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

2. Findings in Doel 3 and Tihange 2

cause of the defects.

2.3.1 Formation mechanism of hydrogen flakes The first cases of defects associated with hydrogen flakes date back to the beginning of the 20th century. After World War II, a lot of research was done related to the problem of hydrogen flaking. [23] It was found that the occurrence of hydrogen flakes is closely related to segregation in the ingot. It is generally known that solidification of a large ingot does not occur homogenously. Due to a difference in solubility of alloying elements and impurities in the liquid and solid phase, segregation will occur. These segregated areas are the latest to solidify and are enriched in these alloying elements, like carbon, and impurities, like hydrogen and sulphur. Furthermore, these areas contain the most inhomogeneities in the ingot. [24] A schematic representation of the macrosegregation is shown in Figure 2.7. [25]

Figure 2.7: Schematic representation of macro-segregation in a large ingot. The “+” indicate positive segregated area’s, whereas “-” indicate negative segregated area’s. [25] The exact mechanism for the formation of hydrogen flakes is not yet fully understood. However, most likely, the formation of hydrogen flakes is related to the difference in solubility for hydrogen between the solidified and non-solidified material. As one can see in Figure 2.8, the solubility of hydrogen varies strongly between the different phases. As the material used for the reactor pressure vessel walls is a low alloy ferritic steel, the material has to make all of the microstructural transitions shown in Figure 2.8. The transitions from the liquid to the ferritic δ -phase and 16