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.

While more than 70 percent of existing nuclear capacity is located in countries that are part of the Organization for Economic Cooperation and Development, nearly 75 percent of the nuclear reactors currently under construction are in non-OECD countries, and half of those are in China, according to the World Nuclear Association’s recent supply chain report.

As China’s economy has grown, so too has its energy output. China’s total energy output reached 7,600 terawatt hours in 2020, a massive increase from 1,280 terawatt hours in 2000, according to the U.S. Energy Information Administration.

“The primary imperative is to meet what has been a staggering growth in demand over the past twenty years,” John F. Kotek, senior vice president of policy development and public affairs of the nuclear advocacy group, the Nuclear Energy Institute, told CNBC. “So they haven’t just been building a lot of nuclear, they’ve been building a lot of everything.”

Currently, nuclear energy accounts for only 5 percent of the total amount of energy produced in the country, while coal still accounts for about two-thirds, according to the International Energy Agency.

But China’s use of coal to meet its surging demand for electricity has caused a secondary problem: dirty air. “With the huge growth in coal use, along with a dramatic increase in private vehicle ownership, has come a dire need for more clean electricity generation,” Kotek told CNBC.

Nuclear energy generation does not release any of the greenhouse gasses that contribute to air pollution and global warming, so China has turned to nuclear as a way to produce large quantities of clean energy fast.

“The Chinese have been pro-nuclear for a long time, but now they seem to have committed to a truly massive scale up to 150 gigawatts in 15 years. And they seem to be on track to meet that goal,” Buongiorno told CNBC.

“This will be the largest expansion of nuclear capacity in history, by far,” Buongiorno said.

China kickstarted its nuclear program by buying reactors from France, the United States and Russia, Luongo told CNBC, and built primary homegrown reactor, the Hualong, with cooperation with France.

One reason for China’s dominance is the government’s strong control over the energy sector, and most of the economy.

“They built a state-supported, financed industry that allows them to build multiple nuclear units at lower cost,” Luongo told CNBC. “They don’t have any secret sauce other than state financing, state supported supply chain, and a state commitment to build the technology.”

China’s focus on building nuclear energy has global climate benefits, but it also poses ge-political challenges.

“China’s prowess and commitment to nuclear is good for the technology, for China’s energy security, grid stability, economy and air pollution, as well as global climate change mitigation,” Buongiorno said. “If they start to export nuclear technology to other countries, the concern is the geo-political-economic dependence on China that such projects will create for those countries. The same logic applies to Russia.”

To wit, the two large new nuclear reactors at the Vogtle Plant in Georgia have become infamous for taking much longer than initially expected and blowing through early budget estimates.

But the U.S. is making moves to regain its previous dominance in the nuclear space.

“The U.S. has reversed its political opposition to nuclear power at home. It now is a rare issue of bipartisan agreement,” Luongo told CNBC.

A recent survey from the Pew Research Center found support for nuclear energy is up among both Democrats and Republicans: 57 percent of Americans report favoring more nuclear reactors to generate electricity, up from 43 percent of Americans who favored nuclear reactors in 2020.

The U.S. is providing subsidies to keep some existing nuclear plants open, selling some large nuclear reactors to eastern Europe. But the country pinning much of its ambition on scaling up the market for small modular and advanced reactor technology and building the associated fuel enrichment capacity.

“The US may catch up if the new technologies being developed here — small modular reactors and microreactors above all — will prove to be technically and commercially successful, which is currently uncertain,” Buongiorno told CNBC.

Smaller nuclear reactors are less expensive because they are smaller, but also because the modular design allows for component parts to be made in a factory and put together on site. That process is faster and cheaper than building each reactor as a boutique one-off.

The NuScale small modular reactor and Westinghouse AP300 are scaled-down light-water reactors, which is the design most conventional nuclear reactors are using, while some other small modular reactor designs are “more exotically fueled and cooled,” Luongo said, like the TerraPower Natruim Reactor or the X-Energy high-temperature gas cooled reactors.

“The U.S. government is pouring billions of dollars into their development and demonstration in the anticipation that they will work, be less expensive than large reactors, and provide the U.S. with a larger market for their export,” Luongo told CNBC. “We’ll see where we are by 2027 when Congress has mandated the demonstration phase. Delays and cost growth in some technologies are already popping up.”

In addition to being smaller and cheaper to build, small modular reactors are well suited for providing heat for industrial processes, Kotek of the Nuclear Energy Institute told CNBC.

Part of the United States’ attempting to re-ignite its nuclear industry is also its desire to be an exporter of nuclear reactor technology.

“The U.S. has decided that it is at a disadvantage in the nuclear export arena and is trying to reposition itself to be a major competitor in the next 15 years. This began with the Trump administration and Biden has amped it up,” Luongo told CNBC. Some of this export business will be large nuclear reactors, like those being sold to Eastern Europe, but “a significant part of this strategy is small modular and advanced reactors,” Luongo said.

Here, again, the U.S. is up against China.

“China rightly views nuclear energy as a strategic industry. They know that nuclear energy exports help build long-term relationships with partner countries. So they have invested heavily in their domestic nuclear energy capabilities and are now seeking to export their reactor designs to other nations,” Kotek told CNBC. China and Russia both offer “very attractive financing” and other kinds of incentives to spread their nuclear industry aboard, Kotek said.

For the United States to win the export business, it must prove it can put steel in the ground in the United States.

“The U.S. is widely recognized to offer world-leading nuclear energy technology, but having great designs on paper is not enough – most other nations want to see that technology demonstrated before they will consider building it in their country,” Kotek told CNBC. “So the U.S. would be wise to incentivize an accelerated build-out of next-generation nuclear energy systems here at home, so that we’re in a position to take proven designs into the global marketplace and take back our position as the world’s top nuclear energy exporter.”

Jockeying for the top spot in the international nuclear industry is going to get more intense as demand for clean energy continues to climb.

“We and our close nuclear energy allies are at what I think is just the start of a fierce competition for supremacy in global nuclear energy export markets,” Kotek said.