EXPERIENCE WITH IMPLEMENTING PART 7 OF THE ASME PRA STANDARD (HIGH WIND): CANADIAN PERSPECTIVE

摘要

Canadian Regulatory Standard S-294 "Probabilistic Safety Assessments (PSA) for Nuclear Power Plants" has forced the Canadian nuclear utilities to rigorously assess external hazards using probabilistic approaches. In support of this standard, AMEC NSS and ARA have worked together over the past three years to complete High Wind Probabilistic Risk Assessments (PRAs) for all five of Ontario's multi-unit CANDU nuclear generating stations (Pickering A, Pickering B, Bruce A, Bruce B and Darlington). This work followed the guidance provided in Part 7 of ASME/ANS RA-S-2009 "Standard for Level I/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications ". To execute these projects within reasonable timelines, a phased approach was applied. Phase 1 utilized simplified approaches and techniques to assess and identify the most important aspects of high wind risk for a particular plant, and Phase 2 utilized enhanced techniques to evaluate these important aspects in greater detail. The enhancements can generally be categorized as follows: First, civil/structural modelling issues; And second, PRA modelling issues. The civil/structural enhancements include items such as detailed fragility analysis of the buildings housing credited equipment, and consideration of the physical layout of equipment within the buildings. PRA modelling issues include items such as the number of wind speed intervals, wind duration and the impact on operator action credits, modelling of the correlation of wind and rain, credit for FLEX/EME and the number of high wind targets. This paper looks at the assumptions and methods followed in the Phase 1 "Simple Approach " as applicable to the PRA items identified above and compares them with the Phase 2 "Enhanced Approaches" along with insights with regard to the impact the enhanced approaches may have on high wind risk based on experience with the Canadian nuclear power plants assessed to date. Topics include the following: 1. Increased Number of Wind Speed Intervals 2. Duration of Storm and Human Actions 3. Credit for Flex/EME 4. Number of High Wind Targets 5. Missile Hit vs. Damage Probabilities for Cable Bridges 6. Importance of Loss of Off-Site Power.