Nuclear energy experts were working on Saturday to establish the chain of events at Japan’s Fukushima Daiichi facility, which suffered an explosion following Friday’s 8.9 magnitude earthquake. Experts outlined the range of possible scenarios, based on information available from official Japanese sources.
What happens to nuclear plants in the event of an earthquake?
All operating reactors measure something called ground acceleration and when anything beyond the slightest tremor is detected, this trips the plant and it is shut down automatically.
What about the fuel?
Even though the plant is shut down, engineers need to make sure that the fuel inside the reactors does not heat up to unsafe levels. There are therefore water cooling systems around the core to keep the water circulating in order to get rid of all the heat, what is termed “decay heat”.
What went wrong at the Fukushima Daiichi facility?
In order to run the cooling systems, you need power. Typically, back-up supplies are provided by diesel generators and batteries. In this case, it appears that the generators failed completely, probably due to the impact of the tsunami.
Malcolm Grimston, an energy expert at Chatham House, the London-based think-tank, said he believed from reports coming out of Japan that the actual fission process had closed down safely.
However, because of the broken down generators, water stopped circulating and began to boil. This led to a rise in pressure in the centre of the reactor and an increasing danger that some of the core could become uncovered.
Japanese authorities said the explosion at the plant was a mixture of hydrogen, from steam escaping the core, and oxygen from the surrounding air.
But why then have there been reports of the radiation levels rising?
The fuel is stored in metal cans that are made out of an alloy of zirconium and these cans may have begun to rupture, releasing some of the fuel, according to Mr Grimston.
He stresses that at this point the fuel is made up of fission products, including iodine and caesium. Some of these materials are radioactive but not for long – for example, all the iodine would be gone within three months. At the same time, the metal cans can react with the steam, producing hydrogen.
How did the hydrogen escape?
The decision by the authorities to release the pressure in the reactor meant that they released the mixture of materials inside: a small amount of the radioactive material and hydrogen. When the hydrogen mixed with the air outside, it exploded.
Mr Grimston said the Japanese authorities would have been aware there could be an explosion.
“It is clear they did not plan the explosion but in case it did occur, they had already evacuated people from around the area,” he said. “The explosion was not desirable but it is not a game-changer,” he added.
Why are they now flooding the reactors with seawater?
Nuclear energy experts on Saturday said the decision to flood the core of one of the reactors at the Fukushima Daiichi facility with seawater is a clear sign the authorities had decided to write it off and that it would never be operational again.
What happens next?
Experts said they expected the seawater should deal with the overheating.
Should we be worried?
The big concerns are that the seawater does not solve the overheating. In the worst case scenario, damaged fuel could lead to highly radioactive products being emitted from the core.
There is also a risk of contamination. Experts said the hope is this will have been small and localised. Detailed analysis will have to take place before anyone is allowed back into the evacuated areas.
How do events so far compare with Three Mile Island and Chernobyl?
“In the case of Three Mile Island, you also had a coolant problem and damaged fuel. But there was a containment structure that worked. You had ruined fuel that was successfully contained. Chernobyl lacked the containment,” said one expert.
“I don’t think under any circumstances have we ever been in Chernobyl territory,” said Mr Grimston, noting that the reactor closed down. “Three Mile Island is realistic but it is the worst case scenario. The best case is that the plant has closed down but without any fuel melting.”(Financial Times)
