SOMA, Japan — The second hydrogen explosion in three days rocked Japan's stricken Fukushima Dai-ichi nuclear plant today, sending a massive column of smoke into the air and wounding six workers. It was not clear how much — if any — radiation had been released.
The explosion at the plant's Unit 3, which authorities have been frantically trying to cool following a system failure in the wake of a massive earthquake and tsunami on Friday, triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.
The blast followed a similar explosion Saturday that took place at the plant's Unit 1, injuring four workers and causing mass evacuations.
Japan's nuclear safety agency said six workers were injured in today's explosion at Unit 3, but it was not immediately clear how, or whether they were exposed to radiation. They were all conscious, said the agency's Ryohei Shomi.
The reactor's inner containment vessel holding nuclear rods was intact, Edano said, allaying some fears of the risk to the environment and public. More than 180,000 people have evacuated the area in recent days, and up to 160 may have been exposed to radiation.
Four nuclear plants in northeastern Japan have reported damage, but the danger today appeared to be greatest at the Fukushima Dai-ichi nuclear complex. Operators have lost the ability to cool three reactors at Dai-ichi and three more at another nearby complex using usual procedures, after the quake killed power and the tsunami swamped backup generators.
Operators have been dumping seawater into Units 1 and 3 in a last-ditch measure to cool the reactors. They were getting water into the four other reactors with cooling problems without resorting to corrosive seawater, which likely makes the reactors unusable.
Edano said residents within about 12 miles of the Dai-ichi plant were ordered to evacuate as a precaution, and the radioactivity released into the environment so far was so small it didn't pose any health threats.
Such statements, though, did little to ease public worries.
"First I was worried about the quake," said Kenji Koshiba, a construction worker who lives near the plant. "Now I'm worried about radiation." He spoke at an emergency center in Koriyama, about 40 miles from the most troubled reactors and 125 miles north of Tokyo.
A higher-than-usual level of radiation was detected at the Dai-ichi plant today, after levels rose and dropped in previous days. Naoki Kumagai, an official at Japan's nuclear safety agency, told the Associated Press that a person at the monitoring site for an hour would get as much radiation as a plant worker typically gets in six months, but added that the levels would be much higher if one of the reactors were on the verge of a meltdown.
The radiation was detected on the grounds, and Unit 1 was the closest reactor, but it was unclear whether that was where the radiation came from, another agency official said.
At a center set up in a gym, a steady flow of people — mostly the elderly, schoolchildren and families with babies — were met by officials wearing helmets, surgical masks and goggles.
About 1,500 people had been scanned for radiation exposure, officials said.
Up to 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan's nuclear agency. It was unclear whether any cases of exposure had reached dangerous levels.
Edano said none of the Fukushima Dai-ichi reactors was near the point of complete meltdown, and he was confident of escaping the worst scenarios.
Officials have declared states of emergency at the six reactors where cooling systems were down — three at Dai-ichi and three at the nearby Fukushima Daini complex. The U.N. nuclear agency said a state of emergency was also declared Sunday at another complex, the Onagawa power plant, after higher-than-permitted levels of radiation were measured there. It said Japan informed it that all three reactors there were under control.
A complete meltdown — the melting of the radioactive core — could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.
The explosion that destroyed the walls and ceiling of Dai-ichi Unit 1's containment building on Saturday was much less serious than a meltdown would be — in fact, it was operators' efforts to avoid a meltdown that caused it.
Officials vented steam from the reactor to reduce pressure, and were aware that there was an explosion risk because the steam contained hydrogen, said Shinji Kinjo, spokesman for Japan's Nuclear and Industrial Safety Agency. The explosion occurred when hydrogen reacted with oxygen outside the reactor.
All of the reactors in the region shut down automatically when the earthquake hit. But with backup power supplies also failing, shutting down the reactors is just the beginning of the problem, scientists said.
"You need to get rid of the heat," said Friedrich Steinhaeusler, a professor of physics and biophysics at Salzburg University and an adviser to the Austrian government on nuclear issues. "You are basically putting the lid down on a pot that is boiling.
"They have a window of opportunity where they can do a lot," he said, such as using seawater as an emergency coolant. But if the heat is not brought down, the cascading problems can eventually be impossible to control. "This isn't something that will happen in a few hours. It's days."