China is not a major customer for American nuclear technology; only about $170 million in nuclear-related sales went to Chinese customers last year. But the announcement on Thursday amounted to a significant setback in cooperative agreements that have had a checkered history since the Reagan administration.

The first openings, noted Jeffrey A. Bader, the senior Asia specialist on President Barack Obama’s National Security Council, came as an incentive to stop Chinese cooperation with Pakistan, which was building its first nuclear weapons. Trade stopped after the Tiananmen Square uprising of 1989, and the Clinton administration had a major standoff with the Chinese over their help to Iran.

But those issues were resolved, usually amid warnings from the American nuclear industry that failing to deal with China on nuclear issues was to open the way for France, Japan and South Korea to supply Beijing with nuclear technology. But today, Mr. Bader noted, “there is not much left in the American nuclear industry to harm, so the administration can indulge their most extreme-case suspicions.”

There is little question that the Chinese have sought to steal nuclear-related technology: The United States has indicted Chinese hackers and insiders in a variety of cases that involved nuclear technology.

Last year, Szuhsiung Ho, an American citizen born in Taiwan, was sentenced to two years in prison for organizing American nuclear experts to help China develop nuclear materials. That indictment named the China General Nuclear Power Company, the largest Chinese nuclear producer, which was seeking American designs for key components for nuclear reactors.

The officials who briefed reporters, who came from three separate United States agencies with oversight of nuclear materials and technology, cited that case and others like it as they explained a new policy under which there would be a “presumption of denial” for Chinese licenses for certain advanced nuclear technologies. Older-style reactors, and common nuclear-related components, especially those widely available on global markets, will not be subject to the new restrictions.

In a speech in July, Christopher Ford, the senior State Department official for nuclear issues, said that the “hard question” in dealing with China is to what extent the United States can participate in its growing nuclear market “without providing China with technological tools that will help it achieve its goal of seizing a geopolitical role for itself that displaces U.S. influence.”