Systematic source term analysis for level 3 PSA of a BWR with Mark-II type containment with THALES-2 code

The THALES-2 code is an integrated severe accident analysis code developed at the Japan Atomic Energy Research Institute (JAERI) in order to simulate the accident progression and transport of radioactive material for probabilistic safety assessment (PSA) of a nuclear power plant. As part of a level 3 PSA being performed for a 1,100 MWe BWR-5 with a Mark-II containment, a series of calculations were performed by THALES-2 to evaluate the source terms for extensive accident scenarios. This paper presents the methods and insights on the dependency of source terms on the core damage sequences and the containment function failure scenarios from this analysis. As for core damage sequences to be analyzed, 5 following representative core damage sequences were selected from the core damage sequences systematically identified by PSA works at JAERI; (1) Loss of containment heat removal with high pressure injection available, (2) Loss of containment heat removal with low pressure injection available, (3) Loss of coolant injection to the core, (4) Station blackout and (5) Anticipated Transient Without Scram (ATWS) with high pressure injection available. As for containment function failure scenarios, this analyses covered the following 4 containment function failure scenarios: overpressure in drywell, overpressure in wetwell, controlled release by unfiltered containment venting and accident termination by drywell spray recovery. All physically possible combinations (15 scenarios) above core damage sequences and containment function failure scenarios were analyzed by THALES-2 to determine the source terms. Results and insights from the analyses were that 1) the calculated release fractions of CsI and CsOH to the environment were in the range of 0.01 to 0.1 for late containment overpressure failure cases, and the release fractions for the containment venting cases were one order of magnitude smaller than that of over-pressure case and those for drywell spray recovery cases where no containment failure occurred were two orders of magnitude smaller than the containment venting cases, 2) the governing factors for source terms of Iodine and Cesium are different depending on whether the containment fails before core melt or not, 3) the containment venting, which is one of the accident management measures, can be expected to reduce source terms of I and Cs if suppression pool bypass is avoided. (author)