Reactor control rod out-of-control lifting accident analysis method

Abstract

The invention provides a reactor control rod out-of-control lifting accident analysis method, which comprises the following steps: providing a first neutron kinetic model, and setting a shutdown protection signal which comprises an overpower protection signal and a periodic protection signal; carrying out power spectrum analysis according to the first neutron kinetic model to obtain transient peak power; based on the transient peak power and the initial thermal parameters of the reactor, obtaining a transient MDNBR; and comparing the MDNBR in the transient state with a safety analysis limit value, and if the MDNBR in the transient state is less than or equal to the safety analysis limit value, executing system optimization operation and then returning to execute power spectrum analysis until the MDNBR in the transient state is greater than the safety analysis limit value. According to the method, only neutron dynamics calculation is carried out, thermal flow heat transfer calculation is not carried out, the calculation process is most conservative, if only the first analysis stage is carried out in the accident analysis process, the analysis process is most concise, and the working efficiency of accident analysis can be greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of nuclear safety, and in particular to a method for analyzing an accident in which a control rod of a reactor is lifted out of control. Background Art

[0002] A reactor control rod uncontrolled lift accident is a key design basis accident. This occurs when the control rod assembly is withdrawn from the reactor due to a malfunction in the control rod control system, a malfunction in the control rod drive mechanism, or human error. This causes the system to continuously introduce excessive positive reactivity into the reactor core, significantly increasing the core's nuclear power. Therefore, an accident safety analysis is required to ensure reactor safety in the event of a zero-power uncontrolled lift accident. However, the safety analysis methods used in related technologies are complex and inefficient in order to ensure accuracy, which cannot guarantee reactor design efficiency. Summary of the Invention

[0003] The present invention provides a method for analyzing an accident of uncontrolled lifting of a control rod of a reactor, which can optimize the analysis process and improve the design efficiency of the reactor.

[0004] In a first aspect, the present application discloses a method for analyzing a runaway control rod lifting accident in a reactor, comprising executing a first analysis phase, the first analysis phase comprising:

[0005] Step A1: providing a first neutron kinetic model and setting a reactor shutdown protection signal; the reactor shutdown protection signal includes an overpower protection signal and a cycle protection signal;

[0006] Step B1: performing power spectrum analysis based on the first neutron kinetic model to obtain transient peak power;

[0007] Step C1: Obtaining the transient MDNBR based on the transient peak power and the initial thermal parameters of the reactor;

[0008] Step D1: Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B1 until the transient MDNBR is greater than the safety analysis limit.

[0009] Optionally, further comprising executing a second analysis phase, step D1 further comprises: if the transient MDNBR is less than or equal to the safety analysis limit, performing a system optimization operation and then returning to executing step B1, or executing the second analysis phase;

[0010] The second analysis phase includes:

[0011] Step A2: providing a first coupled computational model, the first coupled computational model including a second thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model;

[0012] Step B2: Set the forced flow rate and perform power spectrum analysis based on the first coupled calculation model to obtain transient thermal parameters;

[0013] Step D2: Obtaining transient MDNBR based on transient thermal parameters;

[0014] Step E2: Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B2 until the transient MDNBR is greater than the safety analysis limit.

[0015] Optionally, the second analysis phase also includes:

[0016] Step C2: Verify the conservatism of the forced flow in step B2; Step C2 includes:

[0017] Step C21: providing a natural circulation model;

[0018] Step C22: Based on the transient thermal parameters obtained in step B2, a steady-state natural circulation calculation is performed using a natural circulation model to obtain the maximum natural circulation flow rate under transient conditions;

[0019] Step C23: Check forced flow; if the maximum natural circulation flow ≥ forced flow, or , then adjust the forced flow and return to step B2 until .

[0020] Optionally, step C23 includes:

[0021] , then reduce the forced flow rate and return to step B2;

[0022] If the maximum natural circulation flow rate is greater than or equal to the forced flow rate, increase the forced flow rate and return to step B2.

[0023] Optionally, step D2 includes:

[0024] Based on transient thermal parameters, critical heat flux is obtained;

[0025] Outputting the transient core peak heat flux density through the first coupled calculation model;

[0026] The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state.

[0027] Optionally, the third analysis phase is further included. Step E2 further includes: if the transient MDNBR is less than or equal to the safety analysis limit, performing system optimization operations and then returning to step B2, or performing the third analysis phase. The third analysis phase includes:

[0028] Step A3: providing a second coupled calculation model, the second coupled calculation model including a third thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model;

[0029] Step B3: Perform power spectrum analysis based on the second coupled calculation model to obtain transient thermal parameters;

[0030] Step C3: Obtaining transient MDNBR based on transient thermal parameters;

[0031] Step D3: Compare the transient MDNBR with the safety analysis limit; if the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B3 until the transient MDNBR is greater than the safety analysis limit.

[0032] Optionally, step C3 includes:

[0033] Based on the transient thermal parameters in step C3, the critical heat flux is obtained;

[0034] Directly output the transient core peak heat flux density through the second coupled calculation model;

[0035] The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state.

[0036] Optionally, step B1 includes:

[0037] Step B11: Set the power range for switching the shutdown protection signal.

[0038] Step B12: Perform transient calculation on the power interval when the shutdown protection signal is switched to obtain the power interval when the period protection signal is triggered and the power interval when the overpower protection signal is triggered.

[0039] Step B13: Based on the power interval when the period protection signal is triggered and the power interval when the overpower protection signal is triggered, the power interval when the shutdown protection signal is switched is processed in a refined manner.

[0040] Step B14: Compare the full power range when the trip protection signal is switched and the interval threshold. If the full power range when the trip protection signal is switched is greater than the interval threshold, return to step B12 until the full power range when the trip protection signal is switched is less than or equal to the interval threshold; wherein, the full power range when the trip protection signal is switched = the maximum power interval value when the trip protection signal is switched - the minimum power interval value when the trip protection signal is switched.

[0041] Step B15: Obtain a transient operating condition based on the power range when the shutdown protection signal is switched.

[0042] Optionally, step C1 includes:

[0043] Obtaining the transient core peak heat flux based on the transient peak power;

[0044] Obtain critical heat flux based on initial thermal parameters;

[0045] The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state.

[0046] Optionally, the system optimization operation includes at least one of the following:

[0047] Reduce the control rod value and reduce the maximum rod lifting rate;

[0048] Added lifting rod locking protection signal;

[0049] Reduce the protection signal setting value;

[0050] Shorten the delay time for implementing the shutdown protection function.

[0051] In a second aspect, the present application discloses a method for analyzing a runaway control rod lifting accident in a reactor, including executing a second analysis phase, which includes:

[0052] Step A2: providing a first coupled computational model, the first coupled computational model including a second thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model;

[0053] Step B2: setting the forced flow rate and performing power spectrum analysis based on the first coupled calculation model to obtain transient operating conditions;

[0054] Step D2: Obtaining transient MDNBR based on transient thermal parameters;

[0055] Step E2: Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B2 until the transient MDNBR is greater than the safety analysis limit.

[0056] Optionally, further comprising executing a third analysis phase, step E2 further comprises executing a system optimization operation and then returning to executing step B2 if the transient MDNBR is less than or equal to the safety analysis limit, or executing the third analysis phase;

[0057] The third analysis phase includes:

[0058] Step A3: providing a second coupled calculation model for accident analysis, the second coupled calculation model including a third thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model; and setting a reactor trip protection signal; the reactor trip protection signal including an overpower protection signal and a cycle protection signal;

[0059] Step B3: Perform power spectrum analysis based on the second coupled calculation model to obtain transient thermal parameters;

[0060] Step C3: Obtaining transient MDNBR based on transient thermal parameters;

[0061] Step D3: Compare the transient MDNBR with the safety analysis limit; if MDNBR ≤ the safety analysis limit, perform system optimization operations and return to step B3 until the transient MDNBR > the safety analysis limit.

[0062] In a third aspect, the present application discloses a method for analyzing a runaway control rod lifting accident in a reactor, comprising executing a third analysis phase;

[0063] The third analysis phase includes:

[0064] Step A3: providing a second coupled calculation model, the second coupled calculation model including a third thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model;

[0065] Step B3: Perform power spectrum analysis based on the second coupled calculation model to obtain transient thermal parameters;

[0066] Step C3: Obtaining transient MDNBR based on transient thermal parameters;

[0067] Step D3: Compare the transient MDNBR with the safety analysis limit; if MDNBR ≤ the safety analysis limit, perform system optimization operations and return to step B3 until the transient MDNBR > the safety analysis limit.

[0068] The beneficial effects of the present invention are as follows:

[0069] The present application discloses a method for analyzing a runaway control rod lifting accident in a reactor, comprising executing a first analysis phase, which includes:

[0070] Step A1: providing a first neutron kinetic model and setting a reactor shutdown protection signal; the reactor shutdown protection signal includes an overpower protection signal and a cycle protection signal;

[0071] Step B1: performing power spectrum analysis based on the first neutron kinetic model to obtain transient peak power;

[0072] Step C1: Obtaining the transient MDNBR based on the transient peak power and the initial thermal parameters of the reactor;

[0073] Step D1: Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B1 until the transient MDNBR is greater than the safety analysis limit.

[0074] In the first analysis phase, only neutron dynamics calculations are performed, and no thermal flow and heat transfer calculations are performed. The calculation process is the most conservative. If the accident analysis process only performs the first analysis phase, the analysis process is the most concise, which can greatly improve the work efficiency of accident analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The above features and advantages of the present invention will be better understood after reading the detailed description of the embodiments of the present disclosure in conjunction with the following drawings. In the drawings, the components are not necessarily drawn to scale, and components with similar related properties or characteristics may have the same or similar reference numerals.

[0076] Figure 1 It is the accident analysis flow chart of the present invention;

[0077] Figure 2 It is a power spectrum analysis flow chart of the present invention;

[0078] Figure 3 This is a specific example of power spectrum analysis in the first stage of analysis of the present invention;

[0079] Figure 4 This is a forced flow verification flow chart of the present invention;

[0080] Figure 5 It is a flow chart of the system optimization operation of the present invention. DETAILED DESCRIPTION

[0081] The present invention will be further described below with reference to specific embodiments and accompanying drawings. Further details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can obviously be implemented in various other ways than those described herein. Those skilled in the art may make similar generalizations and deductions based on actual applications without departing from the scope of the present invention. Therefore, the scope of protection of the present invention should not be limited by the contents of this specific embodiment.

[0082] It should be noted that these and other subsequent drawings are only examples and are not drawn to scale, and should not be used to limit the actual scope of protection required by the present invention.

[0083] The following combination Figures 1 to 5 Analysis of the accident analysis method for this application:

[0084] The technical solution of this application is primarily targeted at plate-type fuel pool reactors. The operating mode of plate-type fuel pool reactors includes a natural circulation cooling mode at low power. Therefore, after an accident, the power increases rapidly, but its flow rate gradually increases after passing through the core heating fluid, resulting in an increase in density difference. That is, the increase in flow rate lags behind the power. Therefore, the degree of power (heat flow) increase after the accident and the natural circulation flow rate are the key concerns of this accident. However, due to the large uncertainty in the calculation of natural circulation flow rate, and the fact that natural circulation flow rate also affects the calculation of heat flow, the program needs to be fully verified, which will lead to a cumbersome analysis process and cannot guarantee the design efficiency of the reactor. Therefore, this solution proposes a staged accident analysis method for the reactor, which is detailed as follows:

[0085] Execute the first analysis phase. The first analysis phase only performs neutron dynamics calculations and does not perform thermal flow and heat transfer calculations. The calculation process is the most conservative. If the accident analysis process only performs the first analysis phase, the analysis process is the most concise and can greatly improve the work efficiency of the accident analysis. The first analysis phase includes:

[0086] Step A1: Use the RELAP5 program or other programs as the accident analysis system program, provide the first neutron kinetic model for accident analysis, and set the reactor trip protection signal. RELAP5 is a one-dimensional system safety analysis program, a specialized computational tool for simulating and analyzing the thermodynamic behavior and reactor dynamics of nuclear power plants. RELAP5 simulates the flow and heat transfer of fluids such as coolant and steam within nuclear equipment, as well as the nuclear reaction processes in the core. The reactor trip protection signals include an overpower protection signal and a periodic protection signal. The overpower protection signal is used to protect against excessive neutron flux, while the periodic protection signal is used to protect against excessively rapid power increases. The neutron kinetic model is used to calculate post-accident power changes. After an accident, control rods are raised, introducing positive reactivity and increasing power. When the trip protection signal is reached, a trip is triggered, the control rods are lowered, introducing negative reactivity and decreasing power. This calculation process can be completed using the RELAP5 program.

[0087] Step B1: Perform power spectrum analysis on the trip protection signal according to the first neutron kinetic model to obtain the transient peak power during the accident transient process. Step B1 is as follows: Figure 2 and Figure 3 As shown, the details are as follows:

[0088] Step B11: Set the power range when the shutdown protection signal switches; the shutdown protection signal switching is the mutual switching between the overpower protection signal and the periodic protection signal. For example, the power range when the shutdown protection signal switches is 10 -9 FP~10 -5 FP, which is 10 -9 ~10 -5times the rated power, for example, the rated power of the reactor is 100MW, then 10 -9 Times the rated power is 0.1W;

[0089] Step B12: Perform transient calculation on the power range when the shutdown protection signal is switched based on the first neutron kinetic model to obtain the power range when the periodic protection signal is triggered and the power range when the overpower protection signal is triggered; wherein the power range when the periodic protection signal is triggered and the power range when the overpower protection signal is triggered are both within the power range when the shutdown protection signal is switched. For example, for the rated power 10 when the shutdown protection signal is switched, -9 FP, 10 -8 FP, 10 -7 FP, 10 - 6 FP, 10 -5 FP separately carries out transient calculation to determine the power range when the period protection signal is triggered. -9 FP~10 -8 FP, power range when over power protection signal is triggered 10 -7 FP~10 -5 FP.

[0090] Step B13: Based on the power interval when the periodic protection signal is triggered and the power interval when the overpower protection signal is triggered, the power interval when the shutdown protection signal is switched is refined. The power interval when the shutdown protection signal is switched will be reduced after the refined processing. For example, based on the power interval 10 when the periodic protection signal is triggered, -9 FP~10 -8 FP, power range when over power protection signal is triggered 10 -7 FP~10 -5 FP, power range when the shutdown protection signal switches 10 -9 FP~10 - 5 After FP is refined, the power range when the shutdown protection signal is switched is reduced to 10 -8 FP~10 -7 FP; It can be seen that after the refined processing, the power interval when the stack protection signal switches is between the power interval when the periodic protection signal is triggered and the power interval when the overpower protection signal is triggered, and the minimum value of the power interval when the shutdown protection signal switches is the maximum value of the power interval when the periodic protection signal is triggered. -8 FP, the maximum value of the power range when the shutdown protection signal switches is the minimum value of the power range when the overpower protection signal is triggered. -7 FP.

[0091] Step B14: Compare the full power range at the time of the trip protection signal switching with the interval threshold. If the full power range at the time of the trip protection signal switching is less than or equal to the interval threshold, proceed to Step B15. If the full power range at the time of the trip protection signal switching is greater than the interval threshold, return to Step B12 until the full power range at the time of the trip protection signal switching is less than or equal to the interval threshold. The full power range at the time of the trip protection signal switching = the maximum power interval value at the time of the trip protection signal switching - the minimum power interval value at the time of the trip protection signal switching.

[0092] Specifically, combined Figure 2 and Figure 3 As shown in the figure, since transient calculation cannot be infinitely precise, the precision of transient calculation results can be controlled by the interval threshold, for example, the interval threshold is set to 10 -8 FP; After the refinement process in step B13, the power range when the shutdown protection signal switches is 10 -8 FP~10 -7 FP, full power range when the shutdown protection signal is switched = 10 -7 FP-10 -8 FP=9×10 -8 FP, the power range when the shutdown protection signal switches is 9×10 -8 FP> interval threshold 10 -8 FP; then return to step B12, continue to switch the power interval 10 when the shutdown protection signal -8 FP~10 -7 FP performs transient calculations (e.g. for rated power 10 -8 FP, 2×10 -8 FP, 3×10 -8 FP,......10 -7 FP respectively expands transient calculation), obtains the power interval 10 when the period protection signal is triggered -8 FP~5×10 -8 FP, power range when over power protection signal is triggered 6×10 - 8 FP~10 -7 FP; then perform step B13, based on the power interval 10 when the periodic protection signal is triggered -8 FP~5×10 -8 FP, power range when over power protection signal is triggered 6×10 -8 FP~10 -7 FP, power range when the shutdown protection signal switches 10 -8 FP~10 -7 FP is refined to reduce the power range when the shutdown protection signal is switched to 5×10 -8 FP~6×10 -8FP; After refined processing, the power range when the shutdown protection signal switches is 6×10 -8 -5×10 -8 =10 -8 FP, the power range when the shutdown protection signal switches is 10 -8 FP=interval threshold 10 -8 FP, proceed to step B15.

[0093] Step B15: Obtain the transient operating condition based on the power interval when the shutdown protection signal switches. Specifically, select the lower power in the power interval when the shutdown protection signal switches as the initial power of the transient operating condition, and do not trust the short-period signal to obtain a reasonable and conservative transient operating condition; for example, select the power interval 5×10 -8 FP~6×10 -8 Minimum value in FP 5×10 -8 FP is the transient initial power of the transient condition. In the transient calculation, the periodic protection is not considered and only the overpower protection is used. Then, during the transient process of the accident, the initial power of the transient condition is 5×10 -8 The maximum power to which FP can be increased is taken as the transient peak power of the transient condition. The transient initial power and the transient peak power of the transient condition are both determined. The transient peak power of the transient condition obtained in this way is the largest and most conservative.

[0094] Step C1: Based on the transient peak power and the initial thermal parameters of the reactor, obtain the transient MDNBR corresponding to the first analysis stage, including:

[0095] The transient core peak heat flux is obtained based on the transient peak power. In actual accidents, the heat flux is lower than the power. However, since only neutron dynamics calculations were performed in the first analysis phase, a conservative assumption was made that the transient heat flux and power change synchronously. Based on the influence of core structural parameters, core axial power distribution, and radial power peaking factor, a conservative assumption was made that the transient core peak heat flux / reactor initial heat flux = transient peak power / reactor initial power.

[0096] The critical heat flux is obtained based on the initial thermal parameters. Specifically, the core inlet parameters remain essentially unchanged, and no thermal transmission calculations are performed in this phase. Therefore, in the first analysis phase, the initial thermal parameters can be used, conservatively assuming the core flow velocity is zero. These initial thermal parameters are substituted into the zero-flow CHF relationship to calculate the critical heat flux. The zero-flow CHF relationship can be constructed using, for example, the SUDO zero-flow relationship.

[0097] The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state, where DNBR (Departure from Nucleate Boiling Ratio) is the core departure from nucleate boiling ratio, and MDNBR is the minimum core departure from nucleate boiling ratio. Specifically, the transient MDNBR satisfies:

[0098] .

[0099] Step D1: Verify whether the transient MDNBR corresponding to the first analysis stage meets the acceptance criteria, including:

[0100] The transient MDNBR is compared with the safety analysis limit. The safety analysis limit is the safety limit specified in the DNBR acceptance criteria. The DNBR acceptance criteria is a safety criterion in the reactor thermal hydraulic design to prevent the fuel elements from deviating from nucleate boiling. For example, the safety analysis limit is set to a value greater than 1, such as 1.3 or 1.5. If the transient MDNBR is greater than the safety analysis limit, the acceptance criteria are met and the accident analysis is completed. If the transient MDNBR is less than or equal to the safety analysis limit, the system optimization operation is performed and the process returns to step B1, or the second analysis phase is performed until the transient MDNBR is greater than the safety analysis limit. The system optimization operation includes at least one of the following:

[0101] Under the premise of meeting the reactor power regulation requirements, reduce the control rod value and the maximum rod withdrawal rate to minimize the reactivity introduction rate after the accident;

[0102] Added rod lifting lockout protection signal, including setting the maximum one-time continuous rod lifting distance and the rod lifting lockout setting value below the shutdown setting value;

[0103] When the operating range is met, the protection signal setting value should be reduced as much as possible to trigger the shutdown protection as soon as possible after an accident.

[0104] Optimize protection logic, instrumentation, etc. to shorten the delay time for implementing the shutdown protection function;

[0105] When the rod control system meets the normal power increase requirements, the one-time rod lifting distance is limited;

[0106] Optimize the loop structure of the natural circulation model to reduce the loop resistance of the natural circulation model.

[0107] System optimization operations are commonly used optimization designs in the accident analysis process and will not be described in detail here.

[0108] The second analysis phase, compared to the first, simultaneously performs neutron dynamics and thermal-hydraulic calculations and considers reactivity feedback in transient conditions. This is equivalent to taking into account the phenomenon that the post-accident peak core heat flux / initial reactor heat flux is less than the transient peak power / initial reactor power. This eliminates the conservative assumption in the first analysis phase that the post-accident peak core heat flux / initial reactor heat flux = the transient peak power / initial reactor power. Therefore, the conservative margin can be released, and the transient MDNBR obtained is more accurate. Specifically, the second analysis phase includes:

[0109] Step A2: Provide a first coupled computational model for accident analysis. The first coupled computational model includes a second thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model. The second thermal-hydraulic model, the core thermal component model, and the second neutron kinetic model are coupled. The second thermal-hydraulic model is a fixed-flow boundary model with a forced flow. The core thermal component model can be used in conjunction with the second neutron kinetic model and the second thermal-hydraulic model to calculate transient changes in fluid parameters and peak core heat flux density. The second neutron kinetic model contains reactive feedback, which can affect the calculation of transient peak power.

[0110] Step B2: A forced flow rate is set in the second thermal-hydraulic model, and under the forced flow rate set in the second thermal-hydraulic model, a power spectrum analysis is performed on the trip protection signal according to a first coupled calculation model including the second thermal-hydraulic model, the core thermal component model, and the second neutron kinetic model to determine the transient operating conditions during the accident transient process, and then obtain transient thermal parameters corresponding to the transient operating conditions.

[0111] The power spectrum analysis in step B2 may be performed with reference to step B1, and specifically includes:

[0112] Step B21: setting the power range for switching the shutdown protection signal;

[0113] Step B22: First, under the set forced flow rate and in combination with the first coupled calculation model, a transient calculation is performed on the power range when the trip protection signal is switched, to obtain the power range when the period protection signal is triggered and the power range when the overpower protection signal is triggered;

[0114] Step B23: Based on the power interval when the period protection signal is triggered and the power interval when the overpower protection signal is triggered, the power interval when the shutdown protection signal is switched is processed in a refined manner;

[0115] Step B24: Compare the full power range when the trip protection signal is switched with the interval threshold. If the full power range when the trip protection signal is switched is less than or equal to the interval threshold, proceed to step B25. If the full power range when the trip protection signal is switched is greater than the interval threshold, return to step B22 until the full power range when the trip protection signal is switched is less than or equal to the interval threshold.

[0116] Step B25: Obtain the transient operating condition based on the power range when the shutdown protection signal is switched.

[0117] The transient operating condition obtained in step B2 is similar to the transient operating condition obtained in step B1. The two can be the same or different. For example, in the transient operating condition obtained in step B2, the initial power is 5.2×10 -8 FP, peak power is the initial power 5.2×10 -8 The maximum power that FP can reach during an accident transient is provided here for illustrative purposes only. The specific power is subject to calculation results and is not detailed here. Transient thermal parameters include transient power, transient heat flux, transient temperature, and transient pressure. The set forced flow rate affects the final results of these transient thermal parameters.

[0118] Step C2: Verify the conservatism of the forced flow in step B2. Specifically include:

[0119] Step C21: Provide a natural circulation model for analyzing forced flow.

[0120] Step C22: Based on the transient peak power of the transient thermal parameters obtained in step B2, a steady-state natural circulation calculation is performed using a natural circulation model to obtain the maximum natural circulation flow rate under the accident transient condition.

[0121] Step C23: Check the forced flow rate.

[0122] like ; The maximum natural circulation flow is much smaller than the forced flow, and the set forced flow is too conservative, so the forced flow is reduced to release the margin and return to step B2; wherein, the flow conservative threshold is used to control the conservative degree of the forced flow, and the flow conservative threshold can be taken as 20%.

[0123] If the maximum natural circulation flow rate is greater than or equal to the forced flow rate, the set forced flow rate is not conservative, and the forced flow rate is increased to release the margin, and the process returns to step B2.

[0124] like ; The set forced flow is conservative, then the conservative verification of the forced flow is completed and the process goes to step D2.

[0125] Step D2: Based on the transient thermal parameters, obtain the transient MDNBR corresponding to the second analysis stage, including:

[0126] The critical heat flux is obtained based on the transient thermal parameters of the verified forced flow rate in step C2. Specifically, the core flow rate can be conservatively assumed to be zero. The transient thermal parameters of the verified forced flow rate are substituted into the zero-flow CHF relationship constructed using the SUDO zero-flow relationship, etc., to calculate the critical heat flux.

[0127] The first coupled calculation model directly outputs the transient core peak heat flux density. Specifically, the first coupled calculation model performs calculations to output a transformation curve for the transient core peak heat flux density. The transient core peak heat flux density obtained in the second analysis phase is smaller than that obtained in the first analysis phase, but still greater than the actual core peak heat flux density, indicating that the result is conservative.

[0128] The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state, that is,

[0129] .

[0130] Step E2: Verify whether the transient MDNBR corresponding to the second analysis phase meets the acceptance criteria, including:

[0131] Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is greater than the safety analysis limit, the acceptance criteria are met and the accident analysis ends. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B2, or execute the third analysis phase until the transient MDNBR is greater than the safety analysis limit.

[0132] The third analysis phase is executed. Natural circulation calculations are performed in the third analysis phase, and the critical heat flux density under natural circulation flow is considered. This further releases the conservative margin in transient heat flux and critical heat flux calculations. The obtained transient MDNBR is more accurate. Specifically, the third analysis phase includes:

[0133] Step A3: Provide a second coupled computational model for accident analysis. This second coupled computational model includes a third thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model. The third thermal-hydraulic model includes a natural circulation loop. Loop resistance affects the natural circulation flow rate. A greater loop resistance results in a smaller natural circulation flow rate and, consequently, a poorer ability to remove core heat. Loop resistance can be determined using reliable methods such as empirical formulas, computational fluid dynamics (CFD), or hydraulic testing, and will not be detailed here.

[0134] Step B3: Perform power spectrum analysis on the shutdown protection signal according to the second coupling calculation model to determine the transient operating conditions during the accident transient process, and then obtain the transient thermal parameters corresponding to the transient operating conditions. The power spectrum analysis in step B3 can be performed with reference to step B1; specifically, first, combine the second coupling calculation model to perform transient calculation on the initial power interval when the shutdown protection signal switches, and finally determine the power interval when the shutdown protection signal switches, and then obtain the transient operating conditions based on the power interval when the shutdown protection signal switches. The transient operating conditions obtained in step B3 can be the same as or different from the transient operating conditions obtained in step B1. For example, in the transient operating conditions obtained in step B3, the initial power is 5.3×10 - 8 FP, peak power is the initial power 5.3×10 -8 The maximum power to which FP can be increased during the transient process of an accident.

[0135] Step C3: Obtaining the transient MDNBR corresponding to the third analysis stage based on the transient thermal parameters, including:

[0136] Based on the transient thermal parameters in step C3, the parameters are substituted into the zero flow CHF relationship formed by the SUDO zero flow relationship and the like to perform calculations, thereby obtaining the critical heat flux.

[0137] The second coupled calculation model directly outputs the transient core peak heat flux. Specifically, the second coupled calculation model performs calculations to output a transformation curve for the transient core peak heat flux. The transient core peak heat flux obtained in the third analysis phase is smaller than that obtained in the second analysis phase.

[0138] The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state, that is,

[0139] .

[0140] Step D3: Verify whether the transient MDNBR corresponding to the third analysis stage meets the acceptance criteria, including:

[0141] Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is greater than the safety analysis limit, the acceptance criteria are met and the accident analysis ends. If the MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B3 until the transient MDNBR is greater than the safety analysis limit.

[0142] In summary, this scheme proposes a staged accident analysis method for plate fuel pool reactors. Specifically, in the first analysis stage, only neutron dynamics calculations are carried out, avoiding the calculation of natural circulation flow and heat flux. It is conservatively assumed that transient peak power / reactor initial power = core transient peak heat flux density / reactor initial heat flux density. At the same time, it is conservatively considered that heat flux and power change synchronously in the transient state. The analysis process is the simplest and can greatly improve the work efficiency of accident analysis. In the second analysis stage, the first coupled calculation model including the second thermal-hydraulic model, the core thermal component model and the second neutron dynamics model is adopted. Compared with the first analysis stage, the verification of forced flow and the acquisition of transient thermal parameters are added, which can release the conservative margin and improve the work efficiency to a certain extent. In the third analysis stage, the natural circulation flow is calculated to further release the conservative margin.

[0143] In the accident analysis method of the present application, the first analysis stage to the third analysis stage can not only independently complete the analysis of the accident, but also progressively complete the analysis of the accident stage by stage. For example, the first analysis stage to the second analysis stage are sequentially executed to complete the analysis of the accident, or the second analysis stage to the third analysis stage are sequentially executed to complete the analysis of the accident, or the first analysis stage to the third analysis stage are sequentially executed to complete the analysis of the accident. For the accident analysis that is completed step by step from the first analysis stage to the third analysis stage, if the rod-related design requirements in the reactor design are not high, the conservative use of the first analysis stage can also pass the acceptance of the MDNBR in the transient state, which greatly improves the design efficiency; if the first analysis stage cannot pass the acceptance of the MDNBR in the transient state, further system optimization operations can be carried out or the second analysis stage can be entered to release the margin, which can also relatively improve a certain efficiency; if the second analysis stage cannot pass the acceptance of the MDNBR in the transient state, further system optimization operations can be carried out or the third analysis stage can be entered to release the margin, and the third analysis stage requires additional natural circulation simulation verification work; if the third analysis stage cannot pass the acceptance of the MDNBR in the transient state, further system optimization operations can be carried out until the acceptance of the MDNBR in the transient state is achieved.

[0144] In summary, by analyzing the accident in a progressive stage-by-stage manner, while ensuring the accuracy of the accident analysis, the analysis process can be simplified and redundant steps can be reduced, thereby ensuring the design efficiency of the reactor.

[0145] Although the present invention is disclosed above with reference to preferred embodiments, this is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications without departing from the spirit and scope of the present invention. Therefore, any modifications, equivalent variations, and modifications made to the above embodiments in accordance with the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for analyzing an uncontrolled lifting accident of a control rod in a reactor, characterized in that: The method includes performing a first analysis phase, wherein the first analysis phase includes: Step A1: providing a first neutron kinetic model for accident analysis and setting a reactor trip protection signal; the reactor trip protection signal includes an overpower protection signal and a cycle protection signal; Step B1: performing power spectrum analysis on the trip protection signal according to the first neutron kinetic model to obtain transient peak power during the accident transient process; Step C1: Obtaining the transient MDNBR based on the transient peak power and the initial thermal parameters of the reactor; Step D1: Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and then return to step B1 until the transient MDNBR is greater than or equal to the safety analysis limit, wherein the safety analysis limit is greater than 1.

2. The accident analysis method according to claim 1, characterized in that: It also includes executing the second analysis phase, and the step D1 further includes: if the MDNBR in the transient state is less than or equal to the safety analysis limit, then executing the system optimization operation and then returning to executing step B1, or executing the second analysis phase; The second analysis phase includes: Step A2: providing a first coupled computational model for accident analysis, the first coupled computational model including a second thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model; Step B2: setting a forced flow rate in the second thermal-hydraulic model, and performing a power spectrum analysis on the trip protection signal according to the first coupled calculation model to obtain transient thermal parameters during the accident transient process; Step D2: obtaining the transient MDNBR based on the transient thermal parameters; Step E2: Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B2 until the transient MDNBR is greater than or equal to the safety analysis limit.

3. The accident analysis method according to claim 2, characterized in that: The second analysis phase also includes: Step C2: Verify the conservatism of the forced flow in step B2; Step C2 includes: Step C21: providing a natural circulation model for analyzing forced flow; Step C22: Perform steady-state natural circulation calculations on the transient thermal parameters obtained in step B2 according to the natural circulation model to obtain the maximum natural circulation flow rate under the accident transient condition; Step C23: Check forced flow; if the maximum natural circulation flow ≥ forced flow, or , then adjust the forced flow and return to step B2 until .

4. The accident analysis method according to claim 3, characterized in that: The step C23 includes: , then reduce the forced flow rate and return to step B2; If the maximum natural circulation flow rate is greater than or equal to the forced flow rate, increase the forced flow rate and return to step B2.

5. The accident analysis method according to claim 3, characterized in that: The step D2 comprises: Based on transient thermal parameters, critical heat flux is obtained; Outputting the transient core peak heat flux density through the first coupled calculation model; The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state.

6. The accident analysis method according to claim 2, characterized in that: The method further includes executing a third analysis phase, wherein step E2 further includes: if the transient MDNBR is less than or equal to the safety analysis limit, performing system optimization operations and then returning to step B2, or executing the third analysis phase; the third analysis phase includes: Step A3: providing a second coupled computational model for accident analysis, the second coupled computational model including a third thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model; Step B3: performing power spectrum analysis on the trip protection signal according to the second coupling calculation model to obtain transient thermal parameters during the transient accident process; Step C3: Obtaining transient MDNBR based on transient thermal parameters; Step D3: Compare the transient MDNBR with the safety analysis limit; if the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B3 until the transient MDNBR is greater than the safety analysis limit.

7. The accident analysis method according to claim 6, characterized in that: The step C3 comprises: Based on the transient thermal parameters in step C3, the critical heat flux is obtained; Outputting the core peak heat flux density in transient state through the second coupled calculation model; The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state.

8. The accident analysis method according to claim 1, characterized in that: The step B1 comprises: Step B11: setting the power range for switching the shutdown protection signal; Step B12: performing transient calculation on the power range when the trip protection signal is switched according to the first neutron kinetic model to obtain the power range when the period protection signal is triggered and the power range when the overpower protection signal is triggered; Step B13: Based on the power interval when the period protection signal is triggered and the power interval when the overpower protection signal is triggered, finely processing the power interval when the shutdown protection signal is switched; Step B14: Compare the full power range when the trip protection signal is switched with the interval threshold. If the full power range when the trip protection signal is switched is greater than the interval threshold, return to step B12 until the full power range when the trip protection signal is switched is less than or equal to the interval threshold. Here, the full power range when the trip protection signal is switched = the maximum power interval value when the trip protection signal is switched - the minimum power interval value when the trip protection signal is switched. Step B15: Obtain a transient operating condition based on the power range when the shutdown protection signal is switched.

9. The accident analysis method according to claim 1, characterized in that: The step C1 comprises: Obtaining the transient core peak heat flux based on the transient peak power; Obtain critical heat flux based on initial thermal parameters; The transient MDNBR is obtained based on the core peak heat flux and critical heat flux in the transient state.

10. The accident analysis method according to claim 1, characterized in that: System optimization operations include at least one of the following: Reduce the control rod value and reduce the maximum rod lifting rate; Added lifting rod locking protection signal; Reduce the setting value of the reactor protection signal; Shorten the delay time for implementing the shutdown protection function.

11. A method for analyzing an uncontrolled lifting accident of a control rod in a reactor, characterized in that: The method includes performing a second analysis phase, wherein the second analysis phase includes: Step A2: providing a first coupled computational model for accident analysis and setting a reactor trip protection signal; the reactor trip protection signal includes an overpower protection signal and a cycle protection signal; wherein the first coupled computational model includes a second thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model; Step B2: setting a forced flow rate, and performing a power spectrum analysis on the trip protection signal according to the first coupling calculation model to obtain a transient operating condition; Step D2: Obtaining transient MDNBR based on transient thermal parameters; Step E2: Compare the transient MDNBR with the safety analysis limit. If the transient MDNBR is less than or equal to the safety analysis limit, perform system optimization and return to step B2 until the transient MDNBR is greater than the safety analysis limit.

12. The accident analysis method according to claim 11, characterized in that: It also includes executing the third analysis phase, and the step E2 further includes executing the system optimization operation and returning to the step B2 if the transient MDNBR is less than or equal to the safety analysis limit, or executing the third analysis phase; The third analysis phase includes: Step A3: providing a second coupled computational model for accident analysis and setting a reactor trip protection signal; the reactor trip protection signal includes an overpower protection signal and a cycle protection signal; wherein the second coupled computational model includes a third thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model; Step B3: performing power spectrum analysis on the trip protection signal according to the second coupling calculation model to obtain transient thermal parameters; Step C3: Obtaining transient MDNBR based on transient thermal parameters; Step D3: Compare the transient MDNBR with the safety analysis limit; if MDNBR ≤ the safety analysis limit, perform system optimization operations and return to step B3 until the transient MDNBR > the safety analysis limit.

13. A method for analyzing an uncontrolled lifting accident of a control rod in a reactor, characterized in that: This includes conducting the third phase of analysis; The third analysis phase includes: Step A3: providing a second coupled computational model for accident analysis and setting a reactor trip protection signal; the reactor trip protection signal includes an overpower protection signal and a cycle protection signal; wherein the second coupled computational model includes a third thermal-hydraulic model, a core thermal component model, and a second neutron kinetic model; Step B3: performing power spectrum analysis on the trip protection signal according to the second coupling calculation model to obtain transient thermal parameters; Step C3: Obtaining transient MDNBR based on transient thermal parameters; Step D3: Compare the transient MDNBR with the safety analysis limit; if MDNBR ≤ the safety analysis limit, perform system optimization operations and return to step B3 until the transient MDNBR > the safety analysis limit.

Images