Surface Barrier for Tritium Permeation

To apply the surface barrier to reduce hydrogen permeation, the influence of the surface barrier on both the permeation and retention has been investigated considering physical and chemical stability of the barrier in fusion environment. Since energetic hydrogen from the plasma not only impinges directly into subsurface but also removes the front surface barrier, only the back surface barrier works reliably . Oxides, carbide and nitride are candidates as the barrier but their mechanical as well as chemical stability is an important concern, because very large thermal gradient and thermal cycling in fusion environment could enhance the crack initiation and exfoliation of the barrier Therefore an appropriate barrier which is stable under a particular operating condition must be developed The most reliable way to reduce the permeation is to use a metallic layer, but it must be rather thick. It should be noted that the back surface barrier to suppress the permeation inevitably increases the retention Therefore an optimization between the permeation decrease and retention increase is necessary. An alternative way to reduce the plasma or ion driven permeation is to decrease the recombination coefficient at the back surface However, large uncertainty in the observed recombination coefficients does not allow us to rely on the recombination limited process and further work is needed.