United States Nuclear Regulatory Commission - Protecting People and the Environment

Procedures for the External Event Core Damage Frequency Analyses for NUREG-1150 (NUREG/CR-4840, SAND88-3102)

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Publication Information

Manuscript Completed: October 1990
Date Published:
November 1990

Prepared by:
M.P. Bohn, J.A. Lambright
Sandia National Laboratories
P.O. Box 5800
Albuquerque. New Mexico 87185
Operated by Sandia Corporation

Prepared for:
Division of Systems Research
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC FIN A-1384

Availability Notice


This report presents methods which can be used to perform the assessment of risk due to external events at nuclear power plants. These methods were used to perform the external events risk assessments for the Surry and Peach Bottom nuclear power plants as part of the NRC-sponsored NUREG-1150 risk assessments.

These methods apply to the full range of hazards such as earthquakes, fires, floods, etc. which are collectively known as external events. The methods described in this report have been developed under NRC sponsorship and represent, in many cases, both advancements and simplifications over techniques that have been used in past years. They also include the most up-to-date data bases on equipment seismic fragilities, fire occurrence frequencies and fire damageability thresholds.

The methods described here are based on making full utilization of the power plant systems logic models developed in the internal events analyses. By making full use of the internal events models one obtains an external event analysis that is consistent both in nomenclature and in level of detail with the internal events analyses, and in addition, automatically includes all the appropriate random and tests/maintenance unavailabilities as appropriate.

Hallmarks of the methods described here include, first, the use of extensive computer-aided screening prior to the detailed analysis of each external event hazard to which the plant might conceivably be exposed. These screening procedures identify those external events which could contribute to the risk at the plant and thus, significantly reduce the number of events for which subsequent detailed analysis is required. Both qualitative and quantitative screening steps are applied sequentially. Secondly, for the detailed analysis of fires, floods and other location-dependent scenarios, critical area analysis techniques (heavily dependent on computer analyses) are utilized to identify those areas within the plant for which such events could have a risk significant impact on the plant. Experience has shown that the use of such critical area analysis techniques drastically reduces the number of areas which must be considered.

Taken together, these techniques provide a relatively straightforward and, in some cases, simplified set of techniques for the analysis of the full range of external events and provides for both scrutability and reproducibility of the final results. Furthermore, these techniques have been applied to a number of power plants in a considerably reduced timeframe as compared with external event analyses performed in the past.

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