Subcritical control rod value measurement test method

Abstract

The embodiment of the invention provides a subcritical control rod value measurement test method, and relates to the field of subcritical control rod value measurement. The control rod value measurement precision in the subcritical state can be improved. The subcritical control rod value measurement test method comprises the step of controlling the process of starting the pressurized water reactor nuclear power station to reach the critical state, namely a soluble boron pre-dilution stage, a shutdown rod lifting stage, an adjusting rod lifting stage and a soluble boron dilution critical stage in sequence, wherein subcritical control rod value measurement is carried out in the shutdown rod lifting stage and the adjusting rod lifting stage. Two stages of lifting a shut-down rod and pre-diluting soluble boron in the starting process of the pressurized water reactor nuclear power station are exchanged, and the stage of lifting the shut-down rod is placed after the stage of pre-diluting soluble boron, so that the counting rate and the signal-to-noise ratio of a reactor exogenous range detector of the shut-down rod reactor are effectively increased, and the precision of value measurement of the subcritical control rod is improved.

Description

Technical Field

[0001] The present invention relates to the field of measuring the worth of control rods in a subcritical state, and more particularly, to a method for measuring the worth of subcritical control rods in a test. Background Art

[0002] The method for measuring the worth of subcritical rods is the "new generation" method for measuring the worth of control rods in commercial pressurized water reactor nuclear power plants following the dynamic rod worth measurement method. This method is implemented during the test process before the nuclear power plant core reaches criticality, and is in a subcritical state throughout the process, with higher safety compared to traditional low-power physics startup tests. The process of measuring the worth of subcritical rods is highly integrated with the reactor startup to criticality, and hardly occupies the main line time for the reactor to start up and reach criticality. In addition, no dedicated measurement equipment is required, there is less dependence on nuclear power plant equipment and instrumentation personnel, and it is easy to implement in existing and newly built power plants, which can bring significant benefits to the safety and economy of nuclear power plant operation.

[0003] The process of reaching criticality after refueling in a pressurized water reactor nuclear power plant generally needs to go through five stages in sequence: heating and pressurizing, lifting the shutdown rods, pre-diluting soluble boron, lifting the regulating rods, and diluting soluble boron to reach criticality. Before the reactor reaches criticality, the reactor core is always in a subcritical state, and as the process progresses, the subcriticality of the reactor core gradually decreases, the neutron flux level in the core gradually increases, and the count rate of the ex-core source range detector gradually increases. Among them, the count rate of the ex-core source range detector refers to the count generated by the ex-core source range detector per unit time, which is proportional to the neutron flux level it measures. The ex-core nuclear measurement detector is installed in the ex-core channel outside the reactor containment and is used to monitor the neutron flux level during the process of the reactor starting up to full power operation. The subcritical rod worth measurement test is generally implemented after heating and pressurizing, mainly in the stages of lifting the shutdown rods and lifting the regulating rods. The subcritical rod worth measurement requires the use of count rate data of the ex-core source range detector and corresponding core state parameters, including the average moderator temperature, the pressure of the primary loop system, the soluble boron concentration, the control rod position, etc. During the subcritical rod worth measurement test process, since the core is in a subcritical state and the neutron flux level in the core is very low, the count rate of the ex-core source range detector is very small, and it also contains non-negligible background noise. The background noise is generally generated by gamma rays produced after the activation of the core structural material reacting with the sensitive body of the detector to generate a signal, or is the electrical noise generated by the detector and its cables, and is a non-neutron component in the detector signal that does not change with the core reactivity and shows subcritical source multiplication changes.

[0004] The measurement of the subcritical control rod worth requires collecting the stable count rate data of the reactor source range detector under different test conditions, such as the count rate data of the reactor source range detector after a certain group of control rods are quickly withdrawn from the core or after boron dilution. Operations such as quickly withdrawing control rods from the core or boron dilution will introduce positive reactivity, which is a kinetic process. It takes a certain amount of time for the neutron flux level in the core to return to stability, and only then can the count rate of the reactor source range detector be stable. If the time is too short, the count rate data of the reactor source range detector will not be able to truly reflect the neutron flux level in the core under the test conditions. If the time is too long, it will be necessary to increase the main line time for the nuclear power plant to start up and reach criticality, weakening the economic advantage of the subcritical control rod worth measurement. Currently, the subcritical rod worth measurement test does not well evaluate the time required for the control rods to be withdrawn from the core, affecting the application effect of the subcritical control rod worth measurement. Summary of the Invention

[0005] A brief overview of one or more aspects is given below to provide a basic understanding of these aspects. This overview is not an exhaustive survey of all contemplated aspects, and is neither intended to identify key or decisive elements of all aspects nor to attempt to define the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that follows.

[0006] The objectives of the present invention include, for example, providing a method for measuring the subcritical control rod worth, which can improve the measurement accuracy of the control rod worth in the subcritical state.

[0007] Embodiments of the present invention can be implemented as follows:

[0008] Embodiments of the present invention provide a method for measuring the subcritical control rod worth, including: the process of starting up a pressurized water reactor nuclear power plant to reach criticality successively includes the stages of pre-dilution of soluble boron, lifting the shutdown rods, lifting the regulating rods, and diluting soluble boron to reach criticality; performing subcritical control rod worth measurement during the stage of lifting the shutdown rods and the stage of lifting the regulating rods.

[0009] In addition, the method for measuring the subcritical control rod worth provided by the embodiments of the present invention may further have the following additional technical features:

[0010] Optionally, subcritical control rod worth measurement is performed after the neutron flux level in the core is stable.

[0011] Optionally, the subcritical control rod worth measurement performed after the core neutron flux level stabilizes includes: proposing test conditions for the core for each group of control rods, calculating the core neutron flux level and the variation of the core neutron flux level with time in the radial and axial directions of the core, and taking the time when the core neutron flux level and the radial and axial distributions of the core neutron flux level are both stable as the stable time for each group of control rod groups under the test conditions, and performing subcritical control rod worth measurement on each group of control rods after the stable time;

[0012] Among them, the number of regulating rods is multiple groups, the number of shutdown rods is multiple groups, and the control rod group includes a regulating rod group and a shutdown rod group.

[0013] Optionally, the calculation includes: performing a tracking calculation on the current fuel cycle, performing refueling calculation based on the fuel loading scheme of a new fuel cycle of the nuclear power plant, establishing an analysis and calculation model for the stable time of the test conditions, and performing the calculation based on the calculation model.

[0014] Optionally, the step of performing the calculation based on the calculation model includes: through the calculation model, setting the core state parameters as the core state parameters of the nuclear power plant for performing subcritical control rod worth measurement, setting the lifting speed of the control group as the actual lifting speed of the control rods of the nuclear power plant, and according to the rod lifting sequence of the control rods of the nuclear power plant, performing three-dimensional space-time neutron kinetics calculation on each group of control rod groups from fully inserted into the core to fully withdrawn from the core until reaching a stable state, obtaining the core neutron flux level of each group of control rods and the variation of the core neutron flux level with time in the radial and axial directions of the core, and taking the time when the core neutron flux level and the radial and axial distributions of the core neutron flux level are both stable as the stable time for each group of control rod groups under the test conditions;

[0015] Among them, the core state parameters include the average moderator temperature, the primary loop system pressure, the soluble boron concentration, and the control rod position.

[0016] The beneficial effects of the subcritical control rod worth measurement test method according to the embodiments of the present invention include, for example:

[0017] The subcritical control rod worth measurement test method includes controlling the process of starting up a pressurized water reactor nuclear power plant to reach criticality in sequence as the soluble boron pre-dilution stage, the shutdown rod lifting stage, the regulating rod lifting stage, and the soluble boron dilution to reach criticality stage; performing subcritical control rod worth measurement in the shutdown rod lifting stage and the regulating rod lifting stage.

[0018] The inventors found that the reactor core was in a subcritical state. Although the neutron flux level in the reactor core was very low, during the process of the nuclear power plant starting up to criticality, from increasing temperature and pressure, lifting the shutdown rods, pre-diluting soluble boron, lifting the regulating rods, to diluting soluble boron to reach criticality, the count rate of the ex-core source range detectors gradually increased, and the neutron flux level in the reactor core gradually rose. While the background noise was usually a constant during the process of a nuclear power plant starting up to criticality in a certain fuel cycle, the closer the reactor core was to criticality, the smaller the subcriticality of the reactor core, the larger the count rate of the ex-core source range detectors, and the larger the signal-to-noise ratio which was equal to the count rate of the ex-core source range detectors / noise. Therefore, by swapping the two steps of lifting the shutdown rods and pre-diluting soluble boron, and placing the stage of lifting the shutdown rods after the stage of pre-diluting soluble boron, the subcriticality of the reactor core during the process of lifting the shutdown rods was reduced, thereby increasing the count rate of the ex-core source range detectors during the stage of lifting the shutdown rods, further increasing the signal-to-noise ratio, and ultimately improving the accuracy of measuring the worth of subcritical control rods. Brief Description of the Drawings

[0019] After reading the detailed description of the embodiments of the present disclosure in conjunction with the following drawings, the above features and advantages of the present invention can be better understood. In the drawings, the components are not necessarily drawn to scale, and components with similar related characteristics or features may have the same or similar reference numerals.

[0020] Figure 1 Shows the change of the count rate of the ex-core source range detectors at each stage of a certain nuclear power plant before optimization;

[0021] Figure 2 Shows the change of the count rate of the ex-core source range detectors at each stage of a certain nuclear power plant after optimization in the subcritical control rod worth measurement test method provided by the embodiment of the present invention;

[0022] Figure 3 Shows the change of the count rate of the ex-core source range detectors after all the shutdown rods of a certain nuclear power plant are withdrawn from the reactor core in the subcritical control rod worth measurement test method provided by the embodiment of the present invention;

[0023] Figure 4 Shows the change of the neutron flux level in the reactor core after the control rods of a pressurized water reactor nuclear power plant are withdrawn from the reactor core in the subcritical control rod worth measurement test method provided by the embodiment of the present invention. Detailed Description of the Embodiments

[0024] The present invention will be described in detail below in conjunction with the drawings and specific embodiments. Note that the aspects described below in conjunction with the drawings and specific embodiments are merely exemplary and should not be construed as imposing any limitation on the protection scope of the present invention.

[0025] In the description of the present invention, it should be noted that if terms such as "upper", "lower", "inner", "outer", "vertical", etc. are used to indicate the orientation or positional relationship, it is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the invention product is usually placed during use, rather than indicating or implying that the device or component referred to must have a specific orientation or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention.

[0026] At the same time, it should be noted that if terms such as "first", "second", etc. are only used for differential description and cannot be understood as indicating or implying relative importance.

[0027] In the description of the present invention, it should also be noted that unless otherwise clearly specified or limited, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it can be a fixed connection, an integral connection, or a detachable connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or the communication inside two components, etc. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0028] Next, in conjunction with Figures 1 to 4 The subcritical control rod worth measurement test method provided in this embodiment will be described in detail.

[0029] Please refer to Figure 1 and Figure 2 , the embodiments of the present invention provide a subcritical control rod worth measurement test method, including:

[0030] The processes for controlling a pressurized water reactor nuclear power plant to start up to criticality are successively the soluble boron pre-dilution stage, the lifting of the shutdown rods stage, the lifting of the regulating rods stage, and the soluble boron dilution to criticality stage;

[0031] During the lifting of the shutdown rods stage and the lifting of the regulating rods stage, the subcritical control rod worth measurement is carried out.

[0032] The control rods include regulating rods and shutdown rods. The control rods mentioned in the text can be regulating rods or shutdown rods. Similarly, the control rod groups can be regulating rod groups or shutdown rod groups. During the lifting of the shutdown rods stage, the control rod group is the shutdown rod group, and during the lifting of the regulating rods stage, the control rod group refers to the regulating rod group.

[0033] The core of a nuclear power plant reaching the critical state refers to a specific state in which the chain reaction in the reactor proceeds self-sustainingly. It means that the number of neutrons in the core remains stable and does not increase or decrease over time. The reactor is in a self-sustaining chain reaction state and can maintain a stable nuclear power output. The process of the core reaching criticality includes, in sequence, heating up and pressurizing, lifting the shutdown rods, pre-diluting soluble boron, lifting the regulating rods, and diluting soluble boron to reach criticality. After the first refueling of the reactor, all control rods are usually in the position of being fully inserted into the core, and the primary coolant contains a relatively high boron concentration. At this time, almost all neutrons are absorbed, and the reactor is in a deep subcritical state. To make the reactor reach the critical state, it is necessary to gradually withdraw the control rods and dilute the boron in the primary coolant to increase the number of neutrons participating in the reaction in the core.

[0034] For a pressurized water reactor nuclear power plant after optimized adjustment, the process of starting up and reaching criticality is, in sequence: heating up and pressurizing, pre-diluting soluble boron, lifting the shutdown rods, lifting the regulating rods, and diluting soluble boron to reach criticality.

[0035] Heating up and pressurizing: In the initial stage of reactor startup, it is necessary to gradually increase the temperature and pressure of the coolant to ensure the safe and stable operation of the reactor. The process of heating up and pressurizing needs to be strictly controlled to avoid excessive thermal stress and pressure on the reactor structure.

[0036] Pre-diluting soluble boron: Soluble boron is an important component in the reactor coolant and is used to regulate the reactivity of the reactor. Before reactor startup, it is usually necessary to pre-dilute the soluble boron in the coolant to reduce its concentration and decrease the absorption of neutrons. The degree of pre-dilution needs to be determined according to the specific situation of the reactor and the startup plan.

[0037] Lifting the shutdown rods: The shutdown rods are important devices for controlling the reactivity of the reactor. When the reactor needs to stop operating, the shutdown rods will quickly insert into the reactor core to absorb neutrons and thus stop the chain reaction. During the reactor startup process, the shutdown rods need to be lifted to a certain height to reduce the absorption of reactor reactivity and create conditions for the reactor to reach the critical state.

[0038] Lifting the regulating rods: The regulating rods are also devices for controlling the reactivity of the reactor. However, compared with the shutdown rods, their adjustment range is wider, and they can control the reactivity of the reactor more precisely. During the reactor startup process, after the shutdown rods are withdrawn from the core, it is necessary to gradually lift the regulating rods to further adjust the reactivity of the reactor and make it gradually approach the critical state.

[0039] Soluble boron dilution reaches criticality: When the reactivity of the reactor gradually approaches the critical state, it is necessary to continue diluting the soluble boron in the coolant to further increase the reactivity of the reactor. The dilution process needs to be strictly controlled to ensure that the reactor can reach the critical state smoothly. Once the reactor reaches the critical state, that is, the chain reaction can proceed self-sustainingly, it is necessary to stop the dilution operation and stabilize the operating state of the reactor.

[0040] The "signal-to-noise ratio" (abbreviated as SNR) is an important concept in the fields of electronic communication and signal processing. It refers to the ratio of the intensity of the received useful signal to the intensity of the received interference signals (including noise and interference), or it can be understood as the ratio of the signal power to the noise power. As the process of the pressurized water reactor nuclear power plant starting up and reaching criticality progresses, the reactivity of the reactor core gradually increases, the neutron flux level gradually rises, and the count rate of the ex-core source range detector gradually increases. The count rate of the ex-core source range detector refers to the number of counts generated by the ex-core source range detector per unit time, which is proportional to the measured neutron flux level. Since the background noise signal in the count rate signal of the ex-core source range detector generally remains unchanged during a certain fuel cycle, its signal-to-noise ratio increases as the signal strengthens, that is, the signal-to-noise ratio increases as the process of starting up and reaching criticality progresses. It can be seen that the measurement of the subcritical control rod worth is more beneficial to improving the final measurement accuracy as it progresses further.

[0041] The measurement of the subcritical control rod worth mainly occurs in two stages: raising the shutdown rod and raising the regulating rod. During the process of starting up and reaching criticality, from heating up and pressurizing, raising the shutdown rod, pre-diluting the soluble boron, raising the regulating rod to diluting the soluble boron to reach criticality, the neutron flux level in the reactor core gradually increases. By swapping the two steps of raising the shutdown rod and pre-diluting the soluble boron, moving the stage of raising the shutdown rod after the pre-dilution of the soluble boron, the reactivity of the reactor core in the stage of raising the shutdown rod is increased, making the count rate of the ex-core source range detector in this stage higher; increasing the count rate of the ex-core source range detector in the stage of raising the shutdown rod, and at the same time, the signal-to-noise ratio, which is equal to the count rate of the ex-core source range detector / noise, is also larger, thereby improving the measurement accuracy of the subcritical control rod worth.

[0042] Refer to Figure 1 , Figure 1 shows the change in the count rate of the ex-core source range detector at each stage of a nuclear power plant before optimization. Figure 1The abscissa is the startup time T, and the ordinate is the count rate of the in-core source range detector. From T0 to T4, they are the stages of lifting the shutdown rod group, pre-dilution of soluble boron, lifting the regulating rod group, and dilution of soluble boron to criticality in sequence. From T0 to T4, the count rate of the in-core source range detector gradually increases. The count rate of the in-core source range detector in the pre-dilution stage of soluble boron is greater than that in the stage of lifting the shutdown rod group, that is, the neutron flux level in the core in the pre-dilution stage of soluble boron is higher than that in the stage of lifting the shutdown rod group. Figure 2 The variation of the count rate of the in-core source range detector in each stage of a certain nuclear power plant after optimization is given. Similarly, Figure 2 The abscissa is the startup time T, and the ordinate is the count rate of the in-core source range detector. From T0 to T4, they are the pre-dilution stage of soluble boron, the stage of lifting the shutdown rod group, the stage of lifting the regulating rod group, and the stage of dilution of soluble boron to criticality in sequence. From T0 to T4, the count rate of the in-core source range detector gradually increases. The count rate of the in-core source range detector in the pre-dilution stage of soluble boron is less than that in the stage of lifting the shutdown rod group, that is, the neutron flux level in the core in the pre-dilution stage of soluble boron is lower than that in the stage of lifting the shutdown rod group. Since whether it is the dilution of soluble boron, the lifting of the regulating rod group, or the lifting of the shutdown rod group, the reactivity of the reactor is adjusted by changing the neutron absorption rate in the core. When the control rods (including shutdown rods and regulating rods) are inserted into the core, they will absorb neutrons, thereby reducing the number of neutrons and the neutron multiplication rate in the core, and reducing the reactivity of the reactor. On the contrary, when the control rods are withdrawn from the core, the neutron absorption decreases and the reactivity of the reactor increases. Therefore, when the pre-dilution stage of soluble boron is after the stage of lifting the shutdown rod, the count rate of the in-core source range detector in the pre-dilution stage of soluble boron is greater than that in the stage of lifting the shutdown rod group; when the stage of lifting the shutdown rod is placed after the pre-dilution stage of soluble boron, the count rate of the in-core source range detector in the pre-dilution stage of soluble boron is less than that in the stage of lifting the shutdown rod group. The count rate of the in-core source range detector in the optimized stage of lifting the shutdown rod is greater than that in the stage of lifting the shutdown rod before optimization, so as to be able to increase the count rate of the in-core source range detector in the stage of lifting the shutdown rod. Since the background noise signal in the count rate signal of the in-core source range detector generally remains unchanged during a certain test, the signal-to-noise ratio of the count rate of the in-core source range detector in the stage of lifting the shutdown rod is increased.

[0043] In this embodiment, the measurement of the subcritical control rod worth is carried out after the neutron flux level in the core is stable.

[0044] The subcritical control rod worth measurement is highly integrated with the process of a pressurized water reactor nuclear power plant starting up to criticality. In this embodiment, the test conditions refer to the conditions where each group of control rods is withdrawn from the core during the stage of lifting the shutdown rods and the stage of lifting the regulating rods. These test conditions are all the conditions that the nuclear power plant must go through during startup to criticality, and there is no need to separately conduct a control rod worth measurement test, thus saving the mainline time during the startup stage of the nuclear power plant and improving the economic efficiency of the nuclear power plant operation. However, for the test conditions of subcritical control rod worth measurement, such as the conditions after a certain group of control rods is withdrawn from the core, generally, the core has just experienced a process of reactivity introduction, and it takes a certain amount of time for the neutron flux level in the core to stabilize. Only after stabilization can the count rate signal of the ex-core source range detector truly represent the real neutron flux level in the core under this condition.

[0045] Figure 3 It shows the variation of the count rate of the ex-core source range detector with time after all the shutdown rods of a nuclear power plant are withdrawn from the core. Figure 3 In it, the abscissa is time and the ordinate is the count rate of the ex-core source range detector. From Figure 3 it can be seen that after the shutdown rods are withdrawn from the core, the count rate of the ex-core source range detector shows an increasing trend before 4.5 minutes and is in an unstable state. After about 4.5 minutes, the count rate of the ex-core source range detector begins to stabilize. Although there are still small fluctuations after 4.5 minutes, the overall trend is towards stability. Therefore, each test condition requires sufficient stabilization time to collect stable data that truly represents the core state, so as not to affect the accuracy of the control rod worth measurement. Therefore, conducting subcritical control rod worth measurement after the neutron flux level in the core stabilizes can obtain stable data that truly represents the neutron flux level in the core and improve the accuracy of subcritical control rod worth measurement.

[0046] In this embodiment, conducting subcritical control rod worth measurement after the neutron flux level in the core stabilizes includes: for the test conditions where each group of control rods is withdrawn from the core, calculating the neutron flux level in the core and the variation of the neutron flux level in the core with time in the radial and axial directions of the core. Taking the time when the neutron flux level in the core and the neutron flux level in the core in the radial and axial directions are both stable as the stabilization time of each group of control rods under the test conditions, and conducting subcritical control rod worth measurement for each group of control rods after the stabilization time; among them, the number of regulating rods is multiple groups, the number of shutdown rods is multiple groups, and the control rod group includes the regulating rod group and the shutdown rod group.

[0047] It should be noted that: "the neutron flux level in the core" refers to the number of neutrons passing through a unit area per unit time, usually in neutrons / (cm 2Taking the unit of “·s”, it reflects the neutron density and reaction rate in the core and directly affects the power output of the reactor. “The radial distribution of the core neutron flux level” describes the variation of the neutron flux in the radial direction of the core radius; it usually decreases from the core center to the edge and is affected by the fuel arrangement, the position of the control rods, and the reflector. “The axial distribution of the core neutron flux level” describes the variation of the neutron flux along the height direction of the core; it usually shows an approximate cosine distribution, with the highest at the center and decreasing towards both ends, and is affected by the fuel loading and the insertion depth of the control rods. “The core neutron flux level, as well as the radial and axial distributions of the core neutron flux level, are all stable” means that “the core neutron flux level, the radial distribution of the core neutron flux level, and the axial distribution of the core neutron flux level” are all stable.

[0048] During the stage of lifting the shutdown rods, the worth measurement of each shutdown rod group is carried out, and during the stage of lifting the regulating rods, the worth measurement of each regulating rod group is carried out. The worth measurement of the control rods is carried out under the test conditions where each group of control rods is withdrawn from the core.

[0049] The determination of the stable time for each test condition will affect the application effect of the subcritical control rod worth measurement. In addition, due to the differences in the worth of each group of control rods and their arrangement positions in the core, etc., the influence on the counting rate of the ex-core source range detectors when they are withdrawn from the core is quite different, and the required stable times are also different.

[0050] Table 1 shows the influence of data with different stable times on the final measurement accuracy. In reactor nuclear power plant A, for the counting rate data of the ex-core source range detectors with the same stable time of 45 seconds for control rod groups A, B, C, and D, the deviations are -4.62%, 45.46%, 21.92%, and 10.21% respectively, and the deviations vary greatly. In reactor nuclear power plant B, control rod groups A, B, C, and D respectively adopt the counting rate data of the ex-core source range detectors with longer and different stable times, and the obtained deviations are -0.09%, 3.45%, 0.89%, and -0.75% respectively. The deviations are significantly reduced compared with the deviation results of reactor nuclear power plant A with a shorter and the same stable time, indicating that the stable time has a great influence on the final measurement accuracy, and the required stable times for different control rod groups may be different.

[0051] Table 1 Influence of Different Stable Times on Measurement Accuracy

[0052]

[0053] On the other hand, the stabilization time for each test condition should not be too long to avoid excessively increasing the main line time during the startup to the critical stage, which may affect the economic advantages of subcritical rod worth measurement. Therefore, before the subcritical control rod worth measurement test, pre-analysis can be carried out for each test condition to accurately evaluate the stabilization time required for each test condition through calculation. While ensuring the collection of stable data for each test condition, the test time can be minimized to fully reflect the economic advantages of subcritical control rod worth measurement.

[0054] Figure 4 Figure 4 shows the variation of the core neutron flux with time after the control rod is withdrawn from the core. Figure 4 In Figure 4, SD1 is a rod group of the shutdown rod group, and AO is a rod group of the regulating rod group. Figure 4 In Figure 4, the abscissa is time and the ordinate is the core neutron flux. Before 2 minutes, the core neutron flux shows a gradually increasing fluctuating trend. After 2 minutes, the core neutron flux is in a stable state. The corresponding stabilization time can be obtained through the stable time of the core neutron flux.

[0055] Therefore, before the test, pre-analysis is carried out for each test condition to obtain the core neutron flux level and the variation of the core neutron flux level with time in the radial and axial directions of the core, accurately evaluate the stabilization time required for each control rod group under each test condition, minimize the test time while ensuring the collection of stable data for each test condition, and improve the efficiency of control rod worth measurement in the subcritical state to fully reflect the economic advantages of subcritical control rod worth measurement.

[0056] In this embodiment, the calculation includes: performing a tracking calculation for the current fuel cycle, performing a refueling calculation based on the fuel loading scheme of the new fuel cycle of the nuclear power plant, establishing an analysis calculation model for the stabilization time of the test condition, and performing calculations based on the calculation model.

[0057] Use the software for analyzing the stabilization time of the test condition point to perform a tracking calculation for the current fuel cycle, and perform a refueling calculation for the new fuel cycle based on the fuel loading scheme of the new fuel cycle of the nuclear power plant to establish a calculation model required for analyzing the stabilization time of the test condition point.

[0058] In this embodiment, the steps of performing calculations based on the calculation model include: using the calculation model, setting the core state parameters as the core state parameters of the nuclear power plant for performing subcritical control rod worth measurement, setting the lifting speed of the control group as the actual lifting speed of the control rods of the nuclear power plant, and according to the rod lifting sequence of the control rods of the nuclear power plant, performing three-dimensional core neutronics calculations for each control rod group from fully inserted into the core to fully withdrawn from the core until reaching a stable state, so as to obtain the core neutron flux level of each control rod group and the variation of the core neutron flux level with time in the radial and axial directions of the core, and taking the time when the core neutron flux level and the core neutron flux level are stable in both the radial and axial directions of the core as the stable time of each control rod group under the test conditions; wherein, the core state parameters include the average moderator temperature, the primary loop system pressure, the soluble boron concentration, and the control rod position.

[0059] It should be noted that: the "average moderator temperature" refers to the average temperature of the medium (such as heavy water, light water, etc.) used to slow down the neutron speed in a nuclear reactor. During the operation of the reactor, it is necessary to closely monitor and control the moderator temperature to ensure the stability and safety of the reactor. Usually, the moderator temperature is controlled by adjusting the flow rate and temperature of the coolant. The "primary loop system pressure" refers to the pressure of the coolant in the primary loop system of the nuclear power plant. During the operation of the reactor, it is necessary to strictly control the primary loop system pressure. Usually, the stable pressure of the primary loop system is maintained by adjusting the pressure and temperature of the pressurizer. The "soluble boron concentration" refers to the concentration of boron element dissolved in the coolant of the nuclear power plant. During the operation of the reactor, it is necessary to adjust the boron concentration according to the power demand and reactivity change of the reactor. Usually, the adjustment of the boron concentration is achieved by injecting or discharging boron-containing solution into the coolant. The "control rod position" refers to the position of the control rod in the core of the reactor. During the operation of the reactor, it is necessary to adjust the control rod position according to the power demand and safety requirements of the reactor. Usually, the precise control of the control rod is achieved by the rod control and position system.

[0060] Before loading the fuel in a pressurized water reactor nuclear power plant, based on the refueling plan, use analysis software to simulate and calculate the stable process after each control rod is withdrawn from the core and after being withdrawn from the core, and analyze the stable time of the core neutron flux level after each control rod is withdrawn from the core to determine the stable time of each test condition point.

[0061] Specifically, set the core status parameters as the core status parameters for the subcritical control rod worth measurement test actually carried out in the nuclear power plant, including soluble boron concentration, average temperature and pressure of the primary loop core moderator, etc. At the same time, set the control rod group as the actual lifting speed of the control rod group in the nuclear power plant. According to the actual rod-lifting sequence of the nuclear power plant, carry out three-dimensional neutron space-time dynamics calculation of the core for the whole process of each control rod group from being fully inserted into the core to being fully withdrawn from the core until stable, and give the total core flux level within the stable time and the variation of the flux level with time in the radial and axial directions of the core, and give the stable time of this control rod group.

[0062] Finally, during the subcritical control rod worth measurement test, measure the worth of each control rod group according to the calculated stable time. Collect the count rate of the external source range detector, boron concentration, control rod position, average temperature of the primary loop moderator, and primary loop pressure data within a period of time after the stable time. Calculate the core reactivity of each test condition based on the collected measurement data, and finally obtain the control rod worth in the subcritical state.

[0063] In this embodiment, the stable time analysis software includes a three-dimensional neutron dynamics calculation function module, a nuclear power plant burnup tracking calculation function module, and a refueling calculation function module. For the stable time analysis software of the test condition points, it is necessary to have the three-dimensional neutron dynamics calculation function; and have the nuclear power plant burnup tracking calculation function and refueling function to complete the calculation of temperature-time.

[0064] According to a subcritical control rod worth measurement test method provided by this embodiment, the working principle of the subcritical control rod worth measurement test method includes:

[0065] For the measurement of subcritical control rod worth, methods for measurement accuracy and efficiency are proposed from two aspects: optimizing the process from startup to criticality of a pressurized water reactor nuclear power plant and accurately evaluating the stable time of test condition points. In terms of optimizing the process from startup to criticality, it can effectively improve the count rate of the external source range detector at the test condition points of the subcritical control rod worth measurement test and its signal-to-noise ratio to ensure the final measurement accuracy; at the same time, accurately evaluate the stable time of each test condition point to reduce the test time under the condition of collecting stable test data, improve the operation economy of the nuclear power plant, and give full play to the application effect of the subcritical control rod worth measurement.

[0066] The specific steps are as follows:

[0067] First step: Use the stable time analysis software of the test condition points to track and calculate the current fuel cycle, and carry out refueling calculation based on the fuel loading scheme of the new fuel cycle of the nuclear power plant to establish a calculation model required for the stable time analysis of the test condition points.

[0068] Step 2: Select the core state parameters at the test operating condition points, mainly the soluble boron concentration, to ensure an appropriate subcriticality at the test operating condition;

[0069] Step 3: Set the core state parameters as the core state parameters for the subcritical control rod worth measurement test actually carried out in the nuclear power plant, including soluble boron concentration, average temperature and pressure of the primary loop core moderator, etc. At the same time, set the lifting speed of the control rods as the actual lifting speed of the control rods in the nuclear power plant, and carry out three-dimensional neutron space-time dynamics calculation of the core for the whole process of the group of control rods from being fully inserted into the core to being fully withdrawn from the core until the stable state, and give the core neutron flux level within the stable time and the variation of the core neutron flux level with time in the radial and axial directions of the core, and give the stable time of the group of control rods;

[0070] Step 4: Calculate and analyze the stable time for each group of control rods according to Step 3 in the actual rod-lifting sequence of the nuclear power plant;

[0071] Step 5: The nuclear power plant executes in the order of increasing temperature and pressure, pre-dilution of soluble boron, lifting of the shutdown rods, lifting of the regulating rods, and dilution of soluble boron to reach criticality during the process of starting up and reaching criticality, and collect core state parameters such as the count rate of the source range detector outside the core at the stable time given by Step 3 and Step 4 at the test operating condition points selected in Step 2;

[0072] Step 6: According to all the data at the test operating condition points collected, deduce the core reactivity of each test operating condition to obtain the control rod worth under the subcritical state.

[0073] A method for measuring the worth of subcritical control rods provided in this embodiment has at least the following advantages:

[0074] Move the stage of lifting the shutdown rods after the pre-dilution of soluble boron, improve the count rate of the source range detector outside the core during the shutdown rod lifting stage, and at the same time improve the signal-to-noise ratio of the count rate of the source range detector outside the core at the test operating condition during the shutdown rod lifting stage.

[0075] Simulate and calculate the effective stable time for measuring the worth of each group of control rods under each test operating condition, and then obtain the count rate of the source range detector outside the core that can effectively represent the core neutron flux level at the test operating condition point according to the calculated stable time in the subcritical worth measurement test, ensure reducing the test time while collecting stable and effective test data, improve the measurement accuracy of the subcritical control rod worth while compressing the test time, improve the measurement efficiency of the control rod worth under the subcritical state, and improve the economy of the nuclear power plant.

[0076] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for measuring the worth of a subcritical control rod, characterized in that It includes: The process of controlling the start-up of a pressurized water reactor nuclear power plant to criticality successively includes the soluble boron pre-dilution stage, the lifting of the shutdown rods stage, the lifting of the regulating rods stage, and the soluble boron dilution to criticality stage; During the stage of lifting the shutdown rods and the stage of lifting the regulating rods, the subcritical control rod worth measurement is carried out.

2. The subcritical control rod worth measurement test method according to claim 1, characterized in that The subcritical control rod worth measurement is carried out after the neutron flux level in the core is stable.

3. The subcritical control rod worth measurement test method according to claim 2, characterized in that The subcritical control rod worth measurement carried out after the neutron flux level in the core is stable includes: For each group of control rods, the test conditions of the core are proposed, the neutron flux level in the core and the changes of the neutron flux level in the core in the radial and axial directions with time are calculated. The stable time of each group of control rod groups under the test conditions is the time when the neutron flux level in the core and the neutron flux level in the core in the radial and axial directions are both stable. After the stable time, the subcritical control rod worth measurement is carried out for each group of control rods; Among them, the number of the regulating rods is multiple groups, the number of the shutdown rods is multiple groups, and the control rod group includes the regulating rod group and the shutdown rod group.

4. The subcritical control rod worth measurement test method according to claim 3, wherein The calculation includes: Tracking the calculation of the current fuel cycle, performing refueling calculation based on the fuel loading scheme of the new fuel cycle of the nuclear power plant, establishing an analysis calculation model for the stable time of the test conditions, and performing the calculation based on the calculation model.

5. The subcritical control rod worth measurement test method according to claim 4, wherein The steps of performing the calculation based on the calculation model include: Through the calculation model, setting the core state parameters as the core state parameters of the nuclear power plant for carrying out the subcritical control rod worth measurement, setting the lifting speed of the control group as the actual lifting speed of the control rods of the nuclear power plant. According to the rod lifting sequence of the control rods of the nuclear power plant, carrying out the three-dimensional space-time neutron kinetics calculation of the core for each group of control rod groups from fully inserted into the core to fully withdrawn from the core until reaching the stable state, obtaining the neutron flux level of each group of control rods in the core and the changes of the neutron flux level in the core in the radial and axial directions with time. The stable time of each group of control rod groups under the test conditions is the time when the neutron flux level in the core and the neutron flux level in the core in the radial and axial directions are both stable; Among them, the core state parameters include the average moderator temperature, the primary loop system pressure, the soluble boron concentration, and the control rod position.

Images