Effects of process-generated hydrogen on RPV walls

7.2. Pressure during normal operating conditions

explained before, is due to the fact that the validity of Sieverts’ law is not checked for these very high fugacities. However, these concentrations will be used only for the calculation. Later, one can tranform the concentration into pressures to find more realistic values for the hydrogen pressure in the RPV wall. The initial hydrogen concentration profile can be calculated from Fick’s law, as done before. This curve is shown in Figure 7.8. Starting from this concentration, the time dependent profile can be calculated. The concentration profile is simulated over a time period of 50 hours with an interval of 1 hour. The results are shown in Figure 7.9

Figure 7.8: Initial concentration profile equal to steady state hot in-service condition for radiolysis generated hydrogen.

During the cold shutdown, the maximum hydrogen fugacity in the RPV base material is again reached about 29 hours after the initiation of the cold shutdown. This hydrogen fugacity is found to be 2.79 10 35 Pa. This is, as expected, a ridiculously large number. However, it can be converted to a hydrogen pressure using the fugacity coefficient from equation 6.15. The hydrogen fugacity corresponds to a mechanical hydrogen pressure in the RPV base material of 1.652 10 5 atm. This again is a more realistic value for the hydrogen pressure in the RPV base material, however it has to be mentioned again that this pressure is not in the range where the equation is valid and therefore this extrapolation can not be considered as reliable. 87