Diffusional exchange of isotopes in a metal hydride sphere

► A model is developed that simulates exchange of hydrogen isotopes in a spherical metal hydride particle. ► Diffusion depends on the concentration gradients of both isotopes in addition to the site vacancies. ► Thermodynamic coupling is derived from the Gibbs free energy of mixing and the excess chemical potential. ► Unknown diffusion coefficients are determined by fitting to available experimental data. ► Example calculations illustrate times required for isotope exchange.