Influence of repair welding on cyclic thermal shock behaviour of an RPV nozzle corner

In piping as well as in RPV-nozzles and in surrounding parts of the cylindrical vessel shells, repeated cold water injections have been observed leading to crack initiation and subcritical crack growth. In the framework of the German Reactor Safety Research Programme, cyclic thermal shock tests have been carried out at the RPV of the decommissioned HDR (Heissdampfreaktor) near Frankfurt applying extremely conservative temperature transients. The crack propagation under access of oxygenated pressurized water was evaluated by fracture mechanics methods. The validation by non-destructive examination, as well as by destructive testing at a trepan, proved the fracture mechanics results to be conservative. The depression at the nozzle corner caused by the trepan removal has been repaired by 'temper bead' welding without stress relief heat treatment, similar to the 'half bead' technique of the ASME Boiler and Pressure Vessel Code. The subsequent thermal shock stressing (1200 cycles) under similar parameters demonstrated, with respect to weld repair: (1) no material separation extending into the ferritic RPV-wall; (2) the quality of repair welding is comparable to the quality of the as-delivered condition manufactured in the 1960s in a more conventional manner. The positive experience derived from this work may be helpful in the future in dealing with any cases of cracking in thick-walled large diameter vessels which could require repair welding when a post-weld stress relief heat treatment is not possible. (author)