B
Examples of Technology Development Gaps for Advanced Reactors
In general, the technology development gaps identified in Table 2-4 (see Chapter 2) for the higher maturity advanced reactor technologies are related to licensing qualification of unique systems and extension to new performance regimes—for example, development and qualification of unique reactor components, qualification of improved fuels and materials for higher burnup. 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; and integral performance of passive safety systems. The following sections provide a few specific examples where further technology development is required for particular reactor designs.
SMALL MODULAR LIGHT-WATER REACTORS—DEVELOP AND QUALIFY UNIQUE COMPONENTS
Small modular light water reactor (LWR) designs are largely based on current LWR technologies but incorporate passive safety systems to accomplish their safety functions, using smaller and simpler configurations with unique components. To confirm that the safety function is satisfied, the vendor must develop the component design, identify a supplier for these unique components, and work with the supplier to perform qualification testing to confirm acceptable performance before the plant is put into operation.
The NuScale Power Module (NPM) design is a good example of this because it has addressed the most risk-significant scenarios that arise using passive safety systems that rely on design and operational simplicity and redundancy (NRC 2020). Rather than active systems with pumps and valves, each NPM relies on four passive Emergency Core Cooling System (ECCS) valves that open when appropriate safety signals are received or when electric power is lost. Successful natural circulation cooling of the core is provided if one-of-two reactor vent valves open and one-of-two reactor recirculation valves open. The safety performance of this passive ECCS design is highly dependent on ensuring reliable operation of these valves. NuScale is working with the supplier to perform extensive qualification testing of this unique component, with an objective of providing confidence in the ability of the valves to maintain their required performance after extended periods in an operational environment. Such testing considers the possibility of degradation mechanisms such as deposits, precipitates, and fouling over time
in the presence of boric acid in a high temperature and radiation environment. The results of this testing would be used to confirm acceptable performance in the next couple of years prior to issuance of the combined operating license by the U.S. Nuclear Regulatory Commission.
SODIUM FAST REACTOR–METALLIC FUEL QUALIFICATION
Fuel is the heart of all the nuclear reactor systems where the defense-in-depth principles and safety systems are designed based on it. While traditionally treated as a low-cost item as part of the nuclear power plant total cost, nuclear fuel dictates the reactor power density as well as nuclear plant construction requirements. The Natrium sodium fast reactor (SFR) demonstration plant is planned to start operation with a metallic fuel form that has been previously developed and has demonstrated a high-level of technical readiness (TerraPower 2021). This metallic fuel uses sodium inserted in the gap between the fuel cladding and the metallic fuel itself to better control the gap thermal resistance and to minimize peak fuel temperature throughout the fuel lifetime. This fuel has been successfully tested and used in past U.S. SFRs, such as Experimental Breeder Reactor-II (EBR-II) and Fast Flux Test Facility (FFTF). This fuel design also has developed an extensive body of empirical fuel performance data, which can assist in making a solid licensing case for reactor startup.
At the same time, TerraPower is developing an improved fuel design that removes the internal sodium-bond within the gap and uses fuel with an annular hole, which can better compensate for thermal effects as well as fuel burnup. This design change will also allow longer fuel lifetimes and higher plant thermal efficiency. This advanced fuel form needs to be qualified by insertion and testing of fuel pins and fuel assemblies into the reactor core to gather needed fuel performance data for design confirmation and regulatory approval. It will take a few years to convert from the sodium-bonded metallic fuel to this new annular fuel form.
The initial sodium-bonded metallic fuel design was not approved for direct disposal into Yucca Mountain because of the the presence of sodium. While TerraPower and national laboratory researchers feel that there is a technical case for direct disposal of this startup fuel, committee members note that alternatives exist if direct disposal is not approved by the regulator—for example, pyro-processing of this sodium-bonded fuel as is being done for the EBR-II reactor fuel (Hall et al. 2019). The processed fuel material would be acceptable for repository disposal (Ebert 2005). This waste management issue is discussed in detail in the companion National Academies’ study, Merits and Viability of Different Nuclear Fuel Cycles and Technology Options and the Waste Aspects of Advanced Nuclear Reactors (NASEM 2022).
GAS REACTOR AND MOLTEN SALT REACTORS—DEMONSTRATION OF PASSIVE SAFETY SYSTEMS
Advanced reactor designs have incorporated passive safety systems using natural circulation for decay heat removal and long-term cooling. This approach to overall plant design allows for fewer auxiliary components or systems. These designs aim to demonstrate that long-term cooling can be accomplished without the need for AC power and can allow for extended coping times during transients and accidents (such as station blackout). This approach to long-term cooling has been taken in small modular LWRs (e.g., NuScale Decay Heat Removal System) as well as more mature non-LWR designs (e.g., SFR Direct Reactor Auxiliary Cooling System—DRACS; Groth et al. 2015). Such an approach is also part of the high-temperature gas-cooled reactor, the gas fast reactor design, and the various unique molten salt reactor designs, but actual integral testing would be required to demonstrate acceptable behavior for less mature concepts.
Natural circulation loops have been used, relying on natural forces (e.g., gravity and density difference) to efficiently transport heat without the need for active power sources. In advanced reactor designs, they have emerged as a leading focus area for passive safety, offering a solution to long-term core cooling and decay heat removal. These systems have become an integral part of the design, requiring no human intervention during an accident or transient (Lisowski et al. 2014). Of the concepts under consideration, several use water as a primary working fluid where operating conditions are expected to reach saturation. The venting of steam into the atmosphere serves as the ultimate heat sink, and hence these designs operate at low pressures only, requiring a resupply of water to
the system after several days. Experimental test beds at Department of Energy national laboratories have been established for passive decay heat removal systems for these reactor technologies (ANL 2016). Reduced-scale engineering demonstrations are now being conducted and will be conducted over the next couple of years to understand the integrated behavior of the system prior to scale-up to a performance demonstration of commercial size.
REFERENCES
ANL (Argonne National Laboratory). 2016. “The NTSF at Argonne: Passive Safety and Decay Heat Removal for Advanced Nuclear Reactor Designs.” Nuclear Engineering Division. https://www.ne.anl.gov/capabilities/rsta/nstf/index.shtml.
Ebert, W.L. 2005. Testing to Evaluate the Suitability of Waste Forms Developed for Electrometallurgically Treated Spent Sodium-Bonded Nuclear Fuel for Disposal in the Yucca Mountain Repository. Argonne, IL: Argonne National Laboratory. https://publications.anl.gov/anlpubs/2006/01/55340.pdf.
Groth, K.M., M.R. Denman, T.B. Jones, M. Darling, and G.F. Luger. 2015. Proof-of-Concept Accident Diagnostic Support for Sodium Fast Reactors. Zurich, Switzerland. https://www.osti.gov/biblio/1258246.
Hall, N., X. He, and Y.-M. Pan. 2019. Disposal Options and Potential Challenges to Waste Packages and Waste Forms in Disposal of Spent (Irradiated) Advanced Reactor Fuel Types. San Antonio, TX: Center for Nuclear Waste Regulatory Analysis. https://www.nrc.gov/docs/ML2023/ML20237F397.pdf.
Lisowski, D.D., O. Omotowa, M.A. Muci, A. Tokuhiro, M.H. Anderson, and M.L. Corradini. “Influences of Boil-off on the Behavior of a Two-Phase Natural Circulation Loop.” International Journal of Multiphase Flow 60(April):135–148. https://doi.org/10.1016/j.ijmultiphaseflow.2013.12.005.
NASEM (National Academies of Sciences, Engineering, and Medicine). 2022. Merits and Viability of Different Nuclear Fuel Cycles and Technology Options and the Waste Aspects of Advanced Nuclear Reactors. Washington, DC: The National Academies Press. https://doi.org/10.17226/26500.
NRC (U.S. Nuclear Regulatory Commission). 2020. “Design Certification Application—NuScale.” Small Modular Reactors (LWR designs). https://www.nrc.gov/reactors/new-reactors/smr/nuscale.html.
TerraPower. 2021a. “Additional TerraPower Information for Review Committees.” TerraPower, LLC.
