A hydrogen explosion occurred at a nuclear reactor at a power plant in northeastern Japan.

The explosion was heard at around 11 a.m. at the Number Three reactor of the Fukushima Daiichi nuclear power plant in Japan's Fukushima Prefecture, which is about 240 kilometers north of Tokyo.

Japanese chief government spokesman Yukio Edano said at an emergency news conference Monday that the reactor container was not breached despite the explosion.

Edano cited as a cause for the blast a build-up of hydrogen in the building around the Number Three reactor. He said, however, that the likelihood of mass radiation leakages is low.

The operator, Tokyo Electric Power Company, said that eleven workers were injured in the explosion while working at the power plant and are now receiving treatment.

The explosion follows a previous nuclear reactor blast at the same plant on Saturday, the day after the devastating earthquake and tsunami struck northeastern Japan.

The Japanese Nuclear and Industrial Safety Agency has advised residents within a 20-kilometer radius of the plant to take shelter indoors to avoid exposure, but added that the area in which residents need to take shelter will not be expanded.