Master Curve applicability to highly neutron irradiated Reactor Pressure Vessel steels - Results of a BMWi Grant Project

Currently, several countries already have adopted or are in the process of adopting the Master Curve (MC) approach into their reactor pressure vessel (RPV) integrity assessment. Yet, for highly irradiated material some open technical issues like the postulated invariance of the MC shape and the behaviour of inhomogeneous material need resolving to allow the MC approach to be incorporated into the German Nuclear Regulatory Guidelines (KTA). Within the framework of the reactor safety research of BMWi specimens from 3 different RPV steels were scrutinized (IAEA reference material 3JRQ57, 1JFL11 comparable to German RPV steel 22NiMoCr3-7 and Russian WWER-440 type base metal KAB-B), which were neutron irradiated up to 10²⁰ n/cm² (E>1MeV). Additionally, the effects of a recovery heat treatment at 475°C/100h are investigated. Besides microstructure analysis, hardness, tensile and Charpy-V tests, the experimental part focuses on fracture mechanical testing, i.e. J_R curves according to ASTM E1820-06 and MC reference temperature T₀ determination according to ASTM E1921-05 as well as additional analysis methods like SINTAP, multimodal MC method and the Unified Curve. Resistance against ductile crack initiation (J_R curves) remains relatively unaffected by irradiation (J_Q/J_0,2BL according to ASTM E1820 and J_0,2 according to GKSS). The MC provides a precise description of the fracture toughness vs. temperature of unirradiated, irradiated, and recovery annealed material. Especially for the highly irradiated states, the MC describes the K_Jc(1T) values exceptionally well, including data points outside the temperature limit T₀±50K according to Standard ASTM E1921 and even for highly irradiation sensitive and inhomogeneous RPV steel 3JRQ57. Some scatter occurs for low and medium irradiated states, where more points than expected lie below the MC for 5% failure probability. The Unified Curve, which allows for MC shape changes at high irradiation levels, overpredicts the influence of irradiation with 3JRQ57. For none of the 3 investigated RPV steels a shape change of the Master Curve due to irradiation could be observed. Embrittlement prediction formulae according to Reg. Guide 1.99, Rev. 2 and WWER specific VERLIFE procedure tend to underpredict the irradiation-induced changes in material properties.