Research On Source Term Rapid Evaluation Method During Severe Accident For AP1000 Nuclear Power Plant

In severe accidents of nuclear power plant,large amounts of radioactive fission products are released to the containment.If the containment failed,the radioactive fission products would be released to the environment.Timely and accuracy assessment of the released fission products can provide theoretical basis for severe accident mitigation measures and nuclear emergency responses.There are two traditional methods to evaluate the source term,the method utilizing the integral code MELCOR and the rapid method using parameter source term.The severe accident integral code has been usually used in safety analysis for the nuclear power plant.However,it has difficulty in using for nuclear emergency with the reason that the method is time-cosuming.The parametric source term was used in nuclear emergency.However,the parametric source term was initially proposed for the second-generation reactor.Whether it is applicable for the third-generation reactor needs to be further validated.Moreover,it has difficulty in simulating changes in release fraction along with accident processes.In this paper,the AP1000 nuclear power plant has been selected as the research object.The following four aspects has been studied for the source term rapid evaluation,the accumulation of fission products in core,the release of the fission products in the core,the natural removal of the fission products aerosol suspended in the containment and the influence of the decay and ingrowth processes on the release of the fission products.The source term rapid evaluation code RIST has been developed,mainly including the database module,the release module,the natural removal module and the decay module.Each module has been verified by ORIGEN2,MELCOR or rapid evaluation method.Main researches and conclusions are as follows:(1)The development of the source term rapid evaluation systemThe framework of source term rapid evaluation code has been designed and the RIST code has been developed.The RIST system mainly includes database module,the release module,the natural removal module and the decay module.(2)Study on the calculation method for accumulation of fission products in the core based on the power distribution.The traditional calculation method has been improved in this paper.The accumulation of the fission products has been calculated based on the power distribution.The axial and radial orientation power distribution of AP1000 reactor was calculated by the MCNP code.Then the core was devided into several regions based on the axial and radial orientation power distribution.The point depletion code ORIGEN2 has been used to calculate the accumulation for each region.The results calculated by ORIGEN2 are taken as the MCNP's input parameter for the next burnup step.The final results can be obtained through repeating the above steps.The results were compared with those calculated by the traditional method.The improved method is recommended to calculate the accumulation of fission products.The accumulation of fission products for AP1000 has been obtained with the improved method.(3)The release of fission products rapid evaluation module has been developedThe release of fission products rapid evaluation module ISST has been developed through the study on the release of core fission products.The AP1000 nuclear power plant has been selected as the research object.Three other methods have been used to verify the ISST module,including the method using MELCOR,the method utilizing the parameter source term in NUREG-1465 report and the method in IAEA-TECDOC-1127 report.The following conclusions can be obtained.(a)Compared with the results calculated with the IAEA's method,the results calculated with ISST module are closer to those calculated using MELCOR.(b)Among the models integrated into ISST module,CORSOR-M model is more conservative than the CORSOR-BOOTH model,the ORNL-BOOTH model and the Kress-Booth model.(c)The ISST module can model the phenomenon that the water in CMTs and ACCs can obviously delay the release of fission products.Based on the comparative analysis of the results obtained from ISST,MELCOR and NUREG-1465,it can be concluded that ISST can calculate the source term release fraction faster than MELCOR.(4)The natural removal of fission products aerosol in containment for AP1000 rapid evaluation module has been developedThe natural removal of fission products aerosol suspended in containment for AP1000 rapid evaluation module NPR has been developed through the study on the rapid evaluation methods.Both the NPR module and the integral severe accident code MELCOR have been used to study the natural removal of the fission products aerosol in passive containment of AP1000 nuclear power plant.The NPR mudule includes two rapid evaluation methods,the method in NUREG/CR-6189 report and the method through the combination of ISST mudule and the natural removal coefficients in AP1000 design control document.Three typical severe accidents,induced by large break loss of coolant,small break loss of coolant accident and steam generator tube rupture,respectively,are selected for the study on the method utilizing MELCOR.The results obtained by two methods were further compared in the following aspects: efficiency of aerosol remocal by natural processes and the fraction of aerosol suspended in containment.It can be concluded that the method in NUREG/CR-6189 report is conservative;compared with the results calculated by the method in NUREG/CR-6189,the results calculated by the method combinating the ISST mudule and the natural removal coefficients in AP1000 design control document are closer to the results obtained by MELCOR.Two states of the containment,work and failure,are studied in the paper.It can be concluded that the results calculated with the method in NUREG/CR-6189 report are close to those obtained by MELCOR for the failure case of containment,while the results calculated by the method combinating the ISST mudule and the natural removal coefficients in AP1000 design control document are close to those obtained by MELCOR for the work case of containment.(5)The decay module for fission products has been developedThe influence of decay on the source term evaluation has been studied.The decay module DAIC has been developed by FORTRAN language and verified by ORIGEN2.Then,the influence of decay on the fission products release has been studied.The release fraction and suspended fraction were obtained.Through the analysis of the results,the following conclusions can be drawn.(a)Decay has littile influence on the release fraction of long half-time nuclides,whose release fractions are close to those discounting decay.The release fraction is on the rise as a whole along time.(b)Decay has obvious influence on the release fraction of the short half-time nuclides,whose release fraction curves rise first and then decrease.(6)The verification of the source term rapid evaluation codeThe source term rapid evaluation code RIST has been verified in two aspects,calculating results and time.The release fractions of fission products show that the release fractions of Kr,Xe and Cs calculated with RIST code are close to those obtained through the method combing MELCOR code and DAIC module;the release fraction of halogen element iodine is conservative.The suspended fractions of fission products aerosol show that the results calculated by RIST code are basically consistent in the curves ascending stage with those calculated through the method combing MELCOR code and DAIC module.The results calculated by RIST code are conservative in the curves decent stage.It is found through the comparison of the calculating time that the RIST code has a significant reduction in time for source term evaluation.