A report from Japan's Atomic Energy Agency said the China Experimental Fast Reactor stopped generating electricity in October following an accident, Britain's The Daily Telegraph reported Friday.
Concerns of possible radiation leaks affecting Japan or South Korea were made worse by Beijing's failure to report the accident or release details of what occurred, a Tokyo newspaper citing the Japanese report said.
Chinese officials issued a denial of an accident or any coverup.
"CEFR hasn't been operating since July last year so reports that an accident occurred in the autumn are extremely inconsistent with the facts," Wan Gang, director of the China Institute of Atomic Energy, told Chinese media.
While China has never suffered a major nuclear accident, there have been small leaks of radiation from some of its nuclear power stations, the most recent in May 2010 in Shenzhen in southern China's Guangdong province.