Integrity Assessment of a German PWR RPV Considering Loss of Constraint

The brittle failure assessment for five pressurized water reactor pressure vessels (RPV) of German nuclear power plants (NPP) has been revisited according to an advanced state of the art. Besides of recent innovation in fracture toughness curves and reference temperatures being already in the codes, also the effect of loss of constraint had to be considered when fracture toughness values determined from deep cracks in fracture toughness specimen with high multi-axial state of stress were transferred to crack configurations in the component. Thus, the available concepts were compared for their fitness for purpose, i.e. for their ability to give a fracture toughness representative to the crack configuration or flaw postulate in the component. The results of the investigation reveal a significant lower constraint in the component resulting in increased fracture toughness and showing that the brittle failure assessment based on the high constraint fracture toughness from the standard specimens can be very conservative. For consideration of the constraint conditions in the component besides the deterministic T-stress parameter also probabilistic local approach concepts based on the Weibull model were used which have the advantage of considering both the local stress strain field and the material volume under high loading. The loss of constraint was determined for several flaw postulates in the leading situations on the RPV being the coolant inlet nozzle corner and the flange joint. A considerable loss of constraint was demonstrated for flaw postulates with broken clad in the ferritic nozzle corner. Also in the flange joint the loss of constraint is evident for small flaws. In addition, for flaw postulates under the intact cladding the loss of constraint is remarkably higher than with broken postulated cladding. In summary, with the measured material toughness and the significant loss of constraint a considerable inherent margin against brittle failure can be demonstrated for the investigated load cases.