The 2012 in-service vessel inspections performed at Doel-III and Tihange-II Nuclear Power Plant revealed a large number of ultrasonic indications in the forged core shells suggesting nearly-laminar defects attributed to hydrogen flakes induced during the component manufacturing process. These observations have initiated a very large experimental test program not only for reliably and accurately characterizing the ultrasonic indications but also to characterize the mechanical properties in presence of hydrogen flakes. A key point of the Safety Case was the justification of the material properties to be used in the structural integrity analysis, in particular the local fracture toughness of the material ahead of hydrogen flakes. An innovative methodology was used to manufacture fracture toughness specimens, including both compact tension and Charpy size specimens, with a hydrogen flake substituting the standard fatigue pre-crack. As a result, it is possible to measure the fracture toughness directly at the tip of real hydrogen flakes. This paper describes the procedure and the results that were obtained in comparison to standard fatigue precracked specimens.

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