"As long as there's no meltdown of the fuel rods, you're in good shape," said Kirby Kemper, a nuclear physicist at Florida State University. But if pressure from gases in the nuclear core builds up, "a crack could appear in the containment vessel and release radiation," he said.
Tokyo Electric Power Co.'s (9501.TO) Fukushima Daiichi Nuclear Power Station, situated about 150 miles north of Tokyo, was shaken by an 8.9-magnitude earthquake Friday. A tsunami and aftershocks also hit the area.
A portion of the No. 1 reactor's fuel rods has become exposed due to a failure of the cooling system. Tepco officials have been pumping seawater with boron in a last-ditch effort to bring temperatures down to prevent an outright meltdown. The boron helps neutralize some of the radiation.
"I would describe this measure as a Hail Mary pass," said Robert Alvarez, a senior scholar at the Institute for Policy Studies and a former policy adviser for the U.S. Department of Energy.
Fukushima Daiichi has six reactors, all built in the 1970s, and three were operating when the quake happened. The No. 1 unit, the oldest and smallest of the reactors, appears to be the main source of the problems.
A nuclear reactor produces electricity by heating up the radioactive material in its core, which then heats water to produce steam. That steam is used to turn turbines that produce electricity. To shut down a reactor, safety valves are opened to release the steam and water is used to then to cool down the reactor core.
The core can overheat if part of it has been exposed above the water level for some time. Pellets of radioactive material are arranged into long rods of stainless steel that are kept in cylindrical arrays, which are then submerged in water. If the temperature is high enough, the core slumps to the bottom of the reactor vessel and melts through it and onto the floor of the containment unit.
Loss of cooling water resulted in a near meltdown of the Three Mile Island reactor in Pennsylvania in 1979, the worst nuclear incident in U.S. history.
The Fukushima Daiichi No. 1 reactor has a better "containment system than Chernobyl but on the scale of most reactors in this country, it's not as strong as most of them," said Ken Bergeron, a physicist and former scientist at the Sandia National Laboratories, where he worked on nuclear-reactor accident simulation.
"If the containment doesn't survive, we have a worst-case scenario" of a meltdown, Bergeron said in a Saturday conference call with other U.S. nuclear scientists to discuss the situation.
Tepco has said the containment is intact.
The Tepco reactor has been shut down so no nuclear reaction is taking place. "What you have to worry about is the decay heat that is still in the core that will last for days," Bergeron said. "To keep the decay heat in the uranium from melting the core, you have to keep water on it."
The conventional methods for keeping the core in water have failed, "so they are using some very unusual methods," Bergeron said.
In the wake of the earthquake, the back-up generators that are used to pump water into the reactor stopped working so back-up batteries were brought in. Those batteries weren't designed to last very long. "We don't know exactly how they are getting enough water at the core," he said.
Another scenario is that high temperatures can cause "high speed rusting" by the water around the tube. This would convert zirconium into zirconium oxide and release hydrogen. Hydrogen is a flammable.
An explosion Saturday that blew off the roof and walls of a secondary-containment structure around the reactor could have been caused by a buildup of hydrogen. Bergeron said hydrogen combustion typically doesn't occur in containment, but this reactor had to be vented to ease pressure.
"The problem is you don't know exactly what is happening," said Alvarez.
Thousands of residents within a 20-kilometer radius of the reactor have been evacuated as a precaution. But Dr. Kemper of Florida State University said in a separate interview that in the event of a large-scale radiation leak, people should barricade themselves into buildings and seal the windows until evacuation crews arrive.
The specter of huge environmental fallout on the scale of Chernobyl is unlikely, though.
The Chernobyl reactor used carbon to slow down neutrons, a key part of the fission reaction. In that disaster, a fire ignited the carbon and created radioactive soot that was carried afar by winds.
The nuclear core in many modern reactors, including the ones in Japan, is enclosed by a steel containment vessel. Today's reactors also use water instead of carbon to slow down neutrons, so there is no big danger of the emanation of radioactive soot from the Fukushima plant.
A full meltdown at the Japanese facility would still release radioactive gases, but those tend to dissipate in the atmosphere. For example, there was a small amount of radiation released in the 1979 Three Mile Island disaster in Pennsylvania, but post-release assessments suggested it didn't have any real health or environmental impact. About half of the core melted during the early stages of that accident.
If the Japanese nuclear core were to melt, certain radioactive materials, such as iodine, strontium and cesium, would also be released. These particles are one-quarter the size of a grain of salt and can be carried by winds. The larger the grains, the more quickly they would fall out of the air.
Dr. Kemper noted that the wind tends to blow west to east in Japan, and so a good deal of the radioactive particles would drift out to sea. Consequently, there probably wouldn't be much fallout in Japan's densely populated areas to the south.
Unlike an accident that releases chemical toxins, a nuclear-plant disaster has one advantage: Radiation levels can constantly and precisely be measured.
Since the signing of the Comprehensive Nuclear Test Ban Treaty in 1996, various countries have put in place devices that can monitor radiation levels. So if there was any significant wind-borne fallout from the Japan plant, it would likely get noticed quickly.
But in the area surrounding the Japanese facility, the effects of a full-blown meltdown would likely linger. "The cleanup would go on for a year or two," Dr. Kemper said.
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