Strategic Planning for RPV Head Nozzle PWSCC

Life cycle management of RPV heads is a significant issue for utilities with PWR plants. Experience has shown the potential for primary water stress corrosion cracking (PWSCC) of the Alloy 600 CRDM nozzles and/or Alloy 182 J-groove welds which can lead to leaks if allowed to propagate. Non-destructive examinations to detect the PWSCC at an early stage are expensive and potentially time consuming while failure to perform timely inspections can lead to leaks, boric acid corrosion, and the possible need to perform repairs during outages when no provisions have been made for inspections or repairs. Decisions regarding the optimum long term strategic plan can be made based on economic modeling of alternative strategies in combination with probabilistic risk modeling based on plant-specific and industry generic inspection results. This paper describes the key factors in developing a strategic plan that takes into account planned costs, potential repair costs, costs of lost production, and other consequential costs, while ensuring safety. Modeling tools discussed include welding residual stress analyses, fracture mechanics calculations, crack initiation and growth modeling, and probabilistic Monte Carlo simulation.