For years at global climate summits, nuclear energy was seen by many as part of the problem, not part of the solution.

Sama Bilbao y Leon has been attending the annual United Nations climate change talks since 1999, when she was a student of nuclear engineering. And for most of that time, she said, people didn’t want to discuss nuclear power at all.

“We had antinuclear groups saying, ‘What are you doing here? Leave!’” she said.

These days, it’s a very different story.

At last year’s climate conference in the United Arab Emirates, 22 countries pledged, for the first time, to triple the world’s use of nuclear power by midcentury to help curb global warming. At this year’s summit in Azerbaijan, six more countries signed the pledge.

“It’s a whole different dynamic today,” said Dr. Bilbao y Leon, who now leads the World Nuclear Association, an industry trade group. “A lot more people are open to talking about nuclear power as a solution.”

The list of countries pledging to build new nuclear reactors, which can generate electricity without emitting any planet-warming greenhouse gases, includes longtime users of the technology like Canada, France, South Korea and the United States. But it also includes countries that don’t currently have any nuclear capacity, like Kenya, Mongolia and Nigeria.