Evaluation of a Containment Failure Frequency Considering Mitigation Accident Managements for a Japanese PWR Plant

摘要

Institute of Nuclear Safety of NUPEC is carrying out a program of developing methodology of Probabilistic Safety Assessment (PSA) and severe accident analysis. For a Japanese 4-loop PWR plant with a pre-stressed concrete containment, containment failure frequency evaluation methods by point-estimate and uncertainty estimate were established for internal initiating events during full power operation assuming mitigation accident management. In the point-estimate evaluation, core damage sequences were categorized for every plant damage state (PDS) using the results of Level 1 PSA that evaluated core damage frequencies. Sequences both without and with mitigation AM countermeasures were analyzed with the MELCOR code: (1) the natural convection cooling by containment cooling units for normal operation, (2) fire water injection into the containment, (3) the forced depressurization of primary system by pressurizer PORVs, (4) the restoration of containment spray system, and (5) water injection into the primary system by charging pumps. A containment event tree including the AM measures was made, and the simplified reliability evaluation on equipment failure and human factor at AM operation was executed. Severe accident sequence representing each PDS was analyzed with MELCOR code in order to quantify the branch probability of a containment event tree. The quantification of containment event tree was done for each plant damage state. Consequently, in the case of AMs included, the total containment failure frequency was obtained to be 1.1×10~(-7) / reactor year comparing 2.2×10~(-7) / reactor year with without AMs. A dominant sequence of containment failure when the AM plan is implemented is an interface system LOCA sequence that a pipe of the residual heat removal system breaks loaded primary system pressure.