Development of a Technical Basis for Code Case N-914, “Accounting for the Effect of Embrittlement on Fracture Toughness Properties Used in Evaluations of Pressure Boundary Materials in Class 1 Ferritic Steel Components, Section XI, Division 1”

Abstract

Abstract Section XI of the ASME Boiler and Pressure Vessel Code includes methods to analyze the continued operating safety of nuclear power plant components. For the reactor pressure vessel (RPV) a means to account for the effects of neutron irradiation embrittlement on the mechanical properties of vessel steels constitutes a key part of this analysis. Nevertheless, Code guidance on this topic remains imprecise. Over time regulations in various countries have filled this gap. While functional, this practice means that the structural integrity practices for nuclear RPV safety management depend on an amalgam of Code and regulatory documents. This diminishes the clarity with which safety margins are addressed and inhibits adoption of new technologies for fracture toughness and embrittlement characterization. During the February 2019 ASME Code week, the Section XI Working Group on Operating Plant Criteria (WGOPC) initiated an effort to develop a Code Case to address this deficiency. This paper summarizes an Electric Power Research Institute Report (MRP-462) recently provided to the WGOPC as a single source technical basis for the proposed Code Case on this topic. The report reviews relevant previous national and international work, both within the Code as well as licensing applications made using Master Curve techniques. The report proposes a single, unified, method to estimate an irradiated reference temperature (RTIRRAD) that can be used to position a Code-approved bounding curve (either a RTNDT or RTTo indexed KIc curve, or a T0 indexed 5th percentile Master Curve) using whatever input information (Charpy-based or Master Curve-based) is available.