The objective of the present document is to provide guidance on the implementation of external events into an “extended” L2 PSA. It has to be noted that L2 PSA addresses issues beginning with fuel degradation and ending with the release of radionuclides into the environment. Therefore, the present document may touch upon, but does not evaluate explicitly issues that involve events or phenomena which occur before the fuel begins to degrade.
Following the accident at Fukushima, the community has realized that much attention should be given to the areas of operator interventions and accidents that may develop at the same time in more than one unit if they are initiated by one or more common external events. For this reason and to fulfill the PSA end-users’ wish list (as reflected by an ASAMPSA_E survey), the attention is mostly focused on interface between L1 and L2 PSA, fragility analysis, human response analysis and some consideration is given to L2 PSA modeling of severe accidents for multiple unit sites, even though it is premature to provide extensive guidance in this area.
Vulnerability/fragility analyses should be performed with respect to all external hazards and all structures, systems and components potentially affected that could be relevant to L2 PSA (SSCs associated to SAM strategies),
More focus and less confidence should be applied towards human errors following external events. SAM human interventions should be analyzed taking into account the catastrophic external hazard scenarios which caused the core melt accident,
Results presentation should include assessment of total risk measures compared with risk targets able to assess all contributions to the risk and to judge properly the safety. See document D30.5  for recommendations on PSA results presentation,
Because NPPs on multi-units sites are in general not fully independent, verification and reassessment of existing single PSAs is needed before developing multi-units PSA,
Because no established methodologies for multi-unit sites L1-L2 PSA analysis currently exist, it is recommended to use first a simplified method, as outlined within the present report.