Effects of process-generated hydrogen on RPV walls

7. Hydrogen pressure in PWR

(a) Zoom on time dependent concentra- tion profile close to the steel-water inter- face.

(b) Zoom on the time dependent concen- tration profile at the lining-base material interface in the RPV wall.

Figure 7.11: Concentration profile of hydrogen in the RPV wall as a result of a PTS considering only radiolysis generated hydrogen.The lines show the concentration profile for different moments after the start of the PTS with an interval of 1 hour up to 50 hours. during a PTS is found to be lower by a factor of more than 2 compared to that during a cold shutdown. This can be surprising as less hydrogen was able to escape from the wall material. However, the water temperature during a PTS is 40 ◦ C and sets the RPV wall temperature. This is higher compared to the final cold shutdown temperature, 298 K or 25 ◦ C. This difference of only 15 K has a big influence on the hydrogen fugacity due to the Sieverts’ constant temperature sensitivity, as shown before in Figure 6.9. Despite the reduced hydrogen fugacity immediately after a PTS compared to a cold shutdown, its value is still very high. It corresponds to a mechanical hydrogen pressure of 1.625 10 5 atm. Furthermore, one has to realise that a PTS is a very demanding transient for the RPV. The rapid drop in temperature results in big thermal gradients and therefore extra thermal stresses, which can be very large. These will add up to the stresses due to the hydrogen buildup in the material. Therefore, the possibility of crack growth during a PTS can be larger compared to a cold shutdown. 7.3.2 Corrosion generated hydrogen From the analysis above, one can expect similar results for the corrosion generated hydrogen. Therefore, these results will not be discussed in much detail as the discussion above is equally valid for corrosion generated hydrogen. 7.12 shows the hydrogen concentration profile as a function of depth in the RPV wall and time for the worst case corrosion scenario, 150 mol H/yr and 90 % absorption in the wall. Conform to the earlier graphs, lines are shown with an interval of hour and extend 90