Judging by https://en.wikipedia.org/wiki/Critical_mass , californium-252 has the lowest critical mass among known isotopes and isomeres. But it is still very high. Is there a way to reduce it significantly (down to microgramms) or, perhaps, a semi-stable isotope/isomere, which would have such a low critical mass initially? What I am looking for, actually, is possibility (not interested in feasibility) to produce energy from fission in a similar way it is done in internal combustion engines. E.g. if an uncontrolled explosion caused by a chain reaction could be small enough to be contained and harvested for energy in some industrial setting (I imagine a giant turbine or a piston). For simplification, I assume it would be initiated by contact of small two half-spheres (not half-balls, e.g. empty inside) which would make material to get super-critical.