Highly toxic plutonium is seeping from the damaged nuclear power plant in Japan's tsunami disaster zone into the soil outside.
Plutonium has been detected in small amounts at several spots outside the Fukushima power plant for the first time, said plant operator Tokyo Electric Power.
Safety officials said the amounts were not a risk to humans but support suspicions that dangerously radioactive water is leaking from damaged nuclear fuel rods, a worrying development in the race to bring the power plant under control.
"The situation is very grave," Chief Cabinet Secretary Yukio Edano said. "We are doing our utmost to contain the damage."
The tsunami on March 11 destroyed the power systems needed to cool the nuclear fuel rods in the complex, 220km northeast of Tokyo.
Since then, three of the complex's six reactors are believed to have partially melted down, and emergency crews have grappled with everything from malfunctioning pumps to dangerous spikes in radiation.
Radiation seeping from the plant has made its way into produce, raw milk and tap water as far away as Tokyo.
The Yomiuri daily newspaper reported that the Government was considering temporarily nationalising the troubled nuclear plant operator, but Edano and Tepco officials denied holding any such discussions.
The nuclear crisis has complicated the Government's ability to address the humanitarian situation facing hundreds of thousands left homeless by the twin disasters. The official number of dead has surpassed 11,000 and the final figure is expected to top 18,000.
The discovery of plutonium, released from fuel rods only when temperatures are extremely high, confirms the severity of the damage.
Of the five soil samples showing plutonium, two appeared to be coming from leaking reactors and the rest were likely the result of years of nuclear tests that left trace amounts of plutonium in many places around the world, Tepco said.
Plutonium is a heavy element that doesn't readily combine with other elements, so it is less likely to spread than some of the lighter, more volatile radioactive materials detected around the site, such as the radioactive forms of cesium and iodine.
When plutonium decays, it emits an alpha particle, a relatively big particle that carries a lot of energy. When an alpha particle hits body tissue, it can damage the DNA of a cell and lead to a cancer-causing mutation.
Plutonium also breaks down very slowly, so it remains dangerously radioactive for hundreds of thousands of years.
"If you inhale it, it's there and it stays there forever," said Alan Lockwood, a professor of Neurology and Nuclear Medicine at the University at Buffalo.
Radioactive water was pouring from the Fukushima plant, with officials warning it will soon reach the sea. The contaminated samples were found outside the reactor building for the first time, causing concern that soil and sea in the surrounding area will be poisoned.
Tepco was yesterday accused by the Government of making unforgivable mistakes in its handling of the country's worsening crisis, after releasing startlingly high radiation figures which it later retracted.
Several foreign energy firms said yesterday that they had been contacted by Tepco, asking for help in bringing the situation under control.
The International Atomic Energy Agency said over the weekend that the crisis could take months rather than weeks to resolve.
The Government has been accused of allowing an incestuous relationship to develop with power giant Tepco to the detriment of safety standards.
Senior government figures have become accustomed to lucrative posts in the energy sector on retiring from public life and criticism of companies like Tepco has consequently been muted.
Influential nuclear critic and MP Taro Kono suggested the power lobby's influence on the media, where it is among the country's leading advertisers, had insulated it from proper inspection.
- Independent, AP