Japanese officials have taken steps to evacuate residents from five villages outside the exclusion zone around the beleaguered plant, where the severity of the crisis was upgraded to the worst rating possible on an international nuclear disaster scale.
The move, which comes as firefighters worked to extinguish a fire at a seawater sampling station at the plant, puts the incident at the crippled power station on a par with Chernobyl in 1986, the only previous nuclear disaster to be given the highest ranking.
Michael Mariotte, head of the Nuclear Information Research Service, an advocacy organisation, said those living in the villages were leaving for good. “The people are not being evacuated because of the threat of a large fast release, or releases that could cause acute symptoms. It is because radiation in these areas is too high to stay over the long term. It is not an evacuation per se. It is a permanent relocation. That is why they are not rushing it.”
The upgrading resulted in differing opinions from Japanese officials, with the Nuclear Safety Commission estimating the total amount of leaked radiation to be only one tenth of that released at Chernobyl. Meanwhile an official for Tepco, the utility company that runs the plant, admitted the radiation leak could eventually top that of the Ukrainian disaster.
Monitoring stations around the globe have picked up small amounts of radioactive iodine and caesium that have carried on the wind from the power station and landed on the ground and in water courses, but the levels are considered far too low to be dangerous to human health.
“The releases into the atmosphere will travel around the world, but we have incredibly sensitive detection systems. If there had been a major release we would know about it,” said Gerry Thomas, a molecular pathologist and director of the Chernobyl tissue bank at Imperial College London.
“They have re-evaluated the incident on the grounds that there has been leakage into the environment, but this is nothing like Chernobyl,” she added.
But measurements of radiation levels in Europe have led at least one group to raise concerns over safety. A French NGO, the Commission for Independent Research and Information on Radioactivity, said levels of radiation were no longer negligible, a claim Thomas dismissed as “absolutely ridiculous”.
Japan‘s location has helped lessen the effect of radiation from the damaged plant reaching other countries. The prevailing winds are westerlies, so the radioactive plume has typically blown out over the sea and had to cover thousands of miles before it reaches landfall in the US.
Trace amounts of iodine-131 were discovered in drinking water in Idaho and Washington in the US last week, but were so low that the Environmental Protection Agency said an infant would have to drink around 7,000 litres to receive a radiation dose equivalent to one day’s worth of natural background radiation.
Other measurements of the air, drinking water, rainfall and milk have all been far below the levels for concern, according to the EPA. The most contaminated drinking water clearly linked to Fukushima was found in Chattanooga, Tennessee, on 28 March, but even in that case, an infant would need to drink 900 litres of the water to receive a dose of radiation they would normally receive in a day from living in the area.
The direction of the wind currents mean the radioactive plume has had to blow the long way around the world to reach the UK, covering thousands more miles along the way.
In Britain, the Health Protection Agency has begun publishing weekly updates that cover radiation in the air, on grass and in the water from sites around the country. The levels detected in the air last week gave a dose equivalent to roughly one ten thousandth of that received from natural background sources.
According to the Met Office, a complex interplay of factors determine where the radiation goes and how soon it arrives, a situation being modelled continuously by the organisation, using the same computer tools that were brought to bear on the Icelandic volcanic ash cloud that brought air travel to a standstill last summer.
The Met Office simulations feed into the Cobra emergency committee and the chief scientist’s Science and Engineering Advice in Emergencies group and are shared internationally with the IAEA and other meteorological offices that are working to track the spread of the radiation.
The danger posed by the airborne plume falls with time and distance from the power plant. Unlike the Chernobyl disaster, which saw vast amounts of highly radioactive material blasted high into the atmosphere, the airborne leaks from Fukushima have been low down, where the air mixes quickly and wind speeds are relatively low.
As the plume moves out over the ocean it becomes steadily more dispersed. At the same time, the radiation ebbs as the contamination decays. This is more significant for some elements than others. Iodine-131 halves in radioactivity every eight days, leaving more long-lived radioactive substances behind, such as caesium-137.
Computer models of the marine environment show the large amounts of radioactive water that escaped into the sea from Fukushima drifted south along the coast of Japan before being caught in one of the largest warm water currents on the planet, the Kuroshio, which drives out to sea at a rate of 75 miles a day.
Simulations created by ASR Ltd, an international marine consulting firm, suggest that water leaking from Fukushima would have swept many hundreds of miles out to sea in the past few weeks, spreading out and dispersing in huge eddies.
The Kuroshio is comparable to the Gulf Stream that warms western Europe and takes the form of a 70-mile wide body of water that pushes almost directly east from Japan into the vast expanse of the Pacific Ocean.
“As soon as the plume gets entrained in the Korushio current, it gets shot out into the Pacific, but it is unlikely to have reached any other countries. It is still out in the Pacific and being diluted rapidly,” said Jose Borrero, a consulting oceanographer who worked on the model.