"We've never encountered this type of situation in history before," said Joe Cirincione, a nuclear policy expert. "We are beyond a reactor crisis at this point. This is a nuclear system crisis. The entire northern part of the Japanese nuclear power system has been delivered a body blow."
The leak at unit 3 made it difficult to keep the core of the reactor covered with sea water, Dr. Michio Kaku, a physicist, said.
"The situation is getting worse by the hour. We haven't hit bottom yet," Kaku said. "We now have reports that unit 3 suffered perhaps a 90 percent uncovering of the core -- this is unprecedented since Chernobyl."
Japanese officials insist that things are under control at the nuclear plant and that radiation levels are safe.
"They haven't stabilized the sea water yet," Kaku said. "Remember, they're hanging in there right there with the fingernails. This is how close we are to a full-scale meltdown. So it's stable in the sense that you're stable when you're hanging by your fingernails."
A trip to Japan's emergency command center revealed officials re-watching the explosion at the Fukushima plant. A gray plume of smoke could be seen rising from unit 3 on the command center's television.
The explosion of unit 3 injured 11 workers on Monday and could be felt as far as 25 miles away.
The Japanese government continues to test the nearly 180,000 people evacuated from around the Fukushima plant for exposure to radiation.
In the town of Koriyama, lines of families waited to be tested today.
Medical teams wearing white suits used Geiger counters and silver hand-held scanners to check everyone, aiming the scanners at little children holding their hands out.
One little girl clung to her Winnie the Pooh toy before being checked by the medical team.
The mother of the girl told ABC News that her family is OK but that surviving Japan's worst earthquake and massive tsunami has been like living through hell.
At least 160 people tested positive for exposure to radiation.
Radiation detected by U.S. helicopters flying 60 miles from the Japanese shore prompted the repositioning of the USS Ronald Reagan and seven other U.S. Navy ships, military officials said.
There is growing concern about whether radiation released in Japan could reach the United States. So far, winds blowing around the Fukushima plant are westerly or southwesterly winds, meaning any radiation that's released is being blown out to the Pacific Ocean.
The chairman of the U.S. Nuclear Regulatory Commission said officials see "a very low likelihood" and "a low probability" that there's any possibility of harmful radiation levels in the united States, Hawaii or any other U.S. territories.
"The lack of any harmful impacts to the U.S. is simply based on the nature of these reactors and the large distances, obviously, between those and any U.S. territories. So you just aren't going to have any radiological material that, by the time it traveled those large distances, could present any risk to the American public," NRC chairman Greg Jaczko said.
If a nuclear meltdown were to occur releasing significant amounts of radiation, it would take at least five to seven days to reach the west coast of the united States, according to a blog written by Jeff Masters, the director of meteorology for WeatherUnderground.com.
By the time the radiation would reach Hawaii or the western coast of the united States, much of the radiation capable of causing harm to people would have left the atmosphere or turned into precipitation and have been "rained out," Masters wrote in his blog.
Following the 1986 Chernobyl disaster, a cloud of radiation traveled over Europe. The radiation diffused as it traveled by wind and did not cause harm. Those closest to the Chernobyl plant were the ones who suffered the effects of the radiation.
To learn more about nuclear radiation, click here.
ABC News' Martha Raddatz, Luis Martinez, Lauren Pearle, Sunlen Miller and The Associated Press contributed to this report.