The container of the No. 3 reactor of the quake-hit Fukushima No. 1 nuclear power plant is feared to have been damaged and may have leaked radioactive steam Wednesday, emitting high-level radiation, Chief Cabinet Secretary Yukio Edano said.
The radiation level briefly topped 6 milisievert per hour at the plant, the government’s nucler safety agency said.
The explanations were given after smoke was seen rising from the No. 3 reactor since around 8:30 a.m., according to Edano.
Earlier in the day, a fire broke out again at the plant’s No. 4 reactor, where there was already a risk of leaks of high-level radioactive materials, but flames were no longer visible about 30 minutes later.
Tokyo Electric Power Co, the operator of the plant, said it considered spraying boric acid by helicopter to prevent the No. 4 reactor’s spent nuclear fuel rods from reaching criticality again, restarting a chain reaction.
TEPCO said water in a pool storing the spent fuel rods may be boiling and that its level may have dropped, exposing the rods. The government ordered the firm to inject water into the pool ‘‘as soon as possible to avert a major nuclear disaster.’‘
Due to high radiation levels at the reactor, workers have been unable to pour water into the troubled pool. Difficult conditions have led the utility to evacuate around 730 of the 800 workers from the site, according to TEPCO.
‘‘The possibility of recriticality is not zero,’’ TEPCO said Wednesday as it announced the envisaged step to control the situation.
Unless the spent fuel rods are cooled down, they could suffer damage and emit radioactive substances.
An estimated 70% of the nuclear fuel rods have been damaged at the plant’s No. 1 reactor and 33% at the No. 2 reactor, the firm said. The cores of both reactors are believed to have partially melted with their cooling functions lost in the wake of Friday’s magnitude 9.0 earthquake and tsunami.
Around 5:45 a.m., a worker at the plant saw flames on the fourth floor of the building housing the No. 4 reactor, believed to be the same spot where an apparent hydrogen explosion caused a fire at 9:38 a.m. Tuesday. The reactor had been halted for regular checks from before the quake.
TEPCO said it had stopped fire-fighting operations after judging that the Tuesday morning fire had been extinguished. It said it promptly reported the latest incident to firefighters and local governments.
Flames were seen through the two square-shaped holes about 8 by 8 meters created in the walls of the building by Tuesday’s 6:14 a.m. blast, but were no longer visible when workers at the plant tried to confirm them at 6:15 a.m. Wednesday, the utility said.