Fracture Mechanics Assessment of Reactor Pressure Vessels in the French Regulatory Context

Abstract Nuclear reactor pressure vessels (RPVs) are non-replaceable components and, as such, demonstrating their structural integrity is a key point to the viable operation of a nuclear power plant. In the French regulatory context, a detailed reevaluation of the loads, fracture toughness and operating experience associated with a RPV is carried out every 10 years to update the reference values used in calculations and, in particular, to demonstrate the absence of any fast fracture risk. These updates may lead to an increase in the applied loads, while the plant’s life extension increases the irradiated state of the RPV leading to a possible reduction of the material toughness. This work presents an example of the workflow used by EDF to exclude the risk of fast fracture in a RPV based on a deterministic approach. To limit the number of complex calculations and to redirect the computational resources to the more challenging cases, different computational approaches are employed. Firstly, a semi-analytical calculation is used to rank all thermo-mechanical loading combinations according to the safety margins, considering the appropriate safety factors. A second level with detailed modeling based on 3D finite element calculations considering the elastoplastic behavior of the materials may be required if the safety margins are not satisfactory according to the previous approach. In parallel, other physical phenomena such as the WPS effect might be considered in combination with either the semi-analytical approach or finite elements 3D calculations. EDF notes that the methods used to analyze the risk of fast fracture incorporate some requirements that are more strict than international practice. As a consequence, advanced methods currently being developed by EDF and Framatome are evaluated and will be briefly discussed.