EVALUATION OF THE EFFECTS OF IRRADIATION ON THE FRACTURE TOUGHNESS OF BWR INTERNAL COMPONENTS

摘要

Reactor pressure vessel internal (RPVI) components in BWR's (boiling water reactors) are normally made of wrought austenitic stainless steel and are subjected to high levels of radiation during plant operation. Exposure to high levels of radiation over long periods of time can lead to reduction in fracture toughness. In its original condition, the materials used for the core support components are ductile and sufficient fracture margin exists even under limiting seismic events. The reduction of fracture toughness may lead to reduced structural margin. Thus, it must be verified that sufficient structural margin exists even after considering the reduced fracture toughness. This is particularly important for nuclear plants considering extension of operating lifetimes (i.e., >60 years). This paper describes the results of fracture toughness tests performed on irradiated Type 304 stainless steel reactor internal material taken from operating BWR's with fluences ranging from 1 × 10{sup}21 to 6 × 10{sup}21 n/cm{sup}2, E>1 MeV. An example fracture assessment for the BWR core shroud is also provided. The data obtained in this program can be directly applied to the evaluation of highly irradiated LWR (light water reactor) internal components. Based on the test results for this program, a fracture toughness saturation level 55 MPa√m is confirmed for all fluences considered. An example fracture assessment for the BWR core shroud using this data demonstrates adequate fracture margins for the BWR core shroud with limiting load conditions.