The Tokyo Electric Power Company reported that pressure inside a reactor at its Fukushima-Daiichi nuclear plant continued to rise, prompting fears of a radiation leak.

Japan earlier declared a nuclear emergency after reports that the plant's cooling system malfunctioned following the 8.9 earthquake early Friday. The U.S. Air Force has delivered coolant to the nuclear plant, and residents have been evacuated from the area.

The plant is one of of two owned by TEPCO in the Fukushima prefecture, and one of four affected by the quake. The International Atomic Energy Agency confirms that the other three (Fukushima-Daini, Oganawa and Tokai) have been successfully shut down. One of them suffered a fire because of the earthquake. This is Japan's first-ever declaration of a nuclear emergency.

Commissioned in 1971, the Fukushima-Daiichi is one of the largest nuclear power plants in the world. Japanese Nuclear Safety Organization records indicate the plant has a lengthy history of problems, including an accidental 2006 release of tritium-infused steam.

In the present situation, workers at the plant were unable to generate enough electricity to operate the cooling system pumps, thus triggering the emergency. 

As an ABC report explains, overheating fuel could lead to a meltdown, and in turn to a large release of radioactive Cesium-137 into the environment, dwarfing even the Chernobyl disaster.