What might happen at nuclear plant next?
The threat of nuclear disaster in the aftermath of the Japanese earthquake increased last night as three reactor cores at the stricken Fukushima Daiichi plant began to melt.
Despite earlier assurances, engineers were losing their three-day battle to avoid a full-scale meltdown after all efforts to cool the cores appeared to have failed. The authorities were facing the “worst case scenario” of the plant’s reactors exploding and releasing a massive dose of radiation into the atmosphere.
There are now growing fears the situation could escalate beyond a Three Mile Island-style leak and towards a disaster on the scale of Chernobyl.
How did this happen?
The three reactors running at the plant survived the initial earthquake intact and were successfully shut down by inserting control rods into the core, which snubbed out the chain reaction.
However, like embers in a fire, the nuclear fuel rods continued to emit enormous amounts of heat and this needed to be controlled by circulating water around the core, much like a radiator keeps a car engine cool.
Unfortunately, the earthquake knocked the power supply to the cooling system pumps, and back-up diesel generators were then flooded by the tsunami.
Emergency batteries were insufficient to pump enough water and the core started heating up. Even swamping the three reactors with sea water appears to have failed to halt the rise in temperature.
What happens when reactors overheat?
Initially, much of the water covering the rods turns to steam and this puts huge pressure on the outer casing of the reactors.
In order to avoid the pressure bursting them open, the engineers decided to release some of the super-heated steam into the outer housing of the plant. Unfortunately, at such high temperatures the water broke down into highly explosive hydrogen and oxygen. It was this that caused the explosions in units one and three, and last night's third explosion in unit two.
Why were these explosions not catastrophic?
While it looked spectacular, the explosions did not actually cause much damage to the reactor itself. This is because the destroyed outer building is merely there to "keep the weather off". It plays no part in containing the radioactive core.
The radioactive fuel itself is first embedded in fuel rods made of zircalloy, an immensely strong and heat-resistant metal. This, in turn, is surrounded by cooling water and a super-strong six-inch steel "flask". Finally, this flask is encased in more than three feet of concrete.
So why did it not stop there?
The cooling systems to the reactor cores continued to fail and the release of the steam left the fuel rods uncovered.
When the rods reached 2,200C, even the zircalloy began to melt, exposing uranium, and in the case of unit three, plutonium, contaminating anything it comes into contact with.
Does this mean Japan has suffered a meltdown?
Not exactly, or not yet anyway. Scientists are describing it as a partial meltdown as the zircalloy appears to have bubbled.
A full meltdown like the name suggests is when the fuel completely melts and creates a pool of radioactive fuel and metal at the bottom of the reactor core.
Surely the flasks are designed to contain a meltdown?
On paper they are, but we are getting into uncharted territory. Technically, it could be possible for the molten fuel to reach even higher temperatures and to melt through the flask.
Equally there is the problem of pressure building to such a degree that the flask itself explodes, firing radioactive particles into the atmosphere. If this threatens to happen, engineers face a dilemma. By now any steam in the reactor will be highly radioactive and any controlled release would in itself cause nuclear contamination.
Will it explode like a nuclear bomb?
No. Nuclear reactors are not bombs. Explosions at nuclear plants are caused by either a build-up of pressure or a chemical fire, such as when water breaks down into hydrogen and oxygen.
What is being done about it?
Engineers continue to pump sea water into the reactors in the hope this will eventually douse the heat. They are also adding boric acid, which dampens nuclear reactions. The hope is that the radioactive fuel will eventually cool down.
What can Japan do to prepare for a nightmare scenario?
Evacuate the area and hope the wind continues to blow out to sea. At present, more than 200,000 people have been moved from around the plant and a 20km exclusion zone has been set up.
They have also provided 230,000 potassium iodine tablets to evacuation centres as a precautionary measure.
How does this now compare with other nuclear accidents?
Fukushima remains rated four on the International Atomic Energy Agency's seven-point scale, described as an "accident with local consequences". But it is threatening to go higher.
Three Mile Island in Pennsylvania in 1979 was a five. This involved a partial core meltdown, but it was contained by the reactor and little radiation escaped.
So could it be another Chernobyl?
Scientists say no. Chernobyl was a completely different type of reactor and exploded while it was still running. The fire raged for days, sending radioactive particles high into the atmosphere. The authorities in the Ukraine also kept it secret and failed to adequately evacuate and provide medical attention.
Do we need to start worrying now?
No. Even with a catastrophic meltdown and radiation leak, it will be barely detectable 9,600km away in this country.