Japan's damaged nuclear plant in Fukushima has been emitting radioactive iodine and caesium at levels approaching those seen in the aftermath of the Chernobyl accident in 1986. Austrian researchers have used a worldwide network of radiation detectors - designed to spot clandestine nuclear bomb tests - to show that iodine-131 is being released at daily levels 73 per cent of those seen after the 1986 disaster. The daily amount of caesium-137 released from Fukushima Daiichi is around 60 per cent of the amount released from Chernobyl.

The difference between this accident and Chernobyl, they say, is that at Chernobyl a huge fire released large amounts of many radioactive materials, including fuel particles, in smoke. At Fukushima Daiichi, only the volatile elements, such as iodine and caesium, are bubbling off the damaged fuel. But these substances could nevertheless pose a significant health risk outside the plant.
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The level of radionuclides leaking from Fukushima Daiichi has been unclear, but the CTBT air samplers can shed some light, says Gerhard Wotawa of Austria's Central Institute for Meteorology and Geodynamics in Vienna.

For the first two days after the accident, the wind blew east from Fukushima towards monitoring stations on the US west coast; on the third day it blew south-west over the Japanese monitoring station at Takasaki, then swung east again. Each day, readings for iodine-131 at Sacramento in California, or at Takasaki, both suggested the same amount of iodine was coming out of Fukushima, says Wotawa: 1.2 to 1.3 × 10¹⁷ becquerels per day.

The agreement between the two "makes us confident that this is accurate", he says. So do similar readings at CTBT stations in Alaska, Hawaii and Montreal, Canada - readings at the latter, at least, show that the emissions have continued.