Last Updated at 5:49 p.m. ET
An explosion Saturday at a nuclear power station damaged by Japan's powerful earthquake destroyed a building housing the reactor, amid fears that the plant located 150 north of Tokyo could melt down. Later Saturday, Japan's nuclear safety agency reported an emergency at a second reactor in the same complex where the explosion had occurred earlier.
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 seawater into the reactor to avoid disaster and moved 170,000 people from the area.
The Nuclear and Industrial Safety Agency said early Sunday that the cooling system malfunctioned at Unit 3 of the Fukushima Dai-ichi nuclear plant. The agency said it was informed of the emergency by Tokyo Electric, the utility which runs the plant.
No further details of the troubles at Unit 3 were immediately available.
Large amounts of radiation were spewing out of the Fukushima Dai-ichi nuclear power station, and the evacuation area around the plant was expanded to a 12-mile radius, but officials did not know how dangerous the leak was to people. Tens of thousands of people have already been evacuated.
Yoshinori Baba, a disaster official in Fukushima prefecture, confirmed three people being evacuated from an area near the plant have been exposed to radiation, but said they showed no signs of illness.
Shinji Kinjo, a spokesman for the Japanese nuclear agency, could not say how much radiation was in the atmosphere or how hot the reactor was following the failure of its cooling system.
"They are working furiously to find a solution to cool the core, and this afternoon in Europe we heard that they have begun to inject sea water into the core," said Mark Hibbs, a senior associate at the Nuclear Policy Program for the Carnegie Endowment for International Peace. "That is an indication of how serious the problem is and how the Japanese had to resort to unusual and improvised solutions to cool the reactor core."
Tokyo Power Electric Co., the utility that runs the Fukushima Dai-ichi plant, said four workers suffered fractures and bruises and were being treated at a hospital.
Chief Cabinet Secretary Yukio Edano said Saturday's explosion was caused by vented hydrogen gas and destroyed the exterior walls of the building where the reactor is, but not the actual metal housing enveloping the reactor.
"We are now trying to analyze what is behind the explosion," said government spokesman Yukio Edano, stressing that people should quickly evacuate. "We ask everyone to take action to secure safety."
Japanese authorities have informed the International Atomic Energy Agency (IAEA) that the explosion at the Unit 1 reactor of the Fukushima Daiichi plant occurred outside the primary containment vessel (PCV), not inside.
The plant operator, Tokyo Electric Power Company (TEPCO), has confirmed that the integrity of the primary containment vessel remains intact.
As a countermeasure to limit damage to the reactor core, sea water mixed with boron is being injected into the primary containment vessel to prevent possible damage to core. A spokesman for NISA told a news conference that as long as they continue to deliver seawater into the plant, the situation should not get worse.
Japan's Nuclear and Industrial Safety Agency (NISA) confirmed the presence of caesium-137 and iodine-131 in the vicinity of Fukushima Daiichi Unit 1. NISA reported an initial increase in levels of radioactivity around the plant earlier today, but these levels have been observed to lessen in recent hours.
Officials have said that radiation levels 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 explosion was preceded by puff of white smoke that gathered intensity until it became a huge cloud enveloping the entire facility.
Containment remains intact at Fukushima Daiichi Units 1, 2 and 3.
The Japanese authorities have classified the event at Fukushima Daiichi Unit 1 as a Level 4 "Accident with Local Consequences" on the International Nuclear and Radiological Event Scale (INES). (The scale runs from 0 for a deviation, to 7 for a major accident.)
The trouble began at the plant's Unit 1 after the massive 8.9-magnitude earthquake and the tsunami it spawned knocked out power there.
CBS News correspondent Nancy Cordes says the reactors in Japan were built to withstand the tremendous seismic activity that occurs in Japan, with backup systems to the backup systems. "If the electrical grids fail, then they go to diesel generators; if the diesel generators fail, then they go to battery packs," Cordes said on CBS' "The Early Show on Saturday Morning." "So it's really a mystery right now why none of those systems seem to be working properly."
In the wake of the disaster, Toyota suspended operations at 12 sites in Japan Saturday.
Special report: Disaster in Japan
Japanese nuke plant scare (Video)
Seismologist: Japan earthquake magnitude "unusual" (Video)
Tsunami surges across Pacific, batters Calif. coast (Video)
How you can help
Submit your photos and video to CBSNews.com
CBS News correspondent Lucy Craft says the anti-nuclear lobby is saying that this could end up to be something between a Three Mile Island and a Chernobyl, while the government is trying o reassure the public that the accident is being addressed in a sensible way, and that there is a good chance of containing any potential radiation leaks.
"So you have this sort of tug-of-war going on between people saying this is a perfect example of why an earthquake-prone country like Japan should not have nuclear power," said Craft, "and the government - of course, very heavily invested in nuclear power, which provides about a third of this tiny country's energy, and they're very dependent upon it - they're trying to say, 'Please calm down, we've got the situation under control.'"
Speaking to "The Early Show on Saturday Morning," an American in Tokyo in likened the disaster and after-effects to 9/11 said Japan was in "panic mode" over the nuclear reactors.
Meanwhile, also on "The Early Show on Saturday Morning," a spokesman for a nuclear industry group called the possibility of a meltdown "most unlikely."
Friday's double disaster, which pulverized Japan's northeastern coast, has left 574 people dead by official count, although local media reports said at least 1,300 people may have been killed.
Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely.
"It's not a fast reaction like at Chernobyl," he said. "I think that everything will be contained within the grounds, and there will be no big catastrophe."
In 1986, the Chernobyl nuclear reactor exploded and caught fire, sending a cloud of radiation over much of Europe.
The true scale destruction was still not known more than 24 hours after the quake since washed-out roads and shut airports have hindered access to the area. An untold number of bodies were believed to be buried in the rubble and debris.
In another disturbing development that could substantially raise the death toll, Kyodo news agency said rail operators lost contact with four trains running on coastal lines on Friday and still had not found them by Saturday afternoon.
East Japan Railway Co. said it did not know how many people were aboard the trains.
The most troubled nuclear plant, Fukushima Dai-ichi, is facing meltdown, officials have said.
A "meltdown" is not a technical term. Rather, it is an informal way of referring to a very serious collapse of a power plant's systems and its ability to manage temperatures. It is not immediately clear if a meltdown will cause serious radiation risk, and if it does how far the risk would extend.
Pressure has been building up in the reactor -- it's now twice the normal level -- and Japan's Nuclear and Industrial Safety Agency told reporters Saturday that the plant was venting "radioactive vapors." Officials said they were measuring radiation levels in the area.
The reactor in trouble has already leaked some radiation: Operators have detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.
Ryohei Shiomi, a nuclear official, said each hour the plant was releasing the amount of radiation a person normal absorbs in a year.
He has said that even if there were a meltdown, it wouldn't affect people outside a six-mile radius -- an assertion that might need revising if the situation deteriorates. Most of the 51,000 residents living within the danger area had been evacuated, he said.
Wind in the region is weak and headed northeast, out to sea, according to the Meteorological Agency.
Meanwhile, the first wave of military rescuers began arriving by boats and helicopters.
Prime Minister Naoto Kan said 50,000 troops would join rescue and recovery efforts following the quake that unleashed one of the greatest disasters Japan has witnessed -- a 23-foot tsunami that washed far inland over fields, smashing towns, airports and highways in its way.
"Most of houses along the coastline were washed away, and fire broke out there," said Kan after inspecting the quake area in a helicopter. "I realized the extremely serious damage the tsunami caused."
More than 215,000 people were living in 1,350 temporary shelters in five prefectures, or states, the national police agency said. Since the quake, more than 1 million households have not had water, mostly concentrated in northeast.
The transport ministry said all highways from Tokyo leading to quake-hit areas were closed, except for emergency vehicles. Mobile communications were spotty and calls to the devastated areas were going unanswered.
Local TV stations broadcast footage of people lining up for water and food such as rice balls. In Fukushima, city officials were handing out bottled drinks, snacks and blankets. But there were large areas that were surrounded by water and were unreachable.
One hospital in Miyagi prefecture was seen surrounded by water. The staff had painted an SOS on its rooftop and were waving white flags.
Kan said a total of 190 military aircraft and 25 ships have been sent to the area, which continued to be jolted by tremors, even 24 hours later.
More than 125 aftershocks have occurred, many of them above magnitude 6.0, which alone would be considered strong.
Technologically advanced Japan is well prepared for quakes and its buildings can withstand strong jolts, even a temblor like Friday's, which was the strongest the country has experienced since official records started in the late 1800s. What was beyond human control was the killer tsunami that followed.
It swept inland about six miles in some areas, swallowing boats, homes, cars, trees and everything else.
"The tsunami was unbelievably fast," said Koichi Takairin, a 34-year-old truck driver who was inside his sturdy four-ton rig when the wave hit the port town of Sendai.
"Smaller cars were being swept around me," he said. All I could do was sit in my truck."
His rig ruined, he joined the steady flow of survivors who walked along the road away from the sea and back into the city on Saturday. Smoke from at least one large fire could be seen in the distance.
Smashed cars and small airplanes were jumbled up against buildings near the local airport, several miles from the shore. Felled trees and wooden debris lay everywhere as rescue workers coasted on boats through murky waters around flooded structures, nosing their way through a sea of debris.
Basic commodities were at a premium. Hundreds lined up outside of supermarkets, and gas stations were swamped with cars. The situation was similar in scores of other towns and cities along the 1,300-mile-long eastern coastline hit by the tsunami.
In Sendai, as in many areas of the northeast, cell phone service was down, making it difficult for people to communicate with loved ones.
President Barack Obama pledged U.S. assistance following what he called a potentially "catastrophic" disaster. He said one U.S. aircraft carrier was already in Japan and a second was on its way. A U.S. ship was also heading to the Marianas Islands to assist as needed, he said.
Japan's worst previous quake was a magnitude 8.3 temblor in Kanto that killed 143,000 people in 1923, according to the USGS. A magnitude 7.2 quake in Kobe killed 6,400 people in 1995.
Japan lies on the "Ring of Fire" -- an arc of earthquake and volcanic zones stretching around the Pacific where about 90 percent of the world's quakes occur, including the one that triggered the Dec. 26, 2004, Indian Ocean tsunami that killed an estimated 230,000 people in 12 countries. A magnitude-8.8 quake that shook central Chile in February 2010 also generated a tsunami and killed 524 people.