Compared with the land-based nuclear power plant, the operating conditions of offshore nuclear power plant (ONPP) are much more complicated. For example, the barge-mounted platform malfunction, which is as important as the natural events and human events, should be considered in the plant safety analysis,. As a result, a two dimension operating condition coupled with barge and reactor status should be considered in the development of relevant power plant operating procedures. On the other hand, the beyond design basis hazards induced by the combination of unique and unanticipated external events of ONPP may lead to a blind area to both traditional and two dimension procedures mentioned above. Due to the insufficiency of existing operating condition and relevant procedures to tackle with the above events mentioned, an expanded operation strategy, namely the beyond design basis hazards and the extended ultimate response measures, is developed, Injecting sea water into reactor pressure vessel directly after primary system depressurized and venting the containment when necessary, formed the basis of ultimate response measure, which was proposed by Taiwan Power Company after Fukushima Accident. Considering the offshore and barge-mounted features, the ultimate response measure can be extended to include sea water injection into steam generator indirectly through secondary side passive residual heat removal lines and reactor cabin flooding by sea water through Kingston valves, to rebuild a newly, hierarchical one. Finally, the extended ultimate response measures, provided mainly for the plant command staff and operators, are analyzed utilizing thermal-hydraulic integral computer code preliminarily, to prove the effectiveness of the system configuration and operating strategy. It is concluded that injecting sea water into steam generator can remove the decay heat effectively, and the sensitivity study shows that operator intervention is good enough in accident mitigation.

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