Methods for External Event Screening Quantification: Risk Methods Integration and Evaluation Program (RMIEP) Methods Development (NUREG/CR-4839,SAND87-7156)

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Publication Information

Manuscript Completed: February 1985
Date Published:
July 1992

Prepared by:
M.K. Ravindra, H. Banon
NTS/Structural Mechanics Associates
5160 Birch Street
Newport Beach, California 92660

Under Contract to:
Sandia National Laboratories
P.O. Box 5800
Albuquerque. New Mexico 87185
Operated by Sandia Corporation
for the U.S. Department of Energy
Under Contract No. DE-AC04-76DP00789

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

Under Memorandum of Understanding DOE 40-550-75

NRC FIN A-1386

Availability Notice


In this report, the scoping quantification procedures for external events in probabilistic risk assessments of nuclear power plants are described. External event analysis in a PRA has three important goals:

  1. The analysis should be complete in that all events are considered.

  2. By following some selected screening criteria, the more significant events are identified for detailed analysis.

  3. The selected events are analyzed in depth by taking into account the unique features of the events: hazard, fragility of structures and equipment, external-event initiated accident sequences, etc.

Based on the above goals, external event analysis may be considered as a three-stage process:

Stage I: Identification and Initial Screening of External Events

Stage II: Bounding Analysis

Stage III: Detailed Risk Analysis

In the present report, first, a review of published PRAs is given to focus on the significance and treatment of external events in full-scope PRAs. Except for seismic, flooding, fire, and extreme wind events, the contributions of other external events to plant risk have been found to be negligible. Second, scoping methods for external events not covered in detail in the NRC's PRA Procedures Guide are provided. For this purpose, bounding analyses for transportation accidents, extreme winds and tornadoes, aircraft impacts, turbine missiles, and chemical release are described.

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