The media are giving as much attention to the Fukushima I nuclear power plant as they are to the impact of the tsunami, even though the likelihood of measurable health effects from the former is small, and the number of deaths caused by the latter is certain to be very large. This isn’t surprising: nuclear fear, founded on Hiroshima and Nagasaki and reinforced by Three Mile Island and Chernobyl, is not irrational, though it’s worth noting that many more people have been saved by X-rays and radiotherapy than have been killed by radiation of any kind.

What’s happening at Fukushima Dai-ichi Units 1 and 2 is similar to what happened at Three Mile Island Unit 2 in 1979. The Three Mile Island light water reactor scrammed eight seconds after a pump failure. (SCRAM stands for 'Safety Control Rod Axe Man': Norman Hilberry stood with an axe on top of the first nuclear reactor, at the University of Chicago in 1942, ready to cut the rope holding the control rod if radioactivity ran amok.) The Fukushima light water reactors 1, 2 and 3 scrammed automatically in response to the earthquake (reactors 4, 5 and 6 were already shut down for maintenance). Scramming reduces a reactor’s heat production by more than 92 per cent, but heat from radioactive decay means that cooling is still needed.

At Three Mile Island, valves failed; at Fukushima, the problem was with diesel-powered cooling water pumps. At Three Mile Island, the massive onslaught of alarms overwhelmed the operators, measurements were misinterpreted and half the reactor core melted. Water reacted with hot fuel casings to produce hydrogen, which escaped into the containment building and ignited. But the reinforced concrete building, which was designed to withstand a plane crash, was unbreached. At Fukushima, each of the sturdy containment buildings is surrounded by a much flimsier structure. The explosions were probably caused by plant operators releasing hydrogen from the containment buildings to relieve the pressure.

Most studies have found that the negative health effects of Three Mile Island approach zero. But the accident did lasting damage to nuclear power’s reputation, already low – in 1973, E.F. Schumacher wrote that nuclear power is 'the most serious agent of pollution of the environment and the greatest threat to man's survival on earth' – and Fukushima has dented it even further. The hard task for politicians is to square nuclear fear with anxiety about the imminent threat of climate change.