Inherent margin in the brittle failure assessment for RPV
The brittle fracture assessment of the reactor pressure vessels (RPV) of the EON Kernkraft (EKK) pressurized water reactors has been updated to the state of the art. The reference temperatures of the RPV for the design fluence of 5E18 n/cm² are at or below 0°C. The weld joint between vessel flange and cylinder as well as the hot and the cold leg nozzle corners proved to be the prominent and covering situations for the RPV. The fracture mechanics results for these configurations for operational and loss of coolant transients (LOCA) were produced rerunning the assessment procedure established for the licensing of the RPV and additionally with the application of state of the art procedure. Big margin inherent to the older procedure was disclosed with the more realistic but still conservative input. Most of the margin resulted from the application of the fracture toughness based reference temperature RTTo in place of RTNDT, from the constraint representative fracture toughness applied in the nozzle corner situation and from taking into account the results of the non destructive evaluation (NDE) during the manufacture and the in-service inspection (ISI) with defect postulates of allowable size. The results show the load paths with large distance to thefracture toughness curve, in the upper shelf temperature region of fracture toughness and/or with negligible values far below the lower shelf, both demonstrating an excessive inherent safety of the older procedure. In addition, for the LOCA, a multi barrier concept is applied. Besides the flawless state of the vessel and the preclu sion of initiation from postulated flaws, with the arrest of postulated crack initiation and break preclusion for crack extension from postulated rising load after crack arrest two additional barriers against failure were proven. For all load cases the relevant load path temperatures proved to be in the upper shelf regime of fracture toughness, where ductile fracture is the failure mechanism and brittle fracture is precluded.