3 ways Japanese nuclear crisis may end

Best case: beyond Three Mile Island

If all goes according to the most optimistic predictions, Japan would be left with one of the world's largest nuclear power plants in ruins, half of its six reactors destroyed and probably too fouled with radiation to examine for years. But the consequences to human health would be minimal.

The biggest immediate challenge in this scenario is maintaining adequate water levels in the reactors' spent fuel pools. At least one appeared this week to be very low or possibly empty, according to the NRC, and a brief fire that apparently occurred at another led to spikes in radiation levels that interrupted repair work. If workers succeed in efforts to keep the pools filled with more than 30 tons of water sprayed from helicopters and ground hoses, it could forestall the superheating and ignition of the spent fuel — and prevent the resulting releases of radiation into the atmosphere.

At the same time, the plant's skeleton crew of 180 workers would continue to try to prevent a full meltdown by pumping seawater directly into the reactors. That process, ongoing since soon after the plant lost power, appears to be keeping the fuel rods from overheating, though it has triggered periodic explosions caused by the buildup of hydrogen generated by the process.

"The situation in the reactors isn't really bad right now with respect to a worst case scenario, because the (reactor housing) has not failed," said Kenneth Bergeron, a physicist now retired from Sandia National Laboratories, where he managed nuclear reactor accident simulations. "As long as all that fuel remains in an intact reactor, then it will contain the vast majority of the dangerous materials."

Even if the efforts to cool the reactors and maintain the spent-fuel pools succeed, small amounts of radioactive gas still would vent for weeks, perhaps longer. But significant contamination would be limited mainly to the site. Radiation exposures to people in surrounding areas would be slightly above normal, but likely not enough to raise cancer rates significantly, according to radiation safety expert Henry Royal of Washington University in St. Louis.

That scenario would constitute "a worse accident than Three Mile Island" in 1979, Royal said. From a health standpoint, he added, "I think we will see a similar situation," with effects of the small radiation releases on the local population very hard to see.

Middle case: A big dirty bomb

If efforts to refill the spent-fuel pools fail and the used-up fuel rods ignite into a major fire, the resulting release of radiation would be significant. Since those pools already appear to have lost substantial amounts of water, many experts believe a release along these lines is likely.

The plant's pools holds hundreds of rods, totaling some 1,760 tons of spent fuel composed of uranium and other radioactive elements, according to Tokyo Electric. When water levels get too low, the heat from the 12-foot rods generates steam, which can react with the rods' metal casing to create hydrogen explosions. Radioactive plumes from the burning fuel would rise into the atmosphere and be carried by winds, either out to sea, or back towards some of Japan's most heavily populated areas.

That sort of accident "is likely to unfold, perhaps, over a period of weeks," said nuclear safety expert Robert Alvarez, a former Energy Department official and a scholar at the Institute for Policy Studies.

In this scenario, radioactive cesium and other elements move through the atmosphere, where they can be carried back to ground by rain or snow, possibly on to heavily populated areas.

"I hate to say it but this could be potentially way more significant than a 'dirty bomb' scenario, where we are talking about a small mass of extremely radioactive material exploding over a highly populated area," said physicist Ferenc Dalnoki-Veress of the Monterey (Calif.) Institute of International Studies.

The radiation spikes from one or more spent-fuel fires also could force workers to abandon efforts to pump seawater into the reactors to cool their cores. If a reactor had a full meltdown as a result, it's likely that the resulting mass of radioactive material would theoretically remain inside its steel containment structures. And the potentially devastating release of radiation into the environment would be avoided.

"Even in the case of a core melt ... I think that you are looking at a situation where the significant contamination will be limited to the area surrounding the plant," says nuclear engineer Brian Woods of Oregon State University in Corvallis.

Although radiation levels in the countryside would increase, evacuations already ordered within 12 miles of the plant and other precautionary measures would reduce many of the worst health effects from the radiation release, said Charles Meinhold, president emeritus of the National Council of Radiation Protection and Measurements, based in Bethesda, Md.

Already, radiation releases at Fukushima have gone beyond those from Three Mile Island, Meinhold says. "If more occur, the neighboring community could see contamination, probably out to a couple of miles."

Worst case: Another Chernobyl

Since the reactors' fuel rods already have been cooling for a week, the odds of them melting through their containment vessels has diminished, according to nuclear consultant Lake Barrett, a former official of the Nuclear Regulatory Commission. Still, a worst-case scenario could unfold if one or more fires in the spent fuel pools combine with a full reactor meltdown in which some radiation escapes through a damaged containment vessel.

Tokyo Electric suspects that at least one reactor's containment vessel already has been damaged, leading to some radiation leaks from the plant. If plumes from burning spent fuel were to combine with radioactive elements leaking from those suspect containment buildings, contamination could be severe and widespread.

"The Japanese have characterized what they have been doing as desperate and last ditch" efforts, said physicist Edwin Lyman of the Union of Concerned Scientists. "If these measures fail, then they are out of tricks."

Lyman suggested that contamination from a worst-case plume likely would reach to the 50-mile range — or further. If that occurs, it probably would happen in "only a matter of days," Lyman added.

Two things would have to happen in this scenario to maximize its threat to human health, said Bergeron, the former Sandia scientist.

"The cloud gets lifted very high in the air, which would require a lot of heat," such as that generated by a major fire or explosion," he added. Then, there would have to be "a very unfortunate wind direction, maybe followed by rain or snow, which captures the radioactive particles and brings them down to earth, and in the worst case ... in areas where you have a lot of people."

Prevailing winds in Japan generally blow to the East, which would carry a lot of the plumes out to sea. Tokyo, a city of 12.8 million, is about 140 southwest of the plant. Sendai, a city of 1 million people, is about 50 miles to the north of the plant."

It's not likely, Bergeron added, but it can't be ruled out. "That's a chain of a lot of ifs."