Theoretical calculation of hydrogen trapping in helium contained metals

In fusion devices, helium is formed in the first wall by severe α bombardment and by tritium decay. In such a device, helium is also formed by (n, α) reactions caused by the very high fluence of high-energy neutrons (≈14 Mev). Since there are many sources from which hydrogen can be introduced into the pre-demaged metal, the synergistic effects caused by the presence of both helium bubbles and hydrogen atoms in the lattice, should be considered. The purpose of this study is to suggest a model which explains the trapping of hydrogen around or near helium bubbles. According to this model, hydrogen atoms are attracted toward the bubbles due to positive stresses created by the very high pressure (350 kbar) existing inside the bubbles. Theoretical calculations based on the proposed model show that the extreme trapping energy of hydrogen atoms around helium bubbles is 0.71 ev/atom. It is shown that most of the hydrogen atoms are trapped in a very small volume located very close to the bubble surface. The calculations show that the average hydrogen concentration in the volume which contains the bubbles is 1×10⁴ ppm in the case where the hydrogen concentration far from the bubble zone is only 1 ppm. This last result strongly shows that hydrogen trapping around helium bubbles must be treted as a massive factor and not only as a local and negligible one.