The company said firefighters were trying to extinguish the blaze, whose cause was not immediately known.
A fire burned for two hours Tuesday in the pool containing spent nuclear fuel rods at the unit 4 reactor. News services quoted a government official as saying the pool could still be near the boiling point.
The latest blow to an increasingly beleaguered and frightened populace came after Japanese officials, racing to avert a potential nuclear catastrophe, said Tuesday that they were considering a risky plan to spray water from a helicopter to prevent new fires in a pool of spent fuel at the Fukushima Daiichi power plant.
All but about 50 workers were evacuated from the plant, where at least three reactor cores are believed to be imperiled, and Prime Minister Naoto Kan hailed those who remained, saying they “are putting themselves in a very dangerous situation.”
Explosions destroyed the tops of two buildings housing reactors at the plant, one on Saturday and another on Monday. An explosion Tuesday in another reactor, unit 2, “may have affected the integrity of the primary containment vessel,” the International Atomic Energy Agency reported.
In addition to the helicopter operation, Tokyo Electric will also try to spray water from trucks on the ground through a hole in the building around the pool, NHK television reported.
Such drastic measures would be a last-ditch effort to prevent the spent fuel from burning and to keep cesium-137 and other radioactive isotopes from being released into the air, experts said.
“This is scary,” said Luke Barrett, a nuclear engineer who directed the clean-up of the Three Mile Island nuclear facility in Pennsylvania. “The plans in a severe accident are to just get a firehose in there, get any kind of water to keep water in the pool above the fuel. To try to lower buckets of water through the roof with a helicopter is a huge challenge, especially for the pilots.”
With the outer containment building at unit 2 primed for a possible explosion, any helicopter crews hovering over nearby unit 4 would be in grave peril, Barrett said.
During normal plant operations, uranium fuel that can no longer produce enough heat for generating electricity is periodically removed from a reactor and placed into the spent fuel pools located above the reactors. The spent fuel continues to generate heat and radioactive isotopes for many years.
Keeping this material covered with water is sufficient to cool it. But water levels may have dropped dramatically during the crisis, exposing fuel rods in the pools at reactor 4. Exposed fuel can easily burn, which is believed to be what happened during the fire in the fuel pool Tuesday.