Effects of process-generated hydrogen on RPV walls

7.3. Pressure during incidental or accidental conditions

equal to the cooling water temperature.

Figure 7.10: Temperature profile in the RPV wall as a result of a PTS. The lines show the temperature profile for different moments after the start of cooling. The interval between the lines corresponds to 10 minutes. The equilibrium temperature of 313 K is reached in the whole thickness of the RPV wall after approximately 2 hours. As mentioned before, one can expect the fast cooling during a PTS to inhibit the diffusion of hydrogen in the RPV steel. Therefore, the high concentration of hydrogen cannot escape from the steel and will result in an elevated hydrogen fugacity in the material. The concentration profile in the RPV wall due to the temperature dependent diffusion during a PTS is simulated and shown in Figure 7.11. As for the case of a cold shutdown, the lines are shown with an interval of 1 hour and extend up to 50 hours after the start of the PTS. One can see from Figure 7.11, that diffusion of hydrogen is very limited. As expected, the rapid cooling of the RPV steel results in very low diffusion coefficient and therefore a very slow hydrogen diffusion. One can compare Figure 7.11 with Figure 7.9. As both graphs use the same scaling, one can easily see that during a PTS even less hydrogen was able to escape the material compared to a cold shutdown. This, of course, corresponds perfectly with the expectations. Therefore, one might expect the hydrogen fugacity at the lining-base material interface to be even higher for during a PTS compared to a cold shutdown. However, using equation 6.18, one can find a maximum hydrogen fugacity equal to 1.02 10 35 Pa. The hydrogen fugacity 89