Destroyed cars are seen at a factory area devastated by earthquake and tsunami in Sendai, northern Japan, March 16, 2011.  [Photo/Agencies]

"It's more of a surrender," said David Lochbaum, a nuclear engineer who now heads the nuclear safety program for the Union of Concerned Scientists, an activist group. "It's not like you wait 10 days and the radiation goes away. In that 10 days things are going to get worse."

"It's basically a sign that there's nothing left to do but throw in the towel," Lochbaum said.

The government has ordered some 140,000 people in the vicinity to stay indoors. A little radiation was also detected in Tokyo, 150 miles (240 kilometers) to the south, triggering panic buying of food and water.

There are six reactors at the plant, and the three that were operating at the time have been rocked by explosions. The one still on fire was offline at the time of the magnitude 9.0 quake, Japan's most powerful on record.

The Nuclear and Industrial Safety Agency estimated that 70 percent of the rods have been damaged at the No 1 reactor.

Japan's national news agency, Kyodo, said that 33 percent of the fuel rods at the No 2 reactor were damaged and that the cores of both reactors were believed to have partially melted.

"We don't know the nature of the damage," said Minoru Ohgoda, spokesman for the country's nuclear safety agency. "It could be either melting, or there might be some holes in them."

Meanwhile, the outer housing of the containment vessel at the No 4 unit erupted in flames early Wednesday, said Hajimi Motujuku, a spokesman for the plant's operator, Tokyo Electric Power Co.

Japan's nuclear safety agency said fire and smoke could no longer be seen at Unit 4, but that it was unable to confirm that the blaze had been put out.