KR100443369B1 - Lower shifted worth control rod for pressurized water reactor - Google Patents

Abstract

In the lower shifted worth control rod (LSWCR), the present invention solves a problem related to a change in the axial output distribution when the output of a nuclear power plant is changed to perform a load follow operation of the nuclear power plant. It is an object of the present invention to provide a lower bias strength control rod for a pressurized water reactor (PWR) that can improve the power generation efficiency and safety of a nuclear power plant.

According to the present invention, in a control rod for controlling the output of a pressurized water reactor including a pellet as a neutron absorbing material, the strength of the pellet (neutron absorbing force) has a distribution of intensity changes such that the strength of the control rod is relatively large from the top of the axis of the control rod. The control rod for output adjustment is divided into a multipurpose control rod and an adjustment control rod, and the slope of the intensity change of the multipurpose control rod is relatively slower than the slope of the intensity change of the control rod.

Description

Lower shifted worth control rod for pressurized water reactor

The present invention relates to a control rod for changing the output of a pressurized water reactor reactor, and in particular, it is easy to change the output of the reactor to improve the operating flexibility of the nuclear power plant, and to control the axial output distribution during the change of the reactor power nuclear power plant It relates to a lower bias strength control rod for a pressurized water reactor that can increase the safety of the.

Nuclear power plants have limited flexibility in operation compared to other sources because of special safety measures related to the safety of the core. Therefore, in the current grid operation in Korea, nuclear power is used only as a base load, and the change in power production according to the load, that is, load follow operation, is driven by other power sources such as thermal power generation. Is performed. However, as the proportion of nuclear power generation increases in power generation, the necessity for the nuclear power plant to follow the load has increased. As a result, the load following operation capability of the nuclear power plant is considered more important. .

In nuclear power plants, changes in reactor core output are caused by changes in reactivity. The two main mechanisms involved in changing the reactivity in a reactor are control rods and boric acid water. The control rod is a means used for rapid change in reactivity, and is in the form of a cylindrical rod in which neutron-absorbing material is distributed in uniform concentration in the axial direction. These rods are tied together to form several groups. And boric acid water control is a means for controlling by using a boric acid type neutron absorbing material dissolved in the coolant to compensate for the low-speed reactivity change. And coolant temperature control is used as an auxiliary means.

In the drawing, FIG. 1 is a front view (a) of the assembly of control rods used for the reactor, and a cross-sectional view (b) of the control rod, FIG. 2 is a graph showing the target setting range of the upper and lower half output deviations of the reactor core, and FIG. 4 is a graph showing a target setting range of the reactor core axial set point AO, and FIG. 4 shows a graph (a) showing the axial set point AO setting range according to the change of the core output, and a change in the output distribution of the core. Graph (b).

As shown in FIG. 1A, the control rod assembly 2 used in the nuclear reactor has a structure in which the control rods 1 are combined as shown in FIG. 1B. In a nuclear power plant, control rod assemblies 2 as shown in FIG. 1A are assembled to form a group, which is called a control rod bank, and control rods belonging to the control rod bank move together. The reactor houses a plurality of control rod banks, among which control rod banks related to power regulation are divided into part strength control rod banks and full strength control rod banks.

In general, the partial steel control rod banks are P1 and P2, and the full steel control rod banks are R1, R2, R3, R4, and R5. Here, P1, P2, R1, R2, R3, R4, and R5 are names for distinguishing the control rod banks. In each control rod of the control rod bank, a space (3) is filled so that pellets of neutron absorbing material are filled. ), Pellets with uniform neutron absorption (hereinafter referred to as 'strength') are uniformly distributed. Therefore, the strength of the control rod 1 does not change its strength along the axial direction.

On the other hand, during the change of output of a nuclear power plant, the reactor core is in a transient state due to the effect of Xen. Xenon is one of the fission products and has a strong absorption of thermal neutrons. Changes in the core's reactivity are accompanied by changes in xenon concentration and axial distribution, which can cause xenon vibrations, and due to the strong absorption of xenon's thermal neutrons, xenon vibrations control the core Incapacitation can cause the reactor to shut down. Therefore, it is important not to generate xenon vibration when changing the reactor output. For this purpose, it is required to keep the axial output distribution within a predetermined predetermined range.

2 and 3, the axial output distribution of the nuclear reactor core can be represented by a variable called an axial offset (AO), which is expressed by Equation 1 below.

,

Here, ΔI is the output deviation of the upper half and the lower half of the core, P _T is the output of the upper half of the core, and P _B is the output of the lower half of the core.

Briefly described, the axial set point AO is the relative output deviation of the upper half of the core and the lower half of the core. The output of the upper and lower cores required to calculate the axial set point (AO) is obtained from the upper and lower nuclear instruments located outside the reactor vessel.

The concept of constant axial offset control (CAOC), the output distribution control strategy of Westinghouse, USA, is within the control range around any axial setpoint (AO) based on the axial setpoint (AO). To keep. At this time, the reference axial set point (target AO) is a value corresponding to the axial output distribution of the most stable state under the existing core situation. That is, under the rated maximum power, xenon is in equilibrium and the control rod is pulled out, and the axial setpoint AO is selected as the reference axial setpoint target AO.

The target setting range at which the axial set point AO is to be maintained is determined as follows. This target setting range should be narrow enough to take advantage of the lower peaking factor and at the same time wide enough to allow the plant operator to easily change the output. The target output deviation range, and therefore the range around the reference axial set point AO, must satisfy both of the above goals simultaneously. The target setting range that is generally selected is ± 5%, as shown in FIG.

The axial set point (AO) is calculated by dividing the output deviation ΔI by the relative output, so the setting range is ± 5% at 100% output, but as the output decreases, the setting range becomes wider. This is the same as shown in Figure 3, Figure 4 shows the output range of the axial set point (AO) setting range and the core according to the output change.

However, the reactivity change using the existing reactivity change mechanism has many difficulties in keeping the axial output distribution within the set range for the following reasons.

As a first reason, the movement of a conventional control rod with uniform strength in the axial direction is accompanied by a change in the axial output distribution as follows. Inserting the control rod in the upper half of the core shifts the output distribution of the reactor core downward (hereinafter referred to as "negative direction"), and withdrawing the control rod moves the output distribution upward (hereinafter referred to as "positive direction"). Move to. In the lower half of the core, the opposite is true. However, the change in the axial setpoint AO must be maintained within the above-described axial setpoint AO setting range. This core characteristic makes it difficult to change the output of the reactor using the control rods and limits the movement of the control rods.

As a second reason, the change in output using the change in boron concentration also affects the axial output distribution. For example, a decrease in output through increasing boron concentration (borate injection) always tends to shift the power distribution upwards. This is a phenomenon due to the negative feedback effect on the reactivity of the coolant temperature. In addition, the boron concentration change is accompanied by a slow reactivity change has a disadvantage that can not expect a fast output change.

The present invention is provided to solve the problems of the prior art as described above, by solving the problems associated with the change in the axial output distribution and the resulting xenon vibration when changing the output of the pressure-water reactor reactor to facilitate the output change The aim is to provide a lower shifted worth control rod (LSWCR) that can improve the economic efficiency and safety of nuclear power plants.

1 is a front view (a) and a cross-sectional view (b) of a control rod assembly of control rods used in a nuclear reactor,

2 is a graph showing a target setting range of the upper and lower half output deviations of the reactor core;

3 is a graph showing a target setting range of the axial set point AO of the reactor core;

4 is a graph (a) showing an axial set value (AO) setting range according to the change of the core output, and a graph (b) showing a change in the output distribution of the core,

FIG. 5 is a graph showing a change in the axial strength of the first multipurpose control rod LSWCR1 among the lower bias strength control rods for a PWR reactor according to one embodiment of the present invention.

FIG. 6 is a graph showing a change in the axial strength of the second multipurpose control rod LSWCR2 among the lower bias strength control rods for a PWR reactor according to one embodiment of the present invention.

7 is a graph showing a change in the axial strength of the third, fourth, fifth adjustment control rods (LSWCR3, LSWCR4, LSWCR5) of the lower bias strength control rods for the PWR reactor according to an embodiment of the present invention,

8 to 11 are provided with first and second multipurpose control rods LSWCR1 and LSWCR2 and third, fourth and fifth adjustable control rods LSWCR3, LSWCR4 and LSWCR5 having the strengths shown in FIGS. The control rod banks are the load tracking operation result graphs displayed under the output change condition of 100-50-100% of the pressurized water path and 2-6-2-14h load pattern.

♠ Explanation of symbols on the main parts of the drawing ♠

1: control rod 2: control rod assembly

3: space part

According to the present invention for achieving the object as described above, in the control rod for controlling the output of a pressurized water reactor reactor containing a pellet that is a neutron absorbing material, the strength of the neutron absorbing force of the pellet in the downward direction from the axis of the control rod There is provided a control rod for output regulation having a distribution of intensity variations to be relatively large.

In addition, the control rod for output adjustment of the present invention is divided into a multi-purpose control rod and a control rod for adjustment, the slope of the intensity change of the multipurpose control rod is relatively gentle than the slope of the intensity change of the control rod for adjustment.

In addition, the multi-purpose control rod of the present invention is a first multi-purpose lower bias strength control rod that raises the change in the axial offset value (AO) from the core output distribution in the upper direction of the core, and the change in the axial set value (AO) It includes a second multi-purpose lower bias strength control rod for moving the core output in the vertical direction of the core from the core output distribution, the slope of the intensity change of the first multi-purpose lower bias strength control rod relative to the slope of the change in strength of the second multipurpose lower bias strength control rod As gentle as.

In addition, the control rod for the control of the present invention is provided with a plurality of third, fourth, fifth adjustment lower biasing strength control rod for generating a reactivity change, the intensity change of the third, fourth, fifth adjustment lower biasing strength control rod The slope is relatively steeper than the slope of the intensity change of the first and second versatile lower bias strength control rods.

In the following, with reference to the accompanying drawings, preferred embodiments of the lower bias strength control rods for a PWR reactor according to the present invention will be described in detail.

5 is a graph showing a change in the axial strength of the first multi-purpose control rod LSWCR1 among the lower bias strength control rods for the PWR reactor according to one embodiment of the present invention, Figure 6 is an embodiment of the present invention 7 is a graph illustrating a change in the axial strength of the second multipurpose control rod LSWCR2 among the lower bias strength control rods for a pressurized water reactor according to an example, and FIG. 7 is a lower bias strength for a pressurized water reactor according to an embodiment of the present invention. A graph showing a change in the axial intensity of the third, fourth, and fifth adjustment control rods LSWCR3, LSWCR4, and LSWCR5 among the control rods, and FIGS. 8 to 11 show the first, second, and third strengths shown in FIGS. The control rod banks with the second versatile control rods LSWCR1 and LSWCR2 and the third, fourth and fifth adjustment control rods LSWCR3, LSWCR4 and LSWCR5 are 100-50-100% of the pressurized water path, 2-6-2-14h. Under load change condition of load pattern Indicating load-following operation are the result graph.

The lower biasing strength control rod for a PWR reactor according to one embodiment of the present invention is intended to compensate for the problems associated with changes in the axial output distribution during output changes.

As shown in Figs. 5 to 7, lower shifted worth control rods (LSWCR1, LSWCR2, LSWCR3, LSWCR4, LSWCR5) are classified into two types. One type is LSWCR1 and LSWCR2, which are 'multi-purpose control rods', and the other type is LSWCR3, LSWCR4 and LSWCR5, which are 'regulating control rods'.

The multipurpose control rods LSWCR1 and LSWCR2 are relatively strong in the axial setpoint AO when the control rod is present at the bottom of the core, that is, in the direction of the longitudinal axis on the graph of FIG. It performs the function of controlling while moving the output of the core to the upward direction, and provides the required reactivity change in place of the change in boron concentration. And, the control rods (LSWCR3, LSWCR4, LSWCR5) performs the function of causing a change in reactivity as in the conventional control rods.

Hereinafter, multi-purpose control rods LSWCR1 and LSWCR2 having the strengths shown in FIGS. 5 and 6 will be described in detail.

The first multipurpose control rod is LSWCR1, and the change in the axial strength is as shown in FIG. 5, and in order to have such a change in strength, the intensity distribution of the pellets accommodated in the middle of the control rod is varied. That is, pellets having a large strength are distributed under the inner space of the control rod, and pellets having a relatively small strength are distributed in the upper direction of the control rod, so that the strength gradually increases from the top of the LSWCR1 to the lower direction. The same applies to other control rods, LSWCR2, LSWCR3, LSWCR4, and LSWCR5.

LSWCR1 moves mainly in the lower half of the core. And the LSWCR1 causes the axial set point AO to approach the upper boundary of the set range at the initial core. The function and purpose of LSWCR1 are as follows.

The role of LSWCR1 in terms of the control of the axial set point AO is to push the axial set point AO in the positive direction, i.e., in the direction of the vertical axis in the graph of FIG. Move to. In other words, the LSWCR1 should be able to mitigate the distortion of the axial set point AO that occurs in the negative direction on the graph of FIG. 4 by the movement of another control rod. LSWCR1 should also cause a change in reactivity instead of a change in boron concentration. In addition, in terms of responsiveness control, LSWCR1 is used as an auxiliary means compared to LSWCR2 and has a narrow moving range. This is because the most important function and purpose of the LSWCR1 is to push the axial set point AO in the positive direction on the graph of FIG. 4 because the LSWCR1 must always be at the bottom of the core.

The second versatile control rod is LSWCR2, where the change in axial strength is shown in FIG. Unlike LSWCR1, LSWCR2 has a movement range over the whole area from the bottom to the top of the core. LSWCR2's function and purpose are as follows.

The LSWCR2 controls the axial setpoint AO in the positive or negative direction on the graph of FIG. 4. Then, the change in the axial setpoint value in the positive or negative direction due to the movement of the other control rod is alleviated. Another important function of the LSWCR2 is to produce a change in reactivity that is required in place of the change in boron concentration to maintain the set output under the condition that the control rods are all pulled out.

Hereinafter, regulating control rods (LSWCR3, LSWCR4, LSWCR5) will be described in detail.

The three adjustment control rods LSWCR3, LSWCR4, and LSWCR5 have the same change in axial intensity as shown in FIG. These control rods perform the same functions as conventional control rods. These regulating rods are drawn out from the initial full power state, and their main function is to cause a change in responsiveness. In addition, this adjustment control rod differs from the conventional control rod since it is inserted past the center of the core, thereby changing the axial setpoint AO in the positive direction on the graph of FIG. 4 relatively strongly compared to the conventional control rod. This is due to the variation of the control rod strength biased downwards.

The driving relationship of the lower biased strength control rods LSWCRs (LSWCR1, LSWCR2, LSWCR3, LSWCR4, LSWCR5) configured as described above will be described in detail.

First, the movement characteristics of the lower biased strength control rods related to the two factors, the reactivity of the core and the axial set point (AO), are analyzed for the development of the driving strategy. Find the movement characteristics.

In the following, the responsiveness and axial set point (AO) change according to the movement position of the lower biasing strength control rods LSWCR1 and LSWCR2 are shown according to the experimental results.

(A) in Table 1 shows the responsiveness of the LSWCR1 at the initial control rod position and the movement characteristic of the axial set point, and (b) shows the reactivity and axial setpoint of the LSWCR1 when the LSWCR1 is positioned above the initial control rod position. (C) is a table showing the responsiveness and movement characteristics of the axial set point when the LSWCR1 is located below the initial control rod position.

On the other hand, (a) in Table 2 below shows the responsiveness of the LSWCR2 at the initial control rod position and the movement characteristics of the axial set point, (b) is a response when the LSWCR2 is located above the initial control rod position and (C) is a table showing the responsiveness and movement characteristics of the axial set point when the LSWCR2 is located below the initial control rod position.

Second, based on the above analysis results, the driving strategy of the lower biased strength control rod is developed. The rules reflected in the development of driving strategy are as follows.

One). LSWCR1 mainly moves below the reactor core, and in terms of reactivity control, LSWCR1 is used as an auxiliary means compared to LSWCR2 and has a narrow moving range. This is because the most important use of the LSWCR1 is to push the axial set point (AO) in the positive direction, for which the LSWCR1 must always be at the bottom of the core.

2). The LSWCR2, unlike LSWCR1, moves across the core.

Thirdly, using the operation strategy developed as described above, it is applied to change the output of the pressure ramp of the lower bias strength control rod. As an example, 100-50-100% (pattern shown in Figure 4) based on the power demand of the Republic of Korea, was applied to the load pattern of 2-6-2-14hour.

Here, the load pattern of 100-50-100%, 2-6-2-14hour will be described briefly. The load pattern is a load pattern that lowers the core output from 100% to 50% for 2 hours, maintains 50% for 6 hours, then rises from 50% to 100% for 2 hours and maintains 100% for 14 hours. Say

The results of applying such a load pattern are shown in FIGS. 8 to 11.

8 shows the change in relative output of the core over time. It can be seen that the core output closely matches the reference target output. And, as shown in Figure 9, the boron concentration change is fixed to its initial state value and does not change with time. 10 shows the movement of the lower bias intensity control rod during the output change, and the change in the axial set value AO over time is shown in FIG.

Thus, it can be seen that during the output change, the axial setpoint AO is well maintained within the target setting range by the lower biased strength control rod. The above experimental results show that the lower bias strength control rod has good characteristics to control the axial setpoint (AO), and the proper use of the versatile lower bias strength control rod is used to change the axial setpoint (AO) during the output change of the pressure ramp. It shows that it is possible to control within the setting range. In addition, it can be seen from the above results that the output can be changed without reactivity compensation due to the change in boron concentration, and as a result, it is possible to minimize the change in boron concentration.

As described in detail above, the lower biased strength control rod for the PWR reactor has the characteristics to control the axial set point (AO) of the PWR, so that the change in the axial output distribution when the output of the nuclear power plant changes. By alleviating the related problems, the load follow operation of the nuclear power plant is facilitated, thereby improving the economic efficiency and safety of the nuclear power plant.

In addition, it is possible to perform output change without using reactivity change by changing boron concentration when changing output of nuclear power plant, so that problems such as generation of large amount of liquid waste by using boron and corrosion of equipment by boric acid can be overcome. There is an advantage that it can.

Although the technical idea of the lower biased strength control rod for the PWR reactor of the present invention has been described above with the accompanying drawings, this is illustrative of the best embodiment of the present invention and is not intended to limit the present invention. In addition, it is obvious that any person skilled in the art can make various modifications and imitations without departing from the scope of the technical idea of the present invention.

Claims (4)

In the control rod for controlling the output of a pressurized water reactor containing pellets, which are neutron absorbing materials, The control rod for output regulation, characterized in that the intensity change distribution so that the intensity of the neutron absorption of the pellet is relatively large in the downward direction from the upper axis of the control rod. The method of claim 1, The control rod for output control is divided into a multi-purpose control rod and a control rod for adjustment, the output control rod, characterized in that the intensity change slope of the multi-purpose control rod is relatively gentle than the slope of the intensity change of the control rod. The method of claim 2, The multipurpose control rod includes a first multipurpose lower bias strength control rod that raises the change in the axial offset value in a positive direction, that is, from the core's output distribution point of view to an upper direction of the core, and the axial setpoint (AO). A second versatile lower bias strength control rod which shifts the positive and negative directions in a positive and negative direction, ie from the core's output distribution point of view; The intensity change slope of the first multipurpose lower bias strength control rod is relatively gentler than the slope of the intensity change of the second versatile lower bias strength control rod. The method of claim 2 or 3, The control rod for adjustment includes a plurality of third, fourth, and fifth adjustment lower bias strength control rods for generating a change in reactivity, the slope of the intensity change of the third, fourth, fifth adjustment lower bias strength control rod is the first , The control rod for output adjustment, characterized in that the second versatile lower bias strength than the change in the slope of the strength change.