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Japan Nuclear Plant Hit By Second Blast

A screen grab taken from news footage by Japanese public broadcaster NHK show...

Credit: AFP/Getty Images

Above: A screen grab taken from news footage by Japanese public broadcaster NHK shows the moment of a hydrogen explosion at the Fukushima No. 1 nuclear power station number three reactor on Monday.

A second explosion occurred Monday at a nuclear power plant in Fukushima, Japan, and officials now say three reactors at the plant may have experienced a partial meltdown.

Attention is focused now on the situation at the Unit No. 2 reactor. Officials from the Tokyo Electric Power Company say the fuel rods were exposed at some point Monday and a partial meltdown may have occurred. Workers are pumping in seawater into all three troubled reactors in an attempt to cool the nuclear cores.

Monday's blast blew the roof off a structure built around the No. 3 reactor but did not harm the reactor itself or its so-called containment vessel, government officials said.

The containment vessel is a heavily reinforced barrier that surrounds the reactor's core and provides a another layer of protection against the release of radioactive material.

Because the containment structure remains intact, "we believe that the risk that large amounts of radioactive materials have been dispersed is low," said Yukio Edano, Japan's chief cabinet secretary.

The Unit No. 3 reactor is one of six reactors at the Fukushima Daichi plant about 150 miles north of Tokyo. A similar explosion occurred on Saturday at Unit No. 1, and apparently did release some radioactive material.

"We expect an increase in radiation levels similar to the one that followed the hydrogen explosion that occurred in the Unit 1 reactor," Edano said.

Officials say radiation has also been released by periodic venting of steam from the cores of reactors 1 and 3.

Some workers at the plant have apparently been exposed to worrisome levels of radiation. But Edamo said the radiation releases so far should not endanger the public.

Both blasts were blamed on explosive hydrogen gas coming from the reactor cores.

Hydrogen can be produced when a reactor's core overheats, damaging the zirconium tubes that hold the nuclear fuel. Officials say both explosions occurred after hydrogen from the core leaked into the space outside the containment vessels but inside the building that houses them.

Reactors at the Daichi plant have been plagued with cooling problems since the devastating earthquake and tsunami struck Friday.

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