Brittle Failure Assessment of a PWR-RPV for Normal-Anomalous Operation and Loss of Coolant Accident Loads

Abstract

The brittle failure assessment for the reactor pressure vessel (RPV) of a 1300 MW pressurized water reactor (PWR) was revised according to the advanced state of the art. The RPV steel is 22 NiMoCr 37 (A 508 Cl.2). The expected neutron fluence at End of License (EOL) after 32 years of full operation is Φ < 2.3·1018 cm−2. The assessment followed a multi-barrier concept to prove independently exclusion of crack initiation, crack arrest and exclusion of the load necessary to advance the arrested cracks through the RPV wall. For operational conditions, a combination of shut-down and anomalous operation was proven as the leading transient. This transient follows a load path decreasing with temperature and thus producing a warm pre-stressing for flaws in the RPV. Therefore, a warm pre-stress (WPS) effect is to be considered. For loss of coolant accident (LOCA) conditions, a leak size screening was performed to find the leading transients. Enveloping transients were determined and used for the brittle failure assessment. As covering crack postulates circumferential cracks in the flange joint to the vessel and axial cracks in the nozzle corner were investigated both under the assumption of a broken clad. For the LOCA loads, the leading leak size was determined by finite element analyses and fracture mechanics based assessment of different combinations of cold/hot leg leak position and cold/hot leg injection of emergency core cooling (ECC). Leading situations are in the flange joint for the cylinder and in the nozzle corner for the flange. In the nozzle corner, high thermal loading from cross corner cooling produces large plastic deformation and loss of constraint. In this situation, the fracture toughness KIC as determined from deeply cracked fracture mechanics specimens is not representative for the component. Thus, loss of constraint in the component was considered by the application of a constraint modified master curve to determine the constraint representative fracture toughness. For the investigated load cases the relevant part of the load paths is at upper shelf temperatures, with the exception of the anomalous operation starting at 50°C to lower temperatures. The results of the investigation demonstrate preclusion of initiation for normal and anomalous operation as well as for LOCA loads. For LOCA, two further barriers were proven with crack arrest for postulated crack extension well within allowable depth in the RPV wall and with the preclusion of the load necessary to advance the arrested crack through the wall.