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The nuclear industry currently manages approximately 10,000 metric tons of spent fuel waste annually, a byproduct of reactors that generate 10% of the world’s electricity. This established waste management system, which includes on-site storage and the use of dry casks, has proven effective for conventional reactors fueled by low-enriched uranium and cooled by water. However, the emergence of a diverse range of new nuclear reactor designs in the coming years could necessitate significant adjustments to these existing protocols, introducing novel challenges and potential complexities for the long-term disposal of nuclear waste.
The current nuclear energy landscape is dominated by a relatively uniform reactor design: large, centralized facilities utilizing low-enriched uranium as fuel and water for cooling. This homogeneity has allowed for the development of standardized waste management strategies. The industry’s "playbook" for handling nuclear waste primarily categorizes it into two types: low-level waste and high-level waste. Low-level waste, which constitutes the vast majority by volume, includes items like contaminated protective gear from healthcare and research facilities. This material, once its radioactivity diminishes sufficiently through decay, can often be handled with precautions similar to regular waste, and is frequently stored on-site.
High-level waste, conversely, is significantly more radioactive and generates considerable heat. The primary component of this category is spent nuclear fuel. This spent fuel is a complex mixture, containing uranium-235, the fissile isotope essential for sustaining the chain reaction that powers nuclear plants, along with various fission products – the byproducts of atomic splitting that release energy. Experts widely agree that the most secure and sustainable long-term solution for both spent fuel and other high-level nuclear waste is a geologic repository. These are essentially deep, meticulously managed underground facilities designed to isolate radioactive materials from the biosphere for millennia. Finland is at the forefront of this endeavor, with its geologic repository slated for operation on its southwest coast this year.
In the United States, a site for a geologic repository was identified in the 1980s, but political opposition has perpetually stalled its development. Consequently, the nation’s used nuclear fuel is currently stored on-site at both active and decommissioned nuclear power plants. Following its removal from a reactor, spent fuel is typically submerged in water-filled pools for cooling, a process known as wet storage. After an initial cooling period, it is transferred to protective cement and steel containers, referred to as dry casks, for dry storage.
Despite the potential for new reactor designs, many experts believe that the fundamental principles of waste management will remain largely consistent. Erik Cothron, manager of research and strategy at the Nuclear Innovation Alliance, a think tank focused on the nuclear industry, expresses confidence, stating, "The way we’re going to manage spent fuel is going to be largely the same. I don’t stay up late at night worried about how we’re going to manage spent fuel." However, he acknowledges that novel reactor designs and materials may necessitate specific engineering solutions. The sheer diversity of these advanced reactor concepts foreshadows an equally wide spectrum of potential waste types requiring tailored management approaches.
Some of the forthcoming reactor designs will bear a strong resemblance to existing models, meaning their spent fuel will likely be managed using current methods. However, other advanced designs incorporate innovative coolants and fuel types. Syed Bahauddin Alam, an assistant professor of nuclear, plasma, and radiological engineering at the University of Illinois Urbana-Champaign, highlights this potential divergence: "Unusual materials will create unusual waste."
Certain advanced reactor designs could lead to an increase in the volume of material classified as high-level waste. A prime example is reactors employing TRISO (tri-structural isotropic) fuel. TRISO fuel consists of a uranium kernel encased in multiple protective layers, which are then embedded within graphite shells. According to a 2024 report by the Nuclear Innovation Alliance, the graphite matrix surrounding the TRISO fuel would likely be categorized alongside the spent fuel, thereby increasing the overall volume of high-level waste compared to current fuel assemblies. The report indicates that separating these layers would be both difficult and prohibitively expensive, necessitating the entire TRISO package be treated as high-level waste.
X-energy, a company developing high-temperature gas-cooled reactors that utilize TRISO fuel, has already submitted its spent fuel management plans to the Nuclear Regulatory Commission (NRC). Intriguingly, the inherent design of TRISO fuel might offer a waste management advantage. The protective shells eliminate the need for initial wet storage, allowing for direct transfer to dry storage from the outset, according to company statements.
Another emerging reactor technology, liquid-fueled molten-salt reactors, also has the potential to increase waste volumes. In these designs, the fuel is dissolved directly into a molten salt that functions as both fuel and coolant, rather than being kept separate as in conventional reactors. This integration means that the entire volume of molten salt would need to be managed as high-level waste.
Conversely, some reactor designs might produce a smaller volume of spent fuel, but this reduction in quantity does not necessarily equate to a simpler problem. Fast reactors, for instance, achieve higher burn-up rates, meaning they consume a greater proportion of fissile material and extract more energy from their fuel. This results in spent fuel with a higher concentration of fission products and a consequently greater heat output. The management of this intense heat is a critical consideration for waste disposal solutions.
Spent fuel must be maintained at relatively low temperatures to prevent melting and the subsequent release of hazardous byproducts. Excessive heat within a geologic repository could also compromise the integrity of the surrounding rock formations. Paul Dickman, a former official with the Department of Energy and the NRC, emphasizes the significance of heat: "Heat is what really drives how much you can put inside a repository."
The prospect of certain spent fuels requiring chemical processing before disposal, as suggested by Allison MacFarlane, director of the school of public policy and global affairs at the University of British Columbia and a former chair of the NRC, could introduce additional complexity and cost. For example, in sodium metal-cooled fast reactors, the coolant can infiltrate the fuel and fuse to its casing. Separating these materials can be challenging, and given sodium’s high reactivity with water, the spent fuel would necessitate specialized treatment. TerraPower’s Natrium reactor, a sodium fast reactor that recently received an NRC construction permit, is designed to address this challenge. Jeffrey Miller, senior vice president for business development at TerraPower, explained that their plan involves blowing nitrogen over the material before it enters wet storage pools to remove the sodium.
Beyond the material composition of the waste, even changes in reactor size and location could introduce complications for waste management. Some new reactors are essentially scaled-down versions of current large reactors, known as small modular reactors (SMRs) and microreactors. While their waste may be manageable using existing methods, the proliferation of numerous small sites, each hosting its own waste, presents a logistical challenge for countries like the US, where waste is currently stored on-site.
To address this, some companies are exploring the possibility of returning their microreactors and the associated waste materials to a central location, potentially the same facility where the reactors are manufactured. MacFarlane advocates for robust waste planning, asserting that companies should be mandated to consider waste management from the initial design phase and be held accountable for the waste they produce.
She also points out that current waste management strategies are largely based on research and modeling. The true nature and challenges of managing waste from these new reactors will only become fully apparent once they are operational. As MacFarlane aptly concludes, "These reactors don’t exist yet, so we don’t really know a whole lot, in great gory detail, about the waste they’re going to produce." The evolution of nuclear power hinges on successfully navigating these evolving waste management considerations.
