Experts race to contain reactor risk after Japan quake

Amid an unprecedented double whammy of massive earthquake and devastating tsunami, Japan faces days of dealing with nuclear near-disaster.

The question remains how big a risk exists from one reactor said to be close to a meltdown and another facing that possibility.

The disasters that struck Friday severely damaged the reactor units at the Fukushima Dai-ichi plant, leading to venting of radioactive gases. Japan's Chief Cabinet Secretary Yukio Edano had warned that a partial meltdown in Unit 3 of the Dai-ichi plant was "highly possible."

That warning came before today's hydrogen explosion at Unit 3. A similar blast occurred Saturday at Unit 1. Edano said the reactor's inner containment vessel holding nuclear rods is intact, allaying some fears of the risks to the environment and the public.

"We're in a race against time, where no news is good news," says physicist Kenneth Bergeron, author of Tritium on Ice: The Dangerous New Alliance of Nuclear Weapons and Nuclear Power. "The more days and hours those cores go without overheating, the higher the chances of not seeing a meltdown."

Japanese officials also vented gas to relieve pressure at a reactor unit at the nearby Fukushima Daini Nuclear Power Station.

The 1979 Three Mile Island accident in Pennsylvania was a partial reactor meltdown, triggered by a valve releasing coolant from the reactor core, and operators blundering in recognizing the problem. The Japanese reactors now under threat lost water for cooling when the tsunami knocked out power to the plant's cooling system. In a bid to overcome the loss of regular cooling water, engineers introduced seawater to keep the reactor cool.

The failure of the cooling pumps meant that the water level inside reactor units dropped, likely exposing the top of the reactor's fuel rods to air and steam. "They need water to stay cool," says Douglas Chapin of the engineering firm MBR Associates of Alexandria, Va. Although the reactors had been stopped, the fuel rods continued to decay and get hotter. Pressure grew, which led plant operators to vent gas containing hydrogen and some radioactive elements generated from the melting tips of the exposed fuel rods, Chapin says.

In the worst case, the effort to cool those rods with seawater fails, and the fuel rods melt completely to the facility's floor. A bigger worry, although Chapin calls it unlikely, is that the tips of the rods exposed to air melt completely and "slump off" to the reactor's floor. That could release much more radioactive gas.

The pattern of prevailing winds suggests any contamination would be blown out to sea and dissipate safely, says Eric Hall, director of the Center for Radiological Research at Columbia University in New York City. Because the radioactive material would be dispersed, it wouldn't pose a hazard to wildlife, he says.

Cynthia McCollough, a medical physicist at the Mayo Clinic in Minnesota, said the International Atomic Energy Agency rated the danger in Japan as a 4 on a 7-point scale. Three Mile Island rated a 5; Chernobyl a 7 in 1986.