White smoke rose again over a nuclear plant in northeastern Japan Monday after an explosion at a building housing a reactor there, according to CNN.

A buildup of hydrogen in the Fukushima Daiichi plant`s No. 3 reactor building likely caused the blast, authorities said, which injured six people. But the explosion did not damage the reactor or result in significant radiation leakage, Chief Cabinet Secretary Yukio Edano told reporters.

The No. 3 reactor is one of two at the plant where workers have been injecting seawater in a last-ditch effort to cool down fuel rods and prevent a full meltdown after an 8.9-magnitude earthquake and the resulting tsunami Friday disabled cooling systems.

"There is no massive radioactive leakage," Edano said after Monday`s blast.

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.

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 at the Daiichi plant. Authorities have not yet been able to confirm a meltdown, because it is too hot inside the affected reactors to check.

The Tokyo Electric Power Company, which runs the plant, said in a news release 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. 3 reactor began commerical operation five 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 -- 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.

Monitoring of the Daiichi plant has detected several signs that at least a partial meltdown may be occuring, according to Japan`s nuclear safety agency, including high levels of hydrogen inside reactor buildings and radioactive cesium detected outside the plant. This could be caused by the melting of fuel rods inside the reactor, experts said.

Despite such evidence, Noriyuki Shikata, a spokesman for Japan`s prime minister, said Sunday that he would not describe what was occurring in the reactors as a "meltdown," adding that the situation was "under control."

But Cooke, also editor of Nuclear Intelligence Weekly for the atomic-energy community, said she`s not convinced.

"The more they say they`re in control, the more I sense things may be out of control," she said.