Effects of process-generated hydrogen on RPV walls

3. Hydrogen in steel

with binding energies equal to about 20 kJ/mol, while high angle grain boundaries are irreversible hydrogen traps, having a binding energy up to 59 kJ/mol. [39] Furthermore, the interface with a precipitate in the material will also act as a hydrogen trap. Depending on the type of precipitate and its composition, the binding energy will be different. Coherent precipitates will have a smaller binding energy, resulting in a reversible hydrogen trap. An example is small precipitates of NiAl in a steel matrix. These will be coherent and have a relatively low hydrogen binding energy of 27 kJ/mol. On the other hand, there are the incoherent precipitates which will have a much larger binding energy and therefore will be irreversible. MnS is such an incoherent precipitate and has a hydrogen binding energy of 72 kJ/mol. [39] Also neutron irradiation can result in the formation of hydrogen traps and therefore influence the hydrogen solubility and diffusion coefficient. As shown by Krasikov and Amajev [41], low temperature neutron irradiation of RPV steel can increase the hydrogen solubility by several factors. Also, irradiation of RPV steel at 50 – 180 ◦ C results in a reduction of the diffusion coefficient. Both phenomena are related to the formation of radiation-induced lattice defects. These defects, e.g. vacancies and substitutional elements, act as hydrogen traps and therefore will effect both the solubility of hydrogen as the diffusivity in the steel. Furthermore, these vacancies might coagulate to form microvoids in the steel, acting as even stronger hydrogen traps than the individual vacancies. Micro- and macrovoids are another kind of hydrogen traps. The hydrogen will first be bound to the free surface and can then recombine with another hydrogen atom to form molecular hydrogen. The hydrogen atoms are subject to a very strong attraction to the free surface with a binding energy between 70 and 95 kJ/mol. This means such voids are very strong hydrogen traps and consequently the release of hydrogen from these traps is very small, if not impossible. [39] Solano-Alvarez even estimated the binding energy for cracks to be at least 200 kJ/mol. Their conclusion was that “any hydrogen within the cracks can never in practice de-trap”. [42] This is confirmed by Kenik and Busby [40], mentioning studies that measured hydrogen concentrations up to 4000 appm in materials containing either gas bubbles or voids. These high hydrogen levels were found to be retained at least 13 years. The accumulation of hydrogen in these traps can result in a high pressure buildup, which might be high enough to initiate or grow hydrogen cracks in the material as elaborately explained above. 3.4 Conclusion Today, there is general acceptance that the major part of the dissolved hydrogen in steel materials is positioned at hydrogen traps, more than the homogenous solid solution in the interstitial lattice positions. As a result, determining the solubility of hydrogen in steel is a very difficult task. As the concentration of these hydrogen traps depends on lots of parameters, e.g. the chemical composition, the temperature, the history of the material and the irradiation of the material. The chemical composition determines the amount of precipitates that will form, the temperature can result 26