Another reactor at Japan's Fukushima Daiichi nuclear plant lost its cooling capabilities Monday, a government official said.
The problem was detected in the plant's No. 2 reactor Monday afternoon after an explosion rocked the building containing the plant's No. 3 reactor, Chief Cabinet Secretary Yukio Edano told reporters.
"We think that the hydrogen explosion in (the building housing) reactor No. 3 caused the cooling system of reactor No. 2 to stop working," Edano said.
Water levels were falling and pressure was building up inside the No. 2 reactor, he said, and officials were working on a plan to release gas and also inject seawater into that reactor.
Workers have been injecting seawater in a last-ditch effort to cool down fuel rods and prevent a full meltdown at two other reactors at the plant -- No. 1 and No. 3 -- after an 8.9-magnitude earthquake and ensuing tsunami Friday knocked out the reactors' cooling systems.
Japanese officials have said that they are operating under the presumption that there may be a partial meltdown in the No. 3 and No. 1 nuclear reactors. Authorities have not yet been able to confirm a meltdown, because it is too hot inside the affected reactors to check.
There are six reactors at Tokyo Electric Power Company's Fukushima Daiichi plant, located in northeastern Japan about 65 km (40 miles) south of Sendai.
A buildup of hydrogen in the Fukushima Daiichi plant's No. 3 reactor building caused Monday's blast, authorities said, which injured 11 people and sent white smoke billowing above the nuclear plant.
But the explosion did not damage the reactor or result in significant radiation leakage, Edano told reporters.
The explosion blew away the roof and walls of the building housing the reactor, Japan's Kyodo News reported. A similar blast occurred Saturday at the plant's No. 1 reactor.
On Sunday, Edano warned that the same sort of explosion could occur in the No. 3 building.
After Monday's blast, authorities ordered at least 500 residents remaining within 20 kilometers (12 miles) of the plant to stay inside, Edano said. About 200,000 people evacuated the area over the weekend after a government order.
"There is no massive radioactive leakage," Edano said.
Even so, public concern over the possibility remained rampant Monday -- even outside the evacuation zone.
"Everyone is talking about it. There seems to be a real fear about this, an anxiety about it," said CNN's Dr. Sanjay Gupta, reporting from a school serving as an evacuation center about 100 kilometers (62 miles) away from the Fukushima Daiichi plant.
Low levels of radiation were detected at least as far as 100 miles northeast of the plant, according to the U.S. Navy, which repositioned ships and planes after detecting low-level "airborne radioactivity."
The Navy's statement, however, provided some perspective, noting that the maximum potential radiation dose received by any ship personnel when it passed through the area was "less than the radiation exposure received from about one month of exposure to natural background radiation from sources such as rocks, soil, and the sun."
The Tokyo Electric Power Company said in a statement late Sunday that radiation levels outside that plant remained high.
Kyodo, citing the same company, said that there were measurements of 751 microsieverts and 650 microsieverts of radiation early Monday. Both are above the legal limit, albeit less than one reading recorded Sunday.
A microsievert is an internationally recognized unit measuring radiation dosage, with people typically exposed during an entire year to a total of about 1,000 microsieverts.
Authorities early Sunday noted high radiation levels at another plant, located 135 kilometers (85 miles) away in Onagawa. The International Atomic Energy Agency later said that Japanese officials reported that levels had returned to "normal." It also said the increase detected earlier "may have been due to a release of radioactive material from the Fukushima Daiichi nuclear power plant."
Most experts aren't expecting a reprise of the 1986 Chernobyl meltdown, which killed 32 plant workers and firefighters in the former Soviet Union and at least 4,000 others from cancers tied to radioactive material released by the plant.
Analysts said Japan's crisis is unique.
"This is unprecedented," said Stephanie Cooke, the author of "In Mortal Hands: A Cautionary History of the Nuclear Age." "You've never had a situation with multiple reactors at risk."
Japan's 54 nuclear reactors provide about 30% of the country's electricity, according to the World Nuclear Association.
Daiichi's No. 1 reactor -- the oldest of six boiling-water units at the site, according to the nuclear association -- began commercial operation in March 1971. The No. 2 reactor began commercial operation in 1974, and the No. 3 reactor followed two years later.
"Nuclear facilities in Japan ... were built to withstand earthquakes -- but not an 8.9 earthquake," said James Walsh, a CNN contributor and research associate at MIT's security studies program.
The crisis has stoked fears of a full-on nuclear meltdown, a catastrophic failure of the reactor core that has the potential for widespread release of radiation.
Officials are working to prevent such a calamity by injecting seawater and boron into the affected reactors -- even though salt and boron will corrode the reactors, rendering the Daiichi plant inoperable.
"Essentially, they are waving the white flag and saying, 'This plant is done,'" Walsh said. "This is a last-ditch mechanism to try to prevent overheating and to prevent a partial or full meltdown."
The situation -- part of what Prime Minister Naoto Kan called the "toughest and most difficult crisis for Japan" since the end of World War II -- has national and global repercussions as authorities and scientists debate the dangers of nuclear power.
Cooke said that it may take years to fully assess the damage at Japan's worst-hit reactors, much less to get them working again. And authorities may never definitively determine how much radiation was emitted, or how many got sick because of it.
If the effort to cool the nuclear fuel inside the reactor fails completely -- a scenario that experts who have spoken to CNN say is unlikely -- dangerous amounts of radiation could be released into the atmosphere or water. That could lead to widespread cancer and other health problems, experts say.
Authorities have downplayed such a scenario, insisting the situation appears under control and that radiation levels in the air are not dangerous.
The Daiichi plant has a containment vessel, which theoretically would capture radioactive material if a full meltdown occurs.
Edano has said there have been no leaks of radioactive material at any plants. Radioactive steam has been released intentionally to lessen growing pressure in the two Daiichi reactors -- in an amount authorities have described as minimal.
Despite such evidence, Noriyuki Shikata, a spokesman for Japan's prime minister, said Sunday the situation was "under control." His remarks came before Monday's explosion.
Cooke, also editor of Nuclear Intelligence Weekly for the atomic-energy community, said she's not convinced the Japanese are in control.
