Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

6. Hydrogen concentration in PWR wall

wall. It is the fraction of H atoms that will move to the bulk of the RPV wall that of interest for this calculation. This absorption efficiency has been investigated for different conditions but a high variation in results has been reported. Absorption efficiencies as low as 0.02 and up to 90 % can be found in literature. • Smith and Bloom found an absorption efficiency of 90 % for corrosion experi- ments at 300 ◦ C with pH 11. This result was confirmed by Tomlinson, finding an efficiency of 87.5 % for corrosion measurements at 350 ◦ C and a pH of 11. [80] • Gajek and Zakroczymski performed experiments at room temperature, a charg- ing current density up to 30 mA cm -2 and a pH at the steel surface higher then 7. He reported an absorption efficiency as low as 0.02 to 1.6 %. [81] • Escobar et al. investigated the electrochemical charging of hydrogen at high charging currents, between 10 and 50 mA cm -2 , in an acid aqueous environment. They estimated absorption efficiencies between 2 and 20 %. [82] One can see that the lower absorption coefficients are found for high charging currents at room temperature. While higher corrosion rates are found for conditions similar to the conditions in a PWR, being high temperature of 300 ◦ C and high pH. Therefore, it can be expected that the absorption efficiency for H in steel in PWR conditions will be in the range between 10 and 90 %. Using these absorption efficiencies, hydrogen charging rates at the water-steel interface due to corrosion are calculated and shown in Table 6.4. Table 6.4: Hydrogen charging rates of H in the RPV due to corrosion at the steel-water interface.

Hydrogen production rate 50 mol H/yr 150 mol H/yr

Absorption efficiency

8 10 -14 mol cm -2 s -1

24 10 -14 mol cm -2 s -1

10 % 90 %

72 10 -14 mol cm -2 s -1 216 10 -14 mol cm -2 s -1

Equilibrium concentration of corrosion generated hydrogen The constant hydrogen generation at the steel-water interface will result in a con- centration build up of hydrogen in the steel. This hydrogen will diffuse through the steel and leave the steel at the containment side of the RPV wall. Over time, an equilibrium concentration of hydrogen over the complete thickness of the RPV wall will be reached. The steady state diffusion in the RPV wall is given by Fick’s first law:

δc H δx

H = D H

(6.21)

J c

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