Estimates of Rupture Probabilities for Nuclear Power Plant Components: Expert Judgment Elicitation

Abstract

As part of the nondestructive evaluation reliability program sponsored by the U.S. Nuclear Regulatory Commission (NRC), Pacific Northwest Laboratory (PNL) developed a risk-based method for establishing inspection priorities for systems and components at nuclear power plants. In this method, the results of probabilistic risk assessment (PRA) are used to estimate the safety consequences of component failures. The method also requires estimates of the probabilities of structural failures.Since sufficient operating experience data and detailed fracture mechanics analyses are not available, an expert judgment elicitation is conducted to estimate component rupture probabilities. (An expert judgment process is generally adapted from the NRC severe accident risk program.) The plant selected for the detailed evaluation is the Surry nuclear power station Unit 1 (Surry-1). Systems selected for analysis are the reactor pressure vessel, the reactor coolant, the low-pressure injection including the accumulators, and the auxiliary feedwater.Additional technical information is gathered regarding the elicited issues. The data appear to be reasonable, and they generally agree with and reflect Surry-1 plant operating experience. Typical areas of concern correspond to such factors as high stresses (e.g., places where mixing of fluids with large temperature differences occurs) and places where erosion or corrosion effects are active. These results will be used by PNL in an ongoing pilot study based on the PRA results and other relevant information in determining the inspection priorities for systems and components at the Surry power plant.

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