RELAP5 code post-test analysis was performed on one of abnormal transient tests conducted with the ROSA/large scale test facility (LSTF) simulating a PWR station blackout (SBO) transient with the TMLB' scenario in 1995. The TMLB' scenario involves prolonged complete loss of alternating current power and unavailability of turbine-driven auxiliary feedwater as well as malfunction of primary- and secondary-system relief valves. The LSTF test revealed core uncovery by core boil-off took place a little after hot leg became empty of liquid while the primary pressure was kept high. The RELAP5 code predicted well the overall trend of the major phenomena observed in the LSTF test, and indicated remaining problems in the predictions of reverse flow U-tubes in steam generator (SG) during long-term single-phase liquid natural circulation. Sensitivity analyses were performed further to clarify effectiveness of depressurization of and coolant injection into SG secondary-side as accident management measures for core cooling, based on the RELAP5 post-test analysis. SG secondary-side depressurization was initiated by fully opening the safety valve in one of two SGs with the incipience of core uncovery. Coolant injection was done into the secondary-side of the same SG at low pressures considering availability of fire engines. The peak cladding temperature was dependent on the onset timing and flow rate of the SG coolant injection as well as the onset timing of the SG depressurization after core uncovery. The SG depressurization with the SG coolant injection was found to well contribute to maintain core cooling by the actuation of accumulator system during a PWR SBO (TMLB') transient.
2014 The Japan Society of Mechanical Engineers