Isotope effect of hydrogen and deuterium permeability and diffusivity in fusion reactor materials. A literature review

The characterization of hydrogen transport parameters is essential for a good prediction of its behavior and a correct design of the elements that will make up nuclear fusion reactors. It is therefore essential to determine the isotopic ratio of the transport parameters between protium (H), deuterium (D) and tritium (T), both for economic and safety reasons. Classical theory predicts that the ratio of their diffusivities and permeabilities should be proportional to the inverse square root of their atomic mass, while solubility values would be independent of their masses, but there are discrepancies in the literature. Despite this, it has not yet been possible to obtain either a correct value for the ratio or any uniform conclusion on the matter. This article presents a review of the works published in the field of the isotope effect of hydrogen permeability and diffusivity to date, focusing on protium and deuterium isotopes, which are the isotopes that have been most extensively analyzed experimentally. The conclusion is clear: the theoretical value cannot be stated to be correct and the results are very varied, although they seem to be related to the microstructure of the material tested. In addition, some factors that could have an influence on it have been analyzed, which would be interesting to take into account for future studies.