There is enough energy in the nuclear waste in the United States to power the entire country for 100 years, and doing so could help solve the thorny and politically fraught problem of managing spent nuclear waste.

That’s according to Jess C. Gehin, an associate laboratory director at Idaho National Laboratory, one of the government’s premier energy research labs.

The technology necessary to turn nuclear waste into energy is known as a nuclear fast reactor, and has existed for decades. It was proven out by a United States government research lab pilot plant that operated from the 1960s through the 1990s.

For political and economic reasons, the technology has never been developed at commercial scale. Today, there’s an increased urgency to address climate change by decarbonizing out energy grids, and nuclear power has become part of the clean energy zeitgeist. As a result, nuclear fast reactors are once again getting a serious look.

“It feels like it’s real — or realer — than it has ever has been to me,” said Brett Rampal, a nuclear energy expert at Segra Capital Management and Veriten. He did his senior project at the University of Florida on the subject in 2007 and remembers his professors arguing about the future of the technology even then.

There are 93 commercial nuclear reactors at 55 operating sites in the United States, according to Scott Burnell, spokesperson for the Nuclear Regulatory Commission. Twenty-six are in some stage of decommissioning process. All of the nuclear reactors that operate in the U.S. are light-water reactor designs, Burnell told CNBC.

In a light-water reactor, uranium-235 fuel powers a fission reaction, where the nucleus of an atom splits into smaller nuclei and releases energy. The energy heats water, creating steam which is used to power a generator and produce electricity.

The nuclear fission reaction leaves waste, which is radioactive and has to be maintained carefully. There are about 80,000 metric tonnes of used fuel from light-water nuclear reactors in the United States and the existing nuclear fleet produces approximately an additional 2,000 tons of used fuel each year, Gehin told CNBC.

But after a light-water reactor has run its reactor powered by uranium-235, there is still tremendous amount of energy potential still available in what is left.

“Fundamentally, in light-water reactors, out of the uranium we dig out of the ground, we use a half a percent of the energy that’s in the uranium that’s dug out of the ground,” Gehin told CNBC in a phone interview. “You can get a large fraction of that energy if you were to recycle the fuel through fast reactors.”

Fast reactors don’t slow down the neutrons that are released in the fission reaction, and faster neutrons beget more efficient fission reactions, Gehin told CNBC.

“Fast neutron reactors can more effectively convert uranium-238, which is predominantly what’s in spent fuel, to plutonium, so you can fission it,” Gehin said.

The technology for fast nuclear reactors has exited for more than fifty years. A fast reactor plant called the Experimental Breeder Reactor-II (EBR-II), began construction in 1958 and operated from 1964 to 1994, until Congress shut down funding.

“We ran the EBR II reactor out at the site for 30 years, recovered uranium, put it back in the reactor,” Gehin told CNBC. “It’s been proven that it can be done. The trick would be going to commercial scale to ensure that it is done economically. It’s very safe technology. All the basis for the technology has been proven.”

While a fast reactor will reduce the amount of nuclear waste, it does not eliminate it entirely.

“There would still be waste that would have to be disposed, but the amount of long-lived waste can be significantly reduced,” Gehin said.

In the middle of the last century, nuclear energy was seen as a solution to the eventual exhaustion of limited fossil fuel supplies.

At the same time, there were concerns that there would not be enough uranium to fuel the conventional nuclear reactors that the United States would need. Fast reactors were developed as a solution to both problems: They create large amounts of energy and use only minimal amounts of uranium fuel, Gehin told CNBC.

But things changed. “We started discovering there’s actually quite a bit of uranium. And so there wasn’t such a need to use it as as effectively,” Gehin said.

Then, nuclear energy as a whole started falling out of favor, largely because of the nuclear accident at Three Mile Island in Pennsylvania in 1979, Gehin said.

In addition, economics were a factor. Coal, and later natural gas, remained abundant and cheap. Fast reactors were generally thought to be more expensive than traditional light-water reactors, said Gehin, making it an unattractive area for investment.

“The development of the first commercial fast reactors in the U.S. also suffered from cost overruns,” Gehin said.

Fast forward to 2022. With energy prices spiking thanks to Russia’s war in Ukraine, and with the growing public cry to move toward sources of energy that don’t emit planet-warming greenhouse gases, nuclear power is getting another look. At the same time, innovators are looking at redesigning fast reactor technology to make it more cost-effective, Gehin said.

Currently, Russia is the only country producing electricity with fast reactor technology. India and China have plans to build out commercial fast reactors in the future.

In 2019, the U.S. Department of Energy announced it was building its own fast-spectrum test reactor, the Versatile Test Reactor, but it was not funded in the fiscal year 2022 omnibus funding bill. By not having a pilot test facility in the U.S. for almost 30 years, the U.S. is “effectively yielding leadership to Russia, China, and India who have this critical capability,” the Office of Nuclear Energy said in a written statement May.

While the government is moving slowly, start-ups Oklo and TerraPower and energy giant Westinghouse are working on fast reactor technologies.

Even as private companies are working to innovate and commercialize fast reactor designs, there are significant infrastructure hurdles.

Before nuclear waste can be used to power fast reactors, it has to go through reprocessing. Right now, only Russia has the capacity to do this at scale. France, too, has the capacity to recycle used nuclear waste, Gehin said, but the country generally takes its recycled fuel and puts it back into existing light water reactors.

For now, the Idaho National Lab can reprocess enough fuel for research and development, Gehin told CNBC, but not much more.

Private companies commercializing fast reactor technology are pushing for domestic fuel supply chains to be developed. TerraPower says it’s investing in supply chains and working with elected leaders to build political support, while Oklo has received three government awards and is working with the government to commercialize fast reactor fuel supply chains domestically.

The other option to power fast reactors is to create HALEU fuel, which stands for high-assay low-enriched uranium, from scratch, rather than by recycling nuclear waste. (Where conventional reactors use uranium enriched up to 5%, HALEU is uranium enriched up to 20%.)

It’s arguably easier to produce HALEU directly than by recycling spent waste, says Gehin, but ultimately, the cheaper option will win out. “It will be largely be driven by what makes sense economically.” Regardless, Russia is the only country that has the capacity to make HALEU at commercial scale.

Oklo CEO and co-founder Jacob DeWitte says he’s bullish on recycled fuel, even if it comes after commercial-scale HALEU production.

“This looks quite promising to be economically more attractive than fresh fuel,” DeWitte told CNBC. “This process works using electrorefining to electrochemically recycle the transuranics and uranium in the waste into feed material for fuel. We aim for this facility will be operational in the latter part of the decade.”