This paper proposes a novel method for dissociating high‐energy molecular ions, such as H2+, which are injected into a magnetic mirror container. The dissociating medium is a highly ionized plasma consisting of protons and high‐energy electrons ejected in the decay of dense thermal neutrons which exist within a nuclear research reactor. The plasma is accumulated within a weak magnetic mirror container inside the reactor and then transferred magnetically to the site where the H2+injection is to take place. Finally the dissociating plasma is compressed magnetically to a density of 1011/cm3and injection is begun. The plasma provides an efficient means for dissociating the injected H2+ions, but contributes negligibly to charge exchange losses of trapped high‐energy protons.