Ductile fracture behaviour of primary heat transport piping material of nuclear reactors

Design of primary heat transport (PHT) piping of pressurised heavy water reactors (PHWR) has to ensure implementation of leak-before-break concepts. In order to be able to do so, the ductile fracture characteristics of PHT piping material have to be quantified. In this paper, the fracture resistance of SA333, Grade 6 steel — the material used for Indian PHWR — under monotonic and cyclic tearing loading has been documented. An attempt has also been made to understand the mechanism responsible for the high fracture toughness of the steel through determination of the effect of constraint on the fracture behaviour and fractographic observations. FromJ-R tests over a range of temperatures, it was observed that SA333 steel exhibits embrittlement tendencies in the service temperature regime. The fracture resistance of the steel is inferior in the longitudinal direction with respect to the pipe geometry as compared to that in the circumferential direction. Imposition of cyclic unloading during ductile fracture tests for simulation of response to seismic activities results in a dramatic decrease of fracture resistance. It appears, from the observations of effects of constraint on fracture toughness and fractographic examinations, that fracture resistance of the steel is derived partly from the inability of voids to initiate and grow due to a loss of constraint in the crack-tip stress field.