NPP Life Management_vs02

formation. The only theoretical propagation mechanism still considered by the operator is ‘low cycle fatigue’ 3 , although this has been disputed by various corrosion experts. Also the limited experience about the influence of irradiation on flaw propagation in zones with hydrogen flakes is recognized, both by the operator and the regulatory agency. This latter phenomenon is currently still 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” 13 which could cause propagation of the cracks. This, however, is an erroneous conclusion and there could be major questions regarding the stability of the flakes . Not only larger numbers, but also increased average dimensions (Doel 3 & Tihange 2) and a strong increase of maximum dimensions of the “flakes” (i.e. for Tihange 2 upper core shell) have later been reported by the operator, e.g. during various presentations 14 , when inspection results from 2012 and 2014 were compared. Additional tests conducted in 2014 – with adapted equipment detection parameters – revealed 13,047 cracks in Doel 3 and 3,149 in Tihange 2. Reported maximum sizes are now up to 179 mm (vertical) by 72 mm (horizontal) for the Doel 3, and 155 mm (vertical) by 71 mm (horizontal) for the Tihange 2 RPV 15 . It may appear that not all of these changes can be easily explained by higher sensitivity settings or new data interpretation routines. Variations are claimed to be “within the accuracy of the technique” (and the statistical distribution), but this is clearly an item that should be clarified by further investigation and data analyses. 1.3. Amount and Sizes of the Cracks

Hydrogen and NPP Life Management: Doel 3 and Tihange 2

The current size of the voids is also not exactly in line with what is traditionally considered “typical” for hydrogen flaking phenomena (cf. infra).

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