Another reactor explosion ...

(CNN) -- Fresh white smoke rose again Monday from Japan's Fukushima Daiichi nuclear power plant, caused by an explosion at a building tied to the facility's No. 3 reactor.
Japan's Chief Cabinet Secretary Yukio Edano said that, according to the head of the nuclear facility, the container vessel surrounding the reactor is still intact. Initial reports suggest that radiation levels rose following the explosion late Monday morning, but Edano said he does not believe there has been a massive leak.
"We are now collecting information on the concentration of radiation," he said.

A wall of the building collapsed due to the blast, according to Japanese public broadcaster NHK, which showed plumes of smoke above the plant.
The secretary said that water continues to be injected into the plant's No. 3 reactor. That fact, and the pressure levels, has led authorities to believe that the reactor itself remains intact.

The incident is the latest affecting the Daiichi, the hardest hit of several nuclear plants affected since Friday's 8.9-magnitude earthquake and subsequent tsunami.
Officials said that the explosion was likely caused by a buildup of hydrogen gas, similar to what had happened Saturday at the same nuclear plant's No. 1 reactor.

The 600 residents remaining within 20 kilometers (12 miles) of the plant, despite an earlier evacuation order, have been ordered to stay indoors, according to Edano.
Officials earlier said that they were operating on 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 press release late Sunday that radiation levels outside that plant remain high.

Japan's Kyodo newsagency, 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.


On Sunday, Chief Cabinet Secretary Yukio Edano said accumulating hydrogen gas "may potentially cause an explosion" in the building housing the No. 3 reactor at the Daiichi plant.
At another plant, in Onagawa, authorities early Sunday noted high radiation levels. The International Atomic Energy Agency said later -- using information from officials in Japan -- levels returned to "normal" and found no emissions of radioactivity" from Onagawa's three reactors.


"The current assumption of the Japanese authorities is that the increased level may have been due to a release of radioactive material from the Fukushima Daiichi nuclear power plant" located 135 kilometers (about 85 miles) north of Onagawa, the agency said.


Japan's nuclear and industrial safety agency said late Sunday the wind was moving north/northwest -- which could carry airborne radioactive material near the city of Sendai and toward the Onagawa facility.