ROSA/LSTF experiment on a PWR station blackout transient with accident management measures and RELAP5 analyses

Abstract

An experiment on a PWR station blackout transient with the TMLB' scenario and accident management (AM) measures was conducted using the rig of safety assessment/large scale test facility (ROSA/LSTF) at Japan Atomic Energy Agency under an assumption of non-condensable gas inflow to the primary system from accumulator (ACC) tanks. The TMLB' scenario involves prolonged complete loss of alternating current power and unavailability of turbine-driven auxiliary feedwater as well as malfunction of relief valves in primary system and steam generator (SG) secondary-side system. The AM measures considered in this study are SG secondary-side depressurization by fully opening the safety valves in both SGs with the start of core uncovery and coolant injection into the secondary-side of both SGs at low pressures. The LSTF test revealed that the primary pressure started to decrease when the SG primary-to-secondary heat removal resumed soon after the coolant injection into the SG secondary-side. The primary depressurization worsened due to the gas accumulation in the SG U-tubes after the completion of ACC coolant injection. The RELAP5 code well predicted the overall trend of the major phenomena observed in the LSTF test, and indicated remaining problems in the predictions of the SG U-tube collapsed liquid level and primary mass flow rate after the gas ingress. The SG coolant injection flow rate was found to significantly affect the peak cladding temperature and the ACC actuation time through the RELAP5 sensitivity analyses.