The issue at Fukushima isn't criticality, though. All the reactors were safely shut down seconds after the earthquake hit, and no new fission has occurred since

The issue at Fukushima isn't criticality, though. All the reactors were safely shut down seconds after the earthquake hit, and no new fission has occurred since then. The problem is that even a shut-down reactor core generates significant heat due to the decay of fission by-products, and that's what caused the problems. That said, there are other reactor designs which are more inherently safe.