The experience of severe accidents shows that reliable determination of technological process parameters is necessary but not always sufficient to avoid catastrophic consequences. The accident measures should be considered in a broader context that includes the human factor, organization of the nuclear technology, external influences and safety culture.

The anticipated transient without scram (ATWS) events were not considered in the original WWER (Russian PWR) design basis accidents. The design extension conditions scenarios progress in a context which is very uncertain and highly stressful for the operators. If a specific scenario requires some operators’ actions as measures to mitigate, delay or distribute the accident consequences then the dynamics of accident context seem of primary importance for ‘best estimate’ evaluations and enhancing the plant’s capability.

The paper presents the capacities of the Performance Evaluation of Teamwork (PET) procedure for enhancing plant’s capability for design extension conditions based on ’‘best estimate’ context evaluation of human performance in ATWS events. The PET procedure is based on a thorough description of symptoms of various timelines and their context quantification. It is exemplified for different ATWS scenarios of the NPP with WWER-1000 based on thermal-hydraulic simulations with RELAP5/MOD3.2 code and models.

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