Summary

The United States has deployed commercial nuclear power since the 1950s, and as of 2021, nuclear power accounted for approximately 20 percent of U.S. electricity generation. The current commercial nuclear fleet consists entirely of thermal-spectrum, light water reactors (LWRs) operating with low-enriched uranium dioxide fuel in a once-through fuel cycle.¹ In recent years, the U.S. Congress, U.S. Department of Energy (DOE), and private sector have expressed considerable interest in developing and deploying advanced nuclear reactors to augment, and possibly replace, the U.S. operating fleet of reactors, nearly all of which will reach the end of their currently licensed operating lives by 2050. Much of this interest stems from the potential ability of advanced reactors and their associated fuel cycles—as claimed by their designers and developers—to provide a number of advantages, such as improvements in economic competitiveness, reductions in environmental impact via better natural resource utilization and/or lower waste generation, and enhancements in nuclear safety and proliferation resistance.

In the Further Consolidated Appropriations Act of 2020 (Public Law 116-94), Congress directed DOE to contract with the National Academies of Sciences, Engineering, and Medicine to examine two broad issues related to advanced nuclear reactors and nuclear fuel cycles:

1. merits and viability of different nuclear fuel cycles, including fuel cycles that may use reprocessing, for both existing and advanced reactor technologies;
2. waste management (including transportation, storage, and disposal options) for advanced reactors, and in particular, the potential impact of advanced reactors and their fuel cycles on waste generation and disposal.

The Consolidated Appropriations Act of 2021 (Public Law 116-260) expanded the role of the National Academies with additional tasking to examine

3. nonproliferation implications and security risks of fuel cycles for advanced reactors.

In response to these congressional requests, the National Academies assembled a committee of experts (referred to as “the committee” in this report). In addressing the above issues, the committee was tasked with

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¹ A thermal reactor primarily uses slow or thermal energy neutrons for fission of the nuclear fuel; a once-through fuel cycle involves a single use of the nuclear fuel, after which the spent fuel is destined for disposal.

focusing on advanced reactors that could be commercially deployed by 2050 and on technologies being investigated by DOE’s Office of Nuclear Energy (DOE-NE) and by other relevant programs, such as the Generation IV International Forum and the International Atomic Energy Agency. In December 2021, DOE created the Office of Clean Energy Demonstrations, as mandated by the Infrastructure Investment and Jobs Act (Public Law 117-58); this new office has the primary role in managing the two major advanced reactor demonstration projects in DOE’s Advanced Reactor Demonstration Program.²

The Nuclear Energy Innovation Capabilities Act of 2017 (NEICA) (Public Law 115-248) defines an advanced nuclear reactor as “a nuclear fission reactor with significant improvements over the most recent generation of nuclear fission reactors,” where such improvements may include “(1) inherent safety features, (2) lower waste yields, (3) greater fuel utilization, (4) superior reliability, (5) resistance to proliferation, (6) increased thermal efficiency, and (7) the ability to integrate into electric and nonelectric applications.”³ Advanced reactors under development span a broad range of designs.⁴

The advanced reactors currently under various stages of development worldwide differ greatly in their designs and ultimately in their operation. Variations on advanced reactor designs may include (1) different types of coolant besides water, such as liquid metals (e.g., sodium, lead, lead-bismuth), molten chloride or fluoride salts, or helium gas; (2) different neutron energies to induce fission, either thermal neutrons (neutron energy ~0.025 electron volt [eV]) or fast neutrons (neutron energy > ~100 kilo-eV); (3) different fuel forms, such as metals and alloys, TRistructural ISOtropic (TRISO) particles, carbides, and nitrides; (4) different fuel types, such as uranium with 10 percent to just under 20 percent enrichment in uranium-235 (high-assay low-enriched uranium [HALEU]) or thorium to breed fissile uranium-233; and (5) different sizes in terms of generating power, such as microreactors and small modular reactors that would have a smaller plant footprint than large nuclear power plants and a potential for standardized modular factory construction, enhanced safety features, and flexible and incremental power generation. Additionally, the reactors can have either solid, stationary fuels as in conventional LWRs; solid fuels in constant motion, as in pebble-bed reactors; or even liquid fuels dissolved in the coolant and circulated continuously, as in some molten salt reactor designs. Many of the advanced reactor designs are derived from concepts proposed several decades ago, and some have a relatively long history of prior development.

The various advanced reactor design concepts have different fuel cycle options. The U.S. Nuclear Regulatory Commission defines the nuclear fuel cycle as the progression of nuclear fuel from creation to disposal, which includes the front-end steps of mining and milling uranium, chemical conversion of uranium, enrichment of uranium, deconversion of the enriched uranium, and fabrication of nuclear fuel for use in reactors; reactor operation; and the back-end steps of spent fuel storage, any reprocessing or recycling operations, transportation of the spent fuel, and final disposal of spent fuel and nuclear waste. The committee examined potential fuel cycles for existing and advanced reactor technologies, including once-through fuel cycles, as well as partially or fully closed fuel cycles that involve reprocessing and recycling.

Most of the advanced reactors being developed in the United States use HALEU and are small modular reactors, which is defined notionally as having a power output of less than 300 MWe (megawatts electric) and is envisioned for factory construction and modular installation. Almost all developers told the committee that they are planning on an open, once-through fuel cycle for at least the near to intermediate terms. Several developers noted that in the longer term, their technologies have the potential for recycling spent nuclear fuel.

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² This sentence was altered to correct the role of the Office of Clean Energy Demonstrations after release of a prepublication version of the report.

³ This report addresses potential merits of advanced nuclear reactors regarding waste, fuel utilization, and proliferation resistance and focuses the safety assessment on the front and back ends of the fuel cycles. Potential merits related to reliability, thermal efficiency, nonelectricity applications, and reactor operational safety are addressed in the National Academies study Laying the Foundation for New and Advanced Reactors in the United States, which is scheduled to conclude in early 2023. The two studies proceeded independently from one another but had one common committee member. Importantly, the findings and recommendations in each report are the consensus products of each committee acting independently.

⁴ While the U.S. Nuclear Regulatory Commission does not define small modular LWRs as advanced reactors, this report does consider them as such.

No country has yet implemented a fully closed fuel cycle with multirecycling of plutonium and partitioning and transmutation of the minor actinides in a fast reactor, although there are potential benefits of doing so: extracting more energy from fissionable materials, greatly reducing or even eliminating the need for enrichment, reducing the mining or importing of additional uranium, and lessening the radiotoxicity and heat load of high-level waste going to a repository. At the same time, significant costs are required for closing the fuel cycle, the most difficult to surmount being the large and sustained investments in fuel cycle infrastructure (reprocessing and fuel fabrication facilities, non–light water advanced reactors, geologic repository needed for all disposal options) that does not currently exist in the United States. Furthermore, for any advanced reactor and associated nuclear fuel cycle to be deployed, additional proliferation, security, and safety risks will need to be addressed and managed. An important, but often overlooked, aspect is the human capital needed—a trained workforce to support advanced reactors and fuel cycles.

As the committee carried out its work, it appreciated that trade-offs are necessary when assessing potential merits and viabilities of different advanced reactors and associated fuels and fuel cycles. Different designs and associated fuel cycles have different potential benefits. However, not one advanced reactor technology can concurrently provide for all the potential benefits relevant to the scope of this study. For example, use of pebble-bed TRISO fuel could enhance safety with its potential for withstanding high temperatures beyond those of current fuels, but it would significantly increase the volume of irradiated graphite waste produced, creating challenges for waste management and disposal. Similarly, closed fuel cycles with continuous recycling could substantially improve utilization of fissionable materials for energy production in a carbon-constrained world—potentially eliminating the need for uranium enrichment. However, a closed fuel cycle would place significant inventories of potentially weapons-usable materials at security risk in reprocessing and fuel fabrication facilities, in reactors, at storage sites, and during transportation operations. These risks could require significant additional resources for international nuclear safeguards, including physical protection systems designed to secure and prevent diversion of these materials. Furthermore, significant research is in progress to address many of the gaps in understanding of the potential benefits and risks of these new technologies. But until advanced reactor fuel cycle concepts go from paper studies and computer-aided design drawings to demonstration and operating units, it is impossible to understand the myriad trade-offs the different design concepts represent and thereby choose a “best in class.”

As this study was congressionally mandated, its primary focus is U.S. policy makers and the U.S. nuclear industry. This report and its findings and recommendations provide advice to the U.S. government to help inform the decisions on the development of advanced reactors and fuel cycles in the United States. The following sections present the committee’s complete list of findings and recommendations, indicating the topical subject to which they relate. Notably, the findings and recommendations are not presented in order of importance, but rather the order in which they appear in the chapters.

MERITS AND VIABILITY OF THE EXISTING NUCLEAR FUEL CYCLE FOR U.S. LIGHT WATER REACTORS

While the United States has had some experience with commercial reprocessing via monorecycling until the early 1970s, since then it has exclusively deployed the once-through fuel cycle. The relatively low cost of uranium, the abundance of natural uranium, and the deployment of more economical methods for enriching uranium have all contributed to the viability of the once-through fuel cycle for LWRs, not only in the United States but also in most nuclear power–producing countries. Reprocessing of LWR fuel has continued in France and Russia, with varied degrees of commercial success. Presently, the United States has no incentives to undertake monorecycling, largely because of the high costs involved and the decreasing contribution by LWRs to the generation of electricity due to plant shutdowns; substantial challenges (based on past experience) with licensing and construction of spent fuel reprocessing and mixed oxide fuel fabrication installations; security and environmental concerns; and the abundance of natural uranium and uranium enrichment at relatively low costs for the foreseeable future. The clear path for the existing U.S. spent fuel inventory is the once-through nuclear fuel cycle, which is still not being implemented fully because of the political impasse over the Yucca Mountain geologic repository site in Nevada.

Finding 1: Substantial, sustained investments to 2050 and beyond are required to develop technically complex advanced nuclear technologies and fuel cycle facilities and to enable potential commercial success. Notably, France has had a consistent vision on nuclear energy’s role in its energy security for more than five decades, and as a result, the majority of France’s electricity comes from nuclear power; however, after delaying development of fast reactors, it has yet to close the fuel cycle and is still decades from doing so. No matter what fuel cycle option the United States chooses—whether direct geologic disposal or a closed fuel cycle using advanced technologies—long-term vision and significant and sustained financial commitment will be required to execute it.

Finding 2: Continued use of the once-through fuel cycle for the existing U.S. light water reactor (LWR) fleet has several merits: (1) lower cost compared with any fuel cycle that involves reprocessing and recycling, (2) a reliable international market for nuclear fuel services from multiple suppliers (although that could be disrupted by international crises, such as war)⁵; (3) compatibility with the projected available uranium resources; (4) well-understood proliferation resistance of the entire fuel cycle; and (5) theft resistance of spent nuclear fuel. However, the once-through cycle remains incomplete in the United States because there is still no progress toward establishing an operating geologic repository for the spent fuel from nuclear power plants. Pursuing the monorecycling fuel cycle with existing LWRs in the United States would add cost to nuclear power generation but produce no significant benefits, given the projected abundant supply of natural uranium and uranium enrichment at relatively low cost for the foreseeable future.

POTENTIAL MERITS AND VIABILITY OF ADVANCED REACTORS AND ASSOCIATED FUEL CYCLES

Internationally, several countries are pursuing development and deployment of advanced reactor technologies and associated fuel cycles. The Generation IV International Forum (GIF) involves the United States, the European Atomic Energy Community (Euratom), and more than a dozen other nations pursuing development and deployment of advanced reactor technologies. However, the committee has focused its assessment on U.S.-based efforts. Like GIF, DOE’s framework for advanced reactor development emphasizes improved resource utilization, waste minimization, enhanced safety and reliability, and proliferation resistance. In addition, DOE prioritizes versatility more highly compared with earlier generations, especially the ability to provide nonelectrical services, such as desalination, process heat, and hydrogen production. Like GIF designs, DOE’s advanced designs have potential improvements that could manifest themselves in a number of ways, such as inherent or passive safety features, simplified or modular designs for ease of fabrication, scalability and enhanced load-following capabilities to complement sources of renewable energy, increased safety of accident-tolerant materials, and fast neutron spectrums for increased fuel utilization via closed fuel cycles.

Most of the advanced reactors being developed in the United States are small modular reactors. In addition to the potential benefits described by DOE, motivations for the development of small modular reactors include smaller plant footprint, potentially lower operation and maintenance costs and lower up-front capital costs (excluding the to-be-expected higher costs of the first-of-a-kind unit), and improved safety. However, because they have yet to be commercially deployed in the United States, these systems have to demonstrate their operational economic competitiveness compared with larger nuclear power plants and nonnuclear energy systems. The non-LWR small modular reactors will also need to demonstrate that they can meet licensing requirements that include safety assessments by the U.S. Nuclear Regulatory Commission.

Finding 3: Government support to help bring advanced reactor technologies to commercial deployment will take substantial financial and technical resources. Specifically, budget limitations will require the U.S. Department

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⁵ The October 2020 agreement between the U.S. Department of Commerce and Rosatom (the Russian state nuclear energy corporation) allows Russia to continue to export enriched uranium to the United States, but it reduces the proportions from approximately 20 percent of U.S. demand to no higher than 15 percent from 2028 to 2040. The amendment also limits the natural uranium and uranium conversion services from Russia to an amount equivalent to no more than 5 percent of U.S. enrichment demand from 2026 to 2040.

of Energy (DOE) to make difficult decisions about its advanced reactor research and development programs to guarantee support, via industry cost-sharing, for a few promising advanced reactor technologies and associated fuel cycle infrastructure in the next several years. If the Advanced Reactor Demonstration Program is funded consistently and fully by both the government and private industry through completion, information such as costs, reliability, project management, and manufacturing feasibility gained from this program will be key to helping DOE in its decision-making process.

Finding 4: Most of the advanced reactors, especially the non–light water reactors, will confront significant challenges in meeting commercial deployment by 2050. While at least 10 advanced reactor developers currently aim to deploy their technologies by 2050 in the United States,⁶ there are no currently operating fueled prototypes of any of these specific advanced reactor designs in the United States; there are, however, some demonstration and commercial units of similar reactor designs in operation internationally. Moreover, the vast majority of advanced reactors are still in the early design phase. Depending on the maturity of the technology, advanced reactor developers face a range of challenges to bringing the proposed technologies to commercialization, including little or no direct operational experience of some designs at engineering scale; the lack of adequate capabilities to develop, test, and qualify advanced fuels and materials; and as a result, the potential considerable time for regulatory approval.

Finding 5: Of the advanced nuclear reactor technologies currently in development, small modular reactors based on light water reactor (LWR) technologies are furthest along toward being connected to the electrical grid. This is because they can leverage the existing LWR and fuel cycle infrastructure and because these technologies have received government and private-investor financial support for more than a decade.

Finding 6: The common perception that the thorium-232/uranium-233 fuel cycle will generate less plutonium and minor actinides (therefore reducing the radioactive hazard of its spent fuel compared with that from the

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⁶ As of January 2022, this number of developers had submitted applications or preapplications to the U.S. Nuclear Regulatory Commission.

uranium-235/plutonium fuel cycle) is incorrect. Overall, because of the decay of associated actinide products, thorium-based fuels have short- and long-term radiotoxicities (hazards) comparable to uranium-based fuels.

NUCLEAR FUEL CYCLE NEEDS FOR ADVANCED REACTORS

To evaluate the merits and viability of different fuel cycle options, the committee analyzed (1) the once-through cycle for LWRs, (2) monorecycling of uranium and plutonium as mixed oxide fuel in LWRs, and (3) multirecycling of plutonium and minor actinides (americium, curium, and neptunium) in fast reactors.

The committee also analyzed fuel production and fabrication for different fuels that could use HALEU and thorium, and different fuel forms, including TRISO, oxide, pure metal, metallic alloys, nitride, carbide, and liquid fuel salt. Particular emphasis was placed on the supply of HALEU, which almost all of the advanced reactor developers plan to use and for which there is currently no commercial-scale production in the United States. Currently, the only commercial supplier of HALEU is Russia, and as this report was being completed in April 2022, the committee notes that U.S.-based advanced reactor developers’ access to that supply might be prohibited, depending on potential U.S. sanctions on Russia due to the Russian war against Ukraine. For each fuel cycle option, the committee considered resource utilization, waste management, proliferation resistance, security risks, and safety aspects (excluding reactor operational safety). Regardless of the fuel cycle, a geologic repository will be required to dispose of spent nuclear fuel, high-level wastes, and other wastes that contain long-lived radionuclides generated by reactor and fuel cycle operations.

Finding 7: There is no current domestic capacity to supply high-assay low-enriched uranium (HALEU) to meet the projected needs of U.S.-based advanced reactor developers over the next decade. Therefore, if reactor projects requiring HALEU continue to advance, identifying a reliable supply of the material will be crucial. Otherwise, many developers will likely initially acquire HALEU from foreign sources, such as Russia, raising concern about ensuring reliable supply. Reliance on foreign sources of HALEU or HALEU feedstock (as many advanced reactor developers had planned to do prior to the invasion of Ukraine by Russia) without a reliable domestic supply could have serious energy and national security implications if advanced reactors using HALEU are adopted widely.

Finding 8: For a nuclear fuel cycle supporting any reactor technology to be viable, it has to be industrially sustainable. Although many fuel cycle options are possible, most differ dramatically from the current situation in the United States—the once-through fuel cycle. All elements of a sustainable nuclear fuel cycle would have to be fully demonstrated both individually and together, because what works with computer-aided designs would not necessarily translate to industrial-scale deployment. For that reason, an evolutionary, progressive approach is likely more practical than a revolutionary approach that attempts to solve all potential issues at the same time with advanced technologies. The evolutionary approach is more important for commercial deployability and will require the majority of investment efforts; nonetheless, some investments in high-risk, high-reward approaches may be worth pursuing. The committee agrees with the 1996 National Research Council report Nuclear Wastes: Technologies for Separations and Transmutation, which states that advanced fuel cycles will require substantial investment and take between many decades to more than a century of continuous recycling using a separations and transmutation system of appropriate scale, in order to potentially achieve the full benefit of plutonium recycling and partitioning and transmutation of minor actinides.

Finding 9: As proposed for some advanced reactor closed fuel cycles, reprocessing and recycling of spent nuclear fuel introduces additional safety and environmental considerations over the management of open-cycle light water reactor oxide fuels. In assessing the safety and environmental performance of advanced reactors, the risks and environmental impacts will require optimization over the entire fuel cycle, including front-end processes (mining, enrichment, and fabrication), back-end processes (reprocessing and recycling together), and disposal (interim and final). Currently, advanced reactor developers focus primarily on the safety aspects of the reactor and its operation, and put less priority on the safety aspects of other parts of the fuel cycles.

Finding 10: Because of the absence of current commercial operational experience with advanced reactor technologies in the United States, reliable cost data and estimates for these technologies and their associated fuel cycle components are lacking. The costs of advanced reactors and their associated fuel cycles could range from at least several billion dollars—for pilot-scale non–light water advanced reactors and their fuel cycle facilities—to hundreds of billions of dollars—for full deployment of an alternative fuel cycle that would replace the existing once-through cycle and existing light water reactors. Congress and the U.S. Department of Energy will need better understanding of the cost estimates for various scenarios of reactor deployment and supporting fuel cycle requirements to aid their decision making as to what technologies to support in the coming years.

MANAGEMENT AND DISPOSAL OF NUCLEAR WASTES ASSOCIATED WITH ADVANCED REACTORS AND FUEL CYCLES

For its analysis of waste generation from advanced reactors, the committee compared waste issues for four representative advanced reactors—integral pressurized water reactors, high-temperature gas-cooled reactors, sodium-cooled fast reactors, and molten salt reactors—with the once-through cycle for LWRs, which was used as the reference case. Wastes considered were from front-end processes, reactor operations, spent fuel and high-level waste, any processing and/or reprocessing, and decommissioning. The committee also considered the impact of advanced reactors and fuel cycles on geologic disposal.

Finding 11: As the United States nears the 40th anniversary of the Nuclear Waste Policy Act (NWPA) (Public Law 97-425) and its Amendments (Public Law 100-203, Part E), there is no clear path forward for the siting, licensing, and construction of a geologic repository for the disposal of highly radioactive waste (mainly commercial spent nuclear fuel). The United States finds itself in this difficult situation for many reasons, including (1) changes to the original NWPA of 1982 that moved the process of site selection from a consideration of multiple sites to a single site, Yucca Mountain, Nevada; (2) a slowly developing and changing regulatory framework that provided late guidance in the site selection process and the evaluation and comparison of multiple sites; (3) ineffective management

of the Nuclear Waste Fund ($45 billion) by Congress, which treated what was to have been a ratepayer escrow account as if it were taxpayer monies; (4) consequential policy changes occurring with changing administrations; (5) conflicting congressional and executive policies; and (6) insufficient public engagement in decisions concerning the basic strategy for the storage and disposal of the waste. The continued delay in planning and progress has only made the situation more complicated, as the present legal and regulatory frameworks have become outdated and even more limiting. Numerous assessments during the past decade, notably the Blue Ribbon Commission on America’s Nuclear Future (2012) and Reset of America’s Nuclear Waste Management: Strategy and Policy (2018), have outlined a way forward. The committee agrees with common recommendations of these studies to establish a single-mission nuclear waste management and disposal entity, for which models have been proposed that deserve consideration by Congress. The entity could be governmental, partially governmental, or private; as to the latter option, the committee notes that two successful programs are being led by fully private entities: Posiva in Finland and SKB in Sweden. Important attributes of the entity are described in Recommendation G.

• Such an entity should be responsible for “cradle-to-grave” care and disposition of spent nuclear fuel—that is, from its discharge from a reactor plant to its final disposal in a repository. This entity should have continuity of leadership and funding, as well as a consistent disposal strategy. It should also have high technical and scientific competence, be able to organize and lead research programs and large construction projects, and, importantly, be able to engage the public in a way that engenders trust. Finally, the entity should operate effectively over the many decades that will be required to manage the present inventory of nuclear waste, as well as waste generated by future advanced reactors.
• Congress should ensure that funds collected from ratepayers that use electricity from nuclear power plants, now more than $45 billion, are applied to the disposal of the spent fuel generated by nuclear power plants and that collection of funds from all commercial generators of nuclear power resumes. Moreover, funding for the entity should be held in a true escrow account and not be subject to the annual appropriations process.
• The entity should immediately initiate steps to begin the process of site selection. Before sites are considered, a decision-making process with appropriate technical criteria and an acceptable method of public engagement, such as consent-based siting, needs to be defined in collaboration with impacted communities, tribes, and states. Congress should make a decision on what to do with Yucca Mountain, which could include keeping it as a possible site for consideration, depending on the plans of the new entity.

Finding 12: The advanced reactor developers’ presentations to the committee focused on the reactors themselves, with little or no attention to nuclear waste management or disposal of the nuclear waste generated because there is no incentive for them to do so. In the absence of a final geologic disposal strategy in the United States, the expansion of nuclear power using advanced reactors will add to the amount of spent nuclear fuel and associated waste that requires disposal and increase the complexity of this challenge because of the need to dispose of new types of fuels and waste streams.

Finding 13: Presently proposed advanced reactor technologies will initially use a once-through fuel cycle; however, compared with those currently in use, the fuels will have a higher uranium enrichment (e.g., high-assay low-enriched uranium [HALEU]) and a higher burnup; also, they will use new types of fuel materials and designs (e.g., TRistructural ISOtropic [TRISO] fuels). As compared with the disposal of the present uranium oxide spent fuel, these new fuel types may result in changes of (1) the amounts (either in mass or volume), chemical compositions, and radionuclide inventories of the waste to be disposed; (2) the thermal power of fuel assemblies; and (3) the durability of the spent fuel in a disposal environment. More specifically, from the waste management and disposal perspective, it is important to note the following:

• Radiological risks from disposed waste are dominated by the mobility of long-lived radionuclides and not by the radiotoxicity inventory. Therefore, radiotoxicity itself is a poor metric for repository performance and risk to the public from waste disposal. The long-term safety of disposal of actinides in appropriate geologic settings is largely independent of the actinide inventory of the repository, except in the off-normal situation where the geological barrier is bypassed—for instance, by human intrusion. Because the amount of mobile long-lived fission products generated is independent of reactor type, most advanced reactor technologies will have little impact on estimates of long-term repository performance. Key factors for long-term repository performance are the redox conditions of the geochemical environment, waste form stability, groundwater flow rates, and solubility/sorption of radionuclides. A reducing environment is preferred. Advanced reactor technologies will have little or no impact on these factors.

• The total quantities of fission products generated are generally related to fission rate and are largely independent of reactor technologies, although the distributions of different isotopes may differ. Both short- and long-lived fission products are important on the timescales relevant to geologic disposal. Short-lived fission products (e.g., strontium-90 and cesium-137) produce significant heat, while long-lived fission products (e.g., iodine-129 and technetium-99) are extremely mobile in a repository environment. Advanced reactor technologies will, in general, generate a higher amount of fission products in each spent nuclear fuel package because of their higher burnups, resulting in a higher thermal load. Increased thermal loads of waste containers will impact a number of repository design features, such as the size and spacing of waste packages, the size of the repository footprint, and engineering designs, thereby impacting the cost of repository construction.
• Enhanced stability and durability of waste forms in a repository environment can be beneficial to the performance of a repository by limiting the release of radionuclides from the spent fuel. Some advanced reactor technologies propose using advanced fuel designs with the potential to contain radionuclides (e.g., TRISO fuel), but this potential must first be demonstrated by experimental programs that examine the fuel’s long-term integrity in intense radiation fields and at high temperatures.

Finding 14: Conceptually, advanced reactors could be used to reduce the current inventory of transuranics in the approximately 86,000 tonnes of legacy spent fuel to date; this would require considerable resources and time to design, develop, prototype, build, and make operational the required infrastructure. Creating this infrastructure is not practicable in the near future, as long as uranium and enrichment services are readily available.

Finding 15: Most of the advanced reactor types proposed would generate waste streams for which there is little experience or mature technical ability to manage. All additional waste treatment options would entail additional costs not encountered in the management and disposal of spent light water reactor (LWR) fuel. High-temperature gas reactors will produce much larger volumes of spent fuel compared with equivalent energy production from LWRs. It may be possible to reduce the volume by removing graphite from the spent fuel, but those technologies are immature. Dust production from pebble-bed reactors would pose waste and decommissioning challenges. Sodium-cooled fast reactors would produce large volumes of irradiated sodium waste that would require treatment and disposal; sodium-bonded spent fuel is not suitable for direct disposal and would require treatment by methods not yet technically mature at the industrial scale. Molten salt reactors produce two waste streams, radioactive off-gases and the spent fuel salt waste, that would require processing into waste forms suitable for disposal. These treatment methods and suitable wastes forms are in early stages of exploration. Most of these advanced reactors would produce large quantities of irradiated graphite waste—from use as moderators or reflectors—and this material would prove challenging to manage as well. While European researchers have analyzed graphite waste disposal extensively, researchers in the United States generally lack this expertise.

STORAGE AND TRANSPORTATION

The committee also considered storage and transportation requirements for advanced reactor fuels and materials, including HALEU, TRISO and graphite materials, metallic fuels and materials, and molten salt liquid fuels.

Finding 16: Similar to issues with waste management, advanced reactor developers have not adequately examined the back-end operational management (i.e., storage and transportation) of advanced nuclear spent fuel. Consequently, the stability of waste forms and potential issues related to needed processing prior to storage, as well as repackaging that may be required for transportation and final disposal, have not been studied sufficiently.

Finding 17: Secondary waste streams—such as lead, sodium, molten salts, and irradiated graphite (moderators and/or from TRistructural ISOtropic [TRISO]–particle fuel disassembly)—from advanced reactor and fuel cycle operations will need to be stabilized and packaged for storage prior to downstream operations to support disposal. Waste forms for these secondary wastes can be developed to be compatible with storage regulations by the U.S. Nuclear Regulatory Commission; however, some still require research and development to properly characterize performance envelopes.

Finding 18: Because of the higher enrichments of fresh high-assay low-enriched uranium (HALEU) and potential higher burnups of irradiated HALEU fuels, maintaining subcriticality margins and having adequate thermal and shielding protection during transport and storage would most likely require at least some of the following:

• criticality experiments for enrichments above 5 percent to support benchmarking analyses;
• assessment of the feasibility of using type 30B containers for transport of enriched uranium hexafluoride, if needed; and
• criticality, thermal, and shielding assessments for storage and transportation.

NONPROLIFERATION AND SECURITY RISKS

Deployment of advanced reactors and their supporting fuel cycles will involve the production, transportation, storage, and irradiation of nuclear materials with characteristics that differ significantly from the current U.S. LWR fleet fueled with low-enriched uranium. The committee’s evaluation of nonproliferation implications and security risks of fuel cycles for advanced reactors also used the once-through cycle for LWRs as a reference case. For this analysis, the committee grouped advanced reactors and fuel cycles into the following categories: (1) once-through fast reactors using HALEU; (2) pebble-bed reactors using HALEU; (3) once-through molten salt–fueled reactors using HALEU; (4) use of thorium and uranium-233; and (5) closed fuel cycles.

Finding 19: Expanding the global use of high-assay low-enriched uranium (HALEU) would potentially exacerbate proliferation and security risks because of the potentially greater attractiveness of this material for nuclear weapons compared with the low-enriched uranium used in light water reactors. The increased number of sites using and states producing this material could provide more opportunity for diversion by state or nonstate actors.

Finding 20: All of the advanced reactor fuel cycles will require rigorous measures for safeguards and security commensurate with the potential risks they pose. Issues requiring special attention include the following:

• Material accountancy (i.e., tracking and quantification) is more difficult for molten salt and pebble-bed technologies than for reactor systems that use stationary solid fuels because of the technical challenges in performing measurements with online fuel and bulk-handling facilities. Containment and surveillance will also be more challenging to implement for these types of reactors. Thorium/uranium-233 fuel cycles require development of safeguards technology because of the large number of variants in their systems. Moreover, safeguards tailored to traditional uranium/plutonium fuel cycles are not applicable to these systems.
• Fuel cycles involving reprocessing and separation of fissile material that could be weapons usable pose greater proliferation and terrorism risks than the once-through uranium fuel cycle with direct disposal of spent fuel, as the separated fissile material would not be uniformly mixed with highly radioactive fission products. Separated, potentially weapons-usable materials could include fissionable materials other than the “traditional” special nuclear materials of highly enriched uranium, plutonium, and uranium-233. Thus, for these closed fuel cycles, specific safeguard technologies will likely be required to meet the International Atomic Energy Agency’s goal of timely detection.