Effects of process-generated hydrogen on RPV walls

2. Findings in Doel 3 and Tihange 2

stepwise process. After a first crack is formed by the recombined hydrogen gas, the pressure and stress close to the crack are relieved. Diffusion of hydrogen towards the free surface of the crack will lead to a new increase in the local stresses and pressure inside the crack and result in further growth of the crack. Again, more hydrogen will diffuse towards the crack and the process starts again. This continues until the material has reached an equilibrium between the dissolved hydrogen and the pressure inside cracks. The high concentration of hydrogen around the crack results in an embrittlement of the this material. [23] Therefore, the fracture surface of such a hydrogen flake typically has brittle fracture characteristics, e.g. a quasi-cleavage fracture. [26] Figure 2.10 shows a typical fracture surface of a hydrogen flake. Besides the brittle appearance of the fracture, one can also see the diagonal lines, running from the right top to the left bottom, indicating the stepwise growth of the crack. [27]

Figure 2.10: Micrograph of the typical appearance of the fracture surface of a hydrogen flakes. One can recognize a quasi-cleavage fracture structure together with diagonal lines indicating the stepwise growth of the crack. [27]

2.3.2 Parameters influencing the formation of hydrogen flakes It is clear from the above formation mechanism, that the hydrogen content is one of the major parameters influencing the formation of hydrogen flakes. Furthermore, (residual) stresses and the sensitivity of the microstructure also affect the probability for hydrogen flake formation. [24] The goal of this section is not to give an exhaustive list of all possible parameters that can influence the formation of hydrogen flakes, as every parameter which influences the local hydrogen concentration, the stresses or the microstructure will also affect the susceptibility to hydrogen flaking. However, the most important influencing factors are given here. As can be expected, the more brittle the microstructure, the more susceptible it is to hydrogen flaking. As a result, martensitic and bainitic phases must be 18