The events at Fukushima graphically demonstrated that many operating plants are vulnerable to scenarios outside of the envelope previously identified by PSA or deterministic studies. Station Blackout (SBO) has always been considered short term, as recovery of AC power was assumed to be achievable within the duration capacity of the battery power, several hours. At Fukushima the SBO lasted many days and the resulting severe damage proved enormously expensive to the plant owner/operator and caused a hardening of attitutes against nuclear power in any countries world-wide.
In this work, MELCOR severe accident simulation code is used to analyse the long-term SBO scenarios and potential accident management and mitigation measures. The calculaitons are used to estimate a timescale for mitigative measures on-site and emergency measures off-site, and also to identify possible countermeasures and equipment that may be used to slow down or mitigate the accident escalation for a generic BWR and PWR. The investigated accident management measures are, e.g., the re-filling of the steam generator secondary side, and the filtered containment venting.