The magnitude-9.0 earthquake rocked Fukushima Daiichi nuclear power station at 2:46 p.m. on 11 March, but the real emergency began an hour later. A wall of water swept across the site, washing away power lines and the fuel tanks for the emergency backup generators designed to take over if grid power failed. Inside the control room of the unit 1 reactor, the lights went out and the 1970s-vintage analogue gauges drifted to zero.

It will probably be years before anyone knows exactly what happened inside the three reactors at Fukushima Daiichi that seem to have partially melted down in the wake of the tsunami. But from press reports, public statements and interviews with experts, it is possible to work out the most likely scenario. And already it is clear that decisions made in the initial 24 hours by the handful of operators in the control room probably averted a much greater nuclear catastrophe than the one that now faces Japan.