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.
That flow of water is usually maintained by a series of pumps and valves that require electricity to operate. The earthquake knocked out the main source of power, and it appears the tsunami flooded backup diesel generators.
That left steam-powered pumps and a backup battery system, which was later aided by the arrival of replacement generators.
Those measures were not enough to prevent dangerous increases in heat and pressure inside the reactor cores. So Tokyo Electric Power, which operates the plant, ultimately began pumping a mixture of seawater and boric acid into the reactor cores.
Boric acid helps to suppress nuclear reactions.
This last-ditch effort using seawater will badly damage the reactors, but it could prevent further melting of the core.
It is likely that the cores of reactors 1 and 3 have partially melted, government officials said. They spoke after instruments detected a type of radioactive material near the plant that can be released when a fuel rod is melting.
A partial meltdown is what occurred at the Three Mile Island nuclear plant in Pennsylvania in 1979. Copyright 2011 National Public Radio.