There is currently considerable interest in the use of micron scale palladium powder for the separation of hydrogen isotopes ¿ e.g. tritium extraction in connection with fuel for fusion. The rate of isotope exchange is determined by three main factors: The time for the gas to percolate around the particles, the permeation rate through the surface of the particles and the diffusion rate within the particles. We (Salford University in collaboration with AWE) are in the process of developing a model incorporating all three of these contributions, using data from permeation and gravimetric absorption measurements to parameterise the first two contributions and are seeking to obtain reliable data for the isotope dependent bulk diffusion rates, for which QENS is the ideal probe.