Effects of process-generated hydrogen on RPV walls

2.3. Origin of the indications

from the austenitic γ -phase to the ferritic α -phase are the most critical, as these transitions are accompanied by a large decrease in hydrogen solubility. [24] As the ingot solidifies, the hydrogen, together with the other impurities and alloying elements, will be enriched in the liquid phase. In a later stage, the γ - to α -phase transition will have a similar effect, increasing the concentration of impurities in the γ -phase. When this phase transition occurs for the last γ -phase areas, the newly formed α -phase can become supersaturated in hydrogen. As a result, the hydrogen atoms in the supersaturated regions will recombine at so called “trapping positions” to form molecular hydrogen gas, H 2 . The increasing internal pressure from the hydrogen gas together with transformation or residual stresses can result in the formation of brittle cracks, called “hydrogen cracks” or “hydrogen flakes”. [24] [23] Figure 2.9 shows a cross section of a hydrogen flake. The crack is thought to be initiated on the MnS precipitates, shown as the 3 dark grey areas.

Figure 2.8: Solubility of hydrogen in iron as a function of temperature. [24]

Figure 2.9: Light optical micrograph of a hydrogen flake found in a steam generator. The crack is found in the close neighborhood of MnS inclusions. These precipitates act as hydrogen traps and play a crucial role in the initiation of the hydrogen cracks. [24]

As mentioned above, it is the recombination of supersaturated hydrogen that results in the formation of hydrogen flakes. The complete flake formation occurs as a 17