Core control method

Abstract

To provide a reactor core control method that can increase an output adjustment range in daily output adjustment operation suitable for a boiling-water reactor.SOLUTION: A method for controlling a reactor core in a nuclear power plant executing daily load following operation includes: a first step of evaluating a change in the state of the reactor core due to the daily load following operation; a second step of determining excess of the limit of a constraint parameter of the reactor core during restoration from a low-output state to a high-output state and after the restoration; and a third step of setting the amount of control rod operation in a time zone of the low-output state or in a time zone during the restoration to the high-output state.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to a method for controlling a reactor core, and more particularly to a method for controlling a reactor core that is suitable for application to a boiling water reactor. [Background technology]

[0002] The core of a boiling water reactor is loaded with multiple fuel assemblies. Each fuel assembly consists of multiple fuel rods, each containing multiple fuel pellets made of nuclear fuel material (e.g., uranium oxide), an upper tie plate (upper fuel support member) supporting the upper ends of the fuel rods, a lower tie plate (lower fuel support member) supporting the lower ends of the fuel rods, multiple fuel spacers maintaining the spacing between the fuel rods, water rods, and a rectangular channel box with a square cross-section. Nuclear power generation has been recognized as a base-load power source, particularly in Japan, because the nuclear fuel cycle cost accounts for a small proportion of the total power generation cost.

[0003] Even in domestic nuclear power plants, which have previously been recognized as base-load power sources, the large-scale introduction of renewable energy (especially solar power) in recent years has led to a growing demand for their ability to adjust loads through daily load-following operation. One method of daily load-following operation in nuclear power plants is thermal output control, which involves changing the thermal output of the reactor core.

[0004] One of the major challenges in thermal output control is, 135 There is a change in the concentration of Xe. 135 Since xenon is a strong neutron absorber, criticality control needs to be performed in accordance with changes in its concentration. 135 At rated power, Xe production and annihilation are balanced, resulting in an equilibrium state (a constant number density). However, as the thermal output is reduced, the contribution of annihilation decreases, and the concentration rises. Depending on the reactor core, this can occur in approximately 6 hours. 135 The number density of Xe peaks and then begins to decrease. 135The time when the Xe number density peaks (hereinafter referred to as the Xe peak time) is relatively close to the time order of sunlight hours when solar power generation is actively carried out. Therefore, when considering maintaining a low output state in a nuclear power plant for power supply and demand adjustment while solar power generation is in progress, when returning to the rated output, it is very likely that 135 Xe is in an accumulated state. Although it is necessary to maintain criticality when returning to the rated output, 135 due to the accumulated Xe, a positive reactivity has to be inserted compared to the rated operation state. Also, if the rated output is maintained thereafter, 135 the contribution of Xe disappearance increases and conversely 135 the Xe number density decreases and a negative reactivity has to be inserted. Thus, even in the same rated output state, due to 135 the change in Xe concentration due to daily load following, significant criticality control is required. Note that, generally, the control method called "load following" is gradually changing to an environment where operators actively control the output for market entry of power supply and demand adjustment. Hereinafter, it will be referred to as "output adjustment".

[0005] As methods for this criticality control, there are operations of the insertion amount of control rods, core flow rate control by a recirculation pump, feedwater temperature control by adjusting the feedwater heating amount, etc. Control rod operation locally inserts reactivity and causes a large change in the output distribution, so it is generally not used in the high output region (for example, the rated output). On the other hand, flow rate control and feedwater temperature control insert reactivity throughout the core, so the change in the output distribution is relatively small and it can be operated even in the high output region. However, there are limitations such as the P-F map (output flow map) and MCPR (Minimum Critical Power Ratio) for the flow rate control amount, and limitations from the perspective of thermal fatigue of the feedwater nozzle for the feedwater temperature control amount. When the output adjustment range is small, etc., 135 when the change in Xe concentration is small, criticality is basically controlled by flow rate control, but 135When the Xe concentration changes significantly, it can exceed the flow rate limits in the PF map (output flow rate map), especially after returning to high power (e.g., rated power). Therefore, it becomes necessary to use control rod control and feedwater temperature control in conjunction with these constraints. Note that feedwater temperature control may need to be newly implemented in some nuclear power plants.

[0006] Regarding the combined use of flow rate control and control rod operation in output adjustment, there is a technique described in Patent Document 1. In Patent Document 1, the flow rate value (at which the critical point is reached) is shifted by control rod operation before and after output adjustment, reducing the amount of output adjustment by control rod operation and increasing the amount of output adjustment by flow rate control. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2-242198 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the technology described in Patent Document 1 cannot suppress the exceeding of the limits of constraint parameters such as the PF map (output flow rate map) after high output recovery. In other words, there is a risk that the output adjustment range will be limited.

[0009] Therefore, the present invention aims to provide a core control method that can expand the output adjustment range in daily output adjustment operation suitable for boiling water reactors. [Means for solving the problem]

[0010] To solve the above problems, the core control method according to the present invention is a core control method in a nuclear power plant that performs daily load following operation, and includes a first step of evaluating changes in the core state due to daily load following operation, a second step of determining whether the constraint parameters of the core exceed the limit during and after the return from the low output state to the high output state, and a third step of setting the control rod operation amount during the time period of the low output state or during the return to the high output state. The method further comprises, between the second and third steps, a fourth step of determining the feedwater temperature control capability of the nuclear power plant, and a fifth step of setting the feedwater temperature control amount. It is characterized by this.

Effect of the Invention

[0011] According to the present invention, it becomes possible to provide a core control method capable of expanding the output adjustment range in the daily output adjustment operation suitable for a boiling water reactor. For example, in the daily output adjustment operation, even if the output adjustment range is expanded, 135 it is possible to suppress exceeding various constraints of constraint parameters such as the P-F map (output flow map) after returning to high power in critical control accompanying the change in Xe concentration. Problems, configurations, and effects other than those described above will be clarified by the description of the following embodiments.

Brief Description of the Drawings

[0012] [Figure 1] It is a diagram showing the time change of the nuclear reactor thermal output when the low output time is relatively long. [Figure 2] It is a diagram showing the time change of the core flow rate of the cooling water for the case of FIG. 1. [Figure 3] It is a diagram showing the time change of the nuclear reactor thermal output when the low output time is relatively short. [Figure 4] It is a diagram showing the time change of the core flow rate of the cooling water for the case of FIG. 3. [Figure 5] It is a diagram showing the control rod insertion operation when exceeding the flow rate lower limit. [Figure 6] [[ID=3S]]It is a diagram showing the influence on the flow rate by the control rod insertion operation. [Figure 7] It is a diagram showing the control rod withdrawal operation when exceeding the flow rate upper limit. [Figure 8] This figure shows the effect of control rod withdrawal on flow rate. [Figure 9] This figure shows the time change in reactor thermal output when utilizing feedwater temperature control. [Figure 10] This diagram shows the effect on flow rate when using water supply temperature control. [Figure 11] This is a flowchart of the core control method according to this embodiment. [Figure 12] This is a 1 / 4 configuration diagram of a nuclear reactor according to Embodiment 1 of the present invention. [Figure 13] This figure shows the time variation of the required electrical output according to Example 1. [Figure 14] This is a flowchart of the core control method according to Example 1. [Figure 15] This figure shows the time variation of the required electrical output according to Embodiment 2 of the present invention. [Figure 16] This is a flowchart of the core control method according to Example 2. [Figure 17] This is a flowchart of the core control method according to Embodiment 3 of the present invention. [Modes for carrying out the invention]

[0013] The inventors have conducted various studies and, in a daily output adjustment operation with an expanded output adjustment range, have utilized each control method, 135 We have found a control method that can suppress the exceeding of limits for various constraint parameters, such as the PF map (power flow map), after returning to high power, even when implementing criticality control in response to changes in Xe concentration. The results of this study and an overview of the newly discovered control method are described below. In the following, we will describe a typical boiling water reactor core in the first half of the operating cycle, when control rods have been inserted into the core, as an example. However, it is also applicable to cores in the second half of the operating cycle.

[0014] First, let's consider the case of flow control as a base. Figure 1 shows the time change of reactor thermal output when the duration of the low-power state (hereinafter referred to as low-power time) is relatively long compared to the Xe peak time, and Figure 2 shows the time change of the core flow rate of the cooling water for the case in Figure 1. As shown in Figure 2, when the low-power time is long, the constraint of the lower limit of the flow rate at rated output is exceeded. This is because when the low-power time is long... 135 Xe is the parent nuclide 135 Insufficient generation of I occurs, after high power return 135 Since the Xe concentration decreases significantly compared to the steady state, criticality is maintained by drastically reducing the flow rate. Note that the change in flow rate from 14h to 28h in Figure 2 is due to low power. 135 As the amount (concentration) of Xe increases, the flow rate for flow control is increased. 135 The Xe concentration begins to decrease. Therefore, the flow rate is reduced for flow control.

[0015] Next, let's consider the case where the upper limit of the flow rate at rated output is exceeded. Figure 3 shows the time change of reactor thermal output when the low-power time is shorter than in Figure 1, and Figure 4 shows the time change of the core flow rate of the cooling water for the case in Figure 3. As shown in Figure 4, it can be seen that the upper limit of the flow rate constraint is exceeded. This is because during low-power operation 135 Because the Xe concentration returned to rated output before it began to decrease, immediately after the return... 135 The Xe concentration has increased significantly compared to the steady state, and maintaining criticality requires a substantial increase in flow rate. To suppress exceeding the above flow rate limit, criticality control can be considered using control rod manipulation, but the challenge is that control rod manipulation is generally not applicable in the high-power range.

[0016] Therefore, we considered manipulating the control rod position in the low-power range. When the flow rate limit at high power is exceeded on only one side (e.g., the upper flow rate limit), there is often sufficient margin up to the limit on the other side (e.g., the lower flow rate limit). Thus, we thought that if the flow rate limit is exceeded on one side, the control rod position should be changed in the low-power range to utilize the margin up to the limit on the other side. Specifically, for the thermal output adjustment pattern in Figure 1, where the low-power time is long, the flow rate will exceed the lower limit, so as shown in Figure 5, the control rod insertion operation is performed during low-power operation. Figure 6 shows the change in flow rate due to this control rod insertion operation, and it can be seen that after the control rod operation, the flow rate shifts to the high-flow rate side, and the flow rate after returning to rated output falls within the range between the upper and lower flow rate limits (hereinafter referred to as the flow rate limit range). For the thermal output adjustment pattern in Figure 3, where the low-power time is short, the flow rate will exceed the upper limit as shown in Figure 4, so as shown in Figure 7, the control rod withdrawal operation is performed during low-power operation. Figure 8 shows the change in flow rate due to the control rod withdrawal operation. It can be seen that after the control rod operation, the flow rate shifts to the lower flow rate side, and the flow rate after returning to rated output falls within the flow rate limit range. As described above, if the flow rate limit is exceeded in advance through analysis, etc., the flow rate can be shifted and the limit exceedance can be suppressed by operating the control rod in the low output region. Furthermore, in this control system, the control rod position after returning to rated output is different from that before output adjustment. If it is necessary to return to the original control rod position, it is conceivable to do so during a low-power state, when there is sufficient margin for various core constraint parameters, such as when the output adjustment range is small. Also, in this control system, 135 If the change in flow rate due to a change in Xe concentration exceeds the flow rate limit, the excess cannot be suppressed by shifting the flow rate with control rods. In this case, it is necessary to use another criticality control method in combination to reduce the amount of flow rate change. One such method is feedwater temperature control.

[0017] One specific method for controlling feedwater temperature is to control the amount of steam extracted from the turbine to the feedwater heater using a butterfly valve or the like, thereby changing the heating amount and controlling the feedwater temperature. This method can change the void fraction of the reactor core and can therefore be used as a criticality control method. In feedwater temperature control, if the temperature change is large, it is necessary to check for thermal fatigue of the feedwater nozzle (welded part) due to repeated changes in feedwater temperature. In actual operation, from the perspective of thermal fatigue, it is considered that the maximum allowable temperature change should be evaluated in advance and the feedwater temperature control capability should be limited to keep the control within that range. Figure 9 shows the time change of reactor thermal output when feedwater temperature control is utilized, and Figure 10 shows the effect on flow rate when feedwater temperature control is utilized. In Figure 10, the left vertical axis shows the flow rate, and the right vertical axis shows the change in feedwater temperature when feedwater temperature control is used in combination. As shown in Figure 10, when only flow rate control (shown by the dotted line) is used, the flow rate will exceed the upper limit. However, when feedwater temperature control (shown by the solid line) is used in combination, the flow rate can be prevented from exceeding the upper limit by controlling the feedwater temperature to decrease during the time period when the flow rate would otherwise exceed the upper limit. Since feedwater temperature control has little impact on the output distribution, it can be operated not only in low-output states (including during low-output operation and transitions to other output states) but also in high-output states. Furthermore, if the flow rate exceeds the lower limit, the feedwater temperature will be controlled to increase. In the above explanation of control rod operation and water supply temperature control, the example of returning to rated output was used, but it is not limited to rated output.

[0018] As described above, by using control rod control and feedwater temperature control in addition to flow rate control, the output adjustment range can be increased, etc. 135 Even with large changes in Xe concentration, criticality can be maintained while satisfying various constraints of the PF map (power flow map). Figure 11 shows a flowchart of the specific core control method according to this embodiment, which uses these control methods in combination. In the following, the case of the PF map (power flow map) is shown as an example of constraint parameters. As shown in Figure 11, flow rate control is initiated in step S11, and then in step S12, the core state changes due to power adjustment are checked to confirm whether the constraints have been exceeded. For this confirmation, evaluation of Xe transient analysis and dynamic characteristics analysis using a core analysis code is possible (step 1). After this evaluation, in step S13, the flow rate limit is determined to be exceeded in the PF map (power flow rate map) when returning to the high power state and after returning, based on the time change of the flow rate, which is the output result (step 2). If there is no excess, the process proceeds to step S16 to select the control method (power adjustment is performed). If there is an excess, the feedwater temperature control capability of the nuclear power plant is determined in step S14 (step 3). If the feedwater temperature control capability is determined, the process proceeds to step S15 to provisionally set the feedwater temperature control amount (step 4). Specifically in step S15 (step 4), if the flow rate limit is exceeded, the amount of feedwater temperature reduction is set, and if the flow rate limit is exceeded, the amount of increase is set. Then, the process returns to step S12 (step 1) again to evaluate the change in the state of the reactor core due to power adjustment. On the other hand, if the determination in step S14 (process 3) indicates that the reactor does not have feedwater temperature control capability, the process proceeds to step S17, where the control rod operation amount during the low-power state is set (process 5). Specifically in step S17 (process 5), if the flow rate exceeds the upper limit, the control rod withdrawal amount is set, and if the flow rate exceeds the lower limit, the control rod insertion amount is set. Then, the process returns to step S12 (process 1) to evaluate the change in the reactor core state due to power adjustment. Note that in the case of a nuclear power plant that does not have feedwater temperature control capability, steps S14 (process 3) and S15 (process 4) in the reactor core control method flow may not be present. In this flow, if the criticality control associated with the thermal power adjustment of the target reactor exceeds the capacity of flow rate shift and feedwater temperature control by control rod operation, an infinite loop will occur. Therefore, it is necessary to set an upper limit on the number of repetitions of the same process, and if that upper limit is reached, it is determined that the target thermal power adjustment cannot be dealt with. In that case, it would be necessary to review the target thermal output adjustment or consider utilizing other facilities and functions of the nuclear power plant to achieve the target change in electrical output. Examples of other facilities and functions include the use of turbine bypass functions and hydrogen production facilities attached to the nuclear power plant.

[0019] The above judgment flow focuses on exceeding the limits of the PF map (power flow map), which can be a particularly critical constraint parameter in daily power adjustment. However, the judgment flow can also be applied to exceeding the limits of other constraint parameters in core power adjustment, such as MCPR (Minimum Critical Power Ratio), PCIOMR (Pre-Conditioning Interim Operating Management Recommendation), SDRs (Soft Duty Rules), and MLHGR (Maximum Linear Heat Generation Rate). Note that flow control, control rod control, and feedwater temperature control do not need to be controlled individually, but can be controlled simultaneously and used in combination. In actual operation, although the operation of each control is performed individually (for example, flow control and control rod control are performed individually), it is conceivable to use them simultaneously for criticality control at the same time. Furthermore, in step S17 (step 5) described above, the control rod position may be returned to the steady-state position or the initial position before the daily load-following operation.

[0020] In the following, an embodiment of the present invention will be described using drawings, with an Advanced Boiling Water Reactor (ABWR) as an example, but it is not limited to this. Needless to say, it can also be applied to a conventional boiling water reactor (BWR) equipped with a recirculation pump that circulates the cooling water as a moderator by passing it out of the reactor pressure vessel and then flowing it back into the downcomer inside the reactor pressure vessel. Furthermore, in the following section, an embodiment of the present invention will be described using drawings as an example of a constraint parameter, PF map (output flow rate map). [Examples]

[0021] A preferred embodiment of the present invention, a core control method applicable to an improved boiling water reactor (ABWR), will be described with reference to Figures 12 to 14. The nuclear power plant in question is assumed to have a feedwater temperature control function. Furthermore, changes in the electrical output required by the power grid will be addressed by adjusting the core's thermal output.

[0022] Figure 12 is a quarter-configuration diagram of the reactor according to this embodiment. The numbers within the cells in Figure 12 indicate the number of cycles elapsed at the end of the fuel cycle. Region 1 in Figure 12 is the region where control rods are inserted. For example, it is the control cell. In Figure 12, control rods are inserted up to 10, where 0 is fully inserted and 48 is fully withdrawn. Figure 13 is a diagram showing the time variation of the required electrical output according to Embodiment 1. As shown in Figure 13, the time variation of the required electrical output is such that the low-power period is long, and if thermal output is adjusted in the core, the constraint will be exceeded at the lower limit of the flow rate.

[0023] Figure 14 is a flowchart of the core control method according to this embodiment. As shown in Figure 14, flow rate control is disclosed in step S21, and then in step S22, the change in the state of the core due to power adjustment is evaluated (step 7). Following this evaluation, in step S23, the flow rate limit is determined from the time-dependent change in the output flow rate to determine whether the flow rate limit is exceeded in the PF map (output flow rate map) when returning to the high-output state and after returning (step 8). The determination confirms that the flow rate limit is exceeded at the lower limit of the flow rate. In step S24, since it is confirmed in step 9 that there is feedwater temperature control capability (step 9), the process proceeds to step S25, and the feedwater temperature control amount is set (step 10). Specifically, the feedwater temperature rise amount is provisionally set. Note that the feedwater temperature rise amount is set by, for example, trial and error beforehand to determine the set value of the feedwater temperature rise amount. Subsequently, the core state is updated, and the process returns to step S22 (process 7), where the excess of the flow limit in the PF map (power flow map) is checked again in step S23 (process 8). This flow is repeated, and if the feedwater temperature control amount exceeds the temperature change limit determined from the perspective of thermal fatigue of the feedwater nozzle, the control rod operation amount in the low power state is set in step S27 (process 11). Specifically, the insertion amount of the control rods in region 1 is set. After that, the core state is updated, and the process returns to step 22 (process 7), where the excess of the limit is checked again in step 23 (process 8). This flow is repeated until the excess of the limit is eliminated. If the excess of the limit is not eliminated up to the upper limit of the number of iterations, it is determined that the core's thermal output adjustment cannot address the issue. The upper limit of the number of iterations is determined in advance, for example, by trial and error.

[0024] As described above, this embodiment makes it possible to provide a core control method that can expand the output adjustment range in daily output adjustment operation suitable for boiling water reactors. For example, in daily power output adjustment operation, even if the power adjustment range is increased, 135 Critical control in response to changes in Xe concentration can suppress the exceeding of various constraints on constraint parameters such as the PF map (output flow rate map) after returning to high output. [Examples]

[0025] A preferred embodiment of the present invention, a core control method according to this embodiment applied to an improved boiling water reactor (ABWR), will be described with reference to Figures 15 and 16. This embodiment differs from Embodiment 1 described above in that the target nuclear power plant does not have a feedwater temperature control function, and the time variation of the electrical output required by the system (Figure 15) has a short low-power period, and the thermal output adjustment of the core exceeds the constraint at the upper limit of the flow rate. Other aspects of the nuclear power plant configuration are the same as in Embodiment 1.

[0026] Figure 15 shows the time variation of the required electrical output according to this embodiment, and Figure 16 is a flowchart of the core control method according to this embodiment. In Figure 16, since there is no feedwater temperature control capability in the nuclear power plant, there is no process of determining the feedwater temperature control capability and setting the feedwater temperature control amount, compared to Figure 14 of Embodiment 1. The difference in determination is that, since these processes are absent, the only specific control amount to be set is the control rod control amount.

[0027] As described above, according to this embodiment, even without a feedwater temperature control function, the output adjustment range can be expanded by operating the control rods in a nuclear power plant. [Examples]

[0028] A preferred embodiment of the present invention, a core control method according to this embodiment applied to an improved boiling water reactor (ABWR), will be explained with reference to Figure 17. This embodiment differs from Embodiment 1 described above in that the time change of the electrical output required by the grid is not known in advance at the start of the output adjustment operation (start of output reduction). Specifically, for example, in a low-output operation state, when an output increase command is received, it is necessary to increase the output to the target output within a few tens of minutes. This assumes the case of entering the current domestic supply and demand adjustment market and supplying adjustment capacity. If an increase adjustment capacity for a target time period is agreed upon through market trading, it is not known when an increase command will be received or not during that time period, and output adjustment must be made within the response time when a command is received. The configuration of the nuclear power plant and the presence or absence of a feedwater temperature control function are the same as in Embodiment 1.

[0029] When such output adjustments are required, the thermal output adjustment of the reactor is 135 Due to the influence of Xe, there may be cases where the output cannot be increased to the target level, that is, where the agreed-upon adjustment power cannot be provided. Figure 17 shows a description of the flow of the core control method according to this embodiment. The difference between Figure 14 in the above-described embodiment 1 and Figure 17 in this embodiment is that in step S42, among the possible output adjustment patterns, 135The addition of step 12 is setting the pattern that maximizes the change in Xe concentration. 135 By considering the pattern that maximizes the change in Xe concentration and securing adjustment power in the supply and demand adjustment market, it is possible to prevent being unable to increase output to the target level when an output increase command is received. The above example was for an output increase command, but it can be applied similarly to an output decrease command.

[0030] As described above, this embodiment allows for an expansion of the output adjustment range even when entering the supply and demand adjustment market.

[0031] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are included. For example, the embodiments described above are described in detail to make the present invention easier to understand, and are not necessarily limited to those having all the configurations described. Furthermore, it is possible to replace parts of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. [Explanation of Symbols]

[0032] 1…The region where control rods are inserted to perform critical control (control cell)

Claims

1. A method for controlling the reactor core in a nuclear power plant that performs daily load-following operation, The first step is to evaluate the changes in the state of the reactor core due to daily load-following operation, A second step involves determining whether the core constraint parameters exceed their limits during and after the transition from a low-power state to a high-power state, The process includes a third step of setting the amount of control rod operation during the period of low output state or the period of return to high output state, Between the second step and the third step, A fourth step of determining the feedwater temperature control capability of the nuclear power plant, A method for controlling a reactor core, comprising a fifth step of setting a feedwater temperature control amount.

2. In the core control method according to claim 1, A method for controlling a reactor core, further comprising a sixth step, prior to the first step, of setting, by control command, a pattern among the possible output adjustment patterns that maximizes the change in the concentration of 135 Xe.

3. In the core control method according to claim 1 or claim 2, A method for controlling a reactor core, characterized in that the core constraint parameters are an output-flow map that limits the control range of the core's output and flow rate.

4. In the core control method according to Claim 3, In the second step, if the flow rate exceeds the upper limit of the output flow rate map, in the third step, the amount of the control rod to be withdrawn is set. A method for controlling a reactor core, characterized in that, in the second step, if the flow rate exceeds the lower limit of the output flow rate map, the amount of control rods inserted is set in the third step.

5. In the core control method according to claim 3, In the second step, if the flow rate exceeds the upper limit of the output flow rate map, in the fifth step, set the amount of decrease in the water supply temperature. A method for controlling a reactor core, characterized in that, in the second step, if the flow rate exceeds the lower limit of the output flow rate map, the amount of increase in feedwater temperature is set in the fifth step.

6. In the core control method according to claim 1, A method for controlling a reactor core, characterized in that the core constraint parameters are parameters related to one of the following: minimum limiting power ratio, maximum linear power density, PCIOMR, and SDRs.

7. In the core control method according to Claim 1, A method for controlling a reactor core, characterized in that, in the third step, the control rod position is returned to the steady-state position or the initial position before the implementation of daily load-following operation.

8. In the core control method according to Claim 1, A method for controlling a reactor core, characterized in that one of the above steps has an upper limit on the number of repetitions.

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