Japan moved quickly Saturday to contained a crisis at two nuclear power plants damaged when a massive earthquake and tsunami struck the country's northeast coast. Government officials declared a state of emergency and launched a massive, military-led rescue operation.
Japanese officials say the 8.9 magnitude quake and subsequent tsunami knocked out power and caused cooling systems to fail at two plants in Fukushima, about 240 kilometers north of Tokyo.
Officials at first believed there was a possibility that nuclear fuel rods at one of the plants may have begun melting. Later, officials said radioactive pressure was successfully relieved by opening valves and allowing the release of radioactive steam.
But Japanese television broadcast images showing what appeared to be extensive damage to the outer structure of the building that houses the reactor. The paneling on the steel-framed building appeared to blow off in an explosion and smoke could be seen billowing from the structure. Japanese news media reported at least four people injured.
Chief Cabinet Secretary Yukio Edano commented on the incident at a Tokyo news conference.
Edano said the government was investigating what happened at the plant. He urged people to evacuate at least 10 kilometers away from the plant, a distance that was later doubled to 20 kilometers.
Radioactive material has been found outside one reactor, where officials say they are trying to determine if a meltdown has occurred.
The situation at the nuclear plants comes as the government launched a large-scale military-led rescue operation to help victims of Friday's massive quake and tsunami.
Japanese Prime Minister Naoto Kan said he is sending 50,000 troops to aid rescue and recovery efforts. He said 190 military planes and 25 ships have also been sent to the devastated areas.
More than 50 countries have offered Japan help and some has already begun arriving. A team from South Korea touched down in Tokyo on Saturday afternoon. The U.S. military, which maintains several bases in Japan, is also taking part in the rescue effort.
Japan sits on the so-called "Ring of Fire," an area where large numbers of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean.