The approach to equilibrium of a deuteron gas absorbed into a metal is considered in the framework of a model in which the crystal is described in terms of its elementary excitations. The deuteron-deuteron interaction is dominated by the Plasmon exchange; while the relaxation to equilibrium is mainly due to the coupling with the phonons. The particle-hole contribution is smaller than the plasmon contribution, but not negligible. The time evolution of the deuteron gas, after a first stage dominated by quasi-free scattering, is characterized by the relaxation toward the formation of quasi-deuterium molecules. During this evolution toward equilibrium, fusion reactions can take place at an experimentally detectable rate, while at equilibrium the fusion rate is quite small and comparable with the one for free deuterium molecules.