The crisis at the quake-hit Fukushima No. 1 nuclear plant reached a critical phase Tuesday with radiation feared to have leaked after apparent hydrogen blasts at two more reactors, triggering growing fears of widespread contamination.
Prime Minister Naoto Kan urged people living between 20 and 30 kilometers of the plant to stay indoors, after radiation equivalent to 400 times the level to which people can safely be exposed in one year was detected near the No. 3 reactor in the plant.
Residents within a 20-km radius have already been ordered to vacate the area following Saturday's hydrogen blast at the plant's No. 1 reactor.
''The danger of further radiation leaks (from the plant) is increasing.'' Kan warned the public at a press conference, while asking people to ''act calmly.''
Chief Cabinet Secretary Yukio Edano said the high radiation level detected at 10:22 a.m. after the explosions at the No. 2 and No. 4 reactors ''would certainly have negative effects on the human body.''
The plant operator Tokyo Electric Power Co. said the problem could develop into a critical ''meltdown'' situation after part of the No. 2 reactor's container vessel was damaged following the apparent hydrogen explosion at 6:10 a.m.
TEPCO ordered some workers at the site to temporarily evacuate the area, but the utility has been continuing operations to pour seawater into the troubled No. 2 reactor to prevent overheating and further damage to its container.
The possibility of a meltdown, in which fuel rods melt and are destroyed, ''cannot be ruled out'' as the fuel rods have been damaged, the utility said.
Also, a fire occurred around 9:40 a.m. at the plant's No. 4 reactor, where spent nuclear fuels are stored, but it was extinguished later, according to TEPCO. Edano said it was likely caused by another hydrogen explosion.
The nuclear agency said the explosion at the No. 2 reactor may have damaged the ''suppression chamber,'' a facility connected to the reactor's container which is designed to cool down radiation steam and lower the pressure in the reactor. It said a sharp decline in the pressure level of the chamber suggests damage.
Given that the building that houses the No. 2 reactor has already been damaged by Monday's hydrogen blast at the neighboring No. 3 reactor, a spread of radiation outside the plant has become a serious threat, experts say.
In Ibaraki Prefecture, just south of Fukushima, an amount of radiation up to about 100 times the usual level was measured Tuesday morning. In Kanagawa Prefecture, southwest of Tokyo, radiation of up to nine times the normal level was also briefly detected.
The Tokyo metropolitan government also said it has detected a small amount of radioactive materials such as iodine and cesium in the air of the metropolis.
The wind was blowing from north to south when the incidents occurred at the Fukushima plant.
The cores of the No. 1, No. 2 and No. 3 reactors at the plant are believed to have partially melted following Friday's magnitude 9.0 earthquake that hit northeastern and eastern Japan.
The country's biggest recorded quake, which is also one of the largest in global history, caused the three reactors, which were all operating at the time, to automatically shut down. The No. 4 reactor and two others at the plant were not then in service.
Earlier in the day, the government and TEPCO set up an integrated headquarters, headed by Kan, to tackle the nuclear crisis.
''A worrisome situation remains but I hope to take the lead in overcoming this crisis,'' Kan said of the nuclear power plant. ''I will take all measures so that damage will not expand.''
At the headquarters set up at the TEPCO head office, Kan confronted TEPCO officials about their delay in reporting an earlier blast.
The development follows hydrogen blasts at both of the plant's No. 1 and No. 3 reactors on Saturday and Monday.
The cooling system for the No. 2 reactor broke down on Monday, causing water levels to rapidly fall and fully exposing its fuel rods for several hours.
As of 6:28 a.m., the water level had recovered to cover about 1.2 meters of the fuel rods, about one-third of their height, TEPCO said.
Meanwhile, the No. 4 reactor of TEPCO's Fukushima No. 2 nuclear plant, which is adjacent to the No. 1 plant, has successfully cooled down, meaning the plant no longer has a reactor in an emergency situation after three of its four reactors were once in that state after the quake, the firm said.
Japan has asked the United States to provide more cooling equipment to help deal with the crisis, Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, said in Washington. The NRC has already sent two technological experts and is fully supporting Japanese efforts, he said.