Effects of process-generated hydrogen on RPV walls

6.3. In-service hydrogen generation

6.3.1 Dissolved H 2 and radiolytic hydrogen in the primary water In the previous chapter, the concentration of H 2 and H in the primary water were calculated. When one is able to convert these concentrations to equivalent H 2 pres- sures in the primary system, one can use Sieverts’ law to estimate the concentration of H in the RPV steel wall. In the previous chapter, the H 2 partial pressure due to the dissolved H 2 in the primary water was already calculated. It was found to be between 2.29 10 4 and 4.59 10 4 Pa, for dissolved H 2 concentrations of 25 and 50 STP cm 3 /kg, respectively, at a temperature of 300 ◦ C. Performing the same calculation for the typical H 2 concentration as used for the simulations in section 5.4, 35 STP cm 3 /kg, one can find a partial pressure of 3.206 10 4 Pa at 300 ◦ C. The complete temperature dependent partial pressure of H 2 for a concentration of 35 STP cm 3 /kg dissolved in the primary water is shown in Figure 6.1.

Figure 6.1: Equilibrium pressure of H 2 for a concentration of 35 STP cm 3 /kg in the primary water as a function of temperature and a pressure of 150 bar.

As the calculation of concentration of species in the primary water due to radiolysis revealed no change in the H 2 concentration compared to the case without radiolysis, one might think this is sufficient and the equivalent H 2 pressure is equal to 3.206 10 4 Pa. However, this is not completely valid. As hydrogen is absorbed in the steel as atomic hydrogen, the concentration of atomic H in the primary water will also have a big influence on the hydrogen concentration in the steel, as it can be directly absorbed in the bulk of the steel 61