An explosion rocked one of Japan's nuclear power plants Saturday, causing a portion of a building to crumble, sending white smoke billowing into the air and prompting Japanese officials to warn people in the vicinity to cover their mouths and stay indoors.
In what may become the most serious nuclear power crisis since the Chernobyl disaster, the explosion followed large tremors at the Fukushima Daiichi No. 1 reactor Saturday afternoon, injuring four workers who were struggling to get the quake-stricken unit under control.
Earlier, Japan's Nuclear and Industrial Safety Agency (NISA) had warned that the reactor, whose cooling system had been crippled by the giant earthquake Friday, could be nearing a meltdown and that two radioactive substances, cesium and radioactive iodine, had already been detected nearby.
The full extent of the blast remained unclear, but footage on Japanese television showed that the walls of the building housing the reactor crumpled, leaving a skeletal metal frame, according to the Associated Press.
Japan's chief government spokesman, Yukio Edano, later told reporters that the blast occurred when vapor from the reactor's steel container turned into hydrogen and mixed with outside oxygen, the Kyodo news agency reported. Edano said the explosion blew off the roof and walls of the building around the containment vessel but did no serious damage to the container itself. However, authorities widened an evacuation zone to a 12.5 mile radius from the plant and prepared to distribute iodine tablets to people in the vicinity to protect them from exposure to radiation.
The unit, built 40 years ago by General Electric, is one of five reactors severely imperiled by the earthquake and subsequent disruptions in the power supply the reactors use for cooling systems.
Earlier, Japanese authorities had declared a state of emergency for the five reactors at two nuclear power complexes as military and utility officials scrambled to tame rising pressure and radioactivity levels inside the units and stabilize the systems used to cool the plants' hot reactor cores.
Radiation had earlier surged to about 1,000 times the normal level in the control room of one reactor, NISA said. Meanwhile, Tokyo Electric Power Co. said Saturday that the temperatures at two other reactors at a different power plant were rising and that it had lost control over pressure in three reactors there.
The explosion at the reactor is certain to rattle confidence in nuclear power in Japan, victim of the only nuclear weapons explosions and where people have long been sensitized to the dangers of radioactive releases. In the United States, it was likely to deal a severe blow to advocates of a nuclear power renaissance.
The earthquake has led to the shutdown of 11 of the Japan's 55 nuclear power plants, representing nearly 20 percent of the country's capacity. It will deal an economic blow to Japan, which relies on nuclear power for one-third of its electricity generation, and could complicate economic recovery efforts
It's a very serious situation for the reactors and might ultimately render those reactors unusable," said Howard Shaffer, a former Navy submarine engineer and a member of the American Nuclear Society's public information committee.
Japanese authorities initially evacuated about 3,000 residents living within 1.9 miles of the Fukushima Daiichi plant, on the east coast about 150 miles north of Tokyo and south of the heavily damaged city of Sendai. Later they widened that evacuation to a six-mile radius and, after the explosion, extended the evacuation area to 12.5 miles. People within a 16.2-mile radius were told to remain indoors, said the Web site of the International Atomic Energy Agency's Incident and Emergency Center.
The problems at the nuclear plants came in waves, starting with two of the six Daiichi units.
The quake disrupted the electric power the reactors used to run their cooling facilities, which pump water into the reactor core to cool the fuel rods there. The reactors switched to backup diesel generators, but the tsunami then swept in and shut down the generators used for the No. 2 reactor at Fukushima Daiichi. The unit then tapped excess steam in the core to power a turbine and switched to battery power, which would last only a few hours.
"There's a basic cooling system that requires power, which they don't have," said Glenn McCullough, former chairman of the Tennessee Valley Authority, who was tracking the Japan situation.
Japanese utility and government officials raced to get another generator to the site to prevent a possible partial meltdown similar to what took place in 1979 at Three Mile Island in Pennsylvania. By Saturday morning they said they had succeeded. The utility said it had restored power from the grid, but the IAEA said power was restored from "mobile electricity supplies."
Meanwhile, Tokyo Electric said it had decided to vent slightly radioactive steam and gas to relieve pressure that had increased sharply in the containment building at unit No. 1. The company said on its Web site that the increase was "assumed to be due to leakage of reactor coolant." It remained unclear where the leak was. The company said it did not think there was leakage of reactor coolant in the containment vessel "at this moment."
The purpose of a containment building, which surrounds the reactor core, is to contain unplanned releases of steam or gases from the core. If there is not enough water in the reactor core, water turns to steam and is released through special valves into the containment building, nuclear experts said.
That could cause an increase in pressure inside the sealed containment building and ultimately force a release of gas and steam through filters meant to keep most, though not all, of the radiation inside the building.
There were also reports of elevated radiation levels inside the control room of that reactor unit. NISA said levels were 1,000 times the norm. The AP later quoted an official from NISA as saying that a measurement of radiation levels outside the plant was eight times as high as normal. Even that level of radiation still posed little danger to residents, nuclear experts said. They also said the release of steam and gas from containment buildings posed little danger.
But in an update on its Web site, Tokyo Electric said one of its employees working in unit No. 1 was treated for radiation exposure by a special physician. Tokyo Electric also said, shortly before the explosion, that even though it was injecting water into the reactor vessel, levels were still dropping. It said water levels in unit No. 1 were lower than normal but stable.
The status of Tokyo Electric's Daina plants remained unclear. Earlier, they had been said to have completed automatic shutdowns. But Saturday, Tokyo Electric suggested that they were having problems similar to the ones at the other nuclear complex because of disruptions in the power supply needed to run cooling facilities.
"The danger is the very thermally hot reactor cores at the plant must be continuously cooled for 24 to 48 hours," said Kevin Kamps, a specialist in nuclear waste at Beyond Nuclear, a group devoted to highlighting the perils of nuclear power. "Without any electricity, the pumps won't be able to pump water through the hot reactor cores to cool them."
In addition to the efforts to get Tokyo Electric's nuclear reactors under control, Japan's NISA said Friday that a fire had broken out at the Onagawa nuclear power plant but was later extinguished. The three reactors at the Onagawa site remained closed.
The key buildings in the Onagawa plant are about 45 feet above sea level, according to the Web site of Tohoku Electric Power, owner of the plant. The company said that was about twice the height of the previous highest tsunami.
The IAEA said it is seeking details on Japan's nuclear power plants and research reactors, including information on off-site and on-site electrical power supplies, cooling systems and the condition of the reactor buildings. Nuclear fuel requires continued cooling even after a plant is shut down, the IAEA noted.
"This is the most challenging seismic event on record, so it is a severe test," McCullough said. "Clearly the Japanese government is taking this very seriously.
Correspondent Chico Harlan in Tokyo and staff writer Mary Beth Sheridan contributed to this report.