Hydrogen permeability through beryllium films and the impact of surface oxides

Beryllium will constitute the major part of the first wall of ITER, however, several aspects of the tritium retention and recycling in fusion reactors are still open. Studying details of the hydrogen isotope interactions on Be films is in principle easier and more accurate than on the bulk Be metal since a thin (and therefore more permeable) layer of Be film could be deposited on a desired substrate by applying well controlled methods. Results of the hydrogen permeation through 8 micrometer thick Be films deposited by the thermionic vacuum arc method on Eurofer steel membranes with exposed area of 8.4 cm2 are presented. The permeation reduction factor (PRF) at 400 °C varied on six samples from 14 to 135 with respect to the bare Eurofer membrane. The highest PRF value enables expression of the Be film permeability coefficient P by means of a simple model which gives PBe ∼ 2 × 10−15 mol H2/m s Pa0.5. Lower PRF values could be explained by microscopic imperfections which represent parallel hydrogen paths through the Be film and enhance the permeation rate. Some of them were revealed by the SEM while their presence could be confirmed also by observing permeation flux transients recorded after the hydrogen exposure. The two-step process of achieving the steady flux agrees with our numerical simulation. It was found that for unintentionally oxidized samples the extracted regular (eliminated contribution of the pinholes in Be film) permeation rate is almost identical from sample to sample and accounts to j ≈ 1.2 × 10−7 H2/m2 s at 1 bar hydrogen driving pressure due to BeO formation. For a non-oxidized sample this value is several times higher, j ≈ 6.5 × 10−7 mol H2/m2 s. From the latter follows that PBe ≈ 1.9 × 10−14 mol H2/m s Pa0.5, while PBeO ∼ 1 × 10−17 mol H2/m s Pa0.5 can be estimated by assuming a 35 nm thick BeO layer.