“The floating nuke accident scenario also carries with it the potential for molten parts of the reactor core burning through the bottom of the barge to reach the water below,” Mr. Lochbaum wrote in an email. “The water is good for cooling, but not good for containment.”

In the 1970s, an industry consortium called Offshore Power Systems also had plans for floating nuclear power plants in the United States. An electric utility had ordered a plant that would have been moored outside Atlantic City. The facility, which would have been built in Florida, was canceled, Mr. Lochbaum said.

The Army’s floating nuclear reactor in the Panama Canal Zone provided power during the late 1960s and into the 1970s to the grid in what was then United States territory. But China would be placing floating atomic power stations at islands that until recently did not exist in seas claimed by several nations.

The artificial islands built by the Chinese in the past two years in disputed waters of the South China Sea have stoked tensions with neighbors, especially the Philippines and Vietnam, and prompted the United States to assert its right to transit the area freely by sailing navy ships close to the islands, often shadowed by Chinese vessels.

In February, the Center for Strategic and International Studies in Washington reported that the Chinese were probably building radar facilities on several of the islands. That is in addition to airstrips capable of handling large jets like one on Fiery Cross Reef, harbors to berth seagoing vessels, lighthouses and large buildings such as barracks.

All that requires electricity, provided now by generators and augmented by limited solar and wind power, said Gregory B. Poling, director of the Asia Maritime Transparency Initiative at C.S.I.S.

Mr. Poling said it was too soon to tell how a possible deployment of the floating nuclear power stations would play out in the complicated politics of the South China Sea, though he said it was “potentially worrisome.”