Master Curve and Unified Curve applicability to highly neutron irradiated Western type reactor pressure vessel steels

While the Master Curve (MC) method is gradually entering brittle fracture safety assessment procedures world-wide, knowledge is still lacking about its limits of applicability to highly neutron irradiated material. In this paper two reactor pressure vessel (RPV) steels A533B Cl.1 (IAEA reference material code JRQ) and A508 Cl.3 (code JFL) were scrutinized for possible deviations of the postulated invariant MC shape and the MC validity for macroscopically inhomogeneous microstructure. Besides tensile and Charpy-V tests, MC tests were performed on Charpy-size three-point bend specimens in the unirradiated, neutron irradiated with fluences up to nearly 1020 n/cm2 (E > 1 MeV) and recovery heat treated condition. Evaluation procedures include Master Curve reference temperature T0 determination according to ASTM E1921-05 as well as additional analysis methods such as SINTAP, multi-modal MC method (MML) and the Unified Curve (UC). Integrity assessment according to ASME Code Cases N-629 and N-631 has been applied. It is shown that the standard MC concept provides a precise description of the fracture toughness for all conditions, even exceptionally well for the highly irradiated state. No MC shape change could be observed, whereas the UC concept indicates a significant influence of irradiation on the fracture toughness curves for the highly irradiated JRQ.