Details added (the first version was posted at 07:17)
The inner shell of a quake- and tsunami-damaged nuclear reactor in Japan might be damaged and radiation is so high there that employees could no longer enter the complex, the government's top spokesman said Wednesday, dpa reported.
White smoke was seen rising Wednesday from reactor number 3 at the plant in Fukushima that is home to six reactors after a fire broke out for a second day at reactor number 4.
Chief Cabinet Secretary Yukio Edano said there was definitive information about the reactor shell on number 3. It is one of the reactor's three containment structures designed to prevent leakage of radiative materials, and Edano said radioactive steam might have leaked Wednesday.
Radioactive emissions at the plant reached record levels overnight. Measurements of 1,000 millisievert were taken and on Wednesday morning, 600 to 800 millisieverts were measured, Edano said.
Being exposed to 1,000 millisieverts can cause radiation poisoning. That dose is 250 times what people usually receive in a year, but people can experience health problems at a dose of 400 millisieverts.
A spokesman at Japan's nuclear safety agency told the NHK broadcaster that workers could not longer enter reactor number 3's control room for safety reasons.
At reactor number 4, a fire had broken out Tuesday, releasing significant amounts of radiation into the environment. The fire had been believed to be under control Tuesday before a blaze flared again in the same part of the plant Wednesday.
There were concerns that water levels have fallen inside a pool storing spent nuclear rods at reactor 4, leaving the fuel exposed and allowing it to overheat and melt.
Plant operator Tokyo Electric Power Co (TEPCO) said it was considering spraying boracic acid by helicopter - to prevent spent nuclear fuel rods from reaching criticality again and restarting a chain reaction - at reactor 4, but Edano said the plan was considered too dangerous.
TEPCO said Wednesday that the possibility of a chain reaction restarting was "not zero."
Meanwhile, about 70 per cent of the nuclear fuel rods have been damaged at the troubled reactor 1 of the Fukushima 1 plant and 33 per cent at reactor 2, Kyodo reported, citing TEPCO officials. The reactors' cores are believed to have partially melted.
The cooling systems at all six of the power plant's reactors have failed after Friday's magnitude-9 earthquake and tsunami. Technicians have been working frantically ever since to prevent meltdowns as the reactor cores heat up and radiation levels have spiked. Explosions have occurred at some of the reactors, including reactor number 3 on Monday.
Edano said there were no plans to expand the evacuation zone around the plant 250 kilometres north of Tokyo. Authorities ordered people to leave a 20-kilometre radius of the plant and those remaining in a 30-kilometre radius to remain indoors with windows and doors shut.
Winds at the site Wednesday morning were in a south-easterly-easterly direction and blowing away from land and toward the Pacific.