The problem here isn't criticality but waste heat: heat generated by uncontrolled radioactive decay of waste products (fission products and products of neutron activation). There's no way to turn off this heat; it continues uncontrollably for a long time, decaying super-exponentially (here's a graph: [1]).
As far as the nuclear reaction goes, the Fukushima reactors were "off" within seconds of the initial earthquake, and remained so since then. (Though, tangentially, there's been speculation of some low-power criticality events since then, but the evidence is flimsy. It's not particularly important anyway). That's the polar opposite of the Chernobyl disaster, where it was an uncontrolled nuclear reaction which destroyed the reactor building and released vast amounts of nuclear waste in an explosion (and subsequent fire). In Fukushima (and Three Mile Island, and others) there was no uncontrolled reaction; but there was decay heat and its effects. The big explosions at Fukushima were an indirect effect of decay heat: explosions of hydrogen generated by when extremely hot zirconium "burned" in steam. And the large scale fuel failure ("meltdown") was also caused by decay heat -- too much decay heat, and not enough cooling.
As far as the nuclear reaction goes, the Fukushima reactors were "off" within seconds of the initial earthquake, and remained so since then. (Though, tangentially, there's been speculation of some low-power criticality events since then, but the evidence is flimsy. It's not particularly important anyway). That's the polar opposite of the Chernobyl disaster, where it was an uncontrolled nuclear reaction which destroyed the reactor building and released vast amounts of nuclear waste in an explosion (and subsequent fire). In Fukushima (and Three Mile Island, and others) there was no uncontrolled reaction; but there was decay heat and its effects. The big explosions at Fukushima were an indirect effect of decay heat: explosions of hydrogen generated by when extremely hot zirconium "burned" in steam. And the large scale fuel failure ("meltdown") was also caused by decay heat -- too much decay heat, and not enough cooling.
[1] http://en.wikipedia.org/wiki/Decay_heat#Power_reactors_in_sh...