The events unfolding at the Fukushima Daiichi nuclear plant are already worse than any worst case scenario could have predicted. Three reactors are thought to be damaged, but no one knows how badly, because the tsunami knocked out most of the instruments that would have allowed operators to read, among other things, water levels. With the roofs blown off by hydrogen explosions, there is increasing concern over the spent fuel pools inside the secondary containment shells. The spent fuel, recently discharged from the reactor cores, is hot and extremely radioactive. If the fuel disintegrates (or ‘melts’) because of insufficient cooling, there is a chance that it could reach criticality, either inside the core or in the spent fuel pools. In other words, a self-sustaining nuclear chain reaction could start, and fires could spread the radioactivity far and wide – depending on wind and weather conditions, possibly beyond Japan.