Nuclear plant operators were frantically trying to keep temperatures down in a series of nuclear reactors — including one where officials feared a partial meltdown could be happening Sunday — to prevent the disaster from growing worse.

But hours after officials announced the latest dangers to face the troubled Fukushima Dai-ichi nuclear complex, including the possibility of a second explosion in two days, there were few details about what was being done to bring the situation under control.

Chief Cabinet Secretary Yukio Edano said Sunday that a hydrogen explosion could occur at the complex's Unit 3, the latest reactor to face a possible meltdown. That would follow a hydrogen blast Saturday in the plant's Unit 1, where operators attempted to prevent a meltdown by injecting sea water into it.

"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said. "If there is an explosion, however, there would be no significant impact on human health."

More than 180,000 people have evacuated as a precaution, though Edano said 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 (60 kilometers) from the troubled reactors and 125 miles (190 kilometers) north of Tokyo.