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Summary of Advanced Reactor Design Concepts
Advanced reactor systems include small modular light water reactors as well as small modular reactor systems that use non-water coolants, so-called Generation-IV reactors (World Nuclear Association 2020). In the following sections, general characteristics and representative examples of each reactor concept are described.
SMALL MODULAR LIGHT WATER REACTORS
Small modular light water reactors (LWRs) are based on the current large LWR technologies but have smaller and simpler system configurations. These designs are characterized by an electrical generating capacity of less than 300 MWe for a single reactor power module, with the major difference in comparison to large LWRs being the high degree of passive safety. The smaller thermal power allows the designer to simplify the engineering system and to eliminate active safety systems that require AC electrical power. Instead, passive safety systems rely on natural forces of pressure and gravity to provide the required water coolant flow through the reactor core, removing heat under normal operation as well as the residual decay heat after shutdown to guarantee long-term core cooling during off-normal transients. As an additional benefit, these designs can take advantage of the large operating experience of the current operating fleet.
The NuScale Power Module (NPM) design is the most mature small modular LWR design and is targeted for deployment in this decade as a source of electricity (NuScale Power 2021a). The NPM is a self-contained integral pressurized water reactor (PWR) module, and the reactor vessel and containment are designed to be manufactured in a factory, shipped to the plant site, and installed with other NPMs as part of an overall plant configuration. The current fuel assembly design for an NPM is a shortened version of a standard 17×17 PWR fuel assembly, with the uranium-dioxide fuel enriched to less than 5 percent 235U. The current fuel cycle is designed to be once-through with refueling and inspection of the NPM every 18–24 months. Modules are sited below-grade in a seismically robust pool of water that acts as the ultimate heat sink. The NPM functions similarly to current LWRs, and each module contains both the primary and secondary side of the power cycle. Additionally, the NPM relies on natural circulation for core-cooling and heat transport to the steam generators with a Rankine steam-turbine power conversion system, making it more resilient to any accident. The NuScale reactor system design was recommended by the U.S. Nuclear Regulatory Commission (NRC) staff for its Design Certification (DC) for potential construction and operation in this decade (NuScale Power 2021b). The NRC granted the DC granted the DC in July 2022 (World Nuclear News 2022) and published in the Federal Register
in January 2023. In December 2022, NuScale submitted a Standard Design Approval application to NRC for its updated design, which is based on a six-module configuration powered by an uprated 250 MWth (77 MWe) module (NuScale Power 2023). The design employs the same safety case and passive safety features approved by the NRC in 2020 with a power uprate and select design changes to support customers’ capacity needs.
LIQUID METAL–COOLED FAST REACTORS
Liquid metal-cooled fast reactors have evolved from early small demonstration plants (e.g., the sodium-cooled Experimental Breeder Reactor, EBR) (ANL 2020) and are a relatively mature design concept. The leading design concept uses liquid sodium as the coolant, although lead-cooled systems have also been designed and tested internationally. The sodium fast reactor (SFR) operates at high power density given the absence of a moderator and the superior heat removal capability of the liquid sodium coolant. The reactor operates near atmospheric pressure at outlet temperatures (500–600°C) far below the sodium boiling point (~900°C), a safety feature of the design. There are a few potential system designs: modular (50–100 MWe), pool-type (100–1,500 MWe), or loop-type (600–1,500 MWe). The designs under consideration today are small pool-type SFRs with inherent and passive safety features. These designs may include three coolant circuits: a sodium pool that cools the reactor core and an intermediate loop that separates the sodium pool for safety and transfers the heat to a third loop, which contains the working fluid for the power conversion system (e.g., Rankine steam-cycle). SFRs typically have higher thermal efficiencies (35–40 percent) than LWRs. U.S. designs are fueled with a metallic alloy of uranium and zirconium contained in steel cladding, which was successfully tested in the EBR. The metal alloy fuel has the advantage of having little stored thermal energy, and the core is designed to thermally expand under off-normal high-temperature conditions, providing a negative reactivity feedback to limit any power increase. In contrast, international programs (e.g., France, Japan) use uranium dioxide fuels.
Some small modular SFR concepts, like the GE PRISM design (Triplett, Loewen, and Dooies 2012), recycle their fuel and thus operate on a fuel cycle in which the fuel contains both uranium and plutonium. Larger SFR designs that do not contain recycled plutonium and achieve long-lived fuel cycles (e.g., decades) are under development (e.g., Hejzlar et al. 2013). However, these designs require advances in fuel and materials technology to meet performance objectives.
Most recently, TerraPower, with GE-Hitachi Nuclear Energy and Bechtel as major subrecipients, is developing the Natrium reactor (TerraPower 2020). Natrium™ is a pool-type SFR combined with a molten salt thermal energy storage system as an intermediate loop. The design has a 345 MWe nuclear island with thermal energy storage that can increase output to 500 MWe of power for up to five hours when needed. The nuclear reactor and its supporting safety systems are decoupled from the energy storage system and the balance of plant. This type of design can provide flexible power operations and reduce costs by allowing for the use of non-safety-grade components for the balance of plant. The Natrium design can be optimized for specific markets such as electrical energy or process heat applications (e.g., hydrogen production). This plant is planned for demonstration at the end of this decade.
HIGH-TEMPERATURE GAS-COOLED REACTOR
High-temperature gas-cooled reactors (HTGRs) are an evolutionary reactor design of the gas-cooled reactors in operation today, such as the UK Magnox reactors (World Nuclear Association 2021b) and the Chinese HTR-PM (World Nuclear News 2021a), and are a relatively mature design. The HTGR design is a graphite moderated thermal reactor with a low power density and cooled by high-pressure gas. These reactors use prismatic graphite blocks or spherical graphite pebbles as fuel assemblies within a graphite core structure. Major differences in comparison to large LWRs include use of helium (an inert gas that overcomes corrosion issues) as the heat transfer coolant, higher outlet temperatures, and higher thermal efficiencies (~1,000 K), as well as enhanced safety in the fuel design. The higher outlet temperature enables heat to be supplied for a variety of process heat applications. The fuel design uses uranium fuel formed into “poppy seed”–size particles, termed TRi-structural ISOtropic (TRISO) fuel particles. The current TRISO particle fuel is made up of a uranium-oxy-carbide (UCO) fuel kernel encapsulated by three layers of dense carbon and silicon carbide (SiC) (EPRI 2019). These particles provide the primary
containment of the fission products and are then formed into cylindrical compacts in prismatic graphite blocks or spherical graphite pebbles (billiard ball size). The HTGR uses passive safety cooling systems that rely on heat conduction, thermal radiation, and natural circulation to remove residual decay heat after reactor shutdown, rather than active systems used in current large LWR technologies. HTGRs can be designed to operate as a base load or a flexible load on the grid.
The X-energy HTGR design, Xe-100, is targeted for demonstration in this decade (X-energy 2021b). Each reactor module is 80 MWe, with the plant designed for four reactor modules, comprising a 320 MWe power plant. X-energy is partnering with Dow to demonstrate its first plant at one of Dow’s industrial sites in the U.S. Gulf Coast (X-energy 2023). The reactor will be fueled by X-energy fuel, TRISO-X, in the form of thousands of pebbles, which slowly circulate through the core to allow for on-line refueling and on-site fuel storage. The reactor helium coolant outlet temperature is 700°C. The helium coolant transports the reactor heat to a steam generator, and the plant output options include direct steam heat or electricity production through a Rankine steam-turbine power conversion system. The reactor design incorporates factory-produced commercial components and is planned to be factory assembled and road transportable, with an expected construction time of four years. X-energy is currently engaged with NRC staff on preapplication activities (Bowers et al. 2018; NRC 2021b) and with Canadian Nuclear Safety Commission on a Vendor Design Review for demonstration in Canada at the Ontario Power Generation Darlington site (X-energy 2021a).
GAS-COOLED FAST REACTOR
The gas-cooled fast reactor (GFR) design concept has been developed by researchers in the United States (e.g., General Atomics EM2, Choi and Schleicher 2017; Choi et al. 2021) and internationally (e.g., French CEA Allegra, Dumaz et al. 2007). GFR design concepts employ a low-power-density fast reactor cooled by high-pressure helium. To date, a demonstration plant has not been planned to be built at any scale, and the design is not considered a mature concept. Barriers to demonstration include fuel qualification and developing a method to handle gaseous fission product buildup in the fuel. Uranium carbide (UC) fuels (e.g., EM2’s UC fuel in SiC cladding) have been considered for use in this design because they meet the high fissile fuel requirements for a fast reactor with a smaller core volume. To accommodate the large buildup of fission product gases in the fuel, design concepts propose to use vented fuel rods or larger fuel rod gas plenum volumes as in the SFR. Conceptual designs have specified a core that contains fuel and structures that need to accommodate the high operating temperatures (700–850°C). Outlet temperatures for most proposed designs of this reactor type may be as high as 850°C, and the power conversion system can be either a Brayton power cycle (direct or indirect) or a more traditional Rankine steam power cycle.
FLUORIDE-SALT HIGH-TEMPERATURE REACTOR
The fluoride salt-cooled high-temperature reactor (FHR) is a unique thermal reactor concept that uses TRISO particle fuel technology in combination with a liquid fluoride salt coolant at high temperatures (outlet temperatures 650–700°C) and ambient pressure (Forsberg et al. 2015). The fluoride-based salt is a mixture of lithium fluoride and beryllium fluoride (i.e., FLiBe), which has superior heat transfer characteristics compared to helium, resulting in lower fuel operating temperatures. The reactor is designed to operate at a power density at least four times higher than a HTGR. The graphite moderator can be arranged in a prismatic or pebble core design. The FHR is a relatively new concept (2010), but the pebble bed core design has been advanced more than the prismatic concept. A major challenge to be addressed is gaining operational experience with the FLiBe coolant given its chemical composition with beryllium and the need to maintain worker safety. Kairos, the private company advancing this pebble bed fueled and molten salt cooled design (Kairos Power 2021b), plans to build a prototype test reactor (Hermes) sited in Tennessee near Oak Ridge National Laboratory (Kairos Power 2021a). Kairos plans to demonstrate the performance of this design concept with the Hermes test reactor under prototypic operating conditions that allow for integral testing of its safety systems. The Kairos development approach is unique; that is, at each stage of development, an iterative process of “design, build, and test” is employed to obtain key test data that can improve the reactor design.
MOLTEN SALT REACTOR
The molten salt reactor (MSR) is a concept that has been considered viable since a small experimental reactor was operated at Oak Ridge National Laboratory in the 1960s (ORNL n.d.). This type of reactor uses a molten salt as both the coolant and the fuel; that is, the uranium fuel is dissolved in the molten salt. Both thermal and fast MSRs are under development, and all MSR design concepts are quite unique with the common element being dissolved fuel in molten salt. The thermal systems use a fluoride salt composition with graphite or another material as a moderator. The fast reactor concept uses chloride salts without a moderator. Power densities are generally similar to LWRs for thermal reactor designs and to SFRs for fast reactor designs. Fluoride and chloride salts have high melting points; thus, reactor inlet coolant temperature for these systems must be above 500°C to prevent freezing during normal operation as well as operational transients. While a variety of conceptual designs have been proposed (World Nuclear Association 2021a), they are relatively immature and require significant research and development before a demonstration plant is possible, particularly to address issues related to materials compatibility.
Terrestrial Energy is developing a thermal spectrum Integral Molten Salt Reactor (IMSR) that is 195 MWe, with a 44 percent thermal efficiency, and uses uranium enrichments of about 5 percent, which can be supplied by today’s supply chain (Terrestrial Energy 2021). They are also working on a 390 MWe version. Terrestrial Energy focuses primarily on the Canadian market, where their design is under review by the Canadian regulator and has been selected by Ontario Power Generation for consideration for their Darlington site (Terrestrial Energy 2020). An advantage of molten salts is their high heat capacity, and Terrestrial Energy is planning to be able to use 700°C heat for industrial heat applications in addition to power generation (Terrestrial Energy 2021).
TerraPower is also developing the Molten Chloride Fast Reactor (MCFR), which will use HALEU-based fuel. The MCFR development program began in 2013. A future MCFR-Demonstration reactor will operate with HALEU at approximately 180 MWth, while future commercial plants are expected at 720 MWth (310 MWe). MCFR development includes the ARC15 project for the Integrated Effects Test (IET), which is focused on thermal hydraulic performance, and is currently being commissioned. Additionally, MCFR physics performance will be validated as part of the Molten Chloride Reactor Experiment (MCRE), which will be the world’s first fast-spectrum MSR as well as the first chloride MSR. MCRE will be installed and operated at Idaho National Laboratory’s LOTUS facility as a part of ARDP’s Risk Reduction category. The MCRE team includes personnel from Southern Company (prime awardee), Core Power, INL, Orano and 3M.
MICRO-SCALE NUCLEAR REACTORS
Very small reactors with power output less than 10 MWe (so-called microreactors) are now under development for applications away from the main electrical grid. These microreactors could provide electricity and/or heat for a range of microgrid applications, such as remote communities, industrial complexes, and military and government installations that need a secure, resilient energy supply (Buongiorno et al. 2021b). To maximize their utility and minimize their cost, microreactors are being designed with several key features, including transport of the complete system to the site where it will be used, minimal on-site construction activities, operation with minimal on-site staffing, and coupling to compact power conversion systems such as supercritical CO2 Brayton cycles (Buongiorno et al. 2021a). As discussed in Chapter 7, regulatory challenges exist for these microreactor designs—for example, autonomous operations, security issues, and transport of fueled microreactors to and from the site of operations. Microreactor conceptual designs cover the range of larger advanced reactor designs—for example, liquid metal-cooled and gas-cooled reactors, thermal and fast neutron spectrum reactors (Palmieri, Corradini, and Wilson 2021). Some designs, based on space reactor application, use heat pipe technology for cooling the reactor core and transporting heat to the power conversion system. Examples of these microreactors include the Westinghouse eVinci and the Oklo Aurora (Westinghouse 2019; NRC 2020).
The eVinci microreactor, designed by Westinghouse, is targeted for operation in the next decade to generate heat and electricity in remote communities, as well as commercial and government installations that are self-contained on a local microgrid. The reactor is designed to be manufactured and fueled in a factory environment and transported to a proposed site in standard shipping containers. The design has a scalable power generation
ranging from 1–5 MWe. The eVinci concept is an evolutionary design based on the Los Alamos National Laboratory Megapower reactor, which was built and tested for space applications. The reactor core is designed to use conventional uranium oxide fuel (or TRISO-UCO fuel) with the fuel encased in a solid monolithic metal block with minimal moving parts for reactor control and shutdown. This uranium-fueled reactor does not use a bulk primary coolant. Instead, heat is removed from its core using heat pipes, thereby limiting the number of moving parts and providing overall plant simplicity. The heat pipes use a liquid metal as the working fluid at low pressures. They are embedded in the solid monolithic core to transfer the reactor heat from the core region to a Brayton power conversion system. The design uses the inherent safety features in the fuel, moderator, and heat pipes to enhance safety and self-regulation capability—for example, long-term cooling by conduction to the surroundings. The reactor core is designed to operate for more than three years without refueling and maintenance. The reactor module is also designed to require a minimal number of on-site operational personnel with advanced instrumentation that allows for on-site as well as remote system monitoring.
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