Effect of resolution of the Millstone Point Unit 1 Systematic Evaluation Program issues on probabilistic calculations of risk

The US NRC Systematic Evaluation Program (SEP) identifies deviations from current licensing requirements for older nuclear power plants. The Millstone Point Unit 1 (Millstone-1) plant is the first plant in the SEP which has had a probabilistic risk assessment (PRA) performed on it (as part of the Interim Reliability Evaluation Program, IREP). Sandia was requested to calculate the effect on risk of resolving the deviations identified by the SEP for Millstone-1. The results of the IREP Millstone-1 PRA were recalculated incorporating resolution of the issues which were within the scope of the original PRA. Of the 19 issues analyzed in detail, we calculated no reduction in core melt frequency, exposure, and risk for 12 issues, less than 2% reduction for 5 issues, and approx. 15% reduction for 2 interrelated issues if implemented together. These numbers should be viewed in the context of the very large uncertainties in PRA methodology and data. No attempt at cost/benefit analysis was made. The deviations not analyzed were generally those concerning the plant's response to external events such as flooding and earthquakes.