Effects of process-generated hydrogen on RPV walls

7. Hydrogen pressure in PWR

7.4 Discussion During the operation of a PWR, it is clear that hydrogen can build up in the RPV material. When the steel is saturated with hydrogen, the atoms can recombine in the micro- or macro-voids in the steel and result in an increased mechanical pressure in these voids. In this chapter, it has been tried to quantify this pressure increase for different conditions. To start with, normal operating conditions are converted to a model. Also abnormal conditions are simulated with a focus on a very demanding transient condition, the PTS. The first model was set to simulate a hot in-service condition of a PWR. Using the correct boundary conditions, it was found that the hydrogen fugacity as a result of the radiolysis generated hydrogen is as high as 4.9 10 32 Pa. This value is very high, but after conversion to the corresponding mechanical hydrogen pressure this resulted in 1.471 10 5 atm. This is in the order of a few GPa and thus much more physically acceptable compared to the very high fugacity. In order to separate both hydrogen sources, the hydrogen pressure due to corrosion generated hydrogen during hot in-service condition has also been calculated. It was found to be equal to 3.62 Pa for the worst possible conditions, being 150 mol H/yr of corrosion generated hydrogen and 90 % absorption of this hydrogen in the wall. One can see that the mechanical hydrogen pressure in the RPV wall during hot in-service condition is very small. Another normal operation condition is the cold shutdown. In this case, the reactor is stopped and cooled down to room temperature over a period of approximately 25 hours. Due to slow hydrogen diffusion and the large temperature dependency of the Sieverts’ constant, this results in a significant increase of the hydrogen fugacity in the RPV wall. Over a period of 50 hours after the start of a cold shutdown, less than 1 % of the hydrogen can escape from the RPV material. As the hydrogen concentration in the material remains approximately constant and the Sieverts’ constant decreases significantly for a decreasing temperature, this results in a significant hydrogen pressure that can build up. The four different conditions for corrosion generated hydrogen were again simulated. It was found that the maximum hydrogen fugacity during such a cold shutdown is reached just after the material has completely cooled down. Depending on the different corrosion conditions assumed, the hydrogen fugacity varied between 166 and 1.3 10 5 Pa. This is 5 orders of mangnitude higher compared with the hot in-service condition. Furhtermore, one has to consider the radiolysis generated hydrogen. Similar calculations resulted in a maximum hydrogen fugacity of 2.79 10 35 Pa just after the RPV wall has cooled down. Again converting this to a mechanical pressure, it was found to correspond to 1.652 10 5 atm. This is again in the order of a few GPa and is much more physically relevant compared to the hydrogen fugacity. Finally, the RPV must also be able to withstand severe incidental or accidental conditions. One of the most demanding of them, is the pressurized thermal shock, where the RPV is rapidly cooled and an extra pressure buildup in the primary circuit can occur. This results in high thermal stresses in the RPV material and the rapid temperature drop in the wall will even more inhibit the diffusion and escape of the 92