Heavy-Section Steel Irradiation Program: Embrittlement issues

Maintaining the integrity of the reactor pressure vessel (RPV) in a light-water-cooled nuclear power plant is crucial in preventing and controlling severe accidents and the potential for major contamination releases. It is imperative to understand and predict the capabilities and limitations of its integrity. It is particularly vital to fully understand the degree of irradiation-induced degradation of the RPV's fracture resistance which occurs during service, since without that radiation damage it is virtually impossible to postulate a realistic scenario which would result in RPV failure. The Heavy-Section Steel Irradiation (HSSI) Program has been established by the US Nuclear Regulatory Commission (USNRC) to provide a thorough, quantitative assessment of the effects of neutron irradiation on the material behavior, and in particular the fracture toughness properties, of typical pressure vessel steels as they relate to light-water reactor pressure-vessel integrity. Recent results are described concerning the shifts in fracture toughness and crack arrest toughness in high-copper welds, the effects of irradiation on stainless-steel weld-overlay cladding, the unirradiated examination of a low upper-shelf (LUS) weld from the Midland reactor, irradiation temperature effects on Charpy and tensile properties of several LUS welds, initial theoretical studies related to irradiation-rate effects, and the continued investigation into the causes of accelerated low-temperature embrittlement recently observed in RPV support steels