Two reports agree: Diverse advanced reactor fuel cycles can succeed

According to Argonne senior nuclear engineer Taek Kyum Kim, first author of the DOE report, “All told, when it comes to nuclear waste, SMRs are roughly comparable with conventional pressurized water reactors, with potential benefits and weaknesses depending on which aspects you are trying to design for. Overall, there appear to be no additional major challenges to the management of SMR nuclear wastes compared to the commercial-scale large LWR wastes.” (The DOE report puts all three reactors it studied under the SMR umbrella, despite their different coolants, sizes, and claims to modularity; the TerraPower and X-energy designs could also be classed as non-LWRs.)

Confronting timely questions: As expectations have grown in recent years that new nuclear reactors will play a significant role in fighting climate change, much has been said about the safety and benefits of reactor designs. However, authoritative reports on the fuel cycle and waste management aspects of advanced reactor deployments have been lacking.

In that information vacuum, a paper titled “Nuclear Waste from Small Modular Reactors” was published on May 31 in the Proceedings of the National Academy of Sciences (PNAS), contending that “SMRs will produce more voluminous and chemically/physically reactive waste than LWRs, which will impact options for the management and disposal of this waste.” Like the recent DOE report, it attempted to quantify the waste considerations of specific reactor designs, but some of the paper’s methods and conclusions were questioned.

In the DOE’s recent report, Kim, two of his Argonne colleagues, and a researcher at Idaho National Laboratory analyzed many of the same aspects of reactor designs using reactor design and performance parameter information available in open literature (as did the PNAS paper). As stated in the report, “Conducting a credible assessment of the nuclear wastes generated from nuclear power plants requires wide-ranging information. . . . Because reactor concepts are evolving as they are optimized, the nuclear waste attributes should be assessed using the latest design information, emphasizing those designs moving forward toward demonstration or deployment in the near term.” After the DOE authors consulted open literature for the most recent design details, “a draft of the completed report was provided to the vendors of the three SMRs to ensure the open literature information was not misinterpreted.”

According to Kim, in terms of nuclear waste, each reactor studied offers both advantages and disadvantages over large LWRs. “It’s not correct to say that because these reactors are smaller they will have more problems proportionally with nuclear waste, just because they have more surface area compared to the core volume,” he said. “Each reactor has pluses and minuses that depend upon the discharge burnup, the uranium enrichment, the thermal efficiency, and other reactor-specific design features.”

NASEM report—Toward national strategies: The NASEM Committee on Merits and Viability of Different Nuclear Fuel Cycles and Technology Options and the Waste Aspects of Advanced Nuclear Reactors, chaired by Janice Dunn Lee, released its report on November 22. The committee concluded that “importantly, advanced reactors and their associated fuel cycles would not eliminate the requirement for geologic repositories for some radioactive wastes because even advanced reactors will require disposal of radioactive fission products.”

According to the report summary, “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. . . . 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.’”

The 317-page NASEM report contained 20 findings, including the following:

• On geologic disposal: “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.”
• On safety: “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. . . . 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.”
• On reprocessing: “Conceptually, advanced reactors could be used to reduce the current inventory of transuranics in the approximately 86,000 [metric tons] 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.”

The NASEM report also made 15 recommendations, including the following:

• On downselecting designs: “Using data from the Advanced Reactor Demonstration Program and the [DOE’s] research and development programs over the next several years, DOE should select and support, with industry cost sharing, the development of a few promising advanced reactor technologies and fuel cycles that can be potentially deployed by 2050” (original emphasis).
• On reprocessing: “The current U.S. policy of using a once-through fuel cycle with the direct disposal of commercial spent nuclear fuel into a repository should continue for the foreseeable future. The once-through fuel cycle is the baseline, and any new fuel cycles should have advantages over that baseline for them to be deployed. However, so as not to preclude these options in the future, the [DOE] should continue fundamental studies to evaluate the feasibility of using recycling and transmutation for closing fuel cycles.”
• On geologic disposal: “The immediate-future focus of the U.S. nuclear waste management and disposal program should be planning for the geologic disposal of the existing spent fuel that is presently stored at 79 sites in 35 states and the approximately 2,000 metric tons per year being generated by existing commercial reactors.”
• On fuel cycle project costs, including repositories and reprocessing: “Congress and the [DOE] should obtain an independent assessment of cost estimates of various scenarios for potential deployment of advanced reactor technologies and related fuel cycle components. The independent assessor should have expertise in evaluating large-scale construction projects; examining project management challenges; and understanding technological and financial risks, as well as their uncertainties.”
• On federal waste management responsibilities: “Congress should establish a single-mission entity with responsibility for the management and disposal of nuclear wastes.”

DOE report—How the demos stack up: Three reactor designs are expected to have demos producing commercial power by 2030 with cost-shared funding from the DOE: NuScale Power’s 77-MWe modular pressurized water reactor design using conventional LWR fuel that can be deployed in VOYGR plants with up to 12 modules (924 MWe); TerraPower’s Natrium, a 345-MWe sodium fast reactor with high-assay low-enriched uranium (HALEU) metallic fuel with energy storage designed to boost output at times of peak demand; and X-energy’s Xe-100, an 80-MWe high-temperature, helium-cooled reactor fueled with graphite pebbles containing HALEU TRISO fuel particles that can be deployed in a four-pack modular configuration (320 MWe).

The three reactors were compared to a large Generation II 1,175-MWe PWR using selected metrics developed during a comprehensive assessment of nuclear fuel cycles published by the DOE in 2014. These include depleted uranium as a metric for front-end wastes, five different types of metrics for spent fuel, and low-level waste volumes, all normalized per GWe-year of generation. The study compared the smallest unit of each reactor—a 1,175-MWe PWR, a 77-MWe NuScale module, a 345-MWe Natrium plant, and an 80-MWe Xe-100 module—and assumed a 60-year operating life and 90 percent capacity factor for each.

Key to the results for each design and each metric were enrichment, fuel burnup, thermal efficiency. “Many metrics showed only small changes versus the reference reactor,” according to the report conclusions. “This was especially true for the [NuScale design], where masses and volumes of wastes were slightly higher, due primarily to the smaller core size having moderately higher enrichment and slightly lower burnup.”

“Most of the back-end waste metrics for the two non-PWR reactors had notably lower values than the reference PWR, including large reductions in [spent nuclear fuel (SNF)] mass, SNF activity for most timeframes, SNF decay heat and for Xe-100, long-term radiotoxicity,” the report continued. There were two metrics where the generalization did not apply: the long-term activity and long-term radiotoxicity of Natrium spent fuel were both somewhat higher than the reference due to a higher normalized plutonium content, and the normalized SNF volume of Xe-100 was significantly higher because of the volume of graphite in the Xe-100’s TRISO-filled pebbles (which have lower decay heat and radiotoxicity than other spent fuel forms).

The authors concluded that “given the analysis results in this study and assuming appropriate waste management system design and operational optimization, there appear to be no major challenges to the management of SMR wastes compared to the reference LWR wastes,” adding that “the results of this study are only applicable to a once-through fuel cycle. Any of these reactors, including the reference LWR, could be used with fuel recycle, resulting in reductions in most waste attributes.”