China's nuclear safety agency had ordered staffers to be prepared for an environmental emergency the day after a massive earthquake jolted a region that includes several key atomic sites.

France's nuclear watchdog has said some of China's nuclear facilities sustained minor damage in Monday's magnitude 7.9 earthquake, though no Chinese government Web sites viewed Saturday mentioned any damage.

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China's Nuclear and Radiation Safety Center, part of the Ministry of Environmental Protection, activated emergency plans the day after the quake and told all emergency personnel to be on standby in case of nuclear accidents, the center said in an announcement on its Web site.

Officials were in close contact with safety stations throughout the region and were monitoring operational data from nuclear power plants, the undated announcement said. The safety of drinking water was a top priority.

"With the deepening of the relief work, the main task is to prevent secondary environmental disasters and guarantee the safety of the environment in disaster areas," the Ministry of Environmental Protection said in a separate statement Friday.

The French Institute for Radiological Protection and Nuclear Safety said Chinese authorities "reacted well" to the quake and immediately shut down nuclear sites for inspection.

Thierry Charles, the group's director of plant safety, said China's nuclear safety agency, NNSA, had reported no leaks of radioactivity since the quake.

He said Friday the Chinese reported "light damage" to older nuclear facilities that were being dismantled before the quake, noting that seismic construction codes were less strict when those sites were built. China did not specify which facilities had damage, he said.

Phones calls to China's Ministry of Environmental Protection and its Nuclear and Radiation Safety Center went unanswered Saturday. A man who answered the phone at the ministry's Nuclear Safety Department said he had no information.

China has a research reactor, two nuclear fuel production sites and two atomic weapons sites in Sichuan province, where the quake struck, the French agency said. All were 40 to 90 miles from the epicenter.

French authorities do not yet have a full picture of any possible damage at the nuclear weapons sites, where information is more closely guarded, Charles said.

He said he didn't think there were any leaks because it would have been reported and the worst concern was the degradation of buildings.

Nuclear experts said there were several possibilities if any significant damage occurred at the plants, at least one of which is alongside a river. A radioactive leak could cause environmental harm, while internal damage could set back China's nuclear modernization, they said.

Mianyang, an industrial city of 700,000 people that is the headquarters of China's nuclear weapons design industry, was in the disaster area.

A switchboard operator at the site, which has been likened to the U.S. nuclear facility at Los Alamos, N.M., said Saturday that people there were at work.

China's largest plutonium production reactor is also in the quake zone at Guangyuan.

Hans Kristensen, a nuclear arms expert at the Federation of American Scientists, said the Guangyuan reactor is "at the center of China's fissile material production" and damage "would disrupt China's warhead maintenance capabilities.

Matthew Bunn, a senior researcher at Harvard University's Project on Managing the Atom, said the risk of radioactive leaks depended mostly on how the facilities were designed, details of which are known only by the Chinese government.