Japanese officials are struggling with a growing nuclear crisis and the threat of multiple meltdowns, two days after the country's north-eastern coast was savaged by a catastrophic earthquake and tsunami.
A partial meltdown is already "likely" to be under way at one nuclear reactor, a top official said.
Meanwhile, operators are frantically trying to keep temperatures down at the power plant's other units and prevent the disaster from growing even worse.
Chief Cabinet Secretary Yukio Edano said today that a hydrogen explosion could occur at Unit 3 of the Fukushima Dai-ichi nuclear complex, the reactor that could be melting down.
That would follow a blast the day before in another unit at the same power plant, as 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," Mr Edano said.
"If there is an explosion, however, there would be no significant impact on human health."
More than 170,000 people had been evacuated as a precaution, though Mr Edano said the radioactivity released so far into the environment was so small it did not pose any health threats.
A complete meltdown - the collapse of a power plant's systems and its ability to keep temperatures under control - could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.
Up to 160 people, including 60 elderly patients and medial 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.
The severity of their exposure, or if it had reached dangerous levels, was not clear. They were being taken to hospitals.
Mr Edano told reporters that a partial meltdown in Unit 3 of the Fukushima Dai-ichi power plant was "highly possible."
Asked whether a partial meltdown had occurred, Mr Edano said that "because it's inside the reactor, we cannot directly check", but added that officials are taking measures on the assumption that it has.
This comes as Japan attempts to determine the scale of the Friday disasters, including an earthquake - reaching a magnitude of 9.0 according to Japan's Meteorological Agency - the most powerful in the country's recorded history, was followed by a tsunami that savaged its north-eastern coast with breathtaking speed and power.
At least 1,000 people were killed - including some 200 bodies discovered Sunday along the coast - and 678 are missing, according to officials.
Police in one of the worst-hit areas, the Miyagi prefecture, estimated the toll in that one area could eventually top 10,000.Reuse content