A second explosion occurred Monday at a nuclear power plant in Fukushima, Japan, and officials say three reactors at the plant are now having cooling problems in the wake of Friday's devastating earthquake and tsunami.
The latest blast blew the roof off a structure built around the 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 second layer of protection against the release of radioactive material.
It's unlikely that the explosion released large amounts of radiation, said Yukio Edano, Japan's chief Cabinet secretary.
The blast affected reactor No. 3, one of six reactors at the Fukushima Daiichi plant about 150 miles north of Tokyo. A similar explosion occurred Saturday at reactor No. 1, and apparently did release some radioactive material.
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.
When the earthquake hit Friday, the reactors automatically shut down. But even after the nuclear reaction stops, some radioactive materials continue to produce heat and require a constant flow of cooling water to prevent the core from melting.
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