Looking at the table of isotopic masses table it seems that certain atoms, as 64Zn or 58Ni, could be able to capture an electron and then release a positron with an energy higher, in average, that the initial electron. On the other side, some nucleus are able to capture a positron and release an electron, for instance 124Sn or 116Cd. Of course, these are just the atoms wich can undergo double-beta, but here we have an energy input from the initial particle so the decay rate will be higher. We could do a pile of both kinds of material and then try a chain reaction. The energy gain in this kind of reactions is about 1MeV / atom, thus two orders of magnitude lower than fission, and one order lower than fusion, but still orders higher than chemical sources of energy. True, there are dissipative mechanisms against this, the most relevant being neutrino radiation, for which I do not know on any efective trick to recapture :-( In any case, I wonder if someone has anytime read about this mechanism, even if only to dismiss it. Any pointer there? It seems worth to look a little about how to overcome the dissipation problems.