Use of the Failure Assessment Diagram to Evaluate the Safety of the Reactor Pressure Vessel

Abstract

Analysis of multiple failure modes is the key element of the integrity evaluation of the nuclear reactor pressure vessel (RPV). While the simple singlecriterion failure code provides the guidance for structural integrity, the guidance ignores the interaction between fast fracture and plastic collapse. In this paper, the differences between the reserve factor (RF) in the R6 twocriteria failure procedure and the safety coefficient (SC) in the singlecriterion failure code were compared. Based on 3D finite element (FE) analyses, the option 3 failure assessment diagram (FAD) of the beltline of the RPV was established according to the R6 basic route and alternative approaches, respectively. Also, the nonconservation of the secondary stress correction parameter دپ was reviewed. In this paper, it was shown that the effect of crack sizes on the FAD is considered to be limited, and the influence of the thermal stress on the FAD is obvious in the transition region of the failure assessment curve (FAC). The FAD only considering the mechanical load encloses the FAD considering the thermal–mechanical load for the Lr smaller than 1, but it is contrary when the Lr is bigger than 1. It is not enough to just satisfy the requirement in the IWB3612 of the ASME code because the risk of plasticcollapse failure is ignored. And in this study, the maximum nonconservation of the fracture toughness RF is more than 7% due to the approximate value of دپ. Accordingly, the accurate method in the R6 procedure should be used in the integrity assessment of the RPV under the faulted transient.