Nuclear energy offers a pairing of high capacity factors and low carbon emissions unrivaled by other power sources, and advanced reactors and small modular reactors (SMRs) hold promise to deliver that power with even more efficiency and versatility. Researchers in the public and private sectors, backed by government funding through programs like the Department of Energy’s Advanced Reactor Demonstration Program and Advanced Research Projects Agency–Energy (ARPA-E), are bringing that promise closer to fulfillment by engineering solutions to open questions about advanced reactor materials and fuel cycles.
While every power source generates waste, the relatively small volume of waste generated by nuclear fission in light water reactors is contained, secured, and understood. Now, ongoing research in the management of the secured waste streams that will be generated by advanced reactors and SMRs is shaping intelligent design decisions by reactor developers.
A question of waste: A paper titled “Nuclear waste from small modular reactors” was published on May 31 in the Proceedings of the National Academy of Sciences (PNAS). It contends that “SMRs will produce more voluminous and chemically/physically reactive waste than LWRs, which will impact options for the management and disposal of this waste.” Announced in a press release from Stanford University issued the day before the full paper was made public, the paper’s conclusions have been aired by media outlets since.
Lead author Lindsay Krall, now a geochemist at Swedish Nuclear Fuel and Waste Management, researched and wrote the paper while working as a postdoctoral fellow at George Washington University and Stanford University between 2017 and 2020. Krall based her conclusions on an analysis of three of the dozens of advanced reactor and SMR designs—the NuScale iPWR, the Toshiba 4S sodium-cooled fast reactor, and the Terrestrial Energy Integral Molten Salt Reactor (IMSR)—because reactor and fuel cycle specifications for those designs were available in pre-license and patent application materials. Krall relied on public materials with proprietary details redacted, so “gaps in the availability of technical data were filled through explicit assumptions, through reference to similar designs analyzed in the scientific literature, or through derivation using known design parameters.”
In October 2021, after submitting her work to PNAS but before its publication, Krall was invited to present her findings during a public meeting of the National Academy of Sciences, Engineering, and Mathematics Committee on Merits and Viability of Different Nuclear Fuel Cycles and Technology Options and the Waste Aspects of Advanced Nuclear Reactors, chaired by Janice Dunn Lee. Krall's two coauthors—Allison Macfarlane, a professor and director of the School of Public Policy and Global Affairs at the University of British Columbia, and Rodney C. Ewing, Frank Stanton Professor in Nuclear Security and codirector of the Center for International Security & Cooperation at Stanford University—were both members of the committee at that time.
Nonnegotiable physics? Krall’s research emphasized neutron activation effects on structural steel and concrete from what was described in Stanford’s press release as “a problem called neutron leakage” from SMR cores.
Ewing, quoted in the Stanford press release, explained that “the more neutrons that are leaked, the greater the amount of radioactivity created by the activation process of neutrons. We found that small modular reactors will generate at least nine times more neutron-activated steel than conventional power plants. These radioactive materials have to be carefully managed prior to disposal, which will be expensive.”
Neutron leakage is one quantifiable attribute of a reactor core. The probability of neutron leakage is a function of the reactor’s dimensions and the neutron diffusion length, and a smaller core will have more neutron leakage than a similar, bigger core, resulting in less fuel burnup and more neutron activation of surrounding materials. Accordingly, developers of advanced reactors and SMRs tend to make engineering design decisions to maximize neutron efficiency and burnup and minimize low-level waste volumes, such as increasing fuel enrichment and improving neutron economy by including reflectors around the core, using online refueling, or using alternatives to enriched uranium, including natural uranium and thorium.
Nuclear Newswire reached out to Steven Biegalski, nuclear and radiological engineering and medical physics program chair at Georgia Tech, who explained, “Regarding neutron activation of structural material, it must first be clarified that nuclear reactors produce a small amount of waste. Activation-based radioactive waste represents a small portion of the total radioactivity found in the product.”
A systems perspective: Temitope Taiwo, director of Argonne National Laboratory’s Nuclear Science and Engineering Division, delivered a presentation to the NASEM committee in December 2020 titled “Systems Perspective on Advanced Fuel Cycles and Waste Management,” a presentation that introduced work by researchers from Argonne and Idaho National Laboratories on topics ranging from fuel recycling and waste forms to nonproliferation and economics.
Invited by Nuclear Newswire to comment on the relative volumes and complexity of waste from SMRs and non-LWRs, Taiwo first noted that conclusive information would depend on the detailed final design of a reactor, including its containment approach and the dimensions of buildings and equipment. “What is clear,” he said, “is that because the non-LWR systems that are being proposed typically have at least three times the fuel burnup of the current LWRs, the spent nuclear fuel (SNF) masses and likely the volumes for those systems will be about one-quarter to one-half, or so, those for the LWRs.” In the case of a light water SMR with about the same fuel burnup as an existing LWR, “the spent nuclear fuel mass and volume would be about the same as for the LWRs,” Taiwo added.
That conclusion could be significantly different for graphite-moderated systems, he said, because of the low fuel content in the moderating medium. “Consequently, the SNF waste volumes could actually be higher for those systems compared to LWRs.”
Response from vendors: On June 2, Nuclear Newswire published a letter from Jose Reyes, chief technology officer at NuScale Power, to May R. Berenbaum, editor-in-chief of PNAS, regarding the article. Reyes took issue with the authors’ evaluation of a 160-MWt NuScale iPWR, which has been superseded by a larger 250-MWt design.
“The authors mistakenly assert that NuScale [SMRs] will produce significantly more spent nuclear fuel than existing light water reactors,” Reyes wrote. “The basis for this statement is their analysis of the NuScale 160-MW thermal core as opposed to the NuScale 250-MW thermal core implemented in NuScale VOYGR plants.”
Reyes went on to state that “NuScale fuel has an average fuel burnup of approximately 45,000 MWd/t at discharge and that it has a design basis maximum exposure of 62 GWd/MTHM. These values are within the values typically observed in the existing fleet of LWRs. Therefore, the NuScale 250-MWt design does not produce more SNF than the small quantities typically observed in the existing LWR fleet.”
Krall, Macfarlane, and Ewing replied to NuScale’s comments in a letter also reproduced by Nuclear Newswire. “We emphasize that our study focused on the NRC-certified, 160-MWth NuScale iPWR design because it had been reviewed during the certification process and an adequate amount of information was available on the core design,” they wrote. “Based on our present understanding, we anticipate that a future analysis will show that the 250-MWth reactor generates less waste per unit energy-equivalent than the 160-MWth reactor but more than a larger LWR.”
Terrestrial Energy issued its own letter to Berenbaum on June 3, stating that the company was “deeply disappointed” in the article, which “in several instances implies that any single shortfall of any small modular (SMR) system is universal to other SMR designs.” The letter questioned several assumptions or findings in the article before concluding, “Terrestrial Energy is taking waste management very seriously. The approach these authors use is of serious concern. No industry takes such a full accountancy and responsibility for its entire waste stream as the nuclear energy industry. Waste volumes from all nuclear reactors are minute in comparison to other industries. Nations with power reactors, together with their institutions and reactor vendors, remain committed to safe, effective management and long-term storage of used fuel. The IMSR and other advanced reactors aim to supply the world with the clean and affordable energy it desperately needs. Safe, long-term management of long-lived radioactive materials is indeed achievable. In fact, this is a major goal of all Generation IV reactor technologies.”
Research under way: Supporters of advanced reactor and SMR deployment would agree with the PNAS authors’ assertion that a full understanding of the costs and technical requirements of advanced reactor spent fuel and low- and intermediate-level waste management is important. But while Krall and her coauthors were forced to estimate information that was redacted from public versions of design documentation, the reactor designers themselves—as well as their partners in the university and national lab sectors—are and have been conducting research informed by up-to-date and exhaustive design details.
The NASEM committee on Merits and Viability of Different Nuclear Fuel Cycles and Technology Options and the Waste Aspects of Advanced Nuclear Reactors, appointed in 2020 in response to a congressional mandate in the fiscal year 2020 Appropriations Act, is highlighting current research and decades of experience with non-LWRs. Committee members are working on a report that will contain consensus findings and recommendations to advise the DOE, Congress, and other relevant stakeholders.
The committee has heard from more than 80 presenters during 12 public meetings, and among those presenters were many researchers whose past and present work—often funded by federal agencies—addresses the very issues raised by Krall, Macfarlane, and Ewing.
ONWARDS (Optimizing Nuclear Waste and Advanced Reactor Disposal Systems), a new ARPA-E program, has been supporting research and innovations in the management of the back-end of the advanced reactor fuel cycle since 2021, but federally funded fuel cycle and waste management research has an extensive history that predates this program.
The DOE allocated $40 million in funding for ONWARDS in May 2021; in March 2022 $36 million of funding was announced for 11 projects—led by universities, private companies, and national laboratories—investigating reprocessing and waste forms for a range of advanced reactor types. “The ONWARDS program aims to reduce the impact of used nuclear fuel from advanced reactors, thereby accelerating their deployment in support of carbon-free energy. The goals of the program are to facilitate a 10X reduction in advanced reactor used nuclear fuel repository footprint through reprocessing and recycling, with integrated safeguards and high-performance waste forms,” explained Bob Ledoux, an ARPA-E program director.
“Continuing DOE support and public-private partnerships supported by the U.S. government are important to the deployment of the future nuclear energy systems internationally,” Taiwo said, referencing decades of research and development into reactor systems, fuel cycle technologies, and computational tools for safety analysis at Argonne. “In recent times, we are renowned for our work in the development of fast reactors and the pyroprocessing technologies for material recovery and reuse in nuclear reactors,” Taiwo added.
Waste management: The PNAS article authors make the broad claim that “SMRs are incompatible with existing nuclear waste disposal technologies and concepts” and conclude that “future studies should address whether safe interim storage of reactive SMR waste streams is credible in the context of a continued delay in the development of a geologic repository in the United States.”
While the delay in the development of a geologic repository is political and not technical, some advanced reactors offer fueling options that have not been used for light water reactors, including burning plutonium or LWR spent fuel, that could potentially reduce the total volume of high-level waste requiring disposal. Reactors using oxide, metal, and molten salt fuels can be part of once-through, limited, and continuous recycling fuel cycle systems. These options could help support a sustainable uranium fuel cycle and, significantly, would reduce the amount of high-level waste from advanced reactors and the amount of waste that requires a geologic repository.
“For nuclear reactors to be able to contribute in a sustainable manner to the decarbonization of the energy sector,” said Taiwo, “I believe that nuclear fuel recycling is important. Once-through fuel cycles using uranium require the enrichment of the fuel material, which results in significant production of depleted uranium that would not be used in the reactor for energy production. A recycling energy system would enable the use of that material and thus allow over a 100-fold production of energy with the initial uranium material. This translates to a much longer temporal utilization of the fuel material. Recycling will also provide significant benefits to the amount of nuclear waste arising.”
It’s relative: America’s Strategy to Secure the Supply Chain for a Robust Clean Energy Transition, published by the DOE in February 2022 in response to an executive order from President Biden, includes “improve end-of-life energy-related waste management,” among its seven “strategic opportunities.” Significantly, the report considers end-of-life waste management for a range of clean energy technologies, including nuclear energy, energy storage, solar photovoltaics, and wind.
“Concerns about end-of-life management are already a significant issue in land-based wind siting and permitting,” according to the report. In the case of solar, end-of-life management concerns named in the report include cadmium in thin-film solar PV. Among the concerns listed are a “lack of urgency for newer technologies (e.g., solar and wind) that will not reach their end of life at scale for another decade or more.”
Krall and her coauthors chose to emphasize the potential volume of neutron-activated materials—structural materials deep within nuclear power plants that in some cases may provide clean power for decades before they are considered for decommissioning and dismantlement. Once a reactor’s operating life comes to an end, any neutron-activated materials may be safely contained in situ for decades longer, arguably making the disposition of materials from retired wind turbines and solar panels the more pressing concern.
An energy security imperative: “The energy shortage we face in the United States and the world is real,” said Biegalski. “Energy is essential for national security, public health, and the functioning of our economy. We need new nuclear reactors built within the United States as soon as possible to meet our critical needs.
“Vanguard technologies are being developed to both minimize nuclear waste destined for a final repository and utilize nuclear waste as a resource,” he added. Many advanced and SMR developers are also targeting specific operational goals, including high-temperature heat, factory construction, flexible siting, industrial heat applications, or load following; and some developers may even choose to market a reactor that generates slightly more waste per megawatt-hour than an LWR while offering flexibility, reliability, and a small footprint that can’t be matched by other clean energy technologies.
The need for reliable, carbon-free power to deliver the fastest and most cost-effective transition to clean energy is imperative. Advanced reactor developers and others in the nuclear community are working to ensure the safe and secure minimization and disposal of advanced reactor waste streams, but importantly, they are conducting that research without delaying the deployment of the first-of-a-kind reactors that will be essential for national security, public health, and the economy.