Four of the Fukushima Daiichi plant&39's six reactors are shown in this satellite photo from Geoeye via Google Earth. The reactors are the four tall buildings inland of the four longer buildings' the Pacific Ocean to the east is on the bottom of this photo. Reactors four, five, and six had been shut down before the earthquake and tsunami, but plant operators are struggling to cool down units one and three.

Buildup of hydrogen gas at a Japanese nuclear reactor caused an explosion today, but as with an earlier explosion Saturday, the reactor's containment vessel remains intact, the International Atomic Energy Agency said.

The explosion took place at the unit 3 reactor at the Fukushima Daiichi at 11:01 a.m. local Japan time, the United Nations agency said in a statement. And Tokyo Electric Power, which operates the plant, said in its own statement, "The status of the plant and the impact of radioactive materials to the outside environment are presently under investigation.

"All personnel at the site are accounted for. Six people have been injured," the IAEA said. "The reactor building exploded but the primary containment vessel was not damaged. The control room of unit 3 remains operational."

It was the latest in a series of serious problems triggered by Friday's earthquake, now upgraded by the Japan Meteorological Agency to magnitude 9.0, and the resulting series of tsunamis that swept over Japan. The 170,000 residents living within 20 kilometers (12.4 miles) of the Fukushima Daiichi plant have been evacuated, piling another major problem on top of thousands of deaths, extensive property damage, rolling power blackouts, a run on food and water in Tokyo, and economic troubles. Japan's Nikkei 225 stock-market index declined 6.2 percent today despite the Bank of Japan's emergency injection of billions of dollars into the economy.

The nuclear reactors are drawing particular attention. Reactors work when the radioactive decay of uranium fuel pellets inside fuel rods heats water into steam. That steam drives turbines to generate electricity. However, to function, they rely on a constant supply of cool water in which they're immersed.

To control the nuclear reaction and stop it in an emergency, control rods can be inserted between the fuel rods. Control rods absorb the neutrons produced by the uranium's radioactive decay, slowing the reaction because the absorbed neutrons don't trigger more radioactive decay in the chain reaction.

If the fuel rods get too hot, for example if they can't be kept immersed or when the cooling system fails, the uranium can melt. This scenario, called a meltdown, covers a wide spectrum of severity. The extreme heat also can produce dangerous amounts of gas that must be vented into the air to prevent an explosion.

At the Fukushima Daiichi plant, the regular power for the cooling system failed after the earthquake, after which diesel generators kicked in for backup system. Less than an hour after the earthquake, though, the tsunami arrived and the generators failed.

At 4:30 p.m. Saturday, the first explosion occurred at unit 1 of the Fukushima Daiichi plant a few hours after workers began preparations to vent some gases. That explosion didn't damage the reactor's primary containment vessel, but it did damage the containment building outside it.

As an emergency measure, plant operators have been pumping seawater into the unit 1 and unit 3 reactors to cool them.

Meanwhile, workers are trying to safely cool down at another damaged nuclear energy complex, the Fukushima Daini plant, which has four reactors, the IAEA said. Its third unit is safely shut down and cooled. Workers got a cooling system working again at its first unit, which is cooling down. They're working to restore cooling systems at the plant's second and fourth units.

The 30,000 people living within 10km of the Fukushima Daini plant have been evacuated, the IAEA said.