Core control method

Abstract

To provide a control method of a core capable of suppressing increase in a peak of an axial output distribution within a fixed range even when critical control accompanying concentration change in a 135Xe is performed especially at the time of returning to a high output (for example, a rated output) and thereafter in daily load following operation.SOLUTION: A core control method determines a critical control method of a core in a nuclear power plant that performs daily load following operation, and includes the steps of: evaluating a xenon concentration and equilibrium xenon concentration in daily load following; comparing the xenon concentration N1 with the equilibrium xenon concentration N2; determining a position of a peak in an axial output distribution of the core; and determining critical control means by which a magnitude of the peak becomes smaller.SELECTED DRAWING: Figure 6

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 containing multiple fuel rods, which are bundles of fuel pellets containing nuclear fuel material (e.g., uranium oxide). In order to operate the reactor at its rated power, the core must be kept critical. Methods for controlling the criticality of the core include, for example, control rod control, which adjusts the insertion amount of control rods; core coolant flow control, which adjusts the coolant flow rate using a recirculation pump; and feedwater temperature control, which adjusts the feedwater to the boiler or reactor by heating the feedwater using extracted steam from a steam turbine.

[0003] Even in domestic nuclear power plants, which have previously been recognized as base-load power sources (power sources with low generation costs and that can operate continuously and stably regardless of the time of day, such as day or night), the large-scale introduction of renewable energy in recent years has led to a growing demand for their function as a balancing force through daily load-following operation. One method of load-following operation in nuclear power plants is thermal output control, which changes the thermal output of the reactor core. For boiling water reactors, the flow control described above is generally used in the range of approximately 60-100% of the thermal output, and the control rod control described above is used in the range of less than approximately 60% of the thermal output.

[0004] One of the major challenges in controlling thermal output is, 135 There is a change in the concentration of Xe. 135 Xe is produced by the nuclear fission of nuclear fuel materials and is a strong absorber of neutrons that cause nuclear fission reactions. Therefore, criticality control must be performed in accordance with changes in its concentration. 135 For example, 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 takes approximately 6 to 9 hours. 135The number density of Xe decreases after reaching its peak. This 135 The time order of the peak number density of Xe is relatively close to the time order of sunlight hours when solar power generation is actively carried out. Therefore, when considering low-power operation of a nuclear power plant for power supply-demand adjustment during solar power generation, 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 must be inserted compared to the rated operation state. Also, after maintaining the rated output, 135 The contribution of Xe disappearance increases, and conversely 135 The number density of Xe decreases, and a negative reactivity needs to be inserted. Thus, even in the same rated output state, due to load following 135 With the change in the concentration of Xe, significant criticality control is required.

[0005] 135 Regarding the criticality control of the reactor core associated with the change in the concentration of Xe, there is a technique described in Patent Document 1 (Japanese Patent Laid-Open No. 9-304586). The boiling water nuclear power plant described in Patent Document 1 controls the temperature rise by a feedwater heater (feedwater temperature control) for 135 the change in the concentration of Xe during daily load following operation, and controls criticality. When the feedwater temperature is changed, mainly by changing the average void fraction in the reactor core, the inserted reactivity can be controlled. According to Patent Document 1, it is said that a boiling water nuclear power plant capable of appropriately performing daily load following operation without changing the thermal efficiency of the turbine can be obtained.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] In the aforementioned Patent Document 1, in daily load-following operation, the amount of steam supplied to the turbine and the amount of extracted steam are controlled so as to not fluctuate the thermal efficiency of the turbine. 135 It is stated that the system can appropriately perform daily load-following operation in response to changes in Xe concentration. On the other hand, Patent Document 1 does not mention changes in the axial power distribution of the reactor core due to feedwater temperature control, i.e., criticality control itself. In particular, changes in the axial power distribution when returning to high power (e.g., rated power) and thereafter may conflict with constraints such as the maximum linear power density of the plant. Furthermore, envelope deviations on PCIOMR (Pre-Conditioning Interim Operating Management Recommendation) and threshold deviations on SDRs (Soft Duty Rules) should also be considered.

[0008] In view of the above circumstances, the object of the present invention is to provide a reactor core control method that suppresses the peak increase in the axial power distribution during daily load-following operation. [Means for solving the problem]

[0009] One aspect of the present invention that achieves the above objective is a method for determining a method for controlling the criticality of a reactor core in a nuclear power plant that performs daily load following operation, wherein the xenon concentration in daily load following N 1 and equilibrium xenon concentration N The core control method is characterized by comprising the steps of: evaluating 2; comparing xenon concentration N1 and equilibrium xenon concentration N2; determining the position of the peak in the axial power distribution of the core; and determining a criticality control means that reduces the magnitude of the peak. A more specific configuration of the present invention is described in the claims. [Effects of the Invention]

[0010] According to the present invention, in daily load-following operation, 135Even if critical control is carried out in accordance with the change in the concentration of Xe, it is possible to suppress, within a certain range, the increase in the peak of the axial power distribution particularly at the time of return to high power (for example, rated power) and thereafter. Problems, configurations, and effects other than those described above will be clarified by the description of the following embodiments. Problems, configurations, and effects other than those described above will be clarified by the description of the following embodiments.

Brief Description of the Drawings

[0011] [Figure 1] Graph showing the time change of the reactor thermal power [Figure 2A] Is a graph showing the axial power distribution of the core due to the change in flow rate and feed water temperature at the initial stage of the cycle (control in time zone 1) [Figure 2B] Is a graph showing the axial power distribution of the core due to the change in flow rate and feed water temperature at the initial stage of the cycle (control in time zone 2) [Figure 3A] Is a graph showing the axial power distribution of the core due to the change in flow rate and feed water temperature at the end stage of the cycle (control in time zone 1) [Figure 3B] Is a graph showing the axial power distribution of the core due to the change in flow rate and feed water temperature at the end stage of the cycle (control in time zone 2) [Figure 4] Table showing a control method for suppressing the increase in the peak of the axial power distribution in FIGS. 2A, 2B, 3A, and 3B [Figure 5] Graph showing the time change of the reactor thermal power of Example 1 [Figure 6] Flow chart showing the determination procedure of the critical control method of the core in time zones 3 and 4 of FIG. 5 [Figure 7] Graph showing the time change of the reactor thermal power of Example 2 [Figure 8] Flow chart showing the determination procedure of the critical control method of the core in time zones 9 and 10 of FIG. 7 [Figure 9] Graph showing the time change of the reactor thermal power of Example 3 [Figure 10] Flow chart showing the determination procedure of the critical control method of the core in time zones 15 and 16 of FIG. 9 [Modes for carrying out the invention]

[0012] The inventors have conducted various studies and found that, regarding daily load-following operation, particularly during the period when high output (e.g., rated output) is restored and in the period thereafter, 135 We have discovered a control method that can suppress the increase in the peak of the axial output distribution within a certain range, 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.

[0013] As mentioned above, 135 Since criticality control due to changes in Xe concentration is thought to affect the axial power distribution, and given constraints such as maximum linear power density, it is necessary to suppress the peak increase within a certain range, especially during the return to high power (e.g., rated power) and the subsequent period. Here, flow rate control and feedwater temperature control are more easily applicable to continuous criticality control of the reactor core than control rod control. Hereafter, control rod control, flow rate control, and feedwater temperature control, especially during the return to high power (e.g., rated power) and the subsequent period, refer to high power 135 This means changing the state of the reactor core from the control rod position, flow rate, and feedwater temperature in the Xe equilibrium state.

[0014] Regarding flow rate control, for example, when the flow rate is increased to introduce a positive reactivity, the output at the upper axial section increases relatively compared to before the increase. This is mainly because the change in void fraction at the upper axial section is greater than that at the lower section.

[0015] On the other hand, regarding feedwater temperature control, when the feedwater temperature is lowered to introduce positive reactivity, the power distribution increases in the lower axial direction. This is mainly because, as the degree of subcooling increases, the boiling point shifts downstream.

[0016] Thus, it can be seen that flow rate control and feedwater temperature control have different effects on the axial power distribution (for the same reactivity input operation). From this, we considered that by appropriately switching between these controls or using them in combination to achieve criticality control depending on the operating conditions, the peak increase in the axial power distribution can be suppressed within a certain range. Here, operating conditions refer to the progress of the operating cycle and the time elapsed since returning to high power (e.g., rated power).

[0017] In boiling water reactor cores, the axial power distribution generally shifts from the bottom to the top during the operating cycle. This is because, in the early stages of the cycle, the void fraction is low and the fission reaction rate is high in the lower axial direction, but as burnup progresses, the fissile material decreases and the fission reaction rate becomes high in the upper part of the core towards the end of the cycle.

[0018] Furthermore, as mentioned above, after returning from a low-power to a high-power state, it is necessary to control criticality by switching from positive to negative reactivity input over time. For example, in flow rate control, positive reactivity input is used to increase the flow rate, and negative reactivity input is used to decrease the flow rate.

[0019] Figure 1 shows the time variation of reactor thermal output, illustrating the relationship between the time variation of reactor thermal output and the time it takes to switch reactivity control. The control shown in Figure 1 initially operates at high output (rated output), then at low output during the day when solar power output is high, and then at high output again at night when solar power output is low. When switching from low output to high output operation, initially... 135 The Xe concentration is high, requiring positive reactivity input. On the other hand, if the reaction continues, equilibrium will be reached. 135 As the Xe concentration decreases, negative reactivity injection becomes necessary. Time period 1, when positive reactivity injection is required, and time period 2, when negative reactivity injection is required, are analyzed ( 135 Xe transient analysis etc. 135 Xe concentration changes and equilibrium at high power levels 135 Evaluate Xe concentration and load tracking 135 Xe concentration change at high power level 135If the Xe concentration is greater than the time zone, it can be identified as time zone 1; if it is less than the time zone, it can be identified as time zone 2. Note that in Figure 1, time zones 1 and 2 are each shown as separate regions for convenience.

[0020] Figures 2A and 2B are graphs showing the axial power distribution of the reactor core due to changes in flow rate and feedwater temperature during the early stages of the cycle. Figure 2A shows the control results corresponding to time period 1, when positive reactivity is required, and Figure 2B shows the control results corresponding to time period 2, when negative reactivity is required. Figures 3A and 3B are graphs showing the axial power distribution of the reactor core due to changes in flow rate and feedwater temperature during the final stages of the cycle. Figure 3A shows the control results corresponding to time period 1, when positive reactivity is required, and Figure 3B shows the control results corresponding to time period 2, when negative reactivity is required.

[0021] Figure 2A shows that in time period 1, controlling the feedwater temperature (decrease) increases the peak compared to the reference axial power distribution. Figure 2B also shows that in time period 2, controlling the flow rate (decrease) increases the axial peak. The reference evaluation results represent the values ​​for the balanced core at rated output.

[0022] On the other hand, in Figures 3A and 3B, the axial power distribution shifts upward towards the end of the cycle. Figure 3A shows that the axial peak increases with flow rate (increase) control in time period 1, and Figure 3B shows that it increases with feedwater temperature (rise) control in time period 2.

[0023] Figure 4 is a table showing control methods to suppress the peak increase in the axial output distribution in Figures 2A, 2B, 3A, and 3B. Based on the above findings, the combination shown in Figure 4 135It is believed that the increase in the axial power peak can be suppressed by implementing criticality control in response to changes in Xe concentration. Furthermore, since the axial power peak can be kept within a certain range from the perspective of constraints such as maximum linear power density, envelope on PCIOMR, and thresholds on SDRs, exceeding these constraints can be avoided. Therefore, within that range, the control method for a given time period may be combined based on the combination shown in Figure 4. As a specific example, when the axial power peak is at the bottom, in time period 1, the control is based on increasing the flow rate, but feedwater temperature reduction control may also be used simultaneously, within the range where the increase in the axial power peak is acceptable. Also, the control for time period 1 may be implemented in advance under low power conditions.

[0024] Other criticality control methods can also be substituted, taking into consideration their impact on the axial power distribution, similar to flow rate control and feedwater temperature control. For example, feedwater temperature control can be substituted with control rod control. As shown in Figure 4, in the early stages of the cycle, feedwater temperature (increase) control is implemented in time zone 2 to suppress the increase in the lower axial peak by introducing negative reactivity. However, even by inserting control rods from the bottom of the core to introduce negative reactivity, the increase in the lower axial peak can be suppressed within a certain range. The same can be said for feedwater temperature control in the later stages of the cycle. [Examples]

[0025] A preferred embodiment of the present invention, Example 1, which is applied to an improved boiling water reactor, will be described with reference to Figures 5 and 6.

[0026] Figure 5 is a graph showing the time evolution of the reactor thermal output in Example 1, indicating the target change in the reactor core's thermal output. In this example, we consider a typical daily load following with thermal output changes of 100%-80%-100%. We assume that the plant's operation during load following corresponds to the initial stage of the cycle.

[0027] Figure 6 is a flowchart showing the procedure for determining core criticality control during time periods 3 and 4 in Figure 5. In Figure 6, first, in step 5, load following 135 Xe concentration N1 change and equilibrium at rated output state135 The Xe concentration N2 is evaluated by analysis. Next, in step 6, these 135 The Xe concentrations are compared. In step 7, core analysis is used to determine whether the peak position of the axial power distribution is upper or lower. Finally, in step 8, the criticality control method for each time period is determined based on the results of the above steps. Specifically, if step 6 is "True" and step 7 is "lower", flow rate (increase) control is implemented; if step 6 is "True" and step 7 is "upper", feedwater temperature (decrease) control is implemented; if step 6 is "False" and step 7 is "lower", feedwater temperature (increase) control is implemented; and if step 6 is "False" and step 7 is "upper", flow rate (decrease) control is implemented.

[0028] In the scenario of Example 1, during time period 1, step 6 is "True" and step 7 is "lower," so flow rate (increase) control is implemented. During time period 2, step 6 is "False" and step 7 is "lower," so water supply temperature (rise) control is implemented. Note that control methods may be combined within the constraints of the maximum linear power density, etc. For example, during time period 1, in addition to flow rate (increase) control, water supply temperature (decrease) control may also be implemented within the constraints.

[0029] By switching the criticality control method between time zones 3 and 4 in this way, it is possible to suppress the peak increase in the axial power distribution within a certain range. Example 1 is an optimal control method, especially for plants where feedwater temperature control is possible. [Examples]

[0030] A preferred embodiment of the present invention, Example 2, which applies to an improved boiling water reactor, will be described with reference to Figures 7 and 8.

[0031] Figure 7 is a graph showing the time evolution of the reactor thermal output in Example 2, indicating the target change in core thermal output. We consider daily load following at 100%-70%-90%. The plant's operation during load following corresponds to the initial cycle. In Example 2, instead of feedwater temperature control, control rods equipped with FMCRDs that allow for fine adjustment of the control rod drive are used. This allows for, 135 This enables continuous criticality control in response to changes in Xe concentration.

[0032] Figure 8 is a flowchart showing the procedure for determining the core criticality control method during time periods 3 and 4 in Figure 7. The main difference from Example 1 is that control rod control is performed instead of feedwater temperature control. Also, the equilibrium in step 11... 135 The Xe concentration N2 is evaluated based on the value at 90% output when high output is restored.

[0033] In the situation of Example 2, during time period 9, step 12 is "True" and step 13 is "Lower," so flow rate (increase) control is performed. Also, during time period 10, step 12 is "False" and step 13 is "Lower," so control rod (insertion) control is performed.

[0034] The control method in Example 2 is suitable for criticality control in plants that do not have feedwater temperature control. [Examples]

[0035] A preferred embodiment of the present invention, Example 3, which applies to an improved boiling water reactor, will be described with reference to Figures 9 and 10.

[0036] Figure 9 is a graph showing the time evolution of the reactor thermal output in Example 3, indicating the change in the core thermal output targeted for supply and demand adjustment. The daily load following ranges from 100% to 40% to 100%, and is characterized by a larger range of output changes compared to Examples 1 and 2. In terms of plant operation during load following, this corresponds to the end of the cycle.

[0037] Figure 10 is a flowchart showing the procedure for determining the core criticality control method during time periods 3 and 4 in Figure 9. The difference from Example 1 is that instead of flow rate (decrease) control, flow rate (decrease) control + control rod (insertion) control is implemented. This is because in Example 3, 135 This is because a significant reduction in flow rate is necessary to cope with large changes in Xe concentration, which worsens the minimum limit power ratio. It is desirable to increase the (negative) reactivity input amount by using control rod control in conjunction with this method, within a range that does not exceed the standard value. In this case, feedwater temperature (rise) control may be used instead of control rod (insertion) control.

[0038] In the scenario of Example 3, during time period 15, step 18 is "True" and step 19 is "Upper," so feedwater temperature (decrease) control is implemented. Also, during time period 16, step 18 is "False" and step 19 is "Upper," so flow rate (decrease) control + control rod (insertion) control are used in combination. The control method of Example 3 is suitable for plants that perform load following with large output fluctuations at the end of the cycle.

[0039] As explained above, according to the present invention, in daily load-following operation, when the rated output is restored and thereafter 135 Even when criticality control is implemented in response to changes in Xe concentration, the increase in the peak of the axial output distribution can be suppressed to a certain range.

[0040] 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 for the purpose of explaining the present invention in an easy-to-understand manner, 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 configurations from other embodiments to the configuration of one embodiment. In addition, it is possible to add, delete, or replace parts of the configuration of each embodiment with other configurations. [Explanation of Symbols]

[0041] 1, 3, 9, 15... Time periods when positive reactivity input is required; 2, 4, 10, 16... Time periods when negative reactivity input is required; 5, 11, 17... Steps to evaluate the xenon concentration during load tracking and the equilibrium xenon concentration; 6, 12, 18... Steps to compare the xenon concentration during load tracking and the equilibrium xenon concentration; 7, 13, 19... Steps to determine the peak position of the axial output; 8, 14, 20... Steps to determine the control method based on the determinations of each upstream step.

Claims

1. A method for determining a method for controlling the criticality of a reactor core in a nuclear power plant that performs daily load-following operation, A step of evaluating the xenon concentration N1 and equilibrium xenon concentration N2 during daily load tracking, The xenon concentration N １ and the equilibrium xenon concentration N ２ The step of comparing, The steps include determining the location of the peak in the axial power distribution of the reactor core, A method for controlling a reactor core, characterized by comprising the step of determining a criticality control means that reduces the magnitude of the aforementioned peak.

2. The core control method according to claim 1, characterized in that the criticality control means is at least one of flow rate control, water intake temperature control, and control rod control.

3. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is greater than and the peak in the axial power distribution of the reactor core is located at the bottom, the criticality control means is determined to be flow control. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is greater than or equal to the value, and the peak in the axial power distribution of the reactor core is located at the top, the criticality control means is determined to be feedwater temperature control. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is smaller than and the peak in the axial power distribution of the reactor core is located at the bottom, the criticality control means is determined to be feedwater temperature control. The xenon concentration N １ is less than the equilibrium xenon concentration N ２ and when the position of the peak in the axial power distribution of the core is at the upper part, the critical control means is determined to be flow rate control. The method for controlling a nuclear reactor core according to claim 1, characterized by this.

4. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is greater than and the peak in the axial power distribution of the reactor core is located at the bottom, the criticality control means is determined to be flow control. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is greater than or equal to the value, and the peak in the axial power distribution of the reactor core is located at the top, the criticality control means is determined to be control rod control. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is smaller than and the peak in the axial power distribution of the reactor core is located at the bottom, the criticality control means is determined to be control rod control. The xenon concentration N １ The equilibrium xenon concentration N ２ The core control method according to claim 1, characterized in that if the value is smaller than and the peak in the axial power distribution of the core is located at the top, the criticality control means is determined to be flow control.

5. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is greater than and the peak in the axial power distribution of the reactor core is located at the bottom, the criticality control means is determined to be flow control. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is greater than or equal to the value, and the peak in the axial power distribution of the reactor core is located at the top, the criticality control means is determined to be feedwater temperature control. The xenon concentration N １ The equilibrium xenon concentration N ２ If the value is smaller than and the peak in the axial power distribution of the reactor core is located at the bottom, the criticality control means is determined to be feedwater temperature control. The xenon concentration N １ The equilibrium xenon concentration N ２ The core control method according to claim 1, characterized in that if the value is smaller than and the peak in the axial power distribution of the core is located at the top, the criticality control means is determined to be flow rate control and control rod control.

6. The core control method according to claim 1, wherein the position of the peak in the axial power distribution of the core is determined based on the elapsed time of the operating cycle.

7. The core control method according to any one of claims 1, 3, 4, 5, and 6, characterized in that, in the step of determining a criticality control means that reduces the magnitude of the peak, different criticality control means are used simultaneously in addition to the determined criticality control means, within the limits of the linear power density constraint.

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