Master Curve approach for embrittled reactor pressure vessel steels

The structural integrity of the reactor pressure vessel (RPV) has to be maintained throughout the operation time of a nuclear power plant (NPP). The fracture toughness of the RPV steel has to be higher than the stress intensity at the crack tip. The present regulatory-based correlative approaches for the determination of fracture toughness of RPV steels are not consistent because they link technological parameters (RTNDT, Charpy-V) and universal fracture toughness lower bound curves. Safety margins as well as probability estimations cannot be quantified. In contrast, a new “Master Curve” (MC) method enables the direct measurement of the fracture toughness with less uncertainty. The MC approach is based on Weibull statistics and models the cleavage failure of a specimen in the lower ductile-to-brittle transition region.
To maximize the Master Curve’s benefits, it should be incorporated in regulatory evaluation or integrity assessment as a long term objective. Before the Master Curve approach can be implemented in codes and regulations, open issues such as influence of irradiation, loading rate effects, loss of constraint, as well as the effects of inhomogeneous material need resolving.
The Research Centre Rossendorf is currently investigating the invariability of the MC shape for “extreme” irradiation, the applicability in case of partial intergranular fracture, annealing effects, and the correlation of MC and Charpy results. Latest results will be presented.