Summary report of the EBR-II probabilistic risk assessment. Analysis of the risk from internal and external events at EBR-II.

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Abstract

This report summarizes the results from the recently completed EBR-11 Probabilistic Risk Assessment (PRA) and provides an analysis of the source of risk of the operation of EBR-II from both internal and external initiating events. The EBR-II PRA explicitly accounts for the role of reactivity feedbacks· in reducing fuel damage. The results show that the expected core damage frequency from internal initiating events at EBR-II is very low, 1.6x10-6 yr-1 even with a wide definition of core damage, (essentially that of exceeding Technical Specification limits). The probability of damage, primarily due to liquid metal fires, from externally initiated events (excluding earthquakes) is 3.6x10-6 yr-1. Overall these results are considerably better than results for other research reactors and the nuclear industry in general and stem from three main sources: low likelihood of loss of cool ant due to low system pressure and top entry double vessels; low likelihood of loss of decay heat removal due to reliance on passive means; and low likelihood of power/flow mismatch due to both passive feedbacks and reliability of the rod scram capability.