Water containing radioactive material has leaked at an experimental nuclear fuel reprocessing plant in northern Japan, but no radioactivity was released into the atmosphere and no one was exposed to radiation, the operator announced Wednesday.

About 10.5 gallons of water containing plutonium and other radioactive material leaked inside a compound at the plant in Rokkasho on Tuesday, said Yukio Takahashi, spokesman for Japan Nuclear Fuel Ltd. The accident was caused when a robotic arm mistakenly loosened a plug in a container filled with the water, he said.

"It happened inside an enclosed space and there's no danger outside the building," Takahashi said. "Today, it's back to business as usual."

The accident came less than two weeks after the reprocessing plant started test operations after being delayed by an earlier accident and strident public opposition.

The Rokkasho plant, 360 miles northeast of Tokyo, will eventually produce MOX fuel, a uranium-plutonium mixture. The fuel is central to Japan's plans to reduce its dependence on energy imports by building so-called fast-breeder reactors, which produce plutonium that can then be reused as fuel.

Japan's 52 active nuclear plants already supply more than a third of its energy, and the government has said it wants to raise that to nearly 40 percent by 2010.

The country's nuclear power industry, however, has been plagued by safety problems and shutdowns in recent years, including a 1999 reprocessing plant accident outside Tokyo that killed two workers and exposed hundreds to radioactivity.

In 2004, five workers were killed when a corroded pipe at a reactor in western Japan ruptured and sprayed plant workers with boiling water and steam in the country's worst-ever nuclear plant accident.