United States Nuclear Regulatory Commission - Protecting People and the Environment

Estimation of Operator Action Time Windows by RELAP5/MOD3.3 (NUREG/IA-0219)

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Publication Information

Date Published: December 2009

Prepared by:
A. Prošek, B. Mavko, M. Čepin
Jožef Stefan Institute
Jamova cesta 39
SI-1000 Ljubljana, Slovenia

A. Calvo, NRC Project Manager

Published by:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

This report presents the results of analyses performed for the updated human reliability analysis. The analysis estimates time windows available to perform operator action to satisfy the success criteria to prevent core damage. The best-estimate RELAP5/MOD3.3 computer code was used. In the past, the conventional probabilistic safety assessment used a conservative approach to address this factor. However, the current standard for probabilistic safety assessment recommends the use of best-estimate codes. The RELAP5/MOD3.3 best-estimate code calculations were performed for three selected cases in which human actions supplement safety system actuations: (1) small or medium loss-of-coolant accident requiring a manual start of the auxiliary feedwater system, (2) loss of normal feedwater requiring a manual start of the auxiliary feedwater system, and (3) a loss-of-coolant accident requiring manual actuation of the safety injection signal. The analysis used a qualified RELAP5 input model representing a Westinghouse-type, two-loop pressurized water reactor for the calculations. The results of the deterministic safety analysis were examined to identify the latest time that an operator could act and still satisfy the safety criteria. The results show that the time available to perform operator action (i.e., the time window) is greater than the actual time needed to perform the action. The difference is considered additional available time for action. The results of human reliability analysis show that uncertainty analysis of realistic deterministic safety analysis is needed only for significant risk contributors in situations where having additional time available for action makes the difference between considering or not considering recovery operator action.

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