And the fact that the technology is well tested in Russian ships gives critics little solace, given a long history of spills and accidents involving nuclear-powered submarines and icebreakers operated by the Soviet and Russian navies.

In the 1960s and 1970s, the Soviet Union dumped reactors in the Kara Sea, in the Arctic Ocean north of Kola Bay. Russian nuclear submarines sank in 1989 and 2000, while one Russian nuclear icebreaker caught fire in 2011 and the reactor on another leaked radiation that year, according to Bellona, a Norwegian environmental group.

“The question is, would clients of Russia be comfortable with something like this being parked right at a pier in a major city?” Matthew McKinzie, director of the nuclear program at the Natural Resources Defense Council in Washington, said in a telephone interview.

A Greenpeace sailboat tailed the Akademik Lomonosov on its maiden voyage from a shipyard in St. Petersburg to Murmansk, where it will be fueled, flying a banner in English saying: “Floating Nuclear Reactor? Srsly?”

“Like all designs,” Mr. Buongiorno acknowledged, “nothing is perfect.”