Simulation Research Of Core Behavior Under Beyond Design Basic Accident In Pressurized Water Reactors

After Fukushima Daiichi Accident in Japan,the Beyond Design Basic Accident(BDBA)has become a topic among researches in nuclear energy field.Under BDBA in PWRs,the core behavior is very complicated and uncertain.In order to deepen analysis of core damage process during BDBAs and establish mitigation measures for the accident,it is necessary to research on core behavior phenomena and reaction mechanism of core materials further.In this paper,taking the core of PWR as the research object,a program for simulating the core behavior under BDBAs has been developed and coupled with RELAP5 which is the best estimation code for reactor thermal hydraulics at present.The core material behavior under a BDBA initiated by Large Break Loss of Coolant Accident(LBLOCA)has been analyzed in detail.It provides a reference for establishing accident mitigation measures and developing analysis programs of the core behavior.Main researches and conclusions are as follows:(1)A code for analyzing core behavior during BDBAs has been developed and verified.According to the reference manuals and the calculation process of MELCOR1.8.5 which is an integrated code used for the accident transient analysis,the models of core behavior have been built and the Severe Accident Program(SAP)for analyzing the core behavior under BDBAs has been developed.The interface modules have also been provided in main program for coupling the thermal-hydraulic analysis code RELAP5.For adding the properties of new fuel materials,the cladding and the control poison to the program,the tables of materials properties and subroutine for materials property checking have been built and developed,respectively.Taking Korea Ulchin-3&-4 Nuclear Power Reactor as a research object,a BDBA initiated by Total Feed Water Loss has been selected to verify the SAP used MELCOR 1.8.5.The Results of analysis show that the calculation results of SAP match well with the MELCOR 1.8.5,and the SAP can simulate the core behavior with higher accuracy.(2)The coupling between SAP and RELAP5 has been completed.Based on the completion and verification,SAP has been successfully coupled with the thermal-hydraulic analysis code RELAP5.The coupling conditions and scheme,as well as the coupling start-up criteria were raised and discussed to make sure the rationality of this coupling.The variables exchanged between RELAP5 and SAP have discussed in detail.The coupling between RELAP5 and SAP is a beneficial exploration of coupling between the mechanistic and integrated analysis codes.It not only would be helpful for increasing the calculation accuracy with combination of advantages from the codes,but also provide the foundation for the improvement of models in the future.(3)The behavior analysis models of zircalloy cladding in SAP have been ameliorated.Based on the oxidation kinetics of zirconium alloy in high-temperature steam,the oxidation reaction models have been studied.In order to achieve the accuracy and comprehensiveness of the zircalloy cladding behaviors,the verification of the oxidation reaction models with the steam has been performed and both of oxidation models with uranium oxide and effective stress models have been added into the SAP.The QUENCH-06 bundle test has been accepted to verify the oxidation reaction rate equations of the zircalloy with steam.The verification results show that the Cathcart-Pawel&Volchek-Zvonarev correlations have superiority on the simulation accuracy.According to the analysis results of oxidation behavior of the zircalloy,the oxidation reaction inside the cladding has a significant effect on cladding behavior as outside dose.In order to simulate the gap release of cladding,effective stress models of cladding has been added to the SAP.(4)The eutectic failure models of materials in SAP have been amended.In order to improve the eutectic reaction calculations,the commonly used failure mechanisms and the eutectic behavior of the core materials have been analyzed.The models of the failure and eutectic interaction have been added into the SAP program based on the study of the failure mechanism and the eutectic interaction among materials.Many complex eutectic behaviors of core materials which contact with each other in core have been discussed in detail.Meanwhile,the eutectic kinetics and models of the material couple Zr/AgInCd,Zr/SS(Stainless Steel),Zr/Inconel,Zr/B4Cand B4C/SS have been researched.In SAP,the eutectic failure models of materials have been turned from the parameters input to mechanism calculation.According to the discussion about the mixture properties after eutectics,the calculation methods of mixture temperatures and properties have been given.(5)The core behavior under BDBAs has been analyzed with the coupling codes.Taking 600MW Qinshan Phase INPP as the research object,utilizing coupling code of RELAP5 and SAP,the nodalization of the NPP and the core have been set up.Cold leg double-ended guillotine break accident with loss of offsite power has been selected as the initial accident.Studies and analyses on the BDBA have been completed with the thermal-hydraulic conditions provided by RELAP5 during coupling procession.Based on the phenomena mechanism,the effective stress of zircalloy cladding,the oxidation behavior and the eutectic behavior between the cladding and uranium dioxide have been analyzed.The analysis results show that the cladding surface temperature is higher than 1173 K when the gap release;the oxidation behavior on the inner surface of cladding makes the same contribution to the cladding damage as the oxidation reaction on the outer surface.On the basis of the research results of the eutectic reaction and mass transference of the Inconel support grids,the control rod,the cladding components and the fuel components,it is concluded that the failure of a single kind of core materials would lead to early failure of other components around it and accelerate the procession of core damage.