Probabilistic Fracture Mechanics Evaluation of a BWR Feedwater Nozzle

Abstract

Abstract A probabilistic fracture mechanics (PFM) evaluation was performed for the nozzle blend radius and nozzle-to-shell weld of a boiling water reactor (BWR) feedwater nozzle using the PFM methodology in Electric Power Research Institute (EPRI) Boiling Water Reactor Vessel and Internals Program (BWRVIP) BWRVIP-108-A and BWRVIP-241-A, which are the technical basis for inspection relief in ASME Code Case N-702. Using a finite element model of the feedwater nozzle, stress analysis was performed for plant-specific piping loads and bounding transients, which were grouped by severity and projected cycle counts. Monte Carlo simulations were performed using the VIPER-NOZ (Vessel Inspection Program Evaluation for Reliability, including Nozzle) PFM software to determine probabilities of failure for the reactor pressure vessel (RPV) with an inspection population of 25% of the feedwater nozzles every ten years for sixty years of plant operation. The results show that the probabilities of failure for normal operation and low temperature over pressure (LTOP) event meet the acceptance criteria for RPV failure in NUREG-1806 by the U.S. Nuclear Regulatory Commission (NRC). Thus, there is potential to seek regulatory relief to reduce the inspection population of BWR feedwater nozzles from 100% to 25% every ten years using the technical basis of ASME Code Case N-702.