Effects of process-generated hydrogen on RPV walls

6. Hydrogen concentration in PWR wall

The equilibrium constant for this dissociation reaction, K Dis , is given by:

f 2 H f H 2

K Dis =

(6.10)

When one knows the equilibrium constant for this reaction, one is able to calculate the H 2 fugacity in equilibrium with the H fugacity found before. This equilibrium constant is calculated by the HSC Chemistry 5 software. Its variation with temperature is shown in Figure 6.7.

Figure 6.7: The dissociation constant of H 2 as a function of temperature. Calculated by HSC Chemistry 5. H 2 ←−→ 2 H For a temperature of 300 ◦ C, 573 K, the dissociation constant is found to be 3.71 10 -35 atm, equivalent to 3.76 10 -30 Pa. The H 2 fugacity in equilibrium with the 43 Pa H fugacity in the primary sytem is: f H 2 = f 2 H K Dis = 43 2 3 . 76 · 10 − 30 = 4 . 9 · 10 32 Pa (6.11) The equilibrium H 2 fugacity at the beginning of the RPV fuel cycle is found to be 4.9 10 32 atm or 4.9 10 27 bar. At the end of cycle, the equilibrium H 2 fugacity is 4.2 10 32 Pa or 4.2 10 27 atm. These fugacities are incredibly high. Therefore, one might think the model, which calculated the H concentration in the primary water, is wrong. Complete validation 68