2
Advanced Reactor Technologies
Advanced nuclear reactor technologies hold the promise for safer, more efficient, and more nimble designs than currently deployed nuclear technologies. Advanced reactor systems include small modular light water reactors (LWRs) as well as reactors that use non-water coolants and higher 235U enrichments (World Nuclear Association 2020). Many of these systems were considered in the past, and some prototypes were built and operated. However, advances in materials, fuels, and other enabling technologies have been incorporated into these advanced reactor systems that may improve safety or reduce cost. A common characteristic of most of these reactor technologies is the smaller reactor system size compared to current large LWRs; these systems could be as large as a few hundred MWe or as small as a few MWe (so-called microreactors).1 With the smaller size, the intent is to manufacture a major portion of plant components (or entire systems) in a controlled factory setting, which could reduce costs, including on-site construction costs. Multiples of these reactor system modules could be located on the same site to obtain the power capacity required. Installation of these reactor modules could be staggered over time to complement an owner-operator’s integrated resource plan or financing. This chapter examines the spectrum of advanced reactor technologies that are currently under development, including their design and safety attributes, fuels and materials development, and technology readiness and gaps.
DESIGN CHARACTERISTICS
As shown in Table 2-1, advanced reactor technologies can be characterized by their neutron spectrum, the nuclear fuel, and the coolant used for heat transport (Reitsma et al. 2020). The neutron spectrum refers to the kinetic energy of the neutrons in the reactor that causes the sustained fission chain reaction. Fission neutrons are born at high kinetic energies near 1 MeV (i.e., fast) and can be slowed down by collisions with a moderator to kinetic energies near ambient temperature, corresponding to ~0.025 eV (i.e., thermal). Fast reactors do not require a moderator to slow down the neutrons; thus, the fission reactions occur only at high neutron energies. Thermal reactors require a moderator (e.g., water or graphite) to slow down the neutrons, and most of the fissions occur at low energy. The nuclear fuel contains the fissionable material (e.g., uranium) that interacts with the neutrons. The coolant that circulates through the reactor core transfers heat produced in the reactor to the electrical generator
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1 Note that for electricity production, reactor size is quoted in MWe, but in some future applications, the thermal heat from the reactor (MWth) is directly used as an energy product. The thermal efficiency is the ratio of MWe to MWth, typically 33–50 percent.
TABLE 2-1 Classification of Advanced Nuclear Systems by Neutron Spectrum, Fuel, and Coolant
| Reactor Technology | Neutron Spectrum | Nuclear Fuela | Coolant | Example Reactor Designs |
|---|---|---|---|---|
| Small modular light water reactor | Thermal | UO2 | Water | NuScale, GEH-BWRX, Holtec |
| Liquid metal fast reactor | Fast | Uranium-metalb | Sodium | GE–Prism, TerraPower-GEH–Natrium |
| High-temperature gas reactor | Thermal | TRISO UCOb | Helium | X-Energy–Xe-100 |
| Gas fast reactor | Fast | UO2, UCb | Helium | GA–EM2, FMR |
| Fluoride-salt cooled reactor | Thermal | TRISO UCOb | FLiBec | Kairos–Hermes |
| Molten-salt-fuel-cooled reactor | Thermal or fast | UF or UCl saltb | Same as fuel | Terrestrial Energy–IMSR, Moltex, TerraPower |
| Heat-pipe-cooled reactor | Thermal or fast | TRISO or UO2b | Heat pipe | Westinghouse–eVinci, Oklo–Aurora,d BWXT–BANR |
a UO2 = uranium dioxide; TRISO UCO = tri-structural isotropic fuel of uranium oxycarbide; UC = uranium carbide; UF = uranium fluoride salt; UCl = uranium chloride salt.
b These advanced non-LWR designs use high-assay low-enriched uranium (HALEU, <20 percent 235U).
c FLiBe = lithium fluoride-beryllium fluoride molten salt.
d Oklo’s design may have changed.
SOURCE: Committee generated using F. Reitsma, M.H. Subki, J.C. Luque-Gutierrez, et al., 2020, Advances in Small Modular Reactor Technology Developments: A Supplement to IAEA Advanced Reactors Information System (ARIS), Austria: International Atomic Energy Agency, https://aris.iaea.org/Publications/SMR_Book_2020.pdf.
or other systems that directly utilize the heat. Many key reactor features (e.g., operating temperature, pressure, materials) are designed to ensure compatibility with the fuel and the coolant. For example, most existing nuclear plants are thermal fission reactors that use uranium in oxide form clad with zirconium alloys and cooled with ordinary water (“light water”) with structural members composed of steel alloys.
The different design characteristics of these advanced reactor systems are summarized in Table 2-2, and more detailed descriptions can be found in Appendix A. The reactor designs listed as examples are from U.S. developers and include most of the non-LWR design concepts.2 Except for the small modular LWRs, all of these advanced reactor systems are designed to have reactor outlet temperatures higher than those of conventional LWRs, resulting in 10–50 percent higher overall thermal efficiency—that is, thermal efficiencies of 35–50 percent for advanced reactors compared to 31–33 percent for LWRs. Assuming that these advanced designs can achieve comparable capacity factors to current LWRs,3 this design choice is a factor that helps to reduce the cost per unit of energy produced relative to current LWRs. It also reduces the amount of energy that is rejected to the environment and the associated cooling water usage. Water use could be eliminated entirely if dry cooling technology were used. This design choice eliminates the need to be sited close to large bodies of water and presents the possibility of siting in arid regions. However, using dry cooling technology could reduce the thermal efficiency of the plant as the temperature of heat rejection increases. Advanced reactor systems may require higher uranium enrichment levels than the 5 percent 235U currently used in LWRs. This would enable these reactor systems to operate for longer time periods with better fuel utilization. Reactor designers are considering 235U fuel enrichment levels in the range of 10–20 percent (i.e., high-assay, low-enriched uranium [HALEU] fuel), which would enable these fuel systems to achieve fuel burnups up to 2–3 times higher than the current fleet of LWRs. However, this more efficient use of the uranium fuel must be balanced against the higher costs of producing it as well as developing its attendant infrastructure, such as fuel fabrication facilities.
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2 The supercritical water reactor is not considered a viable advanced reactor design and was not considered in this study.
3 LWRs have achieved capacity factors of more than 90 percent. Advanced reactors may have lower initial capacity factors, similar to early LWRs (~50–60 percent), in the initial operation of demonstration plants. Given operational experience and continuous improvements in operations, higher capacity factors would be expected—for example, the EBR-II test reactor achieved capacity factors of ~80 percent.
TABLE 2-2 Selected Technical Characteristics of Advanced Small Modular Reactor Systems
| Reactor Type | Core Outlet Temperature and Pressure | Thermal Efficiency | Fuel Burnupa | Example Reactor Designs |
|---|---|---|---|---|
| Small modular light water reactor | ~560–590 K | ~31–33% | 5–6 atom% using shorter length LWR fuel rods | NuScale, Holtec, GEH–BWRX |
| Liquid metal fast reactor | ~70–140 bar ~750–850 K ~Few bars |
~35–40%b | 7–10 atom% using metallic fuel with recycle; >40% once-through with fuel shuffling | GE–Prism, TerraPower-GEH–Natrium |
| High-temperature gas reactor | ~1,000–1,100 K ~70–100 bar |
~43–50%b | 10–20 atom% using TRISO fuel | X-Energy–Xe-100 |
| Gas fast reactor | ~1,000–1,100 K ~70–100 bar |
~43–50%b | 14 atom% using UC or UO2 fuel in SiC clad | GA–EM2, FMR |
| Fluoride-salt-cooled reactor | ~900–950 K ~Few bars |
~42%b | Similar to HTGR using TRISO fuel with similar burnup | Kairos–Hermes |
| Molten-salt-fuel-cooled reactor | ~900–950 K ~Few bars |
~40–42% | High fissile burnup with dissolved fuel in coolant; burnup limits by reactivity issues | Terrestrial Energy–IMSR, Moltex, TerraPower |
| Heat-pipe-cooled reactor | ~750–800 K Low pressures |
~30% | 5–20 atom% using TRISO fuel | Westinghouse–eVinci, Oklo–Aurora,c BWXT–BANR |
a Atom% designates the percentage of fissile and fertile atoms that undergoes fission in the fuel. Note that 1 atom% is approximately equivalent to 9.5 GW-days/metric Ton U.
b Thermal efficiency depends on outlet temperature.
c Oklo’s design may have changed.
SOURCES: Committee generated using F. Reitsma, M.H. Subki, J.C. Luque-Gutierrez, et al., 2020, Advances in Small Modular Reactor Technology Developments: A Supplement to IAEA Advanced Reactors Information System (ARIS), Austria: International Atomic Energy Agency, https://aris.iaea.org/Publications/SMR_Book_2020.pdf; D. Petti, R. Hill, J. Gehin, et al., 2016, “Advanced Demonstration and Test Reactor Options Study,” INL/Ext-16-37867; D. Petti, R. Hill, J. Gehin, et al., 2017, “A Summary of the Department of Energy’s Advanced Demonstration and Test Reaction Options Study,” Nuclear Technology 199:111–128, https://doi.org/10.1080/00295450.2017.1336029; Nuclear Energy Agency—Organisation for Economic Co-operation and Development, 2021, “Advanced Nuclear Reactor Systems and Future Energy Market Needs,” NEA No. 7566, Nuclear Technology Development and Economics, Paris: Nuclear Energy Agency, https://www.oecd-nea.org/upload/docs/application/pdf/2021-12/nea_7566_arfem.pdf.
SAFETY CHARACTERISTICS
The safety of a nuclear reactor system depends on a set of safety functions that must be satisfied to control the reactor and ensure its safety given the occurrence of an accident. The safety functions include
- Control of reactor reactivity4 during startup, operation, and shutdown.
- Control of heat removal to an ultimate heat sink.
- Containment of radiological materials.
These safety functions should guarantee reactor control under normal operation, shutdown when called on, and removal of residual heat from the reactor for long-term cooling once the reactor is shut down. A full range of possible accidents from internal events must be analyzed to demonstrate that these safety functions are fulfilled. These accidents would include unanticipated power increases, loss of coolant inventory from the reactor core, loss of heat rejection to the ultimate heat sink, as well as internally caused fires or floods. External hazards must also
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4 Reactivity is the term used to express the departure of a reactor system from criticality, which defines a state in which the rate of neutron production from fission equals the rate of neutron absorption. A negative reactivity addition indicates a move toward a power decrease. A positive reactivity indicates a move toward a power increase.
be considered and accommodated. These external hazards include natural events such as earthquakes, tornadoes, external fires, or floods as well as human-made hazards owing to unintentional or intentional acts outside the plant. The objective of these safety analyses is to show via testing and modeling that all the safety functions are satisfied and that the design prevents the release of any radioactive materials into the environment (or limits any releases to acceptable levels).
Nuclear reactors must be designed based on safety criteria that result in safety features and systems that can reliably accomplish these safety functions. Specific design features and operation and maintenance of support systems (electric power, cooling, pressurized air, etc.) may be required. Reactor designs must also provide, where appropriate, physical separation, independence, diversity, and redundancy in the safety systems to reduce the likelihood of common-cause or single-point failures that could lead to failure in executing the required function. Last, designs must use sufficient engineering design margins to cover the possibility that challenges to safety functions could arise from an incomplete understanding of reactor system behavior. The “Defense-in-Depth” concept, meaning that the design includes independent systems to ensure a safety function is satisfied, is an integral part of any reactor design. These design principles are intended to ensure that all safety functions are successfully satisfied, and that the overall reactor system is safe and provides adequate protection to the public and the environment.
In currently operating LWRs, key safety functions are accomplished by a diverse and redundant combination of backup systems (e.g., auxiliary diesel generators for AC-powered electrical systems, additional independent water pumping systems), alternative sources of water, and prescribed operator actions. These systems reduce the likelihood of safety system failure and mitigate the consequences if a failure were to occur. Specific design criteria are established so that system conditions that could drive possible radiological releases are mitigated and controlled for a range of postulated accidents that form the design basis for the specific engineering system: these are called design-basis accidents. This approach has proven successful, and LWRs operate with a high degree of reliability and safety.
Advanced reactor systems would rely more on inherent and passive5 design features than current LWRs. For example, the NuScale small modular LWR design virtually eliminates the need for active systems to accomplish safety functions, relying instead on a combination of passive systems (e.g., safety relief valves driven by gas pressure or spring forces, gravity-driven water flow) and the inherent features of its design geometry and materials (e.g., large water inventory with high thermal capacity). Non-LWR advanced reactor systems have taken a similar approach, accomplishing key safety functions in the system design with a much greater emphasis on inherent and passive features, as noted in Table 2-3. These small modular reactors (SMRs) can also employ integral designs that can incorporate all key components in the primary vessel, thereby reducing the risk from pipe breaks, because the primary coolant remains in the vessel. This configuration is significantly larger than a traditional loop configuration and increases the thermal inertia of the system.
The different fuels, coolants, and moderators (in thermal reactors) used in advanced reactor designs affect the inherent safety of the system through the basic material properties, neutronics designs, and chemical characteristics of system components. Non-LWR advanced reactor designs have the potential to improve safety by a combination of the following safety attributes that minimize the challenges to their systems for a wide range of transients and accidents (see Table 2-3):
- Negative reactivity coefficient for helium-cooled thermal reactor systems and sodium-cooled fast reactor systems causes a reactor power decrease for a reactor temperature increase.
- Single-phase coolants during normal operation with large margins to boiling for liquid coolants keep the reactor core cooling effective over a wide temperature range.
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5 The International Atomic Energy Agency (IAEA) defines active, passive, and inherent safety features as follows: “Active safety features ‘rely on external electrical or mechanical power, signals, or forces to complete a safety function.’ Passive safety features ‘only require natural forces (gravity or gas pressure), properties of materials, or internally stored energy (e.g., mechanical spring forces) to complete a safety function.’ Inherent features ‘rely on fundamental properties (materials or design choices that cannot be changed by internal or external conditions) to complete a safety function’” (IAEA 1991).
- High thermal conductivity and high heat capacity for liquid coolants (e.g., sodium and molten salts) removes heat from the reactor more effectively and reduces the rate of temperature increase.
- High heat capacity for the graphite moderator in gas-cooled and molten-salt thermal systems reduces the rate of temperature increase in the reactor core.
- Low chemical reaction rates of single-phase coolants like helium and molten salts reduce the potential for materials degradation.
- Fuel design with robust tristructural isotropic (TRISO) fuel kernels used in gas-cooled and some molten-salt thermal systems reduces fuel failure rates at high temperatures.
- Strong fission product retention in sodium, molten salts, and graphite moderator reduces the amount of radioactive material release from the reactor system.
TABLE 2-3 Safety Characteristics of Advanced Reactors
| Reactor Type | Passive and Inherent Safety Characteristics | Example Reactor Designs |
|---|---|---|
| Small modular LWR (SMR) (water coolant) |
|
NuScale, GEH-BWRX, Holtec |
| Liquid metal cooled fast reactor (SFR) (sodium) |
|
GE–Prism, TerraPower-GEH–Natrium |
| High-temperature gas reactor (HTGR) (helium coolant) |
|
X-Energy–Xe-100 |
| Gas fast reactor (GFR) (helium coolant) |
|
GA–EM2, FMR |
| Fluoride salt-cooled (FHR) (FLiBe salt coolant) |
|
Kairos–Hermes |
| Molten-salt-fuel-cooled (MSR) (U-F or U-Cl salt coolant/fuel) |
|
Terrestrial Energy–IMSR, Moltex, TerraPower |
| Heat-pipe cooled (low-pressure liquid metal) |
|
Westinghouse–eVinci, Oklo–Aurora,b BWXT–BANR |
a Demonstration is required by integral testing of this safety feature.
b Oklo’s design may have changed.
SOURCES: Committee generated using D. Petti, R. Hill, J. Gehin, et al., 2016, “Advanced Demonstration and Test Reactor Options Study,” INL/Ext-16-37867; and D. Petti, R. Hill, J. Gehin, et al., 2017, “A Summary of the Department of Energy’s Advanced Demonstration and Test Reaction Options Study,” Nuclear Technology 199:111–128, https://doi.org/10.1080/00295450.2017.1336029.
The sodium-cooled fast reactor and the high-temperature gas-cooled reactor have well-developed designs and have confirmed many of their safety characteristics through actual integral testing in prototype reactor plants (Planchon et al. 1987; Kunitomi et al. 1990). In addition, these advanced reactor designs have incorporated passive safety systems that use natural circulation for decay heat removal and long-term core cooling. The overall plant design allows for fewer auxiliary components or systems that may also reduce reactor system costs. These designs have the potential to accomplish safety functions without the need for AC power and can allow for extended coping times during transients and accidents. For example, in a loss of coolant flow accident that may occur in a high-temperature gas-cooled reactor, the reactor temperature increase is much slower, rising over several hours, compared to seconds for current LWRs. Such design attributes also have the potential to significantly reduce the accident source term and may allow for more flexible siting of these reactor systems near population centers.
Regardless of design specifics, the qualitative safety characteristics and design attributes are similar across many non-LWR advanced reactor types, including less mature concepts (e.g., molten-salt reactor). Nevertheless, there is a lack of operational experience with these designs, and trained operators for these new advanced reactor designs will have to be developed. Demonstration of these safety functions still needs to be validated with appropriate operating experience and collection of integral test data at appropriate scales to demonstrate capabilities and to confirm the satisfaction of safety requirements (see Table 2-3).
To improve safety and operational efficiency, advanced reactor designs are likely to incorporate extensive use of digital controls, advanced sensors, and data science capabilities such as artificial intelligence (AI). In current LWRs, the wealth of data involving plant and component performance is now being used to enable enhancements in automation, autonomous risk detection, predictive maintenance, and asset performance management (Al Rashdan et al. 2018; Gohel et al. 2020). For advanced reactors, automation and autonomous risk detection may also have the potential to improve safety by diminishing the demands on operators and reducing the potential for human error. Safety and reliability can be enhanced by continuous monitoring of component status and performing maintenance or replacing components as needed rather than on a time-based schedule. This can avoid unnecessary maintenance that can cause failures. Sophisticated data collection combined with inherent self-regulating physics in microreactors has been suggested as a motivation to consider autonomous control for those designs. However, as more reliance is placed on digital systems and AI, strong cybersecurity principles and clear guidance on when and how to trust AI will become increasingly important. These advanced reactor designs will need to meet the cybersecurity consensus codes and standards that are now being established by regulatory agencies. For example, the U.S. Nuclear Regulatory Commission (NRC, 10 CFR 73.54) requires that nuclear facilities provide high assurance that digital computer and communication systems and networks are adequately protected against cyberattack. The requirements for cybersecurity are discussed in Chapter 9.
Despite these positive design characteristics, safety challenges remain for advanced reactors, particularly to ensure reliable operation and to cope with postulated accidents. These issues will need to be addressed for each specific design given their unique design features. Gas-cooled reactors must control and limit the level of air or water ingress into the reactor system to minimize the amount of graphite oxidation, thereby reducing radioactive releases from the reactor to the surrounding building and the environment. Molten-salt-cooled reactors will require careful chemistry and temperature controls to mitigate material corrosion and salt freezing in piping during operation. Sodium fast reactors must maintain an inert atmosphere and water-free conditions to preclude chemical reactions from sodium leaks. Such events have plagued some past versions of this design and have resulted in extended plant shutdowns. Beyond the typical design-basis accidents that need to be considered for reactor systems (e.g., loss of flow, loss of coolant, power transients), sodium fast reactors must consider the possibility of air or water ingress and the resultant effects of sodium fires, and the safety systems needed to mitigate these effects. Such unique design features for any of the advanced reactors must be considered and analyzed to ensure safe operation.
Therefore, while these advanced reactor designs have the potential to demonstrate that required safety functions can be accomplished, the designs will need to demonstrate that they can meet the standards established by the government regulatory body (e.g., NRC in the United States) by integral testing as well as appropriate supporting safety analyses. The regulatory standards that are being developed specifically for
advanced reactors (e.g., siting near population centers and required emergency planning zones) are discussed in more detail in Chapter 7.
Finding 2-1: Many advanced reactor designs employ a combination of fuel, coolant, and moderator that result in a set of core components with potentially inherent favorable safety characteristics (e.g., physical stability, high heat capacity, negative reactivity feedbacks). The designs also include engineered passive safety systems that incorporate no active components (e.g., pumps, motor-activated valves) and could require no emergency AC power and fewer external operator actions. These inherent and engineered design attributes, if actualized, have the potential to make fulfilling key safety functions (i.e., reactivity control, heat removal, radioactivity containment) simpler, more reliable, more cost effective, and more tolerant of human errors. Employing sophisticated sensing and data collection could further improve safety by increasing component and systems reliability.
Finding 2-2: Reactor designers and owners must demonstrate that key safety functions (i.e., reactivity control, heat removal, radioactivity containment) are satisfied during normal operation, transients, and the full range of possible accidents. (The list of possible accidents considered for new and advanced reactor designs could be different from those considered for current light water reactors [LWRs].) This will require collection of integral test data at appropriate scales and operating experience, supplemented by supporting analyses. The safety risks associated with small and advanced reactors differ from those for conventional LWRs and require new testing facilities and demonstration facilities.
Recommendation 2-1: The Department of Energy should evaluate the need for common experimental facilities that would help provide the required testing to support licensing and long-term operations across multiple reactor concepts within a reactor class (e.g., gas-cooled or molten-salt-cooled concepts).
FUEL AND MATERIALS INNOVATION, DEVELOPMENT, AND MANUFACTURING
The fuel and materials selection process has a major impact on the feasibility of advanced reactor concepts. Many of the advanced reactor concepts would utilize new fuel geometries and/or higher fissile isotope enrichment levels compared to conventional LWR fuel (i.e., solid uranium-oxide pellets with 235U enrichment less than or equal to 5 percent). Therefore, new fuel supply chain systems need to be commercially established and qualified for the advanced reactors to use them to achieve widespread deployment. Issues associated with developing higher enrichment fuels are discussed in a companion National Academies study (NASEM 2022). In general, development of HALEU supply chain infrastructure is a crosscutting challenge for the realization of many of the proposed advanced reactor concepts. Some initial federal funding to establish commercial-scale HALEU feedstock capability has been recently appropriated ($700 million in the Inflation Reduction Act [IRA] of 2022, P.L. 117-169). Furthermore, some of the advanced reactor concepts use different fuel forms such as TRISO particles embedded in a variety of fuel matrix structures (Tables 2-1 and 2-2), which will also require maturation from laboratory-scale to commercial-scale production.
Additionally, because advanced reactors generally operate at higher temperatures and in environments that are different (more challenging) than the existing commercial reactors, the technology gaps for nearly every advanced reactor concept include the need to develop and qualify high-performance materials with improved high-temperature strength and resistance to corrosion and irradiation effects (see Table 2-4). Materials with greater high-temperature strength or high-temperature corrosion resistance would allow for thinner components, improved safety margins, and improved thermodynamic efficiencies and economics for high-temperature reactors (Busby 2009; Zinkle et al. 2016). It should be noted that advanced reactor concepts do not require advanced materials for all reactor components; in many cases, conventional materials would be acceptable. However, some key components in demanding operational environments would benefit from new high-performance materials. One example is advanced cladding materials for sodium-cooled fast reactors that would simultaneously enable high fuel burnups (>15 percent) and thermodynamically favorable high reactor outlet temperatures (>850 K).
TABLE 2-4 Advanced Reactor Technology Experience, Technology Readiness, and Technology Gapsa
| Reactor Type | Technology Experience | Technology Readiness | Technology Gaps |
|---|---|---|---|
| Small modular LWR | Evolution to advanced designs from currently operating LWRs. | High technology readiness with significant similarity to operating LWRs | Development and qualification of unique plant components (e.g., single-failure-proof valves) |
| Liquid metal cooled fast reactor (sodium) | Several small sodium-cooled fast reactors operating worldwide. | High technology readiness for small SFR Low–medium technology readiness for higher burnup breed/burn cores for SFRs |
Qualification of annular metal fuel as a transition from sodium-bonded metal fuel Qualification of advanced steel alloys for use with high fuel burnup breed/burn cores (clad and structure) Source term experiments that reduce conservatisms |
| High-temperature gas reactor (helium) | Several small helium-cooled high-temperature reactors operating worldwide. | Medium–high technology readiness for <1,100 K outlet temperature design | For <1,100 K outlet temperature, qualification of fuel and graphite for use in reactor demonstration plant |
| Low–medium technology readiness for >1,100 K outlet temperature design | For >1,100 K outlet temperature, qualification of materials used in heat exchanger and other components | ||
| Gas fast reactor | No reactor ever built. | Low technology readiness | Qualification of fuel, clad, and structural materials for safety and for radiation damage Demonstration of passive safety systems |
| Fluoride high-temperature salt-cooled reactor (FHR) with FLiBe | FHR was designed; Hermes is a reduced-scale prototype and is planned for demonstration. | Low–medium technology readiness for FLiBe systems | Demonstration of corrosion/control for FLiBe-based salt in the presence of a neutron field Demonstration of materials to show strength, corrosion resistance, and irradiation stability in operation Demonstration of tritium mitigation and radioactivity control in primary system and off-gas systems Demonstration of passive safety systems |
| Molten-salt-fuel-cooled reactor (U-F molten salt coolant and fuel) | ORNL experiments were operated without power conversion systems. | Low technology readiness for thermal and fast systems | Each design is unique but has similar gaps to the FHR: e.g., corrosion control for salts, tritium migration and control, materials to be used for long-term operation or replacement materials Demonstration of passive safety systems |
| Heat-pipe cooled (low-pressure liquid metal) | LANL space reactor demonstrated this concept at reduced power scale. | Low–medium technology readiness for thermal and fast systems | Development of compact power conversion unit (PCU) operation and PCU integration with heat-pipe core cooling Development of autonomous control and instrumentation Demonstration of passive safety systems |
a For all of the non-LWRs using higher enrichment fuel, there remains the need to identify a commercial fuel vendor.
SOURCES: Committee generated based on Generational IV International Forum, 2014, “Technology Roadmap Update for Generation IV Nuclear Energy Systems,” https://www.gen-4.org/gif/upload/docs/application/pdf/2014-03/gif-tru2014.pdf; H.D. Gouger, R.A. Bari, T.K. Kim, et al., 2015, Assessment of the Technical Maturity of Generation IV Concepts for Test of Demonstration Reactor Applications, Revision 2, Technical Report, Idaho National Laboratory, Idaho Falls, Idaho, https://doi.org/10.2172/1236803; A. Sowder, 2015, Program on Technology Innovation: Technology Assessment of a Molten Salt Reactor Design—The Liquid Fluoride Thorium Reactor (LFTR), Technical Report, Electric Power Research Institute, https://www.epri.com/research/products/000000003002005460; D. Petti, R. Hill, J. Gehin, et al., 2016, Advanced Demonstration and Test Reactor Options Study, INL/Ext-16-37867; and D. Petti, R. Hill, J. Gehin, et al. 2017, “A Summary of the Department of Energy’s Advanced Demonstration and Test Reaction Options Study,” Nuclear Technology 199:111–128, https://doi.org/10.1080/00295450.2017.1336029.
Only six alloys are currently code qualified for ASME Boiler and Pressure Vessel Section III, Division 5 (High Temperature Reactors). All of these code-qualified Fe- and Ni-base alloys were developed 35 to 100 years ago and, in general, have properties significantly inferior to more recent commercial alloys. In contrast, dozens of higher-performance alloys have been commercialized (and code-qualified for non-nuclear applications) in the past 20 years for demanding fossil energy and aerospace applications (Viswanathan et al. 2013). If the engineering designs of advanced reactors are limited to these six code-qualified high-temperature reactor materials, significant performance limitations will result (Zinkle et al. 2016; Pathania 2012). Additionally, ASME Section III, Division 5 code qualification does not address any degradation effects associated with corrosion, mass transfer phenomena, or radiation effects in the operating environment. It is generally acknowledged that materials corrosion and radiation effects can impose greater operational restrictions on reactor designs than the “design allowable” restrictions associated with tensile properties, thermal creep, or fatigue/ratcheting effects that are addressed by the ASME code. This suggests there may be value in a coordinated public–private research and development activity for advanced (high performance commercialized) materials that encompasses ASME (or equivalent) code qualification along with reactor-relevant irradiation/corrosion environmental test conditions, as discussed later in this section.
There are at least four potential options to mitigate operational performance gaps associated with using the current limited set of ASME code-qualified materials for advanced reactors: (1) introduce design innovations to reduce the required materials properties, (2) use reduced design margins, (3) introduce advanced high-performance materials and/or advanced manufacturing concepts, or (4) derate the reactor concept operating parameters (temperature, operating lifetime, etc.). Considering the significant improvements in materials properties—such as high-temperature creep strength, high fracture toughness, enhanced corrosion resistance, and improved radiation resistance—that can be attained in specific application-selected materials, option (3) offers considerable promise. For example, recently developed ferritic alloys simultaneously exhibit up to a factor of two increased thermal creep strength and greater than a factor of two improved dose before the onset of deleterious void swelling (Zinkle et al. 2016, 2017). Use of such materials for cladding and fuel assembly ducts for sodium-cooled fast reactors could enable desired deep burnup levels and high operational temperatures to be achieved that could dramatically improve fuel utilization and economics. Similar benefits associated with advanced materials could be realized for high-temperature gas-cooled reactors and molten-salt reactors.
International collaboration (via the Generation IV International Forum or other avenues) might be beneficial for assembling the broad range of experimental data needed to develop an ASME Section III, Division 5 code case (or equivalent alternatives) for one or more high-performance commercial alloys. For example, the Electric Power Research Institute (EPRI) released a high-level roadmap to enable coordination of materials development and validation programs to support the near-term deployment of non-LWR designs (EPRI 2021c). Regulatory acceptance and economic considerations for prospective new materials typically require development of an ASME (or equivalent) code case and understanding of degradation processes associated with corrosion, creep fatigue, and neutron irradiation damage. These historically require a decade or longer to complete, challenging the introduction of higher performance materials. To reduce the time for new materials to gain regulatory acceptance, a staged approach for qualifying advanced materials could be envisioned for introducing new materials.
In the first phase, limited duration testing (up to several years) could be performed to develop an ASME Section III, Division 5 code case for limited lifetimes that might be relevant for a prototype or demonstration reactor system. Longer duration testing could be performed in parallel that could subsequently extend the code qualification to longer lifetimes and higher temperatures. This approach also provides for parallel evaluation of critical degradation mechanisms (corrosion, irradiation, etc.) in advanced reactor environments. The stakeholders in such a staged approach could involve public–private partnership (e.g., DOE-NE and EPRI). The general concept of a staged approach for materials qualification (extending beyond traditional ASME code qualification) has been used several times in the past; a well-known safety-relevant example is the periodic testing of LWR reactor pressure vessel coupons during every fuel replacement outage to ensure safe operation of the pressure vessel.
Advanced manufacturing concepts such as additive manufacturing, powder metallurgy–hot isostatic pressing (PM-HIP), and advanced welding and cladding methods also offer multiple potential benefits. For example, design and fabrication of advanced fuels incorporating TRISO fuel particles based on uranium nitride (UN) kernels have been demonstrated and provide a factor of three higher fissile atom density compared to traditional uranium oxycarbide (UCO)-based TRISO fuel systems (Terrani et al. 2021). Of even greater potential benefit is the possibility
of utilizing in situ diagnostic monitoring during the additive manufacturing build that, combined with advanced data analytics and machine learning methods, could lead to direct quality certification of as-built components without resorting to historical post-build non-destructive or destructive analysis techniques. In addition, design and fabrication of improved high-performance reactor core configurations can be achieved by adapting multi-physics artificial intelligence (AI) techniques to design novel core geometries that can only be constructed using advanced additive manufacturing methods (Sobes et al. 2021). These design innovations enabled by advanced manufacturing have the potential to simultaneously improve safety, reliability, and economics. To take full advantage of these and other benefits of additive manufacturing, the nuclear industry (in concert with federally funded research programs) must address key obstacles, including lack of codes and standards and processing variables. Advanced manufacturing methods such as PM-HIP also provide the advantage of improved material homogeneity, reducing variability and enhancing mechanical performance at elevated temperatures while providing a domestic supply chain for major components. Advanced welding technologies like electron beam welding have the potential to significantly reduce fabrication lead times, improve quality, and avoid embrittlement of welds (EPRI 2021d). Demonstrating these technologies is critical to accelerating their application in new reactors (EPRI 2021b).
Another aspect requiring materials innovation and development for advanced reactors is the ability to maintain reactor systems from a cost and reliability perspective during operation. Many systems will expose components to high temperatures, corrosive environments, and high radiation fluxes. Without thoughtful design and materials selection, some components may need to be replaced frequently, and maintenance may be difficult to execute, either of which could add to cost, complexity, and reactor downtime. To prove survivability of components, many designers are taking new, more agile approaches than those of the past. Some companies are doing rapid testing to gain key data quickly (Kairos Power 2021). Another approach is cyber-physical systems, where simulation and physical components are used together to acquire as much real feedback as possible. In both cases, the goal is to shorten the work and time needed to demonstrate how to conduct proper maintenance and ensure that components will survive to their expected life span in the reactor system environment. It is still unclear if these new and agile approaches will result in systems that sufficiently reduce costs and maintenance.
Finding 2-3: For all the non-light water reactors that require higher 235U enrichment beyond current established levels, a new fuel supply chain system must be qualified and commercially developed. Without this fuel supply chain, widespread commercial deployment of these reactor concepts cannot be achieved. This high-assay low-enriched uranium, while one of many new supply chains that need to be established to support advanced reactors, is critical across many of the advanced concepts.
Finding 2-4: Advanced reactor concepts, while innovative in some aspects of their designs, are generally based on relatively conventional fuels, materials, and manufacturing methods. Such conventional moderate-performance materials (e.g., currently code-qualified structural steels) are suitable for many non-demanding advanced reactor components, such as primary system piping. However, notable improvements in performance and economics could be achieved by more widespread use of better-performing materials for advanced fuels, high-performance fuel cladding materials, and advanced manufacturing (e.g., additive manufacturing). While many of the current concepts plan to move to commercial reactor demonstration with existing materials, optimization of future generations for further improvements in safety, reliability, and economics will require technology advancements.
Recommendation 2-2: The Department of Energy (DOE) should initiate a research program that sets aggressive goals for improving fuels and materials performance. This could take the form of a strategic partnership for research and development involving DOE’s Office of Nuclear Energy and Office of Science, the U.S. Nuclear Regulatory Commission, the Electric Power Research Institute, the nuclear industry, national laboratories, and universities. The program should incentivize the use of modern materials science, including access to modern test reactors, to decrease the time to deployment of materials with improved performance and to accelerate the qualification (ASME Section III, Division 5 or equivalent) and understanding of life-limiting degradation processes of a limited number of high-performance structural materials—for example, reactor core materials and cladding.
TECHNICAL READINESS AND TECHNOLOGY GAPS
Several technological factors can affect the ability to demonstrate and then commercialize an advanced reactor system, including the readiness of the reactor technology to be used; the development activities needed to resolve technology gaps so that a demonstration plant can be designed, licensed, and operated; and the feasibility of full-scale demonstration to confirm that performance and operability meet requirements.6 This section reviews the maturity of different advanced reactor systems, assessing their technical readiness and the technology gaps requiring further development. Table 2-4 summarizes key technical issues that can affect the ability to demonstrate each reactor concept.
DOE has developed a Technology Readiness Assessment Guide to assist the agency in determining the readiness of technologies under development (DOE 2009). In addition, several organizations around the world have examined the technology readiness of advanced nuclear reactor technologies, and each has applied its own specific technology readiness scales in its evaluation (GIF 2014; Gougar et al. 2015; Sowder 2015; Petti et al. 2016). These ratings of technology readiness for demonstration were based on three primary criteria: (1) the extent that further technology development is needed to resolve technical, design, and licensing issues (for fuels, cladding, coolants, or moderators); (2) prior successful operating experience with the reactor system (or similar systems); and (3) the maturity level of the existing safety demonstration of the reactor system or its key subsystems. Based on these criteria, the advanced reactor technologies have been rated on their technology readiness as follows:
- Lowest maturity: gas fast reactor (GFR), molten-salt reactor (MSR-fast), MSR (salt other than FLiBe).
- Low-medium maturity: large sodium fast reactor (SFR), fluoride salt reactor (FHR), MSR (with FLiBe), high-temperature gas-cooled reactor (HTGR) (>1,100 K outlet temperature), microreactors.
- Medium-high maturity: light water small modular reactors, small SFRs, and modular HTGRs (<1,100 K outlet temperature).
In the past, LWR systems have gone through a series of development steps prior to commercialization (Petti et al. 2017):
- Research and development to prove the scientific feasibility of key features associated with fuel, coolant, and reactor system components and configurations.
- Engineering demonstration at reduced scale for proof-of-concept of designs that have never been built, with a goal of demonstrating the viability of the integrated system.
- Performance demonstration to confirm effective scale-up of the system and to gain operating experience to validate the integral behavior of the system resulting in proof of performance.
- Commercial demonstration that leads to the subsequent commercial offerings.
These development stages are being used for more mature advanced reactor technologies (e.g., SFR and HTGR). Advances in fuels and enabling safety technologies have been incorporated into these advanced reactor systems to improve performance and safety. For example, SFR designs employ the operational experience of the EBR-II test reactor to develop a metal fuel for the larger SFR reactor design. The use of metal fuel coupled with the reactor core design allows the SFR to have inherent reactor shutdown capabilities for a range of postulated accidents (ANL 2020). Additionally, taking lessons from the larger Super Phenix SFR, the smaller SFR designs use modular steam generators rather than a large monolithic steam generator to improve operational reliability (albeit at the cost of additional metal needed in the construction). For the HTGR, the TRISO fuel development campaign (Petti 2016) has led to a robust fuel design that will be employed in future HTGR systems. In both these reactor designs, further safety testing will be needed in the demonstration plants to confirm their long-term cooling
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6 Additional, non-technical factors, such as the level of investment by government and private entities in technology development and the consistent bipartisan support of a comprehensive path forward, can also affect the ability to demonstrate and commercialize an advanced reactor system, but are not discussed in detail in this chapter.
with passive decay heat removal capabilities. These reactor development approaches were used to overcome past difficulties experienced with these reactor design concepts.
A similar approach may also be needed for less mature technologies (e.g., FHR and MSR) if the intent is to resolve major technology gaps and complete required technology demonstration activities, gain operating experience, demonstrate effective scale-up of systems performance, and establish required supply chains. These steps are all important prerequisites to offering commercial versions of these advanced reactor technologies. Reactor vendors could also consider developing a prototype plant prior to a full demonstration plant to test their technologies without concern about economic drivers such as outages and staffing levels.
In general, the technology development gaps identified in Table 2-4 for the more mature advanced reactor technologies are related to regulatory qualification of unique systems and extension to new performance regimes—for example, develop and qualify unique reactor components, and qualify improved fuels and materials. While the level of technical readiness is substantial for mature reactor concepts, it is important that the industry not be prone to optimistic schedules and underestimate the time and the associated effort required to qualify these systems and components along with the requirements for full-scale demonstration.
In contrast, the technology development gaps for the lower maturity advanced reactor technologies are related to the viability and performance confirmation of key reactor features—for example, materials compatibility and corrosion behavior in a radiation field; demonstration of materials to show strength, corrosion resistance, and irradiation stability during operation; demonstration of tritium mitigation by control and monitoring; integral performance of passive safety systems. Examples of some specific technology gaps for these advanced reactor systems are provided in Appendix B.
Finding 2-5: The various advanced reactor systems are at different levels of technical maturity. Each reactor design concept requires the completion of certain key technology development activities. The time and effort needed depends, in part, on the technical readiness of the concept and prior operating experience with the specific reactor technology involved. More mature concepts, such as advanced small modular light water reactors, small modular sodium fast reactors, and small modular high-temperature gas-cooled reactors, might be technically ready for demonstration by the end of this decade. Less mature reactor concepts require a range of additional development activities before demonstration can occur (such as qualification of fuel or structural materials for prototypic conditions) and would not be ready for demonstration until after 2030. The success in getting concepts ready for regulatory review, building a demonstration plant in a timely and predictable manner, and proving operational excellence with demonstration plants will determine potential broader commercial deployment.
OPERATION AND MAINTENANCE OF ADVANCED REACTORS
Operation
During the committee’s information gathering, several reactor manufacturers presented on the concept of operations for the various advanced reactor designs. These discussions revealed that, while some operational aspects of the new designs are similar to those of existing commercial reactors, several significant differences will need to be addressed to ensure readiness for broader deployment.
Among those differences is the concept that if a “fleet” of reactors is operating on a site, reactor maintenance could be done in phases such that the overall installation would continue to provide power to the grid. This would be a positive operational aspect regarding grid connectivity and ability to provide continuous power, without losing generation capacity for a few weeks for refueling and maintenance as with current reactor plants. The ability to phase downtime would be applicable only in configurations where multiple reactors would be installed on a site.7 Other operational models might envision mobile reactors that would be returned to a centralized facility for
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7 Note that reactor designs using a pebble bed configuration do not require traditional refueling because refueling would be accomplished while the reactor is operating.
refueling and replaced on site by a new, fully fueled system to ensure continuity of power delivery. This model carries with it significant regulatory challenges for transporting a reactor system that contains spent nuclear fuel (see Chapter 7 for more information on regulatory issues with transportable reactors).
A number of key operations issues impact the safety, cost, operational readiness, and ultimate availability of new designs. These are not addressed in technical detail in this report, but each will require significant benefit/cost assessment, including potential regulatory and schedule risk.
- Automation to reduce workforce requirements. Market viability can be improved with a reduction of onsite personnel. The DOE Light Water Reactor Sustainability program has assessed increased use of automation, and the Advanced Research Projects Agency–Energy’s (ARPA-E’s) program “Generating Electricity Managed by Intelligent Nuclear Assets (GEMINA)” has performed research into better operational options (DOE-NE n.d.; ARPA-E 2019). The ARPA-E program points to the need for digital twins to support transformation of operations and maintenance. Automation also may include adjustable control and protective systems. In some cases, control regimes and setpoints are not fixed but are adaptive based on the performance regime for the system. This adaptability may enhance performance for any design but requires more complex software development and may increase risk for cyber intrusion and interference. Greater automation will also necessitate greater focus on fault tolerant controls. To the extent that reactor developers include the use of AI/machine learning-enabled sensing and controls, monitoring methods will require enhancement and understanding of—as well as regulatory oversight for—AI algorithms.
- Remote monitoring. Coupled with automation, enhanced remote monitoring, including remote regulatory oversight, may reduce the need for onsite personnel. Regulator acceptance of remote monitoring in lieu of regular onsite oversight visits may have market benefit but must be balanced with safety.
- Safety and emergency response. With enhanced automation, increased use of remote sensing, and reduced personnel will come the potential for increased dependency on local emergency response capabilities. In the event of fire, flooding, or seismic events, for example, some immediate response may be shifted from onsite personnel to first responders from the local community. This will necessitate a new training regimen to support response to off-normal events, which will require additional funding and resources.
- Control schema for operating multiple plants from a single control room. Some designers are considering control of multiple plants from a single, possibly remote, control room. Multi-unit operations from a single control room have been demonstrated by the U.S. Nuclear Navy, making this a viable concept. The USS Enterprise had eight reactors that were operated in pairs with summary performance for all eight monitored by a senior engineering supervisor in a central control room. Evaluation of communications pathways, reliability, and cybersecurity for monitoring systems will require analysis and perhaps may lead to increased risk, as noted in Chapter 9.
- Consolidated maintenance. Experience with the existing LWR fleet has demonstrated that there are major savings from well-tailored outage operations to include refueling and maintenance performed by single teams for multiple units. In the case of new advanced designs, this concept may be taken one step further to include the potential for returning some microreactor or mobile (e.g., floating) small modular systems to a central location for refueling and refurbishment. While this new operational paradigm may lead to cost savings, there are major uncertainties in terms of construction standards, transportation safety and security, and nonproliferation risk. The solutions to address these risks may present operational cost uncertainty and require further assessment.
- Operation in non-electricity markets. New reactor designs may allow for use of the reactor for non-electricity purposes, such as providing heat for hydrogen production when the nuclear output is not needed for the grid. This presents new operational considerations with potentially higher operational and market risk.
- Security controls and limits. As with the need to enhance security and cyber protection, remote and automated operations will require enhanced security and proliferation controls, which have implications for operations. Such considerations are discussed in Chapter 9.
- Design-specific operational challenges. New reactor designs may have unique operational challenges, such as the use of molten salts and liquid fuels, as well as novel activation and waste control challenges.
- Regulatory inspection protocols. As noted above, there may be potential for enhanced remote monitoring for operations, which may require or enable changes in the means for regulatory oversight. The regulatory framework needs to mirror the changes anticipated in advanced reactor operations to ensure that opportunities for regulatory efficiency can be exploited, while maintaining current high standards.
While this report does not explore these challenges in detail, the committee notes that assumptions about cost, operational availability, and regulatory risk need to be addressed in the development and deployment plans of any new reactor developer.
Maintenance
Maintenance of any facility and its components is a critical element in safe operations, as well as reliability and sustainability in performance of the physical plant. Configurations of advanced reactors could include a range of designs, such as a fleet of two, four, or eight smaller reactors scaled to MWe requirements or a more conventional single-reactor design with a larger MWe output capacity. One advantage of fleet configuration is that single reactors could be removed from service to perform maintenance. However, in all cases, the balance of plant—that is, the systems that are not part of the nuclear island—would still require typical preventive and predictive maintenance, as with any other conventional contemporary plant. A skilled workforce for such maintenance work exists, and current utility companies and contractors have the personnel capacity in knowledge and numbers to maintain equipment such as steam turbine generators, high pressure piping, cooling equipment, electrical systems, control systems, and other mechanical and electrical components. Thus, it does not appear that balance-of-plant maintenance activities will present any significant complications above what the nuclear industry performs today. In fact, organizations such as EPRI and the Institute of Nuclear Power Operations (INPO) are expected to develop standards for the industry to accommodate any new configurations of reactors and on-site facilities.
FLEXIBILITY AND APPLICATIONS BEYOND ELECTRICITY
Advanced reactor systems are also being designed to provide flexibility in their energy products. These systems can be designed with the capability to produce electricity and/or process heat that can be used for industrial processes at various temperatures. Process heat could be employed to decarbonize industrial processes or heat buildings, while both electricity and heat may be required to produce synfuels or fresh water through desalination (NEA-OECD 2021). Currently operating LWRs have the proven capability to provide low-carbon, flexible electricity generation (Morilhat et al. 2019). For example, the current nuclear power plant fleet in France modifies its electricity supply over the course of a typical day or month (load following) to accommodate system demand as well as the growing variable renewable contribution within France and in neighboring interconnected countries.
Most advanced reactor designers who envision integration to the grid as the significant aspect of their operational model anticipate a future grid with a much higher fraction of renewables and therefore increased variability in revenue based on the increased variability in the demand for power from the nuclear plant. To address this challenge, many point to the ability to load follow and alter the reactor output to meet varying demands, but, as discussed further in Chapter 3, market terms that reward load following are needed to ensure continued economic viability. In the absence of such market terms, energy products in addition to electricity generation may be required for these plants to remain economically viable. See Chapter 4 for a discussion of economic considerations for advanced reactors.
Some designs may vary their power output directly, while others may use a secondary thermal storage and power generation system. For example, in the Natrium design, a 350 MWe plant may heat up a molten-salt storage system attached to a generator that can produce 500 MWe for a specific time span (5 hours), meaning that the integrated system can vary electrical power from 0 to 500 MWe. New deployment scenarios (e.g., microreactors for remote communities or off-grid applications) are also being evaluated (EPRI 2021a). A summary of these advanced reactor concepts is provided in Appendix A, and more detailed discussion of non-electric applications can be found in Chapter 5.
TECHNOLOGY PATH FORWARD
In summary, a wide range of advanced reactor technologies are currently under design and development. Industry teams are moving forward with several advanced reactor designs with a goal of demonstration and eventual commercial deployment. As discussed above, the technical readiness of these different advanced reactor design concepts varies, with some developers aiming for deployment at the end of this decade and others targeting deployment thereafter. Several technological topics must be addressed to improve the technology readiness of the advanced reactor designs under development (see Table 2-4). These reactor designs require not only further investigation of the behavior of physical phenomena (e.g., fuels and materials), but also validation by appropriate test and operational data and associated analysis. Many of the research and development approaches to these technical topics could also accelerate the time to deployment—for example, crosscutting technology programs in additive manufacturing. Compared to large LWRs, advanced reactors will likely use different deployment models, such as providing dedicated electricity to single users, process heat for industry or building heating, or heat for storage for future distribution. The non-technical challenges associated with these new deployment scenarios are addressed in subsequent chapters.
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