Effects of process-generated hydrogen on RPV walls

6. Hydrogen concentration in PWR wall

after adsorption on the surface. Similar to the calculation for H 2 in section 5.2, it is possible to calculate the equivalent pressure for atomic H in the primary system. First, one has to know the Henry’s constant for atomic hydrogen in water. Once this is known, the atomic H fugacity and the equivalent H 2 fugacity in equilibrium with the atomic H fugacity can be calculated.

Henry’s constant for atomic hydrogen As a reminder, Henry’s law is given by: c i = H i · f i

(6.1)

where c i is the concentration of species i in the liquid in mol/m 3 , H i is the Henry constant of species i corresponding to the appropriate liquid in mol/m 3 Pa and f i is the fugacity of gas i in equilibrium with the liquid in Pa. From this law, one can find the partial pressure of gas i in equilibrium with a concentration, c i , in the liquid by rewriting the equation as: f i = c i H i (6.2) A value for the Henry coefficient of atomic hydrogen in water is however unknown in literature. Therefore, it is necessary to estimate this value. As the hydrogen is dissolved in the water as an atomic neutral entity, its behavior can be compared to that of the noble gases. The Henry coefficient of the noble gases have been investigated intensively in the second half of the 20 th century. Himmelblau [70] has found an universal equation relating the relative Henry coefficient to the temperature: log ( H ∗ i ) = 1 . 142 − 2 . 846 1 T ∗ +2 . 486 1 T ∗ 2 − 2 . 486 1 T ∗ 3 +0 . 2001 1 T ∗ 4 (6.3) where H ∗ i = H i /H max i (6.4) and 1 T ∗ = 1 T − 1 T c 1 T max i − 1 T c (6.5) with H i the Henry coefficient of specie i, expressed in atm, at temperature T in Kelvin. As the Henry coefficient is given in atm, the Henry law in this case relates the molfraction to the pressure. H max i the maximum value of the Henry coefficient of the gas vs. temperature for specie i. T max i is the temperature corresponding to the maximum value of the Henry coefficient and T c is the critical temperature of water equal to 647 K. [70]. One can see that equation 6.3 only needs 2 parameters to define the complete function, i.e. H max i and T max i . The equation found by Himmelblau [70], equation 6.3, is shown in Figure 6.2 to- gether with experimental values by Krause and Benson [71], Crovetto and Fernandez- Prini [72] and Potter and Clynne [73] for He, Ne, Ar, Kr and Xe. One can see that 62