Research On Fission Product Release And Transport Under Severe Accident Conditions

When severe accidents(SAs)occur in PWR nuclear power plants(NPPs),large amounts of radionuclides may release with the degradation of the core.Once the containment also fails,these radioactive fission products(FPs)will escape to the environment,which may endanger the environment and public health.By analyzing FP release and transport behavior under different SA conditions and the factors that may influence FP behavior,references could be provided for the formulation of SA mitigation measures,which can satisfy the need for SA management.In addition,achieved results could also be used to confirm SA source terms,which are meaningful for the dividing of emergency planning zone.In this paper,600 MW PWR NPP are selected as the research NPP and its SA simulation model has been built using MELCOR code.By employing this model,related accident progression,thermal hydraulic response,detailed FP release fraction and release location,as well as FP's distribution and chemical forms have been investigated under LBLOCA initiated,SBO initiated and SGTR initiated SA conditions.Besides,the effects of break size,break location,containment sprays,fission product release models and main coolant pump leakage on fission product behavior have also been studied.At last,four types of containment failure modes——containment early failure caused by hydrogen detonation,containment early isolation failure,containment late overpressure failure and containment by-pass accident were selected to simulate and their effects on FP behavior have been investigated.The results reveal that: The total release fractions of noble gas are close under the three types of SA conditions;The total release fraction of non-volatile FP under SBO initiated SA condition is much less than that under LBLOCA initiated SA condition,while the total release fraction of non-volatile FP under SGTR initiated SA condition is between those under the other two SA conditons;The total release fraction of volatile CsI group under SBO initiated SA condtion is the highest among the three conditions.When LBLCOA initiated SA occurs: Break location has less impact on the release and transport features of noble gas;The total release fraction of non-volatile FPs is sensitive to break location;The distribution of volatile FPs is sensitive to break location;For three types of FPs,their total release fractions and their fractions that escape to the containment are all sensitive to break size;The FP release models have a remarkable effect on FP release and transport behavior so that the use of FP release models should be considered seriously.When SBO initiated severe accident occurs,main coolant pump leakage has a notable impact on the total release fractions of the non-volatile alkaline earths group and the volatile CsI group,as well as the distribution of Cs I group.The release process of noble gas and CsI group displays a longer duration under containment early isolation failure mode,while the release process of non-volatile alkaline earths group lasts for a relatively longer period of time under containment late overpressure failure mode.