Effects of process-generated hydrogen on RPV walls

7.4. Discussion

hydrogen. As the temperature drop during a PTS is very fast, it was simulated by an instant temperature drop at the inner wall surface to the water temperature of 40 ◦ C. Again, the hydrogen concentration in through the depth of the RPV wall is simulated as a function of time and the corresponding hydrogen fugacity was calculated. If one only considers radiolysis generated hydrogen, the maximum hydrogen fugacity was found to be equal to 1.02 10 35 Pa. For the corrosion generated hydrogen, the resulting hydrogen fugacity ranges between 60 and 4.73 10 4 Pa, depending on the assumed conditions. This might be surprising as it is lower compared to the hydrogen fugacity during a cold shutdown, however this is explained by the higher final temperature of the RPV wall and therefore higher value for the Sieverts’ constant. The mechanical pressure corresponding to this radiolysis generated hydrogen fugacity is equal to 1.625 10 5 atm. This is the same order of magnitude as the mechanical pressure after a cold shutdown, but one has to realize that on top of this pressure buildup also large thermal gradients are present during a PTS. The resulting thermal stresses will add up to the mechanical pressure. The total stress in the RPV material may therefore be higher during a PTS compared to a cold shutdown. If one wants to relate the mechanical hydrogen pressures calculated above to actual crack growth in the RPV material, one needs to know the minimum pressure necessary for crack growth. Some studies have been investigating this for steel. Chu et al. [85] found that the minimum mechanical pressure in the steel has to be of the order of a few GPa to start a further growth of the cracks. In the above calculations the mechanical hydrogen pressure due to radiolysis generated hydrogen was found to be in the order of 10 GPa. This is exactly on the limit to start crack propagation in the RPV material. Therefore, there is a possibility of crack growth due to the hydrogen pressure buildup in the cracks.

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