Fukushima suffers setback as officials detect signs of nuclear fission | World news | The Guardian

Fukushima suffers setback as officials detect signs of nuclear fission | World news | The Guardian: "The utility dismissed the possibility of a "major criticality accident", in which a sustained nuclear reaction occurs, but has not ruled out localised criticality inside the reactor."

MOX = plutonium plus uranium (natural, reprocessed or depleted) = weapon grade plutonium (therefore it is explosive) -Wkipedia

Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material. MOX fuel contains plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium (LEU) fuel used in the light water reactors that predominate nuclear powergeneration. For example, a mixture of 7% plutonium and 93% uranium reacts similarly, although not identically, to LEU fuel.

One attraction of MOX fuel is that it is a way of utilizing surplus weapons-grade plutonium, 

Fukushima = nuclear fission, what is nuclear fission?-Wikipedia

nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing freeneutrons and photons (in the form of gamma rays).