



Nuclear experts have been saying for days that Japan was underplaying the severity of the nuclear crisis, which later Friday the prime minister called "very grave."


Hidehiko Nishiyama of Japan's nuclear safety agency said the rating was raised when officials realized that at least 3 percent of the fuel in three of the reactors at the Fukushima Dai-ichi plant had been severely damaged, suggesting those reactor cores have partially melted down and thrown radioactivity into the environment.



Later, Prime Minister Naoto Kan urged the nation to unite.






"We see it as an extremely serious accident," Yukiya Amano, the head of the U.N.'s International Atomic Energy Agency, told reporters Friday in Tokyo. "This is not something that just Japan should deal with, and people of the entire world should cooperate with Japan and the people in the disaster areas."


One week after the twin disasters — which has officially left more than 6,500 dead and over 10,300 missing — emergency crews are facing two challenges in the nuclear crisis: cooling the reactors where energy is generated, and cooling the adjacent pools where used nuclear fuel rods are stored in water.










The U.S. has also now conducted overflights of the reactor site, strapping sophisticated pods onto aircraft to measure airborne radiation, U.S. officials said. Two tests conducted Thursday gave readings that U.S. Deputy Energy Secretary Daniel B. Poneman said reinforced the U.S. recommendation that people keep away from a 50-mile (80-kilometer) radius around the Fukushima plant.




After meeting with Kan and other senior officials, the U.N.'s Amano complained that his agency had not been receiving critical information. He said, for instance, the IAEA wanted to know what kind of radioactive elements were being released but could not get the data.

"This kind of information is needed in a timely way, and we hope the Japanese government will provide it. We hope everything will be better," Amano told reporters.



But even once the power is reconnected, it was not clear if the cooling systems were intact and will still work.

Workers were completing laying cables around Units 1 and 2 on Friday, a power company official said, and hoped to reach more units Saturday. Even so, experts will have to check for anything volatile to avoid an explosion when the electricity is turned on.

"There may be sparks, so I can't deny the risk," said Teruaki Kobayashi.






The actions authorities are taking to cool the reactors are the best ones available, experts say. Eventually, the plant may be entombed in concrete, as was done hastily after the 1986 Chernobyl reactor accident.

But pressures and temperatures must be controlled before then, said Mario V. Bonaca, an adviser to the U.S. Nuclear Regulatory Commission. Otherwise, he said, overheated nuclear fuel will melt or burst through the sand, cement or other covering and release more radiation.