Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

internal fissures seen in large forgings due to segregated hydrogen) were well known from the past and their possible formation is particular dangerous for parts fabricated from large ingots. A potential remedy is to use vacuum ladle degassing methods to decrease the content of hydrogen to 2 ppm, which should avoid or mitigate flake formation. Not all forged components of the Doel 3 and Tihange 2 RPVs contain the same amount of flaws. Based on an analysis of the ingot size and the combined sulfur and hydrogen content, there appears to be a good correlation with the intrinsic susceptibility to hydrogen flaking and the amount of flakes found in each forged component. The key question remaining is about the possible evolution over time of these so- called “hydrogen flakes”. The position of the regulatory authorities and the operator, so far, has been that the defects found in the Doel and Tihange RPV “are usually associated with manufacturing and are not due to aging” and that it is “improbable” that the flaws have evolved since their formation. The only theoretical propagation mechanism still considered is ‘low cycle fatigue’ 4 , while also the limited experience about the influence of irradiation on flaw propagation in zones with hydrogen flakes is recognized. This phenomenon is currently under investigation. One of the main reasons for concluding that it is unlikely there has been a significant evolution of the voids over time is the claim that “there is currently no source of hydrogen anymore” which could cause propagation of the cracks. This, however, is an erroneous conclusion. The phenomenon described above is very reminiscent of well- known ‘hydrogen blistering’ or hydrogen induced fracture phenomena from corrosion in the chemical and petrochemical industries. Hydrogen blistering can occur when hydrogen enters steels as a result of the reduction reaction (hydrogen evolution via water and/or proton reduction) on a corroding metal surface. In this process, single-atoms of “ nascent ” hydrogen diffuse through the metal until they react with another atom, usually at inclusions or defects in the metal. The resultant diatomic hydrogen molecules are then too large to migrate through the metal lattice and become trapped. Eventually, a gas blister or internal crack builds up and may split the metal as schematically illustrated in Figure 2. Practical examples are shown in Figure 3. Water chemistry, corrosion effects and hydrogen

Figure 2: Schematic diagram of hydrogen diffusion and blister formation.

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