Stress tests performed in Europe after accident at Fukushima Daiichi also required evaluation of the consequences of loss of safety functions due to station blackout (SBO). Long-term SBO in a pressurized water reactor (PWR) leads to severe accident sequences, assuming that existing plant means (systems, equipment, and procedures) are used for accident mitigation. Therefore the main objective was to study the accident management strategies for SBO scenarios (with different reactor coolant pumps (RCPs) leaks assumed) to delay the time before core uncovers and significantly heats up. The most important strategies assumed were primary side depressurization and additional makeup water to reactor coolant system (RCS). For simulations of long term SBO scenarios, including early stages of severe accident sequences, the best estimate RELAP5/MOD3.3 and the verified input model of Kr？ko two-loop PWR were used. The results suggest that for the expected magnitude of RCPs seal leak, the core uncovery during the first seven days could be prevented by using the turbine-driven auxiliary feedwater pump and manually depressurizing the RCS through the secondary side. For larger RCPs seal leaks, in general this is not the case. Nevertheless, the core uncovery can be significantly delayed by increasing RCS depressurization. 1. Introduction Following the accident at the nuclear power plant Fukushima Daiichi in Japan the “stress tests” had to be performed in European countries [1]. Stress tests required evaluation of the consequences of loss of safety functions from any initiating event (e.g., earthquake or flooding) causing loss of electrical power, including station blackout (SBO), loss of the ultimate heat sink, or both. SBO scenario involves a loss of offsite power, failure of the redundant emergency diesel generators, failure of alternate current (AC) power restoration and the eventual degradation of the reactor coolant pump (RCP) seals resulting in a long-term loss of coolant. In the literature there are many examples of station blackout analyses, using severe accident codes for simulations of station blackout scenarios with core damage in the first 24 hours, while several days (long-term) long simulations were very rare due to the scenarios selected, in which the heat sink was not assumed for longer periods of time. For example, in study [2] a TMLB hypothetical scenario of station blackout with no recovery of auxiliary feedwater at a 4-loop Westinghouse pressurized water reactor (PWR) was used to compare MELCOR, MAAP4, and SCDAP/RELAP5 severe accident codes. It was
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