On the DELTA-T-41 criterion for irradiation shift

Current methodologies for assessing the effects of neutron irradiation on the ductile to brittle transition in reactor pressure vessel steels include the use of the Charpy-based Delta T-41 irradiation shift criterion. This criterion is commonly used in the assessment of irradiation shift for setting limits to the pressure/temperature operating diagram and in defect assessment methodologies. In both of these cases the Charpy-based temperature shift is assumed to produce an equivalent shift in the fracture toughness/temperature curve. Based on the known behaviour of steels, this study shows that this assumption may not be justified when it is applied to static fracture toughness measurements. The results of this analysis indicate that the Charpy-based shift will underestimate the shift in fracture toughness by an amount proportional to the hardening produced by irradiation. Application of the analysis to the USA database on plate and forging materials shows that the experimentally measured differences between Charpy-based and fracture toughness-based temperature shifts are consistent with the anticipated effects of irradiation hardening on fracture behaviour. A yield stress augmented Charpy-based shift criterion improves the equivalence in shift values as determined by the Charpy-based and fracture toughness-based approaches. The implications for the use of the current Delta T-41 criterion in safety assessments are discussed. (author)