During UK HPR1000 Generic Design Assessment (GDA), a shortfall about In-Vessel Retention (IVR) strategy is identified after a detailed timeline analysis according to UK nuclear safety requirements “Safety assessment principles for nuclear facilities” (SAP): the reactor pit injection manual action available time is less than the required time. Four possible modification schemes are proposed and analyzed. After ALARP (as low as reasonably practicable) analysis, the design of reactor pit injection isolation is modified to make the reactor pit injection manual action available time more than the required time : a remote manual permissive action in the main control room by a switch in the Emergency Control Panel replaces the local administrative isolation. Improvements for advanced nuclear power plant R&D, design and future operation are suggested, including enhanced requirement management and configuration management, especially for the design basis of functional requirements and sizing requirements; systematic identification of time-critical actions (TCA), performing analysis, preliminary validation and integrated validation of TCA during design and construction phase, implementing a TCA programme during operation phase; improving Severe Accident Management Guidelines (SAMG) implementation approval process by adding “the point of entry to the use of SAMGs is reached” to reduce the time of SAMG implementation approval process.

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