Effects of process-generated hydrogen on RPV walls

3.3. Hydrogen trapping

in breaking the hydrogen–trap bond. [39]

Figure 3.4: Hydrogen traps present in an industrial steel component. a) Normal solid solution of hydrogen at an interstitial lattice site. b) Hydrogen atom physically bound to a substitutional atom in the iron lattice. c) Hydrogen bound to a dislocation. d) Grain boundary with an accumulation of hydrogen atoms. e) Accumulation of hydrogen atoms at the interface of a precipitate. f) Accumulation of atomic hydrogen at a void surface, recombining to molecular hydrogen in the void. [37] First of all, as elaborately explained above, the hydrogen can be present as a solid solution in the interstitial lattice sites. This is not considered as a real hydrogen trap, but is one of the positions a hydrogen atom can be when dissolved in a steel structure. In the lattices, one can find zero-dimensional lattice defects, like substitutional atoms or single vacancies. These defects can bind a hydrogen atom to it. These type of traps however are fairly weak. Only a very limited increase in temperature is enough to release the hydrogen atom from the trap. Dislocations are another type of lattice defect, capable of trapping hydrogen. However, for dislocations, one has to discriminate two types of hydrogen traps. The dislocation stress field in the close surroundings of the dislocation and the dislocation core. The dislocation stress field is considered to be a reversible trapping site with binding energies up to 20 kJ/mol. However, the dislocation cores are much stronger hydrogen traps with binding energies of 59 kJ/mol. These are in general considered to be irreversible hydrogen traps. [39] In materials containing irradiation damage like black spots, dislocation loops and dislocation networks, local concentrations up to several hundred appm of hydrogen were measured. The hydrogen atoms were presumed to be trapped by these defects. [40] Similarly to the dislocations, one can discriminate two types of hydrogen traps for grain boundaries. The small angle grain boundaries are reversible hydrogen traps 25