Effects of process-generated hydrogen on RPV walls

7. Hydrogen pressure in PWR

efficiency and the corrosion rate. For radiolysis, an equilibrium hydrogen activity in the steel, equal to 3.16 10 15 ppm has been found. Using Sieverts’ law, these concentrations can be converted to internal hydrogen fugacities in the steel. From 6.18, Sieverts’ constant at a temperature of 300 ◦ C is 0.1425 ppm Pa -1/2 . Therefore, the maximum equilibrium hydrogen fugacity in the steel wall is: f H = x H k S 2 = 3 . 16 · 10 15 0 . 1425 ! 2 = 4 . 9 · 10 32 Pa (7.1) The maximum equilibrium hydrogen fugacity due to the radiolytic hydrogen in the steel wall is found to be 4.9 10 32 Pa. This is, as expected, exactly equal to the effective internal hydrogen fugacity in equilibrium with the atomic hydrogen concentration in the primary water. Conversion to the effective hydrogen pressure, using the fugacity coefficient from equation 6.15, gives 1.471 10 5 atm. Doing the same calculation for the corrosion generated hydrogen, one can find: f H = x H k S 2 = 0 . 271 0 . 1425 2 = 3 . 62 Pa (7.2) The hydrogen fugacity corresponding to the maximum hydrogen concentration in the ferritic steel is found to be 3.62 Pa. One can see that the maximum internal hydrogen pressure due to radiolysis is very high and certainly can result in a growth of the hydrogen cracks. 7.2.2 Cold shutdown The second normal operating condition that needs to be considered is the cold shutdown. At the end of each cycle of the reactor, a part of the fuel needs to be replaced by new enriched fuel rods. To do this, the reactor is slowly cooled down and the pressure released. The cooling of the reactor coolant is performed slowly over a time period of approximately one day. The cooling path followed for a typical pressurized water reactor is shown in Figure 7.1. As can be seen, the temperature is decreased stepwise to limit the thermal stresses. After a first fast cooling of the coolant, the temperature is held constant at 80 ◦ C for 12 hours. Subsequently, the temperature is slowly lowered to 25 ◦ C. The complete cooling path takes about 25 hours to cool the reactor coolant from a temperature of 300 ◦ C down to 25 ◦ C. Similar to the reactor coolant, the RPV will cool down. The temperature of the coolant determines the RPV temperature at the steel-water interface. Knowing the thermal conductivity, heat capacity at constant pressure and the density of the RPV material as a function of temperature, one can calculate the temperature in the RPV as a function of time during shutdown. A simple simulation in COMSOL Multiphysics is performed to do this calculation. The material properties are taken 78