‘Worst Case’ Nuclear Disaster Hangs on Unlikely Events

Worst Case Nuclear Disaster Japan Hangs on Unlikely Events
A file photo shows a rod containing plutonium-uranium mixed oxide fuel being loaded into the No. 3 reactor in a pool at Tokyo Electric Power Co.'s Fukushima Dai-Ichi nuclear power station in Okuma Town, Fukushima Prefecture in 2010. Photographer: Tomohiro Ohsumi/Bloomberg

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 depends on cooling the fuel 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, which scientists say isn’t likely, may occur if overheating 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 phone 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.

Fuel Rods

Tokyo Electric has struggled to keep the reactors flooded with water to prevent them becoming so hot that they melt through their steel casing. The partially submerged fuel rods are generating heat, turning water into high-pressure steam inside the core of the plant.

About 70 percent of the uranium-plutonium fuel rods at the plant’s No. 1 reactor and a third of the No. 2 reactor’s fuel may have been impaired, Tokyo Electric said.

If hot enough, the rods may start a chemical reaction with their protective coating which produces a small amount of radioactive byproducts, and increases pressure within the core. That pressure is released by letting this combination of steam and gas flow into external chambers, one of which, in reactor unit 2, 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 Toshihiro Bannai, director of international relations at Japan’s Nuclear and Industrial Safety Agency.

Not Fully Submerged

Rods inside three of the reactors have been partially exposed, 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.

Venting Vapor

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 agency said yesterday. Radiation peaked at 400 millisieverts per hour earlier in the day, the International Atomic Energy Agency said yesterday.

The temperature inside the core is “likely to be stable,” said Bannai at Japan’s 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-plutonium fuel 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.

Fourth Blast

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.

IAEA Director General Yukiya Amano said the new information about damage to reactors 1 and 2 is “worrying.” Reactor No. 2’s core damage may be 5 percent while reactor damage at unit No. 1 is below 5 percent, Amano said at a press briefing yesterday. The suppression chamber in reactor No. 2 also appears to be damaged, he said.

Improve Information

The IAEA still doesn’t know the cause of a fire at the cooling pond of reactor No. 4, where spent fuel rods are stored, and is asking the Japanese government to improve the information it shares with the Vienna-based agency, Amano said.

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, where the contents of the reactor core were exposed and on fire, said Steve Crossley, a radiation physicist based in Perth, Australia, who worked in the U.K. nuclear industry from 1996 to 2002.

Chernobyl Accident

“The design of the reactors in Japan prevent this kind of incident,” Crossley said in an e-mailed statement sent via the Australian Science Media Centre in Adelaide. “Boiling water reactors -- the kind at Fukushima -- are cooled and moderated by water which, unlike the graphite core at Chernobyl, cannot burn.”

A more likely worst-case is a loss of large quantities of fission products into the atmosphere, 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.

“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,” Fletcher said. “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.

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