Cooling system must be brought under control to prevent meltdown

Nuclear expert Walt Patterson tells Michael McCarthy of the extraordinary complexity of the challenge engineers face

Several "alarming scenarios" face the engineers who were last night struggling to prevent meltdowns of two atomic reactors damaged in Japan's tsunami disaster, one of Britain's leading nuclear experts said yesterday.

Disturbing possibilities range from another explosion of hydrogen gas, like that which destroyed one reactor's outer shell on Saturday, to an accidental restart of the nuclear chain reaction inside the damaged plants, to a rupture of the steel pressure vessels holding the reactors' highly-radioactive cores, according to Walt Patterson, a veteran authority on nuclear policy.

"Even if this were happening under normal conditions it would be a huge challenge, but it's happening in circumstances of extreme complication because of the earthquake and the tsunami," said Dr Patterson, an associate fellow of the foreign policy think-tank Chatham House. "There are clearly a number of alarming scenarios that they're trying to cope with."

The nightmare facing Japan is of nuclear meltdown in the middle of its greatest catastrophe since the Second World War, with two of the six atomic reactors at the Fukushima power plant in dire trouble – as their water-cooling systems, essential to carry away the immense heat the reactors produce, have been knocked out by the earthquake and subsequent tidal wave. The danger is that the uranium fuel assemblies in the reactor core will overheat, melt and eventually release large amounts of radioactivity.

Engineers at Fukushima have been flooding the threatened reactors with seawater treated with boron, which inhibits nuclear reactions, but they could not prevent a build-up in the more severely damaged No 1 reactor of hydrogen gas, produced when the zirconium coating of the fuel rods began to melt and react with the water. The hydrogen was "vented" from the pressure vessel, the solid steel casing housing the reactor core, but on Saturday the gas exploded, blowing the outer containment walls of the No 1 reactor to bits. Now there are fears the same could happen with the No 3 reactor, which is also in trouble.

"They have certainly indicated that they're afraid that something similar might happen at unit 3, if they try to vent, that is, try and reduce the pressure inside the reactor vessel by opening valves, because that will let what will be very concentrated hydrogen up into the air," Dr Patterson said.

He went on: "The other consideration that caught my eye is that they have made a point of stating that they're using boric acid, that they're using borated water. This suggests to me that they're afraid the fuel damage is sufficiently bad that the stuff might collapse back into a critical configuration and start up the chain reaction again – which would really put the cat among the pigeons."

This would mean dealing with much more heat that is being dealt with at the moment, which comes from the "fissions products", the radioactive substances such as caesium, strontium and iodine produced inside the fuel by the reaction itself. "They're now obviously wanting to make sure that they don't have the additional compound problem of suddenly having a chain reaction start again," Dr Patterson said.

The worst case, he said, would be if one of the steel pressure vessels broke apart. "They're old," he said. "The No 1 vessel has been there since 1971 and was actually due to be decommissioned later this month.

"These pressure vessels are almost a foot thick, they're cast steel because they have to be, because of the high pressure they're containing.

"But they're also under bombardment from neutrons for, in this case, about 40 years, and you really do not know what condition of that metal is – and particularly if you then have violent shocks both of pressure and temperature, if the reactor is moving outside its normal operating conditions."

In the Chernobyl disaster in 1986, the reactor – of a completely different design to those at Fukushima – overheated so violently that it blew off its pressure vessel's 2,000-tonne lid, allowing the escape of vast amounts of radioactivity.

If a pressure vessel ruptures at Fukushima, Dr Patterson said, "all bets are off".