The nuclear accident at an earthquake-damaged nuclear plant in Japan can now be classed as level six out of an international scale of one to seven, experts said Tuesday.
France's ASN nuclear safety authority's assessment came after Japanese Prime Minister Naoto Kan said radiation had spread from the four stricken reactors of the Fukushima Dai-ichi nuclear plant along Japan's northeastern coast.
Two reactors exploded on Tuesday at the plant after days of frantic efforts to cool them.
Level seven was used only once, for Chernobyl in Ukraine in 1986. The 1979 accident at the Three Mile Island nuclear power plant in Pennsylvania was rated a level five.
"It is very clear that we are at a level six, which is an intermediate level between what happened at Three Mile Island and Chernobyl," ASN president Andre-Claude Lacoste told a news conference in Paris. "We are clearly in a catastrophe."
On Three Mile Island, the radiation leak was held inside the containment shell — thick concrete armor around the reactor. The Chernobyl reactor had no shell and was also operational when the disaster struck. The Japanese reactors automatically shut down when the quake hit.
The International Atomic Energy Agency (IAEA) imposed a 30-km (18-mile) no-fly zone around the Fukushima site Tuesday.
Earlier, Japanese officials told the IAEA that a fuel storage pond had caught fire — an area where used nuclear fuel is kept cool — and that radioactivity was "being released directly into the atmosphere."
Long after the fire was extinguished, a Japanese official said the pool might still be boiling, though the reported levels of radiation had dropped dramatically by the end of the day. The radiation releases prompted Japanese officials to issue orders for 140,000 people to seal themselves indoors Tuesday.
The IAEA said about 150 people had received monitoring for radiation levels and that measures to "decontaminate" 23 of them had been taken.