Nuclear experts have warned the next few days will be crucial in determining exactly how bad the fallout from the Fukushima power plant disaster could be.
They say advanced Japanese engineering at the 40-year-old facility will avoid a Chernobyl-style disaster, but any radiation leak could still have disastrous consequences.
During Friday’s megaquake most of Japan’s 50 nuclear power stations shut down as expected, but at Fukushima’s Daiichi nuclear plant the system failed.
A hydrogen blast at its number one reactor has destroyed part of the building but did not prompt a major radiation leak.
However, experts have warned there could be a second explosion at the plant’s number three reactor.
Reactors convert the energy stored in nuclear fuel rods into electricity, and in doing so generate immense heat.
Water is circulated through the reactor core to keep the fuel rods from overheating.
In case of an emergency each power station has a back-up system to keep reactors cool.
But during the quake – which has been upgraded to 9 on the Richter scale – power at the Fukushima facility was lost and the back-up system failed.
Diesel generators should have kicked in to provide emergency cooling, but they were also damaged and coolant stopped circulating.
The remaining water is likely to eventually boil away, exposing the fuel rods.
If a cooling system is not restored, it could lead to what is known as a meltdown – when the core melts and radiation escapes into the atmosphere.
Officials were now pumping seawater into reactor number three to keep its temperature down.
The Tokyo Electric Power Company (TEPCO) said radiation levels at the Fukushima plant had risen above the safety limit but there was no “immediate threat” to humans.
Despite this, a 12m exclusion zone has been set up around the facility and some 140,000 people have been moved from the area.
Evacuees were being tested for radiation at screening centres and authorities prepared to distribute iodine to protect people from any radioactive exposure.
Gerry Thomas, director of the Chernobyl tissue bank at Imperial College London, explained why iodine is needed in the body.
“The thyroid actually takes up iodine to make the thyroid hormones. It remains in the gland and the tissues in the thyroid,” she said.
“It is important to get stable iodine into the thyroid gland to prevent the uptake of radioactive iodine.
“It is extremely unlikely there will be a significant release (of radioactive iodine from the Fukushima plant).”
In small doses, such as during an X-ray, radiation causes no harm to humans.
But if radioactive particles should enter the body in large doses, health risks range from vomiting, hair loss and in extreme cases, cancer.
But Professor Thomas said the Japanese appeared to be monitoring the situation closely and taking precautionary measures.
“We won’t see any problems from this reactor. The release is tiny and likely to remain so, so I don’t think we need to worry,” she said.
“Chernobyl was a very big accident and this is certainly not on that scale.
“You need quite a large release of radioactive iodine to do any significant damage.”
In 1986, the explosion of reactor number four at Ukraine’s Chernobyl plant was the world’s worst nuclear incident, immediately contaminating 200 people and killing 32 within three months.
Hundreds of thousands of people are thought to have suffered the after-effects of the leak.
The accident was only revealed after a giant radioactive cloud was registered moving across northern Europe.
It was marked at the maximum level seven on the IAEA’s scale of nuclear accidents.
Further contamination was reported from Chernobyl in 1995 during the removal of fuel from one of the plant’s reactors.
Professor Robin Grimes, from the Centre for Nuclear Engineering, told Sky News the Chernobyl plant was an old Russian design which had a completely different structure to Fukushima.
“The plants in Japan are light water reactors so they work on a very different principle,” he said.
“The type of problems that one might anticipate will be quite different to Chernobyl.”
He added that the Fukushima incident was more on the scale of the 1979 accident at Three Mile Island in Pennsylvania, which was registered at five.
Then, 140,000 people were evacuated after the reactor’s core suffered a partial meltdown.
Although there was contamination within the plant, there was none outside and no casualties.
Japan has experienced the only two deadly nuclear accidents since Chernobyl – one in Tokaimura in 1999 which killed two workers and another in Mihama in 2004 which resulted in four deaths.
Tokaimura is Japan’s worst nuclear accident to date, exposing more than 600 people to radiation.
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