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

contact with an aqueous solution (primary coolant), this is clearly an incredible and erroneous conclusion.

Hydrogen segregation during the steel production and manufacturing process as being the only cause of the current cracks is also put to discussion by e.g. Boonen et al. 11,12 The so-called ‘Safety Case Report’ for Doel 3 states that the concentration of hydrogen decreases from 1.5 to 0.8 ppm during cooling of the steel. A first estimate for the high-density flaw region of the lower shell shows that this would equal to about 61 mL H 2 in total. In order to obtain an approximation of the amount of hydrogen needed to generate all the flaws in this section of the RPV, Boonen et al. have carried out a number of linear elastic finite element simulations to estimate the total flaw volume. Based on this theoretical calculation, they come up with a totally needed H 2 volume of 604 mL, which is approximately 10 times the stated released amount of hydrogen from the steel. These calculations appear to be quite realistic, since they would also result in a calculated average flaw/crack opening width of 0.25 to a few micrometer. The conclusion from this study is that traditional “hydrogen flaking” cannot be the only cause of the present flaws. Either another cause has resulted in the flaking, or the flaws have been growing over time. Boonen et al. 11,12 have also severely questioned the validity of the fracture mechanics approach used by the operator. Interaction of the many multiple cracks as seen in the current case is not covered by the ASME approach. The flaking phenomenon described above is very reminiscent of the well-known ‘hydrogen blistering’, ‘water 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 (H atoms) 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 4. Practical examples are shown in Figure 5. In the presence of already existing cracks (cf. the “hydrogen flakes”) the newly generated hydrogen may be responsible for further crack growth in two ways: either through the pressure build-up by molecular hydrogen, or through the concentration of hydrogen atoms and embrittlement phenomena at the crack tips and on grain boundaries. Both processes will have the same deleterious effect on the metal structural properties. Water chemistry, corrosion effects and hydrogen sources Hydrogen from cathodic partial corrosion reaction