In the face of the climate crisis—arguably the biggest challenge humanity has ever faced—an enterprising “all hands on deck” spirit has emerged for developing energy technologies, both new and old. As such, support for nuclear energy has undergone a dramatic U-turn: most experts agree that short-term decarbonization of the global economy will be impossible without developing nuclear power. Nuclear fission has a very low carbon footprint, is weather-independent, can directly replace fossil-fuel-powered plants, and is more scalable than renewables—factors that have led many prominent energy investors such as Bill Gates to push for nuclear power as an essential component of the solution to the climate crisis.
The types of nuclear power plants that Gates and others are backing are, however, fundamentally different from the old generation of light-water reactors, whose designs date back many decades. New designs, termed advanced reactors, vary greatly and offer many benefits, including better safety and a reduced cost and waste footprint. While the private industry pushes forward with building these new reactors, the federal government has been working in a coordinated effort to support the successful development and deployment of this advanced technology.
In this article, we highlight some of the research that is underway as part of two Department of Energy (DOE) Advanced Research Projects Agency–Energy (ARPA-E) programs related to advanced reactors: ONWARDS (Optimizing Nuclear Waste and Advanced Reactor Disposal Systems) and CURIE (Converting UNF Radioisotopes Into Energy).
Nuclear power already provides about 20% of the electricity in the US, or about half the nation’s carbon-free energy. However, many US nuclear reactors are shutting down prematurely: 12 have permanently closed since 2012, with many more at risk of retirement in the coming years. This decline is not new—the number of nuclear units has been decreasing since 1990. This is because they are comparatively cost-intensive in some markets—the cost of nuclear energy has spiked in recent years, peaking in 2012, in part from increasing regulations to address public safety concerns—and also because they have been maligned for perceived environmental impacts as well as potential terrorism risks. Unfortunately, when a reactor is shut down, the lost electricity is usually replaced by fossil fuel sources. A recent article in Vox described how the Indian Point reactor in New York state, which closed down in spring 2021, was replaced largely by natural gas. With each nuclear reactor shutdown, we make backwards progress on our essential carbon-reduction commitments.
The problems causing the decline are far from intractable, however. The majority of reactors in the current US reactor fleet are of Generation II design (1965–1996). Generation III (1996–2016) and IV (2016–present), also known as advanced reactors, offer significant improvements which tackle safety, cost, and waste issues. Specifically, these include improvements in fuel technology with reduced waste, higher thermal efficiency, significantly enhanced safety systems (including passive nuclear safety), stronger reinforcement against improbable aircraft attacks, longer operating lifetimes, and standardized designs intended to reduce maintenance and capital costs.
Advanced reactor designs can be broadly grouped into two types: those cooled by water, such as the small modular reactors, and those that are not, such as molten salt reactors (which use fluoride and chloride salts), sodium-cooled reactors, high-temperature gas-cooled reactors, gas-cooled fast reactors, and micro-reactors.
What are the problems with advanced reactors?
Although advanced reactors promise to solve many of the problems associated with conventional nuclear plants, they also create new challenges that need to be addressed, varying from the specific fuel details through to global issues concerning infrastructure. For instance, some advanced reactor designs use conventional fuel but at higher enrichment: high-assay low-enriched uranium (HALEU), containing 10–20% uranium-235. There has been little investment in HALEU production infrastructure in the US and Europe, and the main supplier is currently Russia. Lack of investment in the domestic market comes down to a chicken-and-egg situation— investors want to see robust demand first, but advanced reactor technology is stalled without a consistent supply of the fuel.
Another question regards fuel recycling, or reprocessing. Our used nuclear fuel— currently 86,000 metric tons stored in spent fuel pools and dry casks at more than 70 reactor or former reactor sites across the country—is destined for permanent disposal, even though more than 90% of its energy remains. Although the US does not currently recycle spent fuel, several foreign countries do (notably France, Russia, and Japan), creating mixed-oxide (MOX) fuel, a mixture of fissile plutonium and uranium oxides. This increases the efficiency of the original uranium fuel by 25–30% and reduces the volume of high-level waste to about one-fifth. It also reduces overall radioactivity levels in the waste. Despite these benefits, it has been argued that reprocessing increases proliferation risk by encouraging increased separation of plutonium from spent fuel in the civil nuclear fuel cycle. For this reason, President Carter
banned the technique in 1977 after India demonstrated nuclear weapons capabilities using reprocessing technology in the previous year. Although Carter intended the US to set example for the world, discouraging domestic reprocessing has had minimal effect on the policies of foreign countries.
In 1981, President Reagan lifted the moratorium. However, reprocessing continued to be viewed unfavorably by following administrations and no investment was made in the technology until 1999 when the DOE commissioned a MOX fabrication facility at the Savannah River Site, which was designed to reprocess Cold War-era plutonium pits extracted from decommissioned weapons. The facility was terminated in 2021 due to escalating costs and missed construction deadlines. To date, although three civil reprocessing plants were built before 1977, the US has not yet recommitted to recycle spent nuclear fuel and thus close the US nuclear fuel cycle.
Waste disposal: Advanced reactors change the equation
Waste disposal is one of the most persistent challenges in nuclear energy. Although waste generated by conventional light-water reactors is a known quantity and has an established disposal path, this path is widely criticized as inefficient and unsustainable, and a final repository location for high-level waste has yet to be decided on in the US. Advanced reactor technologies, however, change the equation. On the one hand, they provide new opportunities for innovation that could lead to lower waste volumes and better management. The ONWARDS program, for instance, aims to reduce waste by a factor of tenfold compared to conventional nuclear reactors and provide waste forms for new fuel cycles. On the other hand, these technologies may potentially create new problems. One study from Stanford researchers in 2022 indicated that some prototype small modular reactor designs could create nine times more neutron-activated steel than conventional power plants due to neutron leakage. Although this is not the final conclusion for advanced reactor technology, it highlights the need for research and development in the back end of the nuclear cycle.
ARPA-E funding programs
In 2018, MIT released an influential study, The Future of Nuclear Energy in a Carbon-Constrained World, emphasizing the need for coordination between federal government and industry to successfully develop nuclear energy in the 21st century. To this end, DOE has recently provided billions of dollars in funding to support development and deployment of advanced reactors in support of carbon-free energy, particularly focused on problems which are not easily solved by industry alone. As part of this effort, ARPA-E has established a suite of complementary programs:
|MEITNER 2018||Awards funds to enable designs for lower cost and safer advanced reactors.|
|GEMINA 2019||Supports projects that use advanced computing techniques to model reactor operations for existing nuclear plants to bring down fixed operations and maintenance costs.|
|ONWARDS 2021||Examines the back end of the nuclear cycle, aiming to reduce waste volumes from advanced reactors and address related issues such as reprocessing, safeguards, and waste forms.|
|CURIE 2022||Seeks to develop technology relating to used nuclear fuel reprocessing (including chemical separations and facility designs), material accountancy, and online monitoring techniques.|
ONWARDS ambitiously aims to reduce waste from advanced reactors by a factor of tenfold compared to light-water reactors, either as a total of waste volume generated or as reduction in size of repository footprint. The program targets both open (once-through) and closed (reprocessing) fuel cycles, investigating issues relating to reprocessing, recycling, safeguards, and waste forms. Specifically, funding has been given to projects that reduce waste volumes, increase fissile fuel use, improve accountability of nuclear materials and their intrinsic resistance to proliferation, bolster advanced reactor commercialization, and develop high-performance waste forms suitable for all advanced reactor classes.
In March 2021, ONWARDS provided $36 million in funding for 11 new research projects, led by universities, private companies, and national laboratories: General Electric Global Research, TerraPower, Citrine Informatics, Rutgers University, Rensselaer Polytechnic Institute, Orano Federal Services, Brigham Young University, Idaho National Laboratory, Oklo, Stony Brook University, and Deep Isolation. Three of these projects are highlighted below.
Idaho National Laboratory
Researchers at Idaho National Laboratory (INL) aim to develop a waste recycling method for metallic fuels used in several advanced reactor designs. These metallic fuels are similar to those used in early experimental fast reactors, such as Clementine (plutonium metal; see Actinide Research Quarterly Second Quarter 2022) and EBR-II (zirconium-uranium alloys). The researchers conceived their idea when they examined historical data and noticed that when this type of spent fuel was heated, there was a natural phase separation between the layers, separating actinide, lanthanide, and alkaline earth components, along with precipitation of alkali metal solids.
By using a zone-refining furnace, which works using magnetic induction, used fuel can be melted with precision control to achieve phase separation and extraction of the high-level radioactive waste. A small-scale device has already demonstrated proof of concept with simple uranium slugs, but the team will expand this work to examine used fuel surrogates and look at ways to improve the technique, as well as gauge its economic feasibility.
Reprocessing and proliferation: What’s the risk?
The primary proliferation concerns associated with civilian nuclear programs come from uranium enrichment, which gives the most straightforward access to fissile materials that can be used in a nuclear weapon. Nevertheless, reprocessing still represents a risk in the eyes of many experts, as it often involves separation of plutonium, and some maintain that the best protection of the back-end of the fuel cycle is to forego reprocessing entirely.
About 1% of used fuel from light-water reactors is made up of plutonium, and around one-half to two-thirds of this is fissile (plutonium-239 and plutonium-241). However, a significant amount of the remainder is made up of plutonium-240: reactor grade plutonium is defined as plutonium with 19% or more of plutonium-240 (sometimes known as “civil plutonium”). This isotope is extremely problematic for weapons use as it has a high rate of spontaneous fission, with accompanying neutron emission. As recognized during the Manhattan Project, a weapon made from high plutonium-240 would result in a high neutron flux when triggered, causing a predetonation, or a “fizzle.” However, a lower yielding “fizzle bomb” could still cause significant damage in an urban area. Therefore, the IAEA conservatively classifies all isotopes of plutonium as “direct-use” material, that is, “nuclear material that can be used for the manufacture of nuclear explosives components without transmutation or further enrichment.”
In response, treaties and safeguards have been put in place by the IAEA to protect the back end of the fuel cycle from proliferation risks (safeguards are activities that allow the IAEA to verify compliance of commitments not to use civil nuclear programs for weapons purposes). MOX is widely used in light-water reactors in Europe and Japan (40 reactors in Europe and 10 in Japan). China and Russia meanwhile are new countries to embark upon MOX use, albeit with a focus on fast reactors. All of these reprocessing facilities are government-run entities that adhere to IAEA protocols.
A breeder reactor is a type of advanced reactor that generates more fissile material than it consumes. Specifically, fast breeder reactors generate plutonium from a uranium-238 blanket that surrounds a MOX or high-enriched uranium (HEU) core. At present, there are only two commercially operating breeder reactors worldwide, both Russian sodium-cooled reactors, but three of the six Generation IV reactor designs currently under development are fast breeder reactors.
Some breeder reactor designs can include schemes which separate plutonium-239 (i.e., weapons-grade plutonium) using the PUREX process, which could present significant security and safeguards challenges—far more serious than those posed by reactor-grade plutonium—leading to potential problems with nonproliferation treaties. New reprocessing methods are however being designed that do not isolate fissile plutonium. These methods are specifically targeted by the DOE ONWARDS and CURIE programs where no pure fissile stream is generated throughout the processing.
Reprocessing is no different in principle to any scheme that separates metals from mineral ore mixtures. There are three overall types of metallurgical treatments used at smelters and refineries:
Hydrometallurgy. This type of reprocessing method uses aqueous solutions to dissolve metals and sometimes also employs electrolytic cells to separate them (e.g., zinc production, copper refining). The PUREX process is a hydrometallurgical process.
Pyrometallurgy. Heat is used to separate metals from their mineral ore (e.g., copper smelting to produce blister copper, lead smelting).
Electrometallurgy. Often called pyroprocessing because it occurs at high temperatures, this uses electric current to separate metals (e.g., alumina smelting to produce aluminum). Electrometallurgical techniques are the main focus of interest for developing future nuclear fuel reprocessing methods, which recover all actinide ions together (i.e., uranium and plutonium) and therefore reduce the risk of proliferation.
In summary, although reprocessing of used nuclear fuel poses some proliferation risk, particularly for transportation, that risk is relatively minor for reactor-grade plutonium and can be reliably safeguarded. Fast breeder reactor schemes that propose using the PUREX process to isolate plutonium-239 are being replaced with improved reprocessing methods that separate actinide fuel components without isolating plutonium. Furthermore, recovery and recycling of plutonium from long-lived waste before deposition eliminates the possibility of plutonium-239 being extracted from used fuel, which may not be reliably safeguarded. As such, reprocessing may actually increase the proliferation resistance of the fuel cycle.
Using the largest of the ONWARDS grants ($8.6 million), TerraPower and its collaborators (New Mexico State University, and Idaho and Savannah River national laboratories) aim to develop an experimental method for the recovery of uranium from used nuclear fuel by harnessing the volatility of chloride salts at high temperatures. Chlorination of used nuclear fuels, either oxide-based or metallic, is possible using chlorine gas or carbon tetrachloride at elevated temperatures. The resulting chloride salts have varying levels of volatility at high temperatures, which may allow for bulk separation of uranium from fission products and plutonium, either for recycling or reduced volume waste disposal. This proposal builds on previous chloride-based volatility studies conducted during the Manhattan Project through to the early 1960s, with a new angle of aiming to reduce waste footprints.
By adjusting chloride-based volatility parameters and separating uranium, waste volumes could be reduced by factors of as much as 10–20 times, according to TerraPower. The team aims to improve chlorination rates by optimizing basic process parameters in a way that would also be suitable for scale-up in a commercial-scale facility. They will start with surrogate oxide used nuclear fuels to synthesize chloride salt mixtures and then later will demonstrate the method using actual used nuclear fuel.
Deep Isolation, in partnership with the University of California, Berkeley, Lawrence Berkeley National Laboratory, and NAC International, received a $3.6 million grant to develop a novel universal canister system for advanced reactor waste streams. This canister will be suitable for storage, transportation, and long-term geological disposal of high-level waste, eliminating the difficulties and cost of repackaging between sites.
The design will account for the nuclear industry’s current dry storage and transportation infrastructure and will meet various waste acceptance constraints across a range of geologic repository options. This includes both conventionally mined tunnels and Deep Isolation’s own proposal: a deep borehole repository that leverages directional drilling in which 18-inch holes suitable for accepting the waste canisters could be configured horizontally, vertically, or slanted.
Deep Isolation is also involved with another ONWARDS-funded project, a $4 million joint venture with Oklo Inc., and Argonne and Idaho national laboratories that aims to perform R&D to develop the first nuclear fuel recycling and disposal facility in the US. The electrorefining facility will ultimately produce fuel for metal-fueled advanced reactors, closing the fuel cycle. Deep Isolation will identify waste forms that will take the waste stream from the electrorefining facility to a deep borehole repository.
The CURIE program aims to develop innovative reprocessing technologies that substantially reduce the volume, heat load, and radiotoxicity of waste requiring permanent disposal. In this regard, the program targets separations technologies and safeguards, including material accountancy and online monitoring technologies. Furthermore, it also seeks to develop a closed fuel cycle in which these technologies provide a sustainable fuel feedstock for advanced fast reactors. To achieve this, CURIE is funding designs for a reprocessing facility that will enable UNF recycle without the generation of pure plutonium streams, incorporate in situ process monitoring, minimize waste volumes, enable a low fuel cost for advanced reactor fuels, and maintain low disposal costs.
In 2022, CURIE provided $38 million in funding for a dozen projects, similar in scope to the ONWARDS portfolio. Seven of the twelve grants explore methods for recycling used nuclear fuel: University of Alabama at Birmingham, Argonne National Laboratory (two grants), University of Utah, CurioTM, INL, and Mainstream Engineering. The remainder focus on safeguards for advanced reactors (GE Research, University of North Texas), materials accountancy (NuVision Engineering, University of Colorado, Boulder), and recycling facility design (EPRI).
As part of a coordinated federal effort to assist the development and deployment of advanced nuclear reactor technology by the nuclear industry, the DOE ARPA-E division has recently established a suite of four complementary grant programs. Each focuses on a different aspect of the technology: MEITNER funds projects for advanced reactor designs that reduce cost and increase safety; GEMINA supports efforts that reduce reactor operations costs using advanced computer modeling; ONWARDS aims to reduce waste volumes from advanced reactors and address related issues such as reprocessing, safeguards, and waste forms; and CURIE seeks to develop technology relating to used nuclear fuel reprocessing (including chemical separations and facility designs), material accountancy, and online monitoring techniques. A total of $165 million has been awarded so far by these ARPA-E programs, whose recipients include academic institutions, national laboratories, and private industry.
Many thanks to Bob Ledoux and Jenifer Shafer for their expertise.