A Generalization of the FAVOR Code to Include BWR Geometries and Heat-Up Transients

Abstract

The FAVOR computer code, developed at the Oak Ridge National Laboratory (ORNL), under United States Nuclear Regulatory Commission (NRC) funding, has been and continues to be extensively applied by analysts from the nuclear industry and regulators at the NRC to apply established fracture mechanics and risk-informed methodologies to assess / update regulations designed to insure that the structural integrity of aging and increasingly radiation-embrittled nuclear reactor pressure vessels (RPVs) is maintained throughout the life of the reactor. Earlier versions of FAVOR were primarily developed to perform probabilistic fracture mechanics (PFM) analyses of RPVs subjected to thermal hydraulic transients associated with accidental pressurized thermal shock (PTS) conditions and therefore were limited to modeling internal surface breaking flaws and / or embedded flaws near the RPV internal (wetted) surface. For cool-down transients, these flaws are particularly vulnerable, because at the inner surface the temperature is at its minimum and the tensile stress and radiation-induced embrittlement are at their maximum. Tensile stresses tend to open existing cracks located on or near the internal surface of a reactor pressure vessel (RPV). These earlier versions of FAVOR did not have the capability to model external-surface breaking flaws and / or embedded flaws near the RPV outer surface which are the primary flaws of concern for heat-up transients, such as those associated with reactor start-up. Furthermore, earlier versions of FAVOR were limited to the calculation of applied stress intensity factors (applied KI) of internal surface breaking flaws in RPVs with an internal radius to wall thickness (Ri / t) ratio of approximately 10:1, characteristic of pressurized water reactors (PWRs). This limitation is because the stress intensity factor-influence coefficients (SIFICs), applied by FAVOR to calculate applied KI for surface breaking flaws, were applicable only to internal-surface breaking flaws in RPV geometries characteristics of PWRs. Most boiling water reactors (BWRs) have an (Ri / t) ratio of approximately 20:1, although a few BWRs in the United States have an (Ri / t) ratio of approximately 15. Work has recently been performed at ORNL to generalize the capabilities of the next version of FAVOR, and its successors, such that it will have the capability to perform deterministic and PFM analyses of cool-down and heat-up transients on all domestic commercial PWR and BWR RPV geometries. This paper provides an overview of this generalization of the FAVOR fracture mechanics computer code.