"When this crisis started we compared it to what Americans are familiar with the 1979 Three Mile Island crisis and the 1986 Chernobyl crisis," said Joe Cirincione, a nuclear policy expert who is president of the Ploughshares Fund and an ABC News consultant. "This is way past Three Mile Island and we are heading into Chernobyl territory."
Just 50 of the plant's 800 workers remained at the plant Tuesday, fighting to keep the reactors cool by pumping sea water into them. Officials ordered most of the workers to leave the plant after the initially high levels of radiation were reported.
The explosion at Unit 2 has experts worried because the roof of Unit 2 did not suffer damage. In two previous hydrogen explosions at the plant, the roofs of those reactors were partially blown apart, giving an indication that the reactor's inner containment vessel remained intact and the radioactive fuel rods were still protected.
It's now believed the trapped pressure cracked Unit 2's containment vessel around the reactor's core, allowing radioactive material to seep out.
"What we're seeing is unprecedented in nuclear power history. It's made worse by way the Japanese build their reactors, they cluster them together. ... Many of the plants in Japan have four or more units, it's very efficient, so what it means is a disaster at one can avalanche into a complicated disaster next door," said Cirincione.
Following the explosion of unit 2, a fire broke out in the fuel pond of unit 4, a reactor that had not been of concern to officials. The fire was contained, but now radiation is believed to be leaking from that unit too.
"It's just as dangerous as the reactor core and they have the same kind of water level problems. If you can't cover them with water, those fuel rods will catch fire and they are even more radioactive," Cirincione said.
"If I had the ear of a Japanese prime minister," said Michiko Kaku, a theoretical physicist at the City University of New York, "I would suggest that he exercise the Chernobyl option, that is, put the Japanese Air Force on standby, assemble a fleet of helicopters, get sand, concrete and sandbag these reactors like what they did at Chernobyl.
"Sandbag the reactors ... entombing it in a sarcophagus of concrete that was the last resort that the Soviets used in 1986 and the Japanese may have to use that final option."
Hours after the explosion and fire at Daiichi, elevated levels of radiation were detected in Tokyo, 175 miles away, though government officials said there was no health risk there.
Naval personnel on the USS George Washington, a U.S. aircraft carrier assisting in the recovery efforts, detected low levels of radiation, prompting military personnel to take precautions that included limiting outdoor activity.
Radiation was detected Monday by U.S. helicopters flying 60 miles from the Japanese shore, prompting the repositioning of the USS Ronald Reagan and seven other U.S. Navy ships, military officials said.
On Monday, the Japanese government formally asked the U.S. Nuclear Regulatory Commission and the IAEA for help in stabilizing its troubled nuclear reactors in the wake of the country's massive 9.0 earthquake and tsunami.
Officials from the Nuclear Regulatory Commission said that Americans should not be concerned about the spread of radiation to the West Coast of the United States and Hawaii.