Inclusion of embedded flaws in pressurized thermal shock analyses of nuclear reactor pressure vessels

摘要

Regulatory Guide 1.154 provides guidance to utilities regarding an acceptable methodology for performing structural integrity evaluations of aging nuclear reactor pressure vessels (RPV) subjected to pressurized thermal shock (PTS) transients.Probabilistic fracture mechanics (PFM) analysis is a major element of this methodology. RG 1.154 currently specifies that when performing a PFM analysis, all flaws shall be considered as surface flaws that penetrate the inner surface of the RPV wall.The U.S. Nuclear Regulatory Commission (NRC) is currently investigating options for future guidance on RPV integrity evaluations where PTS is the principal consideration. One possible option under consideration is a revision to RG 1.154 to provideguidance for two acceptable methods of per-forming a PTS analysis. The method-1 is a conservative flaw tolerance approach, whereas the method-2 is a more detailed probabilistic risk-assessment approach. The method-l type of PTS analysis would continue tospecify that all flaws are surface-breaking flaws. The method-2 type of PTS analysis relaxes the method-1 conservatism by including the uncertainty associated with the location of the flaws within the RPV wall, i.e., the inclusion of embedded as well assurface-breaking flaws for welded regions. The intent is that the licensee will have the option of performing a method-2 analysis if the method-1 analysis results do not satisfy the RG 1.154 acceptance criterion.This paper describes a methodology currently under consideration to include in the PFM analysis the uncertainty associated with the location of the flaws within welded regions of the RPV wall. This methodology utilizes the RR-PRODIGAL computer code, anexpert-based system that predicts flaw densities, flaw sizes, and the distribution of the flaw locations within the RPV wall for welded regions. A software interface has been developed such that output data files from RR-PRODIGAL can be used as input data into a specialized version of the FAVOR (Fracture Analysis of Vessels: Oak Ridge) fracture analysis computer code, which performs PFM analyses.This paper compares some illustrative PTS analysis results generated by the two PTS analysis methodologies currently under consideration by the NRC.