No deaths were reported after the explosion at the Tomsk-7 chemical separation plant, and no one was evacuated from the contaminated region.
Authorities said that the wind blew the radiation away from the Tomsk-7 plant and the nearby city of Tomsk, which has about 500,000 inhabitants.
The accident happened on Tuesday, almost seven years after the explosion at Chernobyl in Ukraine, which immediately killed 33 people and sent a cloud of radiation over
Eduard Gismatullin, of Russian Greenpeace, said a cloud of deadly uranium and plutonium had apparently been released. 'It is too early for the officials to be optimistic. We simply do not know how bad this might turn out to be.'
Dmitri Tolkatski, also of Greenpeace, said a cloud of radiation passed north of Asino, a town of about 30,000 people 120 kilometres (75 miles) northeast of Tomsk, and was moving towards 11 Siberian villages, each with a few thousand inhabitants. Russian Air Defence Command said the cloud was moving north-east at a height of two kilometres (1.2 miles) at 36 kilometres (22 miles) per hour, towards the Yenisei river.
Reports from the area near Tomsk said workers were clearing away contaminated snow and earth. But a full picture was hard to piece together as the area is closed to foreign correspondents.
It is believed that a steel tank containing a mixture of uranium and plutonium blew up after nitric acid was added to extract the plutonium. The blast started a fire which was put out by 25 firefighters; one received a dose of 0.6 REM (Rontgen-Equivalent- Man), which officials said was not high for someone in the nuclear industry. According to international norms, it is dangerous for a person to receive more than five REM in a year.
The Russian Atomic Energy Ministry classed the accident as a 'serious incident' and gave it a three-point rating on the seven-point international scale. It said it was nothing like as bad as Chernobyl, which notched up a full seven points. 'I will not call it a catastrophe because I know it is not a catastrophe,' its spokesman, Georgy Kaurov, said.
'It is practically an accident without danger, but an accident all the same. You simply cannot compare it to Chernobyl. About 80 million curies of radioactivity were released at Chernobyl. Here we do not have a single curie so it is 80 million times less.'
The extent of contamination near Tomsk remains unclear, but Mr Kaurov spoke of an area of 1,000 square metres ajoining the plant where radioactivity was registering 0.4 milliroentgen per hour, and of two 'dirty spots' about 10 kilometres away where radiation was 10 times higher than normal. Tomsk-7, whose existence was once top-secret, was built in the 1950s to produce plutonium for nuclear weapons , but officials say it has stopped military work.
John Gittus, director-general of the British Nuclear Industry Forum, a trade association for the industry, said: 'There can be no comparison between this incident and what happened at Chernobyl seven years ago. It was a near-accident with a small release of radioactivity and public exposure was minimal.'
Many experts believe Russia's political chaos makes industrial disasters more likely. But the government is pressing ahead with an ambitious programme for a network of new atomic power plants.