Q: Was there a meltdown?

A: Not according to Ichiro Fujisaki, Japan’s ambassador to the United States, who spoke on CNN. But other officials said two of the reactors at the plant may have experienced meltdowns. Engineers have not been able to get close enough to the plant to rule that out, said Toshihiro Bannai of Japan’s Nuclear and Industrial Safety Agency.

“We see the possibility of a meltdown,” he said on CNN.

Q: Have civilians been exposed to radiation?

A: Yes, though not many, officials say. Japanese officials have scanned thousands of evacuees for radiation exposure, and at least nine of them have tested positive, although they do not appear to have any immediate health problems. Japan’s Nuclear and Industrial Safety Agency estimated that as many as 160 people may have been exposed.

The threat to civilians was diminished by the evacuation, and the fact that the prevailing winds at the site generally blow out to sea. They have been dispersing the small amount of radiation that has already been released.

Q: What about the safety of the workers at the plant?

A: The risk is certainly greater for them, said radiation biologist Jackie Williams of the Center for Biophysical Assessment and Risk Management Following Irradiation at the University of Rochester Medical Center. All the immediate casualties at Chernobyl were among workers at the site who were exposed to heavy doses of radiation.

So far, however, there is no evidence that has happened at Fukushima. Four workers at the plant were hospitalized for injuries suffered in the explosion, but not for radiation burns. Random tests on three other reactor workers showed they had received some exposure.

Q: Could radiation make its way to the United States?

A: A catastrophic meltdown in Japan would probably not threaten the health of Americans living in Hawaii or on the West Coast, experts said.

Any radiation that escaped from the plant would be diluted by air currents as it traveled over the Pacific Ocean. Should a meltdown occur, the U.S. Departments of Energy and Defense would surely track the movement of any radiation plume.

Q: Has radiation already escaped from any of the other reactor sites?

A: An official from Japan’s Nuclear and Industry Safety Agency said Saturday that a small amount of radioactive cesium was detected outside at least one other plant.

Q: How long will the crisis last?

A: One expert said cooling down all the reactors will “take days, not hours.” But even if circumstances improve, conditions can still turn negative again.

Q: Exactly what is a meltdown, and why is it potentially dangerous?

A: A meltdown occurs when a reactor’s radioactive core, which holds its uranium fuel, gets so hot that it begins to melt. A complete meltdown can breach a reactor’s steel pressure vessel and other protective barriers – and spread radioactive byproducts like iodine and cesium into the surroundings.

That endangers the environment and nearby residents. However, a reactor will not explode like an atomic bomb.