Radioactive waste will continue to be generated for as long as nuclear technologies, including nuclear reactors, are employed. Large amounts of spent nuclear fuel have been reprocessed during the past 80 years in various countries, and various other forms of radioactive waste have been generated and stored in tank farms and other engineered storage facilities. The International Atomic Energy Agency (IAEA) recommendations classify radioactive wastes into six categories according to the specific radionuclides and their concentration present in the waste—with categories requiring more stringent management being those that pose the greatest risk to humanity and the environment (International Atomic Energy Agency 2009). Specific delineation of these categories is up to the individual IAEA member countries.

In general, the waste classification categories come with recommended minimum disposal requirements. High-level waste (HLW), for instance, is considered the most hazardous waste category; it is highly radioactive, generates significant heat from radioactive decay, and contains long-lived radionuclides. HLW originates mostly from the fuel cycle of nuclear power plants, as fission and activation products are created from nuclear chain reactions associated with neutron activation and subsequent alpha-, beta-, and gamma-decay processes. (Neutron activation is the process in which neutrons are captured by atomic nuclei to induce radioactivity, whereas radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation.) Both used (irradiated) nuclear fuel, as well as reprocessed waste from fuel, are normally assessed as HLW (occasionally as intermediate-level waste or ILW). Liquid and solid so-called “legacy” defense wastes—created by the chemical processes used to extract plutonium for nuclear weapons—are usually also considered HLW. In rare cases, HLW can be produced from other sources, notably during nuclear accidents and sometimes nuclear medical isotope production. HLW from spent fuel from the operation of all types of nuclear reactors accounts for less than 1 percent of the global volume of radioactive waste but consists of about 95 percent of the total activity of radioactive waste (International Atomic Energy Agency 2022).

The disposal of HLW is recommended in a deep geological repository, of which none currently exists in the world, though several repositories are in advanced stages of licensing and commissioning as of 2024, such as in Finland, Sweden, and France (International Atomic Energy Agency 2022; US Nuclear Waste Technical Review Board 2024). The immobilization of the radionuclides in a chemically and mechanically durable, heat- and radiation-resistant solid matrix known as the “waste form” is essential to the geological disposal concept (Lutze and Ewing 1989).

The waste form is a component of a multibarrier disposal system that relies mainly on geologic isolation to prevent radionuclides from reaching the biosphere. The waste form acts as the first barrier, whose primary function is to provide near-field radionuclide containment and isolation. General performance requirements for the waste form for its use in geologic disposal have been previously identified (e.g., Ewing and Weber 2011). Performance criteria include a high waste loading to minimize waste form volume and thereby reduce repository space requirements, a high chemical durability to minimize aqueous dissolution and subsequent radionuclide release under repository-relevant conditions, and a high tolerance to the effects of self-irradiation, particularly those that result from alpha‑decay events.

This article tells the story of the history of ceramic and glass-ceramic waste forms for the immobilization of high-level waste and their latest developments. It also highlights some of the essential contributions of professor Rod Ewing (1946-2024) in the areas of radiation damage, natural analogues, and nuclear waste forms.

Waste forms for nuclear waste immobilization

Before treatment for long-term immobilization and disposal, the form of high-level waste can vary considerably, from solid used fuel (containing solid and gaseous radionuclides (Bruno and Ewing 2006)) to caustic or acid radioactive liquids. Many argue that used (or “spent”) nuclear fuel is a sufficient waste form in itself when combined with engineered barriers of standard repository systems (Ewing et al. 2016; Lutze et al. 1990); however, this is a specific case that only applies to used fuel. Tailored waste forms that have been extensively studied for other HLW include ceramics, glasses, and glass-ceramics (see Figure 1), whereas others—such as cements and related materials—are important for lower-level wastes (Caurant et al. 2009; Donald 2010; Ewing 1999; National Research Council 2011; Ojovan and Lee 2005).

Radionuclides incorporate differently in different types of waste forms. In ceramics, for instance, radionuclides may occupy specific atomic positions in the periodic structures of constituent crystalline phases as a dilute solid solution. (Periodic or crystal structures describe an ordered arrangement of atoms or ions. Within this ordered arrangement, certain atoms or ions may be replaced with radioactive species while maintaining the crystal structure and this is termed a “solid solution.”) Rod Ewing himself noted the examples of long-term stability of ceramic materials through the presence of radionuclides in billion-year-old minerals. These naturally occurring materials remained stable despite the presence of radiation-induced processes (Ewing 1976; Ewing et al. 1995). By contrast, glasses incorporate radionuclides in disordered positions depending on the glass structure, resulting in considerable flexibility for incorporation (Donald et al. 1997; Weber et al. 1997). The third waste form type, glass-ceramics, allows the incorporation of specific waste elements in both glass and crystalline sites (Caurant et al. 2009; McCloy and Goel 2017; National Research Council 2011). Glass-ceramics essentially combine the chemical and processing flexibilities of glasses to accommodate processing impurities and excellent chemical durability of ceramic phases to host specific radionuclides.

The vitrification of HLW to produce borosilicate-based glass waste forms has received the greatest attention worldwide. Since the 1960s, several countries have developed vitrification technologies, equipment, and remote operation of the processing. Industrial deployment was first achieved at the Atelier de Vitrification de Marcoule (AVM) in France, which started full operation in 1978 (Damette et al. 1985). By the 1980s, glass waste forms and vitrification had already reached high technical maturity.

The first proposal for the use of minerals (ceramics) as nuclear waste forms was made as early as 1953 (Hatch 1953). However, it was not until the 1970s that scientists led by McCarthy (1977) and Roy (1977) at Pennsylvania State University made innovative ceramic waste form proposals. These scientists subsequently argued that glass was comparatively less stable thermodynamically than crystalline materials and that using synthetic minerals would be more advantageous. At around this time, Ted Ringwood and colleagues at the Australian National University and the Australian Nuclear Science and Technology Organisation (ANSTO) developed a multiphase ceramic based on titanate minerals known as synroc, a contraction of “synthetic rock” (Ringwood 1978; Ringwood et al. 1979). Importantly for geologic disposal, ceramic waste forms indicated a superior intrinsic resistance to groundwater leaching (the release of radionuclides from the waste form following exposure to solutions that represent those expected in a geological repository); for example, synroc was demonstrated to have release rates of the contained hazardous waste elements and radionuclides that were several orders of magnitude slower compared to glass (Lutze and Ewing 1989). See Table 1.

In his pioneering work at the University of New Mexico, Rod Ewing was already thinking about metamict (amorphous) minerals (Ewing 1976; Ewing et al. 1987; Ewing and Haaker 1980; Haaker and Ewing 1981) and natural glasses (Ewing 1979; Lutze et al. 1985; Malow et al. 1984; Ewing, 2001) as being effective natural analogues for assessing the very long-term behavior of waste forms as modified by radiation and aqueous weathering. Such minerals demonstrated stability over millions of years in conditions characteristic of a deep geological repository while retaining incorporated radioactive chemical species. Rod contributed substantially to the research and development of waste forms that occurred internationally throughout the 1970s and 1980s. This period saw the design of a variety of single and multiphase ceramics and new glass compositions—all potentially suitable for different waste disposal applications and with differing radioactive waste immobilization performances. This intense period of research and development ended with controversial reports comparing the properties of ceramic and glass waste forms (e.g., Kerr 1979; Carter 1979). In 1981, the US Department of Energy led a major review of alternative waste forms, which concluded in the “Atlanta shoot-out”—an expert panel review tasked with selecting the most promising waste forms (Garmon 1981).

The “Atlanta shoot-out”

In 1979, the US Department of Energy’s Office of Nuclear Waste Management empaneled a group of esteemed scientists and engineers to evaluate candidate waste forms and recommend a smaller number for further detailed development and demonstration, ultimately leading to deployment at the Savannah River Site in South Carolina (Hench 1979). Researchers in the field referred to the meeting as the “Atlanta shoot-out,” a term borrowed from the headline of a local newspaper article about a panel meeting that occurred in Atlanta. The panel, which included Rod Ewing, selected nine waste form types for further consideration and then evaluated them against present scientific merits, research priority, and engineering practicality (Hench 1980). The scientists and experts recommended further research on these nine waste forms focused on developing standardized tests and reference materials, comparative long-term performance metrics, studies of performance under radiation conditions, and effects of microstructure on long-term performance. In May 1981, the panel made its final recommendations, ranking the waste form options from highest performance to lowest: borosilicate glass, synroc, porous glass matrix (highly-enriched silica glass), tailored ceramics, pyrolytic carbide- and silicon carbide-coated particles, tailored autoclaved concretes formed under elevated temperatures and pressures (FUETAP), metal matrices, and plasma spray coatings (Hench 1981).

In 1982, the Department of Energy’s national HLW program reviewed the results from the expert panel review (Hench 1981), a product performance evaluation (Cheung 1982), a processability analysis (Dunson 1982), and independent reviews from the defense waste contractors at Savannah River (Stone 1982; Stone 1979), Hanford (Schulz 1980), and Idaho (Post 1981). The department then assigned composite scores to each waste form (see Table 2).

The Department of Energy selected borosilicate glass and synroc as the reference and alternative forms, respectively, for immobilizing defense HLW (Bernadzikowski et al. 1982), citing the ease of continuous vitrification process and adequate performance for borosilicate glass and the high performance and adequate processability for synroc. This recommendation was followed by a final environmental impact statement (US Department of Energy 1982a and 1982b) and a record of decision (US Department of Energy 1982c). This decision set the course for HLW treatment at other sites throughout the United States.

At West Valley in New York, an analysis of the terminal waste form selection for the West Valley demonstration project selected borosilicate glass, citing the “more developed borosilicate glass technology being relatively less complex and less expensive to implement at this time” when compared to synroc (Hannum 1983). The stated basis for selecting borosilicate glass at the Hanford site in Washington was its previous selection at the other two US sites as well as in Germany, France, and Japan (US Department of Energy 1987). In 1990, the Department of Energy reevaluated this decision and concluded again that borosilicate glass was the most suitable waste form for all Hanford HLW, citing previous development and no significant change in any of the criteria used for that decision (US Department of Energy 1990). The decision was punctuated by the statement: “The evaluation of alternative waste forms that reached its peak in the late 1970s has largely abated because of the advanced state of development and utilization of borosilicate glass.”

The Defense Waste Processing Facility (DWPF) was built at the Savannah River Site and has been vitrifying HLW since 1996 (Chew 2019). The West Valley Demonstration Project (WVDP) mission was completed between 1996 and 2002 and is currently undergoing decommissioning (Palmer 2004). Low-activity waste vitrification at Hanford is scheduled to start in 2025, with HLW vitrification to follow soon after (Schubick 2023).

Research into alternative waste forms was severely reduced following the Atlanta shoot-out, though small, independent efforts continued (Lutze and Ewing 1988). Even without government support, Rod continued basic research on natural analogues for ceramics and glasses. His research focused on zirconolite and pyrochlore minerals as natural analogues for crystalline nuclear waste form phases (Lumpkin and Ewing 1988), samarskite minerals (Warner and Ewing 1993), and natural basaltic glasses as analogues for nuclear waste glass behavior in natural systems (Grambow et al. 1985). It was during this time that Rod’s work on radiation damage in natural zircon appeared on the cover of Science magazine (Chakoumakos et al. 1987). See Figure 2. Work on synroc also shifted to an emphasis on the treatment of waste types that are problematic for glass matrices or existing vitrification process technologies (Gregg et al. 2020a).

Impact of radiation damage on waste forms

Self-irradiation from the decay of incorporated radionuclides can affect the waste form properties and subsequent performance. Radiation damage must therefore be evaluated in the context of waste form durability.

For HLW, the principal sources of radiation are the beta decay of the fission products (e.g., cesium 137 and strontium 90) and the alpha decay of the actinide elements (e.g., uranium and plutonium). The beta and alpha decays affect the waste form through interactions of beta particles, alpha particles, energetic recoil nuclei, and gamma rays with the host solids primarily through two broad categories: the transfer of energy to electrons (ionization and electronic excitations) and the transfer of energy to atomic nuclei, primarily by ballistic processes involving collisions (Weber et al. 1997; Weber et al. 1998).

Rod’s interest in radiation effects in nuclear waste forms was founded during his years of graduate research on metamict minerals, which are radiation-damaged by radioactive decay of incorporated uranium and thorium (Ewing 1974). He proposed that such minerals are natural analogues to nuclear waste forms and can provide scientific and technical insights into the behavior of nuclear waste forms over geologic time scales (Ewing 1976). See Figure 3.

This led to early collaborative work with William Weber (a co-author) at Pacific Northwest National Laboratory to compare radiation damage in natural minerals with synthetic crystalline ceramics doped with short-lived actinides (curium 244 or plutonium 238) that underwent similar alpha decay as uranium and thorium in the natural minerals but at a substantially faster rate.

The most detailed study compared the accumulation of radiation-induced amorphization in the mineral zircon (zirconium silicate, with chemical formula ZrSiO₄) with crystalline zircon doped with plutonium 238. (Amorphization or metamictization is the gradual destruction of a mineral’s atomic-scale ordered crystal structure that results in the mineral becoming an amorphous, unstructured solid.) Excellent agreement in the amorphization and swelling behavior of zirconium silicate was found irrespective of how the radiation damage was generated (Murakami et al. 1991; Weber, Ewing, and Wang 1994). This work was followed by ion beam irradiation studies that provided a definitive understanding of the kinetics of radiation effects in zircon (Weber, Ewing, and Wang 1994) and led to a predictive model on the temperature-dependence of radiation effects in zircon and other ceramic waste forms over geologic time scales (Weber, Ewing, and Meldrum 1997; Weber and Ewing 2002).

Given the extreme chemical durability and aqueous corrosion resistance of undamaged and radiation-damaged natural zircons, Rod and co-workers proposed zircon as a host phase for the immobilization of plutonium that the Department of Energy would declare as having in “excess” after the Cold War (Ewing, Lutze, and Weber 1995)—a proposal that received a patent for a method to immobilize plutonium in zircon (Ewing, Lutze, and Weber 1996). Pyrochlore was another natural mineral and ceramic of interest as a nuclear waste form, and while many pyrochlores are susceptible to radiation-induced amorphization (Ewing, Weber, and Lian 2004), Rod and co-workers discovered a class of rare-earth zirconate pyrochlores that were resistant to such amorphization (Wang et al. 1999). In 2001, this discovery of radiation-resistant ceramics for the immobilization of plutonium and other actinides was recognized by the US Department of Energy’s Office of Science as one of the top 101 scientific innovations of the last 25 years (Ewing and Weber 2001).

Rod’s contributions to understanding radiation effects in nuclear waste forms (glass and ceramic) are well-documented in several highly cited critical reviews (Ewing, Weber, and Clinard 1995; Weber et al. 1997; Weber et al. 1998; Weber et al. 2019). The main conclusion of these reviews was that radiation effects in glass waste forms are minimal, resulting in small changes in structure and chemical durability. While crystalline ceramics are often more sensitive to radiation effects, they exhibit greater chemical stability, even when highly damaged. As Rod would note in conversations with Weber, crystalline zircon is the most durable mineral in the Earth’s mantle, and metamict (amorphous) zircon is likely the second-most durable mineral.

Plutonium immobilization and ceramic waste form development

At the end of the Cold War, Russia and the United States agreed to reduce their nuclear weapons stockpiles through the Strategic Arms Reduction Treaty I (START I) and then Strategic Arms Reduction Treaty II (START II). The weapons’ retirement would result in large quantities of weapons-usable separated plutonium that, according to the US National Academy of Sciences, posed a “clear and present danger” (National Research Council 1994). In the United States, the Department of Energy managed the decision on the best path forward. Whereas Russia preferred to reuse the plutonium as “mixed-oxide” (or MOX) fuel for their Soviet-era water-water energetic reactors (most commonly known as WWER or VVER, which are pressurized, light-water-cooled and -moderated reactors similar to Western pressurized water reactors). In the United States, the choice was between MOX fuel for light-water reactors and “immobilizing” the plutonium in a solid form—either glass or ceramic. This resulted in a renewed interest in crystalline nuclear-waste forms (Oversby et al. 1997; Ewing 1999), with now a focus on mineral phases such as those of the apatite group, the monazite group, zirconolite, zircon, and pyrochlore group (Ewing et al. 1995).

Further agreements between Russia and the United States on plutonium disposition clarified methods, types of facilities, and timelines to be used to make this material less risky. Both countries offered to dispose of 34 metric tons of their stockpiled weapons-grade plutonium. In 1997, the US Department of Energy decided to follow a hybrid plan in which they would use up to 33 metric tons of clean plutonium metal (including 25 metric tons covered in the agreement with Russia) to fabricate MOX fuel for use in civilian light-water reactors and 9.5 metric tons of impure plutonium metal and oxide that would be immobilized in a can-in-canister method. The can-in-canister method was designed to meet the “Spent Fuel Standard,” in which a method of storing plutonium proves to be as resistant to theft and proliferation as with plutonium in highly radioactive spent nuclear fuel. In the can-in-canister method, plutonium would be immobilized in either glass or ceramic in cans placed in a rack in a larger canister, and with high-level nuclear waste glass poured around the cans, providing a radiation barrier like that of spent fuel.

Later in 1997, the US Department of Energy decided to use ceramic to immobilize the plutonium based on four criteria: the durability of the waste form in a repository, the lower dose to workers producing the waste form, the lower costs for ceramic production due to lower worker doses and high concentrations of plutonium possible in ceramic, and the greater proliferation resistance of ceramic over glass. While the exact type of ceramic waste form itself was still up for discussion (see discussion in Macfarlane 1998), the Department of Energy was considering a waste form that included pyrochlore, zirconolite as well as brannerite and rutile. Rod also weighed in on this discussion, proposing zircon as a viable phase (Ewing 1999).

In 2000, the Department of Energy abandoned the hybrid approach to plutonium disposition and solely pursued the MOX option (US Department of Energy 2002a). This approach fell victim to rising costs when in January 2016, the Obama administration announced the United States would stop the construction of the almost-completed MOX fuel fabrication facility (for background, see Lubkin 2017) in favor of diluting plutonium with an “adulterant” (an undescribed material added to dry blend with plutonium to produce a mixture that is stated by the Department of Energy to be more secure). By October of that year, Russia suspended its side of the bargain, claiming that the US shift in disposition method violated the US-Russia Plutonium Management and Disposition Agreement by no longer being strongly proliferation resistant (Dolzikova 2016). In 2024, the Department of Energy formalized its plan to follow a “dilute-and-dispose” approach (plutonium down-blending) of managing the 34 metric tons of plutonium declared as excess to nuclear weapons needs (National Nuclear Security Administration 2024)—thereby confirming the United States is no longer pursuing the isolation of plutonium in a ceramic waste form.

Glass-ceramic waste form development: combining the benefits

Glass-ceramics are composed of a fine-grained mixture of both crystalline phase(s) and a glass matrix. Celsian (an uncommon feldspar mineral, barium aluminosilicate of chemical formula BaAl₂Si₂O₈) glass-ceramics were first developed at the Hahn-Meitner Institut, and radiation effects in curium 244 and plutonium 238-doped samples were investigated in the late 1970s (Turcotte et al. 1982; Routbort et al. 1982; Malow et al. 1980).

Glass-ceramics are considered versatile advanced waste forms with some apparent advantages over conventional borosilicate glasses and multi- or single-phase ceramics. They combine the chemical and processing flexibilities achievable with glass waste forms with the high chemical durability of ceramic waste forms (Zhang et al. 2022; McCloy and Goel 2017). The potential use of glass-ceramics for the immobilization of actinide-bearing radioactive wastes was considered for the treatment of impure plutonium-bearing wastes containing substantial quantities of process impurities and/or glass-forming components (Hobbs et al. 2012, Stewart et al. 2013). Preferential partitioning of the actinides into the ceramic phase (zirconolite), while the glass-forming impurities resided in the glass was crucial to the design of the glass-ceramic waste form. In this way, the excellent durability and criticality control of ceramic waste forms can be maintained for the actinide within the glass-ceramic. Importantly, the chemical durability of the resulting glass-ceramic, as assessed from short-term standard test protocols, showed similar plutonium release rates to those found from fully crystalline ceramics (Zhang et al. 2017).

Similarly, a glass-ceramic waste form was developed to immobilize partly calcined powders resulting from reprocessed naval reactor fuel in the United States. These calcines are currently stored in silos at Idaho National Laboratory. Initially, an 11-member committee, including Rod, evaluated several options for the management of this waste. The committee was not able to recommend one final waste form without further information stating that “it is the committee’s firm view that the interim storage of calcine in the bins should be maintained at least until such time as it becomes clear (1) where the material can be sent, (2) what disposal form(s) is/are acceptable, and (3) that an approved transportation pathway to the disposal site is available.” The committee further recommended that aggressive efforts be made to determine where the waste was ultimately to be disposed of, how the waste was to be transported, and what waste form requirements would result (National Research Council 2000). Different preferred options were subsequently identified by the state of Idaho (vitrification into borosilicate glass) and the Department of Energy (a treatment option suitable for disposal at a yet-to-be-selected site) (US Department of Energy 2002b).

In 2009, the Department of Energy issued a record of decision to pursue a glass-ceramic waste form with waste loading as high as 80 weight percent designed with hot-isostatic pressing (HIP) processing as the preferred treatment option (Amended Record of Decision 2009). However, a 2016 independent analysis of alternatives recommended that a “final decision regarding the processing technology should be deferred until the disposal path is better defined, as well as its expected regulatory framework, and resulting waste form performance requirements” (Case et al. 2016), which was a similar conclusion to an earlier National Research Council (2000) study.

Though these studies highlight some of the benefits of a glass-ceramic waste form for HLW there are further important considerations, for example, the impact of radiation-induced damage and the possibility of microcracking due to radiation-induced differential volume expansions of ceramics and glasses. This was highlighted and discussed in Ewing and Weber’s highly cited review (Weber et al. 1998).

Current developments and future perspectives

Vitrification is a mature technology that has operated at an industrial scale for decades in several countries—including in the United States, France, the United Kingdom, Germany, Belgium, Russia, India, and Japan—to treat certain types of HLW into glass waste forms for future geological disposal. The process technology employs either joule-heated ceramic melters (a melter in which electrodes immersed in the glass generate heat by the joule heating due to electric current passing between electrodes through glass) or hot-wall induction melters to directly heat mixtures of glass frit (porous enough for gas or liquid to pass through) and nuclear waste to a molten state at temperatures up to 1150 degrees Celsius. The glass melt is then poured into stainless steel canisters where it slowly cools and solidifies to yield the glass waste form ready for disposal.

Recent progress has seen the development of alternative melter options, such as the cold-crucible induction melter. This was commissioned in France and provides higher-temperature processing and therefore accepts a broader range of wastes, including more refractory glass compositions. Portable or rapidly transportable melters, such as types of in-container vitrification are also being developed in several countries, and their future application promises to make decommissioning of legacy nuclear sites faster and more efficient (McCloy et al. 2024).

Despite their potential, ceramic waste forms are not currently produced industrially anywhere in the world. Though existing melters have been investigated for their production (e.g., Amoroso et al. 2014), in general, more specialized processing technologies are being developed for ceramic and glass-ceramic waste forms, including pressureless sintering and pressurized sintering. (Sintering is a process of compacting and consolidating material by thermal treatment.) The conventional pressureless route is already deployed in the nuclear fuel cycle to reaction-sinter uranium dioxide or uranium-plutonium MOX fuel pellets. Hot-isostatic pressing technology is being developed at ANSTO in Australia as a key element of its synroc technology. HIP processing consolidates the ceramic into a dense monolith within a stainless-steel canister at high temperatures (typically 1000 to 1300 degrees Celsius) and pressures (between approximately 30 and 100 megapascals). ANSTO is currently commissioning a first-of-a-kind Synroc Waste Treatment Facility (SWTF) for the treatment of their intermediate-level waste, or ILW, streams. This facility is fully automated and transforms the liquid ILW and waste-forming additives into a tailored granular powder that is subsequently consolidated through hot-isostatic pressing into a dense monolithic waste form. HIP technology is highly flexible and can be applied to produce all three classes of HLW waste form: glass, glass-ceramic, and ceramic waste forms, and offers distinct advantages in terms of waste loading and suppressing volatile losses. To date, several waste form selection activities have explicitly listed the technical maturity of the process as a key reason for selecting glass. The full-scale operating experience provided by the SWTF is likely to sway this decision-making moving forward. Other novel methods, such as microwave sintering and spark plasma sintering, are also under development at lower technology readiness levels in various countries for the fabrication of ceramic materials.

Though vitrification is well-established for HLW treatment with substantial international scientific and engineering maturity, many HLWs that currently exist or are being projected from future nuclear technologies or fuel cycles will challenge the use of glass waste forms and/or its production technology. Ceramic and glass-ceramic waste forms provide feasible solutions to such wastes and are therefore considered complementary technologies that join vitrification to provide a portfolio of waste treatment options to the international community, including for actinide wastes, molten salt wastes, and technetium and iodine wastes.

Most major nuclear power producers—such as the United States, Russia, France, the United Kingdom, and Japan—have policy strategies involving a nuclear solution (i.e., irradiation-induced transmutation in a reactor or by an accelerator) for their growing inventories of separated civilian plutonium; however, immobilization in a ceramic waste form and subsequent disposal in an appropriate geologic environment is still under consideration (Weber et al. 2019; Corkhill et al. 2024). An alternative ceramic phase is a modified or “low-specification” MOX fuel, particularly given the technical maturity of MOX production. This was first proposed by Macfarlane and co-workers (2001) and more recently investigated for the UK plutonium stockpile (Corkhill et al. 2024).

Rod’s more recent work called for more consideration to be given to the management and disposal of nuclear waste streams from small modular reactors (SMRs) (Krall et al. 2022). Rod and co-workers noted that, despite their size, water-, molten salt-, and sodium-cooled SMR designs increase the volume of nuclear waste requiring management and disposal per unit of power produced. This is partially due to the use of chemically reactive fuels and coolants in SMR designs, such as metallic sodium, metallic uranium, and uranium tetrafluoride. The advanced Generation IV reactors currently being commercially developed, together with their associated fuel cycles, also present significant waste treatment challenges (Bonano et al. 2023).

A recent study by the International Atomic Energy Agency (2019) emphasized potentially problematic wastes expected to be generated from innovative reactor designs and their corresponding nuclear fuel cycles. Before disposal, these materials will require treatment and conditioning and both ceramic and glass ceramic waste forms may provide feasible future solutions. For example, hot‐isostatically pressed sodalite glass-ceramic waste forms for lithium chloride/potassium chloride eutectic salts were developed in the United States (Lewis et al. 1993; Morss et al. 1999) and Australia (Vance et al. 2012). Similarly, alkali fluoride salts from fluoride molten salt reactors can be immobilized in hot‐isostatically pressed fluorite glass-ceramic waste forms (Gregg et al. 2020b). Though technetium 99 is a low energy (0.29 megaelectronvolt), beta-emitting fission product, its long half-life of 211,100 years, environmental mobility, and volatility make it challenging to immobilize. Hot‐isostatically pressing to consolidate technetium 99 into durable synroc ceramic waste forms demonstrated promising results (Vance et al. 1997). Finally, with regards to radioiodine, iodine 129 is of particular concern due to its long half-life (1.6 x 10⁷ years), contribution to dose consequences, high aqueous solubility, and high mobility in most geological environments. Several ceramic and glass-ceramic waste form designs are also currently under consideration (Asmussen et al. 2022; Riley et al. 2016).

Although multiphase ceramic and glass-ceramic waste forms have shown promise for the immobilization of challenging nuclear wastes in both current and future fuel cycles, an area requiring attention is the establishment of long-term corrosion models that predict their performance, particularly that of their radiation-damaged state, in a geological repository over extremely long timelines. Such models are already in place for glass waste forms—a result of dedicated research over decades by the international community—and are essential in the qualification of the waste form for repository acceptance (Gin et al. 2013; Grambow 1985). Collective work is now required by the international community to establish such models for ceramics and glass-ceramics to unlock their true potential in HLW treatment.

Concluding remarks

Rod Ewing worked in nuclear waste forms for almost 50 years and was involved in several important and well-recognized advances in the development of highly durable waste forms. A major concern that he continued to share with the community was that the waste form, mineralogy, and materials science occupied a role of secondary importance to the geological barriers in strategies concerning waste form disposal (Ewing 2001).

“It is my opinion that this secondary role is very much a result of the method of analysis used in evaluating the performance of a nuclear waste repository.”

He noted that the impact of important improvements in the materials properties of nuclear waste forms (e.g., low rate of corrosion or high resistance to radiation damage) were lost in the uncertainty generated in other parts of the repository-system analysis. Nonetheless, Rod’s fundamental belief was that ceramics could provide superior physical-chemical performance to glass-based nuclear waste forms and that studies of natural analogues and systems provided useful complementary information to laboratory-based studies. Rod dedicated much of his time and efforts to highlighting and resolving such issues with emphasis on the materials science and engineering, but he was also critical of the progress of the US nuclear waste disposal program, as described in the following quotes:

“Meanwhile, the US program is an ever-tightening Gordian Knot—the strands of which are technical, scientific, logistical, regulatory, legal, financial, social and political—all subject to a web of agreements with states and communities, regulations, court rulings and the Congressional budgetary process. There is no single group, institution or governmental organization that is incentivized to find a solution, nor is any single institution entirely responsible for the failure of the US Program.” (Benson et al. 2018)

“[T]he scientific community should reflect on how to improve and expand the role of scientists and engineers. The disposal of nuclear waste is more than just normal science and engineering projects strung together in the chapters of a safety analysis that is thousands of pages long. Success requires original thinking and an entirely different approach than the one we have all participated in during the past 50 years.” (Ewing 2021)

Rod’s contribution to advancing the fundamental understanding and practical aspects of radiation effects in materials extends beyond nuclear waste forms, across many classes of materials and applications. Many of his students and post-doctoral researchers went on to make considerable contributions to the physics of radiation damage in materials, both from a fundamental science perspective and an applied perspective about nuclear waste form research and development. Perhaps Rod Ewing’s most endearing (and enduring) quality was his never-give-up attitude in taking on one of the most challenging problems that modern society faces in the immobilization and long-term disposal of nuclear wastes. As scientists who have shared this field with him, we will miss Rod’s expertise, support, and unwavering dedication.