Résumé : The 2H(d,p)3H, 2H(d,n)3He, and 2H(d,γ)4He reactions are studied at low energies in a multi-channel ab initio model that takes into account the distortions of the nuclei. The internal wave functions of these nuclei are given by the stochastic variational method with the AV8' realistic interaction and a phenomenological three-body force included to reproduce the two-body thresholds. The obtained astrophysical S-factors are all in very good agreement with experiment. The most important channels for both transfer and radiative capture are identified by comparing to calculations with an effective central force. They are all found to dominate thanks to the tensor force.