The use of master curve method for statistical re-evaluation of surveillance test data for WWER-1000 reactor pressure vessels

The surveillance fracture toughness test data for WWER-1000 reactor pressure vessel materials from Ukrainian nuclear power plants were re-evaluated using the Master curve methodology. It has been shown that experimental temperature dependence of fracture toughness parameters and a scatter of K_Jc values are in a good agreement with a Master curve shape and 5 and 95% tolerance bounds for materials in unirradiated condition and after neutron irradiation up to fluence 41. 2·10²² n/m² (E > 0.5 MeV). For the Khmelnitsky nuclear power plant unit 1 reactor pressure vessel an analysis has shown that normative approach PNAé G-7-002-86 underestimates essentially the measured fracture toughness of unirradiated weld metal. The reference temperature T₀ calculated according to the Master curve method was compared with a critical brittleness temperature T_K0 for reactor pressure vessel materials in unirradiated condition. It has been found that temperature T₀ is much lower than T_K0 . Furthermore a difference between T₀ and T_K0 values varies essentially from one material to another. A correlation between temperatures T_{28 J} defined from Charpy energy curve and T₀ values calculated from precracked Charpy specimens test was obtained. The analysis has shown that the results based on precracked Charpy specimens can provide nonconservative assessment of fracture toughness for WWER-1000 reactor pressure vessel materials.