Deals announced by Google and Amazon

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last week to power their artificial-intelligence businesses with mini nuclear plants mark a new frontier for so-called small-module reactors. The planned new generation of compact power units are faster and less expensive to manufacture than conventional ones, and are simpler to operate, their advocates say.

But the announcements may complicate an already vexing question that has bedeviled the industry since the dawn of the atomic age: what to do with the unending stream of spent fuel and other radioactive waste that are the byproduct of nuclear power.

The U.S. has so far failed in its decadeslong effort to build an underground repository for reactor waste to be stored in perpetuity, leaving it instead to collect on the grounds of reactor complexes.

Some experts who have studied designs for the small-module reactors, or SMRs, say they will produce more potent waste than their larger-scale older siblings—and more of it. They question whether SMRs’ spent fuel can be safely stored at the aboveground reactor sites.

A report last year by National Academies of Science, Engineering and Medicine cited a finding that units planned by the nuclear company working with Amazon, X-Energy, would produce almost 24 times as much spent fuel per year as many existing plants for an equivalent amount of energy.

“It’s irresponsible to go forward and build a whole lot more nuclear power without solving the waste problem,” says Allison Macfarlane, who served as chairwoman of the U.S. Nuclear Regulatory Commission during the Obama administration.

Amazon Web Services chief executive Matt Garman told Barron’s that he hadn’t studied the waste implications of his company’s deal for nuclear power involving X-Energy.

X-Energy’s chief executive, J. Clay Sell, says his company has estimated that its units would produce less waste than the nation’s existing fleet of full-size reactors.

“The waste issue is important,” he says. “But it’s not something that we materially change.”

Google, which is a unit of Alphabet

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, didn’t respond to a message from Barron’s seeking comment about the waste impacts of its deal with San Francisco Bay Area-based Kairos Power.

Kairos spokeswoman Ashley Lewis said in a statement that waste from its reactors can be safely handled using established methods.

“The U.S. nuclear industry has safely managed used fuel for decades at sites nationwide,” she said “Kairos Power will use the same proven methods to store spent fuel securely until the United States finalizes a specific strategy for management and ultimate disposal.”

When spent fuel is removed from a nuclear plant’s reactor core, it spends several years cooling off in pools of water until it can safely be transferred into storage canisters known as dry casks. The casks were designed to serve as an interim method for storing spent fuel on plant property until its disposal.

Under 1980s legislation, the Energy Department was made responsible for that disposal step. It was instructed to develop a geologic repository, a deep-underground pit where spent fuel and other waste could be held in perpetuity. Nuclear waste can retain hazardous levels of radioactivity for thousands of years.

The department started collecting a fee from plant operators to handle the disposal work in 1998, but it never began picking up the reactors’ radioactive trash because it had no place to take it.

Work on a geological repository in Nevada’s Yucca Mountain region, about 100 miles northwest of Las Vegas, was defunded in 2010 by then-President Obama at the urging of late Nevada Sen. Harry Reid. By then, the government had spent $9 billion on community outreach, engineering studies and other planning steps for the project.

Without an underground option, the dry casks that were designed for interim use have effectively become the industry’s permanent storage solution for nuclear waste.

Macfarlane, who now directs the School of Public Policy and Global Affairs at the University of British Columbia, says that strategy is unsustainable. The casks have an uncertain life span and require continued maintenance from plant operators, even in the cases where their plants were long-ago decommissioned, she says.

As of the end of 2022, there were more than 90,000 metric tons of spent nuclear fuel in cooling pools and casks across more than 100 sites in 39 states, according to the Department of Energy. That exceeds the capacity of some large transcontinental container ships.

As of September 2023, the federal government had paid about $10.6 billion in judgments and settlements for court claims over its failure to handle plants’ radioactive waste, according to the Energy Department.

“It’s frustrating that there’s been no movement on the permanent disposal,” says Dale Klein, a professor of nuclear engineering at the University of Texas at Austin who worked on planning for the Yucca Mountain site as NRC chairman under President George W. Bush.

An Energy Department spokesperson had no immediate comment.

The need for spent fuel storage appeared to lose urgency in recent years, as nuclear power took a back seat to newly plentiful natural gas and greener-seeming renewable sources. The 2011 Fukushima nuclear disaster in Japan reinforced overall safety concerns about atomic power, resulting in delays and uncertainties surrounding new plant projects.

But that trend could now be reversing, with the AI requirements of tech firms—along with the needs of increasingly electrified homes and cars—stoking demand for emissions-free generating capacity, says David Brown, a director with energy consultancy Wood Mackenzie.

Atomic power also got a boost from last year’s $2.7 billion congressional appropriation for the domestic enrichment of nuclear fuels, including the “high-assay low-enriched uranium” type to be used by advanced reactors such as the small-module units, says Brown.

The new outlook for nuclear power began coming into focus in September, when Microsoft

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announced that it had signed a 20-year deal with Constellation Energy

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to purchase power from the currently shuttered Three Mile Island nuclear plant.

Then, last week, both Google and Amazon announced that they would work with small-module reactor developers to help bring the new technology to fruition.

Google said it aimed to begin drawing power from Kairos-designed SMR units starting in 2030, while Amazon announced an agreement to join in a $500 million financing round for X-Energy to develop its next-generation SMR units and fuel technology.

Klein, the University of Texas professor, says the commercialization of SMRs will inevitably result in more waste, but that the on-site cask-based storage should be able to accommodate that growth for the foreseeable future.

Others are less certain.

According to a 2021 paper co-written by Macfarlane, the compact size of the small modular reactors may translate into more – and more hazardous—waste than what has been generated by existing, full-size units, which could strain operators’ ability to manage their storage on-site.

The authors of the paper, published in the Proceedings of the National Academy of Sciences, didn’t specifically examine the Kairos and X-Energy reactors, but those models share relevant characteristics with the units that were part of their survey.

The potential disposal complications stem from design and operational adjustments that SMR builders plan to implement to compensate for fuel-efficiency losses that are inherent to smaller reactor vessels.

One of these adjustments involves the use of more highly enriched fuel than what powers the pressurized water reactors that make up most of the nation’s existing nuclear fleet.

Because of those higher enrichment levels, spent fuel from SMRs may have concentrations of certain isotopes that make dangerous-if-uncontrolled “recriticality” reactions “a significant risk,” the paper’s authors wrote.

Spent fuel from the reactors surveyed by the paper’s authors, which include models planned by NuScale, Toshiba, and Terrestrial Energy, wouldn’t just be potentially less stable. The reactors would also produce up to 5.5 times as much spent-fuel volume as do existing pressurized water reactors, they wrote.

The separate 2023 report by the National Academies of Science, Engineering and Medicine, meanwhile, featured an analysis of the graphite-wrapped fuel pebbles that Kairos and X-Energy plan to use in their SMRs.

It cited a finding that X-Energy’s Xe-100 reactor would produce almost 160 cubic meters of spent pebbles per year to generate the same amount of energy as a large pressurized water reactor yielding 6.8 cubic meters of spent fuel. The study featured no comparable estimate for Kairos’s unit.

Because of limitations that come with their smaller size, the SMRs require additional components that are exposed to radiation, increasing the need for future permanent storage, according to the authors of the 2021 study.

They estimated that the SMRs they examined would generate up to 35 times more such contaminated structural material as a pressurized water reactor producing a similar amount of energy.

“We’ve been working on the waste problem for so many decades and it’s not solved,” says Macfarlane. “Maybe it’s not solvable.”

Avi Salzman contributed reporting to this article.

Write to Jacob Adelman at jacob.adelman@barrons.com