Japanese officials warned of a possible second explosion at a nuclear plant crippled by the earthquake and tsunami as they raced to stave off multiple reactor meltdowns, but they provided few details about whether they were making progress. More than 180,000 people have evacuated the area, and up to 160 may have been exposed to radiation.

Four nuclear plants in northeastern Japan have reported damage, but the danger Monday appeared to be greatest at the Fukushima Dai-ichi nuclear complex, where one explosion occurred over the weekend and a second was feared.

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 knocked out power and the tsunami swamped backup generators.

Chief Cabinet Secretary Yukio Edano said Sunday that a hydrogen explosion could occur at Dai-ichi's Unit 3, the latest reactor to face a possible meltdown. That would follow a hydrogen blast Saturday in the plant's Unit 1.

"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."

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