Japan's ongoing crisis is putting new scrutiny an energy source once trumpeted by Obama. Tara McKelvey on why nuclear power may not be as safe as we've been told—and D.C.'s scramble to tamp down the crisis. Plus, see history's 10 worst nuclear accidents.

For all the scary headlines generated by the explosion at one of Japan's nuclear plants, the situation is far from dire, at least for now.

It is a Level 4 emergency, and scientists say that on a scale of 1 to 7, that is serious but manageable. The rating is bestowed by the Vienna-based International Atomic Energy Agency and has been given to the Japanese nuclear power plants, Daiichi and Daini, after an explosion, based on the agency's Nuclear and Radiological Event Scale. The nuclear accident at Chernobyl, a power plant in Ukraine, in 1986, the worst ever, was Level 7, and the 1979 Three Mile Island accident in Pennsylvania was rated Level 5.

The assessment comes as U.S. officials are carefully monitoring the aftermath of the Japanese earthquake and tsunami, although President Obama had no public involvement on Saturday. "Senior officials and technical experts from the Department of Energy continue to be in close contact with other agencies as well as with our Japanese counterparts as we work to assess what is a very serious and fluid situation," says spokeswoman Stephanie Mueller. "The United States will continue to work closely with the Japanese government and will provide whatever assistance they request to help them bring the reactors under control."

Level 4 is no picnic, especially in a country that has just suffered a devastating natural disaster.

"The technology causes a lot of dread," says David Albright, president of the Washington-based Institute for Science and International Security, and those feelings have spread well beyond Tokyo. To offset the dread and anxiety, officials in Washington have been scrambling to stay on top of things and to reassure Americans that the nuclear emergency has been contained.

One of the problems with building a nuclear power plant is that scientists have a hard time planning for worst-case scenarios.

Secretary of State Hillary Clinton, for example, said the U.S. Air Force was delivering "some really important coolant" to one of the plants in Japan, which did not sound very reassuring, either to people in Japan or here in the United States. "Excuse me, coolant is water," says Randall Larsen, director of the Institute for Homeland Security, a consulting firm based in Alexandria, Va. Later, administration officials said that she had misspoken.

Aside from bringing out comical aspects of Washington officials in crisis mode, the situation in Japan raises important questions—about the wisdom of building a nuclear power plant in place that is vulnerable to earthquakes, for instance, as well as whether or not these facilities are as safe as they are cracked up to be. A former staff scientist for the Federation of American Scientists, Albright visited the nuclear facilities in Japan in the late 1990s and believes that Japanese officials may now be understating the risk, partly to avoid panic among the public—and also because "they are struggling with not knowing what's going on."

One of the problems with building a nuclear power plant, as Albright points out, is that scientists have a hard time planning for worst-case scenarios. When they create a system for a plant, they examine a "design basis accident," which basically means, as he says, "How bad can it get,'" and then make decisions about the type of safety precautions that are needed. "But nature has a way of showing that enough is sometimes not enough," he says. The power plants in Japan, beset by an earthquake, he adds, "are having a problem that nuclear scientists said couldn't happen."

Fortunately, the explosion at the Japanese plant is unlikely to have serious, long-term effects on the health of the people who live in the area or in Tokyo, even if the risk is slightly higher than the authorities are now describing. In the midst of the natural disasters, officials still managed to react quickly to the troubles at the nuclear power plants—and evacuate hundreds of thousands of residents—which means they have succeeded in controlling the damage. Still, the problems at the power plants will undoubtedly to have an impact on the way people see these facilities.

"The public perception of nuclear power in Japan may have a meltdown," Albright says.

Tara McKelvey, a frequent contributor to The New York Times Book Review, is the author of Monstering: Inside America's Policy of Secret Interrogations and Torture in the Terror War (Basic Books).