Countries that do not have nuclear weapons will still need lots of electricity, said John Gilleland, chief executive of TerraPower, and “we would like to see them build something that allows us to sleep at night.”

But no one disputes that this is a very long-term bet. Even optimists say it would take until at least 2030 to commercialize the technology. What the competition would look like then — wind, solar, natural gas or some other technology — is not clear. If the idea can be commercialized, it is not even clear that TerraPower could do it first.

The engineers working for Mr. Gates acknowledge the enormous challenges but say they are convinced that he, and they, are chasing the solution not only to energy and weapons proliferation but also to climate change and poverty.

“If you could pick just one thing to lower the price of — to reduce poverty — by far you would pick energy,” Mr. Gates said as he introduced the reactor idea in a speech in 2010. “Energy and climate are extremely important to these people, in fact, more important to them than anyone else on the planet,” he added, referring to killer floods, droughts and crop failures driven by carbon dioxide given off in energy production. He illustrated his talk with a photo of schoolchildren doing their homework under street lamps.

Doug Adkisson, TerraPower’s senior vice president for operations, said Mr. Gates had “a very humanitarian but very cold assessment” about nuclear power and what it could do. What drives him to nuclear power, he said, are the questions “What have you got, and what can you do to raise the living standard of a whole lot of people?”

Despite its difficulties, some outside experts applaud Mr. Gates for trying.

“If you’ve got a huge amount of money, for whatever reasons, you are willing to make a long-term bet, which is not typical of what venture capitalists do,” said Burton Richter, a Nobel laureate in physics. “It’s hard to get a 20-year thing from the standard venture capital world,” he said, adding that financing projects like TerraPower’s is more typical of governments or sovereign wealth funds.