Japan's nightmare goes nuclear after reactor building explodes
More than 200,000 residents are evacuated over fears of meltdown
Engineers were last night fighting a desperate battle to prevent a meltdown at an earthquake-damaged nuclear reactor after a massive explosion at the plant.
More than 200,000 residents in a 12-mile radius around the plant were evacuated following the blast at the Fukushima Daiichi nuclear power plant. The Japanese government said it was to issue iodine tablets to those in the surrounding area in a bid to protect against the harmful effects of radiation.
The accident, which is thought to have been caused by a build of up hydrogen gas inside the building housing the plant's nuclear reactor unit 1, has been rated as among the five worst nuclear accidents in history after the 1986 Chernobyl disaster and the Three Mile Island accident in the US in 1979.
Sea water was being pumped into the reactor last night in an attempt to cool down the radioactive core. A catastrophic build-up of hydrogen gas inside the building that housed the reactor had sparked an explosion earlier in the day, destroying the structure and throwing radioactive debris into the air.
Specialist teams were monitoring the radiation levels as southerly winds threatened to carry some of the radioactive dust towards surrounding settlements.
At least three workers and three residents evacuated from the town of Futaba-machi have been exposed to radiation and monitoring around the plant revealed that radiation levels had reached 1,015mSv after the explosion.
A dose of 50mSv is considered to be the safe exposure limit in a year.
Radiation levels around the power plant, in Futaba, north east Japan, immediately after the explosion were measured at 20 times the normal safety limits, but experts insisted that the amount of radiation released was still unlikely to pose a health risk.
Public-health authorities in Japan were also preparing to distribute potassium iodine tablets last night to residents in the areas around the explosion-hit plant and the nearby Fukushima Daini nuclear power plant, which was also threatened with a meltdown following the earthquake.
Fears of a full-blown nuclear disaster following the explosion were eased after an inspection revealed the containment vessel that surrounds and protects the reactive nuclear core was still intact.
Tokyo Electric Power Company, the private company that owns the plant, said it was to fill the reactor with sea water in an attempt to cool down the nuclear fuel cells in the core.
If allowed to overheat, the fuel cells can melt down and break apart, leading to large amounts of radioactive material leaking into the reaction chamber and potentially into the surrounding environment.
Japan's chief cabinet secretary, Yukio Edano, said it would take between five and 10 hours to fill the reactor core with sea water and around 10 days before the 40-year-old reactor could be made safe. He revealed that boric acid -- which can control the nuclear breakdown of the uranium fuel -- would also be added to the sea water.
"The nuclear reactor is surrounded by a steel reactor container, which is then surrounded by a concrete building," said Mr Edano.
The concrete building collapsed. We found out that the reactor container inside didn't explode.
"We've confirmed that the reactor container was not damaged. The explosion didn't occur inside the reactor container. As such, there was no large amount of radiation leakage outside."
The move to add sea water to the reactor was last night described as a "drastic action" by some nuclear-energy experts.
It is believed that the explosion occurred after Friday's earthquake knocked out the power supply that operated the cooling systems for the three active reactors at the Fukushima Daiichi power plant and two further reactors at the Fukushima Daini plant.
Under normal circumstances, water is pumped through the reaction chamber to cool the nuclear fuel cells. Without this cooling mechanism, the uranium fuel pellets can overheat, causing the ceramic and metal casing that surrounds them to melt and crack, allowing radioactive material to leak out.
An emergency back-up power supply was only able to restore the cooling system to one of the reactors at Fukushima Daiichi, sparking fears of a meltdown in the remaining reactors as temperatures inside increased uncontrollably.
Pressure inside the reaction containment vessels increased as the water was converted to steam, forcing officials at the Fukushima Daiichi plant to vent some of the gas into the buildings contain the reactors.
It is thought that in the rising temperatures at the core, which would have reached more than 1,000 degrees Fahrenheit, the zirconium metal tubes that contained the nuclear fuel caused the breakdown of water in the reactor to hydrogen gas, which was vented along with the steam into the building.
Inside the building, the hydrogen will have mixed with oxygen from the air and a single spark could then have triggered the explosion.
Dramatic television pictures captured the moment when the building exploded, throwing a cloud of dark dust, smoke and debris into the sky. Later, just the shell of the building could be seen still standing.
"The reactors shut down automatically and the power supplies for the cooling systems failed," said Ian Hone Lacey of the World Nuclear Association.
"The key is to keep the reactors cool and prevent any fuel damage which can lead to a melt down. Over time, the heat of the reaction will decrease and about 24 hours after shutdown, the heat will have fallen to 0.5 per cent."
Fears that the overheating reactor core could trigger a Chernobyl-style disaster faded as the sealed reaction vessel that contains the radioactive fuel was not ruptured.
Japan's nuclear safety agency said the accident was rated four on the International Nuclear and Radiological Event Scale. The Chernobyl disaster, the world's worst civilian nuclear accident, was rated as a seven, while the Three Mile Island accident in Pennsylvania in 1979 was rated as a five on the scale.
But John Large, an independent nuclear-energy consultant, said the Japanese accident was probably more serious than the incident at Three Mile Island, where a partial meltdown caused extensive damage to the reactor but only low levels of radiation was released into the environment.
He accused the Japanese authorities of deliberately downplaying the seriousness of the incident for fear of sparking panic among the public.
Radiation specialists were working with meteorologists to monitor how the radiation might spread. The wind is currently blowing from the south, taking it away from Japan's biggest centres of population including Tokyo -- but there are fears it could blow in the opposite direction, potentially putting millions at risk.
"At these levels, the radiation affects on health will be very low," said Professor Simon Pimblot, an expert in radiation chemistry at Manchester University's Dalton Nuclear Institute.
"You would have to be exposed for a very long time -- we are talking about life-long exposures -- for there to be an appreciable risk."