The monitoring of neutron induced embrittlement of nuclear power plants is provided using Charpy impact test in the surveillance program. However structural integrity assessments require the fracture toughness. Some empirical formulas have been developed but no direct relationship was found. The aim of our study is to determine the fracture toughness of a Reactor Pressure Vessel steel from instrumented Charpy impact test using local approach to fracture. This non-empirical method has been applied in the brittle domain as well as in the ductile to brittle transition for an A508 C1.3 steel. In the brittle domain, fracture occurs by cleavage and can be modeled with the Beremin model. Fracture toughness has been successfully determined from Charpy impact tests results and the influence of several parameters (mesh design, Beremin model with one or two parameters, number of Charpy impact tests results) on the results was considered. In the ductile to brittle transition, cleavage fracture is preceded by ductile crack growth. Ductile tearing has been accounted for in the simulations with the Rousselier model whereas cleavage fracture is still described with the Beremin model. The determination of fracture toughness from Charpy impact tests gave encouraging results but finite element simulations have to be refined in order to improve predictions.

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Within framework of 5th EC European SMILE project, some R&D actions have been conducted to demonstrate, to model and to validate the beneficial warm pre-stress (WPS) effect regarding the risk of brittle failure in a RPV assessment. An important experimental work has been conducted including classical WPS type experiments on CT specimens on usual RPV steels, and one PTS type transient on a large cracked cylinder. All experimental results confirm the beneficial effect of warm pre-stress, with a significant increase of the materials resistance regarding the risk of brittle failure. The experiments have been analyzed using fracture mechanics, including both engineering methods (Chell, Haigh, Wallin) and more refined analyses based on local approach to fracture (‘modified Beremin’ model). Following a short description of WPS concept, the paper summarizes the main experimental results and presents a synthesis of mechanical analyses involving engineering approaches and numerical analyses.

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Some components of the main primary circuit of PWR nuclear power plants contain nickel-base alloy 600 parts (steam generator (SG) tubes, steam generator partition plates, lower internal radial supports). It is well known that this alloy is prone to stress corrosion cracking in the primary water environment. In 2002, surface cracks were discovered for the first time in SG partition plates of EDF 900 MWe NPP. The integrity of the SG containing these cracks must be demonstrated for all operating conditions, including accidental conditions. Due to the high tensile consolidation rate and the high fracture toughness of alloy 600, this was proved using limit load analysis. However, for a thorough demonstration, an experimental program was launched at EDF/R&D to better understand the behaviour of cracks in this high fracture toughness material. Centre Cracked Tensile (CCT) specimens were selected for this experimental program, being closer to the industrial case than conventional CT specimens. Two tests have been conducted at room temperature on large CCT specimens containing a semi-elliptical crack. The paper presents the design of the CCT tests, the material characterisation, the main results of the tests and their numerical interpretation.