The situation at one of Japan's crippled nuclear power plants seemed to go from bad to worse as an explosion Saturday destroyed a building that houses one reactor and on Sunday another reactor began experiencing problems with its cooling system. An estimated 170,000 people who live within a 12-mile radius of the plant have been evacuated as a precaution in case the worst-case scenario occurs — a meltdown followed by the release of radioactive ash. An estimated 30,000 people have been evacuated within a six-mile radius of a nearby plant.

Here's a look at where things stand.

What caused the explosion?

A buildup of hydrogen in the disabled cooling system of one of the reactors at the Fukushima No. 1 (Daiichi) plant about 150 miles north of Tokyo. The explosion damaged the building the houses the reactor and its cooling system. However, the reactor containment vessel, which houses the radioactive fuel rods, remained intact, officials told theInternational Atomic Energy Agency, the U.N.'s nuclear monitor.

Was there a meltdown?

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.

How serious is it?

The explosion itself does not appear to have caused radiation to leak into the environment. But experts said the problems stemming from Friday's magnitude 8.9 earthquake and the resulting tsunami rank as the third most severe nuclear accident in recent history, following the 1986 Chernobyl disaster and the 1979 accident at Three Mile Island.

Water from the tsunami disabled diesel generators supplying power to emergency cooling systems for five reactors at the Fukushima No. 1 plant and a nearby plant known as Fukushima No. 2 (Daini). Without power from the generators, the cooling system pumps cannot circulate cool water around the radioactive fuel rods. As a consequence, the water will boil off and the fuel rods will overheat and melt, which could lead to a massive escape of radiation. Already, some radiation has apparently escaped from one reactor at the No. 1 plant, as well as some hydrogen, which was the source of the explosion.

Authorities said the cooling system at a sixth reactor, located at Fukushima No. 1, broke down Sunday morning.

What measures were taken after the explosion?

Officials of Tokyo Electric Power Co., which owns the plants, began pumping seawater into the disabled reactor to keep it cool. The company reported to the IAEA that this action had reduced the release of radiation. Some experts called that an act of desperation that seemed likely to fail.

"I would describe this measure as a Hail Mary pass," said Robert Alvarez, an energy expert at the Institute for Policy Studies. Alvarez spoke at news conference Saturday organized by the environmental group Friends of the Earth.

Others, however, said the reduced levels of radiation at the site indicate the plan is working. "I've been encouraged by what I've been hearing," said Ron Chesser, director of the Center for Environmental Radiation Studies at Texas Tech University.

Why didn't they pump in seawater before?

The salty water is very corrosive and can damage the pipes in the cooling system, so it has never been used for cooling a nuclear core, experts said. But when the explosion destroyed the system for circulating freshwater through the core, company officials thought they had little choice left but to inject cold seawater into the containment vessel as a last-ditch measure to cool it. At this point, damage to the pipes is inconsequential.