Improvement of Fracture Resistance Calculation Procedure for NPP Primary Equipment Components

The procedure of fracture resistance calculation for WWER primary equipment components has been improved. In particular, this refers to the reactor pressure vessel (RPV) and steam generators (SG) under normal operating conditions and emergencies. The developed calculation procedures and software make it possible to determine the significant effect of such factors as deformation stress history, residual technological heredity, analysis of temperature dependence of stress intensity factors for the postulated crack, regularity and density of the finite element mesh in the crack front on the assessment of WWER-1000 RPV fracture resistance. The paper proposes the methodology for justifying the place and orientation of the postulated crack to obtain the most conservative assessment of fracture resistance in the area of RPV inlet nozzles. It is shown that elastoplastic calculations in the thermal shock simulation can help to improve estimates of RPV strength and lifetime.  It was established that not taking into account the elastoplastic deformation history, residual technological stresses after heat treatment and corrosion effects can result in non-conservative assessment of fracture resistance of coolant header welding to SG PGV-1000M shell under normal operating conditions and emergencies. The calculation methodology and software for assessing stress-strain state of in-vessel internals were improved taking into account state-of-the-art approaches to modeling of radiation-induced swelling deformations and dependence of metal mechanical peculiarities on exposure doses and temperature.