‘Worst Case’ Nuclear Disaster in Japan Hangs on Unlikely Events
For Tokyo Electric Power Co.’s stricken nuclear reactors to release catastrophic amounts of radioactive material into the atmosphere, a rare chain of events needs to happen.
Averting a full-scale meltdown — which scientists say isn’t likely — depends on cooling the uranium-containing rods at Fukushima Dai-Ichi’s Reactor No. 2, said S.K. Malhotra, a scientist at India’s Department of Atomic Energy in Mumbai. A worst-case outcome may occur if over-heating in the reactor culminates in the rupture of the steel lining protecting radioactive material.
“In the worst scenario, an explosion could occur inside the steel pressure vessel, fuel bundles melt down and the radioactivity is exposed,” Malhotra said in a telephone interview. “I would say there is a 10 percent probability still.”
Japan, which has no significant oil and gas resources, is struggling to avert a meltdown at the power plant after the earthquake on March 11 caused a tsunami that disabled critical cooling systems.
Prime Minister Naoto Kan said the danger of radiation leaks increased at the nuclear facility, located 135 miles (220 kilometers) north of Tokyo. That sent the nation’s Topix stock index to its biggest two-day drop since 1987 as concern grew over the government’s ability to contain the crisis.
Tokyo Electric has struggled to keep the reactors flooded with water to prevent them becoming so hot that they melt through their steel casing. Nuclear rods not immersed in water produce heat, turning water into steam that can build inside the core of the nuclear plant. Pressure is released by letting gas flow into external chambers, one of which was rocked by an explosion at about 6 a.m. local time yesterday.
The explosions are frustrating cooling efforts at the nuclear facility, and may have damaged a key containment chamber, said Toshihoro Bannai, director of international relations at Japan’s Nuclear and Industrial Safety Agency.
Rods inside three of the reactors were partially exposed as at 1 p.m. Tokyo time, according to Tokyo Electric. The fuel rods in reactor No. 2 were not fully submerged in water for at least 5 1/2 hours at that time as the utility reduced the number of workers because of increased radiation risks, the company said.
“What we are looking at is a long-term cooling problem,” John Prince, a former member of the Safety Policy Unit of the U.K. National Nuclear Corp., told reporters in Adelaide, Australia.
The cooling process stopped after diesel generators pumping water to the plant were disabled by the tsunami, according to information Japanese authorities shared with the World Association of Nuclear Operators.
As the water supply stopped, temperatures inside the core rose, causing a buildup of pressure steam inside a containment area. Some of the vapor was vented to relieve the pressure, leaking a small amount of radioactive material into the environment.
Radiation outside the plant dropped to 0.6 millisieverts per hour from 11.9 millisieverts per hour, the International Atomic Energy Agency in Vienna said yesterday. Radiation peaked at 400 millisieverts per hour earlier in the day, it said.
The temperature inside the core is “likely to be stable,” said Bannai at the safety agency, adding that most of the measuring equipment was “debilitated.” Engineers have used secondary generators to pump seawater and boron into the core of the 40-year old boiling water reactor.
Without cooling water, the rods heat up. At about 1,100 degrees Celsius, the water and the zirconium metal encasing the uranium reacts to create hydrogen. Some of the hydrogen generated in this process was vented, leaked into the reactor building and exploded on contact with oxygen on at least three occasions.
A fourth blast that occurred at Reactor No. 2 yesterday may have damaged one of the layers designed to contain both the core and the radioactive material, according to Tokyo Electric. Any serious breach of the containment layers can allow potentially dangerous radiation leakage.
Material released through a serious breach in the defenses of the nuclear core, or the containment units, would travel through the atmosphere, depending upon weather conditions. An explosion would spread them further, as would heavy winds. The French, German and Chinese embassy in Tokyo recommended their citizens leave the city.
There are no indications so far that any radioactive material will reach Tokyo, said Gerald Laurence, an adjunct professor of chemistry at the University of Adelaide.
Three of six reactors were operating at the time of the temblor. Nuclear fission in the functioning reactors stopped within 90 seconds of a power outage caused by the earthquake. Nuclear material can take weeks to cool down completely after the plant is shut down, the nuclear operators association said.
It’s unlikely nuclear fuel will be released in an event reminiscent of the accident at the Chernobyl Nuclear Power Plant in Ukraine, said David Fletcher, an adjunct professor of chemical and biomolecular engineering at the University of Sydney, who studied the 1986 disaster while working for the U.K. Atomic Energy Authority.
“The worst you can conceive of is losing a lot of fission products into the atmosphere,” Fletcher said. “People think of this scenario where the fuel melts and fails the vessel, and you’ve got this pile of radioactive fuel in a molten state underneath it. I don’t think for a moment that could happen in a modern reactor because they have sufficient cooling to stop that happening.”
Radiation exposure hospitalized one Tokyo Electric worker and the company has reduced the number of engineers working on cooling reactor No. 2 because of increased radiation readings, said Bannai. Eleven more workers were injured in this morning’s explosion.