Prolonged available time and safe states - State of the art review

This report presents a state of the art review on long time windows in PSA and important related topics. The report consists of the results of a literature review and a questionnaire for the stakeholders of the NPSAG project 53-003 PROSAFE. Topics covered in the report include safe, stable state; success criteria; mission times; recoveries and repairs; HRA methods; risk and reliability analysis methods; reliability data; and epistemic uncertainty. The literature related to long time windows appears to be very limited, because PSA is typically limited to the mission time of 24 hours. Scenarios with long mission times are generally recognized as a challenging area that needs to be studied more.

Answers to the questionnaire highlight spent fuel pool accidents and human reliability analysis in long mission time scenarios as important topics to be studied. Longer time windows also bring in the need to model repairs that are usually neglected in normal scenarios with mission time of 24 hours. Modelling of different time windows and time-dependent success criteria could also make PSA models more realistic. In addition, there has been some concern over the applicability of normal failure data to long mission time scenarios.

There is significant variety in both the definitions of safe, stable end state found from the literature, and the questionnaire answers concerning the definition and its application. More consistent and realistic consideration of safe end states could improve PSA analyses, e.g. by making mission times and success criteria more realistic, but it is not considered the most important development area according to the questionnaire answers.