodiments, it goes without saying that the present invention is not limited to such embodiments, and that various design modifications can be made without departing from the spirit of the present invention. .

[Brief explanation of drawings]

Figure 1 is a diagram of the main parameters in power control of a pressurized water reactor using only a group of control rod assemblies in which the neutron absorption capacity of the control rods is set equally in the upper and lower axial directions.
Figure 2 is a diagram of the main parameters in power control using a control rod assembly group for power distribution control, Figure 3 is a neutron absorption capacity diagram of the control rod assembly group for power distribution control, and Figure 4 is a pressurized water type atom FIG. 2 is a structural layout diagram of a reactor core and control rods. A, B, C, D: Control rod assembly group (control bank), SA, SB: Control rod assembly group (stop bank),
P: Control rod assembly group for power distribution control (power control bank).

Claims (1)

[Claims] 1 A plurality of control rod assemblies inserted into a reactor core formed by a large number of nuclear fuel assemblies are composed of a group of control rod assemblies for power control and a group of assemblies for power distribution control, and The control rods of the control rod assembly group for control are freely inserted and removed over the entire height of the reactor core, and the neutron absorption capacity is made uniform over the entire length, while the control rods of the control rod assembly group for power distribution are controlled. The rods are fully inserted into the core only during partial power operation, and are fully withdrawn outside of the same partial power operation, and the distribution of their neutron absorption capacity is changed to the upper half of the control rods smaller than the control rods of the control rod assembly group for power control. A pressurized water reactor characterized by being divided into a lower half that is lower than the upper half.