Japan's nuclear fears intensify at two Fukushima power stations
Authorities scramble to control overheating reactors at one plant, as state of emergency declared at second nearby
Ian Sample, Science correspondent guardian.co.uk,
Sunday 13 March 2011 19.39 GMT
Fears of a major nuclear accident in Japan have intensified as authorities scramble to bring under control several overheating reactors at one power station, and declared a state of emergency at another, where radiation levels soared above normal limits.
Workers at the Fukushima 1 nuclear power plant in the north-east of the country pumped seawater into three reactors in a last-ditch attempt to make them safe after emergency cooling systems failed to stabilise the radioactive cores.
More than 200,000 people were evacuated as officials imposed a 20km exclusion zone around the power station and a 10km zone surrounding the Fukushima 2 power plant nearby. Japanese officials told the International Atomic Energy Agency they would distribute potassium iodide pills as a precaution against an increased risk of thyroid cancer from radiation.
The most urgent crisis centred on Fukushima 1, where officials were braced for an explosion similar to one that blew the roof off the building that housed reactor 1 yesterday morning, after hydrogen escaped from the reactor as engineers vented steam from the pressurised vessel.
The failure of the cooling system caused uranium fuel rods to overheat and split the cooling water into hydrogen and oxygen. The hydrogen exploded, devastating the building, but the containment vessel around the reactor was undamaged, Japan's chief cabinet secretary, Yukio Edano, said.
Desperate engineers were forced to vent steam from two other reactors at the power station before pumping in seawater, despite the risk of triggering further explosions.
Edano said a partial meltdown of fuel rods in reactor 1 was possible and that engineers were pumping seawater into the others to prevent the same happening there. He said the fuel rods in reactor 3 might be deformed, but a meltdown was unlikely. Reactor 3 uses a mixed-oxide fuel which contains plutonium, but the plant operator Tokyo Electric Power Company (Tepco) said this did not present unusual problems.
"The use of seawater means they have run out of options," said David Lochbaum, director of the Union of Concerned Scientists nuclear safety project.
Two of three diesel generators used to drive cooling pumps stopped working at the Tokai 2 nuclear power plant, 75 miles north of Tokyo in Ibaraki prefecture.
The intentional release of steam from the nuclear reactors caused levels of radiation around the Fukushima 1 power station. Levels measured at the power station boundary reached 500 microsieverts an hour, a quarter of the dose the general population receives from natural background radiation in a year.
Tepco officials said radiation around the Fukushima 1 station had risen above the safety limit, but it did not mean an "immediate threat" to human health.
The radioactive substances released from the site suggest at least part of the core in reactor 1 has broken down.
Japan's nuclear and industrial safety agency confirmed that radioactive caesium-137 and iodine-131 had been detected in the vicinity of the Fukushima power station. These radioactive isotopes are produced by fission reactions inside the core and can only have contaminated the cooling water if fuel rods overheated and melted the cladding surrounding them.
A state of emergency was declared at the Onagawa nuclear plant north of Sendai after radiation rose to 400 times the usual level, but Japanese officials said the increase was due to radiation from the Fukushima plant and was not a risk to health. The reactors at Onagawa were said to be under control.
Paddy Regan, a nuclear physicist at Surrey University, said it could take several days to cool the reactors and make them safe. If any of the reactors continued to heat up, the core could go into complete meltdown. This raises the danger of a major release of radiation if the molten core breaches the containment vessel.
The sheer scale of the crisis in Japan has stretched emergency services to breaking point and prompted criticism that authorities were ill-prepared for a natural disaster that struck first with an earthquake and next with a devastating tsunami.
The nuclear power stations shut down automatically when Friday's earthquake rocked the region, but emergency generators crucial for cooling the reactors were knocked out in the following tsunami. The decision to wait until Sunday before filling two of the reactors with seawater has been criticised as a potentially serious delay.
An anti-nuclear energy group in Japan said the government and industry should have foreseen the danger. "A nuclear disaster which the promoters of nuclear power in Japan said wouldn't happen is in progress," the Citizens' Nuclear Information Centre said. "It is occurring as a result of an earthquake that they said would not happen."
Damn Japanese!
