How China became king of new nuclear power, how U.S. could catch up

China is the breakaway global leader in new nuclear construction.

China has 21 nuclear reactors under construction which will have a capacity for generating more than 21 gigawatts of electricity, according to the International Atomic Energy Agency. That is two and a half times more nuclear reactors under construction than any other country.

India has the second largest nuclear buildout right now, with eight reactors under construction that will be able to generate more than six gigawatts of electricity. Third place Turkey has four nuclear reactors under construction with a presumed capacity of 4.5 gigawatts.

The United States currently has one nuclear reactor under construction, the fourth reactor at the Vogtle power plant in Georgia, which will be able to generate just over 1 gigawatt. (For the sake of comparison, a gigawatt is about enough to power a mid-sized city.)

“China is the de facto world leader in nuclear technology at the moment,” Jacopo Buongiorno, professor of nuclear science and engineering at the Massachusetts Institute of Technology, told CNBC.

China is “the determined and pacing leader in global nuclear ambition at the moment,” agrees  Kenneth Luongo, president and founder of the Partnership for Global Security, a nuclear and transnational security and energy policy non-profit. China is “leading, even racing ahead,” Luongo said.

It hasn’t always been that way.

The United States’ existing fleet of nuclear reactors is a testament to its prior dominance.

The United States has 93 nuclear reactors operating with capacity to generate more than 95 gigawatts of electricity, according to the IAEA That is more than any other country by far. Many of those reactors should be viable for some time to come, as nuclear reactors can be licensed to operate for 60 years and in some cases for as long as 80 years, the World Nuclear Association said in a recent report on the nuclear supply chain.

The country with the next most operating nuclear reactors is France, with 56 and a capacity for generating more than 61 gigawatts, according to the IAEA. China comes in third with 55 operating reactors and capacity of over 53 gigawatts.

“It is generally agreed that the U.S. has lost its global dominance in nuclear energy. The trend began in the mid-1980s,” Luongo told CNBC.

China was just getting started as the United States nuclear industry began to take a back seat.

“China began building its first reactor in 1985, just as the U.S. nuclear build-out began a steep decline,” Luongo told CNBC.