The surveillance test specified in JEAC4201 requires the fracture toughness evaluation of base metal at 1/4-T thickness and T-L orientation, where the initial fracture toughness generally is the minimum in whole thickness at any orientation. In the present study, actual through-wall fracture toughness distribution from a commercial RPV base metal on both of T-L and L-S (actual through crack orientation) orientations were evaluated from Zion Unit 1, which was decommissioned after 25 years (15EFPY) commercial operation. A base metal block of 40mm(T) × 216mm(L) × 216mm(S) was retrieved from belt line region by effort of ORNL and provided to CRIEPI under the USA - Japan collaborative agreement on civil nuclear research framework called CNWG. Through-wall fracture toughness distribution was characterized by means of the Master Curve method utilized by 4mm thickness C(T) “Mini-C(T)” specimens. Near surface material possessed significantly high fracture toughness at amount of 40 degrees Celsius lower reference temperature (To) than those in center thickness locations, despite of higher neutron fluence gained during service operation. Pressurized thermal shock evaluation by probabilistic fracture mechanics code PASCAL4 demonstrated that through-wall fracture probability can be remarkably lowered by considering through-wall fracture toughness distribution.

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