The Study That Said Small Reactors Mean More Nuclear Waste, and the Lab That Disagreed

On October 15, 2021, Lindsay Krall, a researcher studying nuclear waste, brought an estimate of small modular reactor waste to a National Academies audience in a slide deck titled “Nuclear waste from small modular reactors”. One slide listed three small modular reactors (SMRs): a 160 MWth NuScale integral pressurized water reactor, Terrestrial Energy’s 400 MWth integral molten salt reactor (IMSR), and Toshiba’s 30 MWth 4S. The yardstick for all three was a conventional pressurized water reactor rated at 3,400 MWth.

The work appeared in the Proceedings of the National Academy of Sciences (PNAS) on May 31, 2022, as “Nuclear waste from small modular reactors,” by Krall, a former MacArthur postdoctoral fellow; Allison Macfarlane, who had been chairperson of the U.S. Nuclear Regulatory Commission from 2012 to 2014 and was a professor and director at the University of British Columbia at the time of publication; and Rodney Ewing, the Frank Stanton Professor and co-director of Stanford’s Center for International Security and Cooperation (CISAC). Their abstract compared the three designs with an 1,100-megawatt-electric pressurized water reactor, the same reference plant as the slides, expressed in electric rather than thermal output. It reported increases in waste volume across a range of 2 to 30 times, and it stated that range without tying it to the three reactors studied.

That missing qualifier is the thread of the story. A result drawn from three designs was quoted as a verdict on small modular reactor waste generally, and a later laboratory analysis of three different designs reached a different conclusion.

The stakes are concrete. Small modular reactor waste would include spent fuel, activated structural metal, and used coolant, all of which have to be managed, and utilities choosing designs, regulators, and state legislators weighing new plants want to know whether smaller reactors make that job larger. The abstract also said that SMRs “will not reduce the generation of geochemically mobile 129I, 99Tc, and 79Se,” nuclides that matter for repository performance because they move relatively easily through rock and groundwater.

Why a small core loses neutrons

The physics at the center of the paper is sound. A reactor core sustains a chain reaction only if enough fission neutrons cause new fissions before they are absorbed or escape through the core’s surface.

Escape is called leakage. Krall’s slides stated that the “probability of leakage” depends on reactor radius and neutron diffusion length, scaling roughly as 1/R². Diffusion length is, in plain terms, the typical distance a neutron travels through the core material before it is absorbed. A core whose radius is not many diffusion lengths wide loses a larger share of its neutrons through the boundary.

Designers have ways to compensate. The paper listed them: increase the initial fissile loading, add a neutron reflector to redirect a fraction of leaked neutrons back into the core, or forgo a moderator or use graphite rather than water.

Each fix has consequences for the back end of the fuel cycle. The slides linked leakage to lower burnup, meaning less energy extracted per unit mass of fuel, and to more U-235 left unburned at discharge. The abstract attributed the extra waste volume “to the use of neutron reflectors and/or of chemically reactive fuels and coolants,” and warned that the higher fissile content “will demand novel approaches to evaluating criticality.” The same abstract noted that volume “is not the most important evaluation metric.”

All of these results come from models. No SMR waste has been measured.

What the codes could, and couldn’t, see

The models were not equally grounded. According to the slides, the team used the SCALE/ORIGEN depletion codes for the light-water reactors and for the “thermal-spectrum IMSR.” The Toshiba 4S relied on “literature data,” beside a note that read “need more data for better accuracy.”

Krall described part of the reason in a June 17, 2022, interview with the Bulletin of the Atomic Scientists. She said designers would only speak in generalized terms about an ideal SMR fuel cycle, and that the information was marked proprietary. In the same interview she said that “on the larger side of the SMR spectrum, the waste will be more similar to those of existing reactors.” She now works at Sweden’s nuclear fuel and waste company, SKB.

The paper itself acknowledged its sample. “Although the analysis focuses on only three of dozens of proposed SMR designs,” the authors wrote, they still concluded that most designs would be inferior to large reactors on waste.

Rebuttals from industry and a competing designer

The response began the day of publication. On May 31, 2022, the blog Neutron Bytes, which leans toward advocacy, posted a critique of the study. The blog also reported that NuScale had written a letter to the National Academy of Sciences by June 1, 2022, and that Terrestrial Energy responded further on June 3.

Also on June 1, Kirk Sorensen, an advocate for a competing thorium-fueled design, published a rebuttal at Energy From Thorium. He called leakage “a valid concern, but it is a design-specific concern.”

The design versions were also out of step. The paper used NuScale’s 160 MWth module, as Atomic Insights noted, while NuScale’s chief technology officer, José Reyes, wrote in ANS Nuclear Newswire in June 2022 that “the NuScale 250-MWt design does not produce more spent nuclear fuel” than the existing fleet. Atomic Insights also said the Toshiba 4S “has not been under active development for at least half a decade.”

Argonne’s small modular reactor waste analysis

On November 18, 2022, Argonne National Laboratory, working with Idaho National Laboratory, released its own SMR waste analysis. It examined NuScale’s VOYGR, TerraPower’s Natrium, and X-energy’s Xe-100, and found SMR waste “roughly comparable” to that of large reactors. Argonne said the outcome depends on “discharge burnup, the uranium enrichment, the thermal efficiency and other reactor-specific design features.”

“It’s not correct to say that because these reactors are smaller they will have more problems proportionally with nuclear waste,” Taek Kyum Kim of Argonne said in the laboratory’s release.

The two studies are not a head-to-head test of small modular reactor waste, because the laboratory’s designs mostly differ from the PNAS designs. Krall answered in Utility Dive on November 23 that the laboratory study included a 345-MWe TerraPower reactor that “does not even meet the definition of a ‘small’ reactor.”

How a case study becomes a verdict

Stanford’s own summary of the work carried the qualifier the abstract left out. It described the increase as “per unit of energy generated for the reactors in our case study.” The 2-to-30-fold range therefore applies per unit of energy generated for the studied reactors.

The study went on to be cited in policy settings. Testimony submitted to an Oregon House committee in 2025 cited the figures and kept that case-study phrasing.

The leakage-versus-size argument is real physics, but whether it decides small modular reactor waste outcomes depends on reactor-specific design features such as discharge burnup, enrichment, and thermal efficiency.

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About the sources: The NuScale and Terrestrial Energy letters to the National Academy of Sciences are described as Neutron Bytes reported them. The paper’s wording comes from the authors’ uploaded copy and an abstract reposted from the proof. Reyes’s comments are cited as relayed by Utility Dive and linked to ANS Nuclear Newswire.