Updated throughout with new details at approximately 11:10 a.m. PT on Sunday.
A Saturday explosion at a closely monitored nuclear power plant in northern Japan and the expectation of another explosion at a second reactor are further distracting rescue efforts of thousands of stranded and missing in the aftermath of one of the largest earthquakes on record.
Officials initially reported that a meltdown at the first crippled reactor was not imminent, but on Sunday admitted that partial meltdowns may have occurred at two reactors, reported The New York Times. By Sunday afternoon local time, The Washington Post reported that as many as seven reactors had built-up pressure.
The first explosion took place at reactor No. 1, a 439-megawatt boiling water reactor built by General Electric 40 years ago and scheduled to be shut down on March 26, The Washington Post reported. By Sunday night, Japan braced for another explosion at one of the seven reactors that workers are trying to keep cool--No. 3 in the same facility--that also lost its emergency cooling function, Reuters reported.
Meanwhile, what was originally a 2-mile evacuation radius has expanded to 12.5 miles and involves some 210,000 residents, while government officials and executives of plant operator Tokyo Electric Power have yet to be entirely clear about whether a partial meltdown occurred, The New York Times reported. Experts told Reuters that the health risks in Japan due to radiation at this point remain low and that contamination is not likely to blow overseas.
The initial explosion occurred as Tokyo Electric Power took emergency measures Saturday to avert a meltdown after a power outage left a cooling system unable to supply water to cool reactor No. 1. Representatives reported to the media that their efforts seemed to be effectively lowering the pressure inside the container, but the seawater workers poured over the fuel rods to cool them formed hydrogen, which--when released to relieve pressure--reacted with oxygen to cause the explosion.
Japan's chief cabinet secretary, Yukio Edano, reported in a news conference Saturday night that the explosion did not take place inside the reactor but rather in a structure housing turbines near the reactor, and that the container was not damaged. He urged everyone to remain calm, and said that the reactor remaining intact improved chances of continuing to cool the core and prevent a full meltdown.
Robin Grimes, a professor of materials physics at Imperial College London, said in a statement to the press that "despite the damage to the outer structure, as long as that steel inner vessel remains intact, then the vast majority of the radiation will be contained. At the moment it does seem that they are still contained and it's a release of significant steam pressure that's caused this explosion. The key will be the monitoring of those radiation levels."
The zirconium casings of the fuel rods are hot enough at 2,200 degrees Fahrenheit to react with the cooling water and create hydrogen, which seems to have happened and caused the explosion. At 4,000 degrees, the uranium fuel pellets inside the rods could start to melt, which could lead to full core meltdown, according to Forbes.com.
The events in Japan are already influencing the ongoing debate about the safety of nuclear power. More than 60,000 people gathered in Germany on Saturday for a previously arranged protest of the government's plans to prolong the life of its nuclear reactors, prompting Chancellor Angela Merkel to tell reporters: "We know how safe our plants are and that we do not face a threat from such a serious earthquake or violent tidal wave, but we will learn what we can from the events in Japan."
Tokyo Electric Power has confirmed in a press release that one worker at reactor No. 1 has died of injuries after becoming trapped in the crane-operating console of the exhaust stack, and that eight workers have been injured and two are missing. The company adds that an employee working on that reactor is being treated for radiation exposure by a special physician.
More than 83,000 people live within 10 kilometers of the two plants under evacuation orders, according to Japan's Nuclear and Industrial Safety Agency. And the BBC reported that an estimated 210,000 people have been evacuated from the surrounding countryside.
Japan's nuclear and industrial safety agency said that as many as 160 people may have been exposed to radiation. Japanese authorities have been handing out iodine to residents in the area, which experts believe can help head off minimize effects of radiation exposure, reported The New York Times.
The Atlantic is among several news outlets drawing comparisons to the Three Mile Island disaster of 1979, during which a partial meltdown occurred due to a loss of coolant.
The massive 8.9 magnitude earthquake off the coast of Honshu, Japan, was officially upgraded to 9.0 on Sunday by the Japanese Meteorological Agency, effectively doubling its recorded power. The earthquake triggered not only the power outage that prompted the country to declare a nuclear emergency, but also a large tsunami resulting in at least hundreds of drownings. Italy's National Institute of Geophysics and Volcanology reports that the Earth's axis shifted 9.8 inches, and the U.S. Geological Survey reports that the main island of Japan moved 7.8 feet. (The largest earthquake on record was a magnitude 9.5 quake in Chile in 1960, which killed 1,655, left 2 million people homeless, and resulted in a tsunami that killed 61.)
Meanwhile, rescue efforts continue around Japan, with 100,000 troops joining boats and helicopters, and dozens of countries offering aid, including longtime rivals China and South Korea. U.S. President Barack Obama says the U.S. has sent at least two American aircraft carriers to the region.
The U.S. Geological Survey is also reporting hundreds of aftershocks off the eastern coast of Japan on Saturday and Sunday, more than two dozen of which are greater than magnitude 6, which is the size that damaged New Zealand in February.
The official count of the dead has jumped to 1,700, The Telegraph reported, with another 10,000 feared dead from the fishing port of Minamisanriku, where the tsunami swept six miles inland. Sunday afternoon police reported recovering another 200 bodies along the coast. At least 215,000 people are now spread across 1,350 temporary shelters in five prefectures, according to the Japanese national police agency, with some 500,000 without power and in many cases without electricity.
Google's 2011 Japan Earthquake Person Finder now includes 213,000 records.