Effects of process-generated hydrogen on RPV walls

6.3. In-service hydrogen generation

Figure 6.9: Sieverts’ constant for hydrogen in ferritic steel as a function of temperature. Notice the logaritmic scale and therefore its high temperature dependency. This results in a H activity at the steel-water interface equal to: a = 0 . 1425 √ 4 . 9 · 10 32 a = 3 . 16 · 10 15 ppm This again is an incredibly high activity. Unlike the fugacity coefficient for H 2 at high pressures, the activity coefficient of H in steel at high concentrations is unknown. Therefore, these activities can not be translated to physical concentrations in the steel. However, it has been shown that for increasing concentrations of species, the activity coefficient for Henry’s law can become very high. [79] Therefore, the physical validity of this calculation is not necessarily undermined. 6.3.2 Corrosion generated hydrogen The other source of process generated hydrogen in a RPV is due to corrosion of the RPV steel wall. The hydrogen generation rate over the complete surface of the the RPV was calculated to be between 50 and 150 mol H per year. This H is adsorbed on the metal surface. Subsequently, two different reactions are possible for the adsorbed H. It can recombine with another adsorbed H atom on the metal surface or it can move to a subsurface position in the steel lattice and diffuse in the bulk of the RPV 73 (6.20)