Effects of process-generated hydrogen on RPV walls

6.3. In-service hydrogen generation

for a concentration of 35 STP cm 3 /kg in the primary water at 300 ◦ C was calculated before and found to be 3.206 10 4 Pa (see Figure 6.1). One can see both values differ one order of magnitude. This is very reasonable if one takes into account all the approximations and assumptions which have to be made to find these values. Especially a the Henry coefficient of atomic H carries a big uncertainty, as it had to be estimated from the Henry coefficient from the noble gases. As this calculation for the absence of radiation has proven that the model has a reasonable accuracy, another explanation must be found for the excessive equivalent H 2 fugacity in the primary system for typical conditions in a PWR at full power. To explain these very high fugacities in the primary water, one has to be aware that a fugacity is the thermodynamical equivalent of the pressure. For this very high fugacity, the difference with pressure becomes significant. The relation between the fugacity, f i and pressure, P i for a specie i is given by: f i = νP i (6.14) where ν is the fugacity coefficient for specie i at a specific temperature and pressure. Shaw and Wones [46] have reviewed lots of experimental measurements of the fugacity coefficient for H 2 over a large range of conditions. They reported an empirical equation for the fugacity coefficient of H 2 between 0 and 300 ◦ C and up to 3000 atm. ln( ν ) = C 1 P − C 2 P 2 + C 3 [exp( − P/ 300) − 1] (6.15) with C 1 = exp( − 3 . 8402 T 1 / 8 + 0 . 5410) C 2 = exp( − 0 . 1263 T 1 / 2 − 15 . 980) C 3 = 300 exp( − 0 . 11901 T − 5 . 941) where P is the pressure expressed in atm and T is the temperature in K. The relative error of this equation, when one compares it to the experimental values, is very low, about 1%. Using this expressing, one can find the pressure corresponding to the fugacity of H 2 in the primary system for a PWR at full power, 4.2 10 27 atm. Using an iterative method, it has been found that a H 2 pressure of 1.471 10 5 atm corresponds to a fugacity 4.2 10 27 atm. As such the actual equilibrium equivalent pressure of H 2 in the primary system is more realistic. However, it has to be mentioned that this pressure is out of the range for which equation 6.15 has been developed. Therefore, the accuracy of this extrapolation is unknown, but it gives a good indication of the order of magnitude for the pressure. Equilibrium H concentration in the RPV wall As the fugacity of H 2 in equilibrium with the atomic H concentration in the primary water is known, it is possible to calculate the equilibrium concentration of H in the RPV wall steel. Sieverts’ law gives the relation between both entities. a H = k S q f H 2 (6.16) 71