Authorities say another reactor at the quake-hit nuclear plant was in trouble after its cooling system also failed.
Source: Al Jazeera
Cooling systems have failed at another nuclear reactor on Japan's devastated coast, hours after an explosion at a nearby unit made leaking radiation, or even outright meltdown, the central threat to the country following a catastrophic earthquake and tsunami.
The Japanese government said radiation emanating from the plant appeared to have decreased after Saturday's blast, which produced a cloud of white smoke that obscured the complex. But the danger was grave enough that officials pumped sea water into the reactor to avoid disaster and moved 170,000 people from the area.
Japan's nuclear safety agency reported on Sunday an emergency at another reactor unit, the third in the complex to have its cooling systems malfunction.
"All the functions to keep cooling water levels in No. 3 reactor have failed at the Fukushima No. 1 plant," plant operator Tokyo Electric Power (Tepco) said, adding that pressure was rising slightly.
Japan's nuclear safety agency rated the accident at four on the international scale of zero to seven. The 1979 Three Mile Island accident in the United States was rated five, while the 1986 Chernobyl disaster was a seven.
Japan dealt with the nuclear threat as it struggled to determine the scope of the earthquake, the most powerful in its recorded history, and the tsunami that ravaged its northeast on Friday with breathtaking speed and power.
The explosion at the nuclear plant, Fukushima Dai-ichi, 274 kilo metres northeast of Tokyo, appeared to be a consequence of steps taken to prevent a meltdown after the quake and tsunami knocked out power to the plant, crippling the system used to cool fuel rods there.
The blast destroyed the building housing the reactor, but not the reactor itself, which is enveloped by stainless steel 6 inches (15 centimeters) thick.
Inside that superheated steel vessel, water being poured over the fuel rods to cool them formed hydrogen.
When officials released some of the hydrogen gas to relieve pressure inside the reactor, the hydrogen apparently reacted with oxygen, either in the air or the cooling water, and caused the explosion.
Officials declined to say what the temperature was inside the troubled reactor, Unit 1. At 2,200 degrees Fahrenheit (1,200 degrees Celsius), the zirconium casings of the fuel rods can react with the cooling water and create hydrogen. At 4,000 F (2,200 C), the uranium fuel pellets inside the rods start to melt, the beginning of a meltdown.
Chief Cabinet Secretary Yukio Edano said radiation around the plant had fallen, not risen, after the blast but did not offer an explanation.
Virtually any increase in dispersed radiation can raise the risk of cancer, and authorities were planning to distribute iodine, which helps protect against thyroid cancer. Authorities ordered 210,000 people out of the area within 12 miles (20 kilometers) of the reactor.
It was the first time Japan had confronted the threat of a significant spread of radiation since the greatest nightmare in its history, a catastrophe exponentially worse: the 1945 atomic bombings of Hiroshima and Nagasaki by the United States, which resulted in more than 200,000 deaths from the explosions, fallout and radiation sickness.
Officials have said that radiation levels at Fukushima were elevated before the blast: At one point, the plant was releasing each hour the amount of radiation a person normally absorbs from the environment each year.
The Japanese utility that runs the plant said four workers suffered fractures and bruises and were being treated at a hospital.
Nine residents of a town near the plant who later evacuated the area tested positive for radiation exposure, though officials said they showed no health problems.
Although the government played down fears of radiation leak, Japanese nuclear agency spokesman Shinji Kinjo acknowledged there were still fears of a meltdown, the collapse of a power plant's systems, rendering it unable regulate temperatures and keep the reactor fuel cool