Workers battling to prevent nuclear meltdown at the stricken Fukushima Daiichi plant were temporarily evacuated on Wednesday morning after radiation levels became too dangerous for them to remain.
The withdrawal hampered efforts to secure safety at the atomic power plant and avert a major radiation leak. Staff returned to the plant after about an hour once radiation levels fell.
Its operator, Tokyo Electric Power (Tepco) said it was considering using helicopters to spray the crippled No 4 reactor with water and boric acid - a fire retardant - in an attempt to prevent more radiation leaks.
The 50 or so engineers, working around the clock in harsh conditions, spent Wednesday morning trying to put out a fire at one reactor and to cool others at risk of overheating and reaching criticality, according to The Guardian.
Meanwhile, 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, Newser reports.