An innovative gas-cooled fast reactor concept being investigated in the United States is the General Atomics Fast Modular Reactor (FMR). The FMR includes a reactor vessel cooling system (RVCS) that is designed to remove decay heat from the reactor pressure vessel via natural circulation in the event that primary cooling is unavailable. Because of its potential role in maintaining the reactor within design limit temperatures, a series of complementary safety assessments were performed to elicit potential failure modes of the system; these included a component failure modes and effects analysis (FMEA), a functional FMEA (FFMEA), and development of a master logic diagram (MLD). The assessments were performed on an analog of the RVCS with design insights derived from the reactor cavity cooling system of the steam cycle-high temperature gas-cooled reactor concept. A model-based systems engineering approach was used to conduct the safety assessments on the analog RVCS via development of a SysML model that captured design features, interfaces, and primary system functions to inform the results of the assessments. Using the SysML model, potential component-level failures and functional failure modes were identified from the component FMEA and FFMEA, respectively. The SysML-facilitated MLD provided complementary results to the FFMEA. The results of the assessments indicate that the RVCS performance is dependent on system parameters that influence natural driving heads. The risk-significance of individual failure modes on RVCS performance can be further evaluated by probabilistic approaches in the future.

This study investigates the transient fuel performance of General Atomics Fast Modular Reactor (GA-FMR) during accident scenarios, focusing on the behavior of its innovative fuel system that combines high-assay low enriched uranium dioxide (HALEUO2) fuel with SiGA (R) ceramic matrix composite silicon carbide cladding. The preliminary fuel design's response was analyzed during reactivity-initiated accidents (RIA) and loss of coolant accidents (LOCA) using BISON fuel performance analysis code, which included both the diffusion enhanced and BISON-FASTGRASS coupled UO2 models. The RIA analysis demonstrated that effective reactivity control reduced fuel temperature, though with transient fission gas release resulting in additional tensile stress state on the cladding. LOCA simulations revealed differing predictions between the two models: the BISON UO2 model showed more transient fission gas release but minimal pellet expansion, while the BISON-FASTGRASS UO2 model predicted less pronounced fission gas release but more fuel swelling and thermal expansion, potentially leading to pellet-cladding mechanical interaction. These findings highlight critical areas for fuel design optimization and identify knowledge gaps requiring further experimental and computational investigation to advance GA-FMR fuel development.

The helium turbo-compressor is one of the core technologies for small modular nuclear reactor (SMR), where the helium gas is a primary coolant of the next generation high temperature gas reactor. General Atomics Electromagnetic System (GA-EMS) is developing a 100-MW thermal Gas-cooled Fast Reactor (GFR) as a part of the Department of Energy (DOE) ARC-20 program, and the helium turbo-compressor is a part of power conversion unit coupled to 44MW permanent magnet generator developed by GA-EMS. The power cycle is a closed loop Brayton cycle with a recuperator. The design speed of the turbo-compressor is 11,400rpm with compressor inlet pressure of 3.1MP and pressure ratio of 2.25. The turbine inlet temperature and pressure are 800 degrees C and 7MPa, respectively. This paper presents novel design features of the helium turbo-compressor with very high aerodynamic efficiency and unique mechanical architecture, which incorporates compact modular design concept with active magnetic bearings (AMBs). The compressor is designed as a two-module (low-pressure compressor and high-pressure compressor) with an intercooler between them for improved cycle efficiency. The entire compressor module and turbine module are designed with their own sets of AMBs and flexible coupling connecting the two rotors for improved rotordynamics and easy maintenance. For effective axial load balancing and thermal management, the axial AMBs were designed with an integrated balance piston. The design point operating speed is below the first bending mode for both compressor and turbine rotors allowing reliable operation of the AMBs for entire operating speed range.

The University of Wisconsin (UW) is part of a team supporting General Atomics Electromagnetic Systems (GA-EMS) in its development of a 100-MWth gas-cooled fast modular reactor. In particular, the FMR uses the reactor vessel cooling system (RVCS) to passively remove the decay heat from the reactor pressure vessel. UW has developed an RVCS test facility for the GA-EMS modular high-temperature gas-cooled reactor and has performed past experiments to demonstrate its performance. This RVCS facility has been updated, and we have performed a series of repeatability tests under ISO (International Organization for Standardization) 17025 standards to validate the existing data set and our previous work. In addition, the UW team has developed a MELCOR model of the UW RVCS facility to analyze these tests and to simulate the two-phase natural circulation flow.This paper presents the recent tests and associated analyses that demonstrate good agreement with the data, except for the larger flow oscillation. The effects of the heat loss in the loop and the water tank pressurization were investigated, but no significant impact was shown. The detailed investigation revealed that the MELCOR simulation results in a higher void fraction (10% to 70%) compared to the experiments (5% to 40%) and an earlier start of flashing. We hypothesize that the water needs to be superheated to flash in the UW RVCS facility due to the slightly colder pipe structures with heat loss.

The accelerated fuel qualification (AFQ) methodology is applied by simulating accelerated fuel tests of the General Atomics Electromagnetic Systems' fuel system for its 44-MW(electric) gas-cooled, fast-spectrum fast modular reactor (FMR). This fuel is comprised of UO2 pellets in SiGA (R) cladding, a silicon carbide ceramic matrix composite. Fast reactors, like the FMR, offer many benefits, including high fuel utilization and flexibility, but may require a lengthy material design process if tests are performed using fast neutron irradiation alone.A thermal neutron irradiation can instead be used to rapidly test how well key components of the current material models extend to high burnup. Thermal neutrons produce a different radial power distribution within the pin than fast neutrons. However, the temperature and burnup values for the two neutron types are comparable, and the differences between the simulated fuel responses are relatively small, demonstrating the weak sensitivity of the physics-based fuel model calculations on the neutron type and the irradiation rate. Furthermore, the deformation of the SiGA cladding saturates after about 1 displacement per atom for both neutron spectra.In an accelerated fuel test, the irradiation time required to reach the target fuel burnup can be reduced by a factor of 3 by using a small rodlet with a 45% smaller pellet diameter while maintaining the same linear power. Therefore, the time for data collection up to high burnup can be significantly reduced while maintaining the same temperature profile, which largely determines the material response. Tests of fuel rodlets of standard and compact size will be carried out in the Idaho National Laboratory's Advanced Test Reactor (ATR), including full size and compact rodlets with varying gap sizes.By applying physics-based mechanistic modeling and simulation in accordance with the AFQ methodology, this type of compact rodlet testing in a thermal test reactor captures the necessary phenomena to test fuel material models up to high burnup and to simulate the expected impact of fast neutron radiation on the fuel in FMR operations. This approach to testing fast reactor fuels in existing thermal test reactors, paired with advanced physics-based mechanistic modeling and simulation, is expected to be applicable to a range of advanced fuels and will decrease the overall fuel qualification timeframe from decades to years.

This paper presents the development of a benchmark for predicting thermal striping through simulation. This work utilized large eddy simulation and will be used to benchmark future models. The testing domain was created using both the STRUCT and the Reynolds-averaged Navier-Stokes turbulence models and is based on an earlier design of the General Atomics Fast Modular Reactor upper plenum. The plenum features two adjacent, identical hexagonal bundles each with a center-placed axial rod drive, with a hot left coolant stream and a cold right coolant stream. The simulation solves the nondimensional Navier-Stokes equations, with temperature accounted as a passive scalar. First- and second-order flow statistics were obtained after 600 convective time units of averaging. The first-order statistics reveal that the hot jet is damped by a recirculatory flow from the near wall. At the same location, the second-order statistics show strong oscillations both in velocity and temperature. The power spectral density was utilized to determine that a low-frequency oscillation occurs here that is within the range of interest for thermal striping. Furthermore, proper orthogonal decomposition was used to identify coherent structures that confirm the oscillatory behavior, indicative of thermal striping. Overall, this benchmark can aid in the development of future models for predicting thermal striping in nuclear reactors, potentially leading to improved reactor safety and performance.

In van der Waals heterostructures (vdWHs), the manipulation of interlayer stacking/coupling allows for the construction of customizable quantum systems exhibiting exotic physics. An illustrative example is the diverse range of states of matter achieved through varying the proximity coupling between two-dimensional (2D) quantum spin liquid (QSL) and superconductors within the TaS2 family. This study presents a demonstration of the intertwined physics of spontaneous rotational symmetry breaking, hidden magnetism, and Ising superconductivity (SC) in the three-fold rotationally symmetric, non-magnetic natural vdWHs 6R-TaS2. A distinctive phase emerges in 6R-TaS2 below a characteristic temperature (T*) of approximately 30 K, which is characterized by a remarkable set of features, including a giant extrinsic anomalous Hall effect (AHE), Kondo screening, magnetic field-tunable thermal hysteresis, and nematic magneto-resistance. At lower temperatures, a coexistence of nematicity and Kondo screening with Ising superconductivity is observed, providing compelling evidence of hidden magnetism within a superconductor. This research not only sheds light on unexpected emergent physics resulting from the coupling of itinerant electrons and localized/correlated electrons in natural vdWHs but also emphasizes the potential for tailoring exotic quantum states through the manipulation of interlayer interactions. 6R-TaS2 is a natural van der Waals heterostructure formed by 1H- and 1T-phase TaS2 layers, which can individually exhibit Ising superconductivity, correlated states and charge density waves. Here, the authors show experimental evidence of emergent nematic Ising superconductivity with simultaneous hidden magnetism (extrinsic anomalous Hall effect and Kondo screening) in 6R-TaS2 under 30 K.

This manuscript presents the fuel performance analysis results of the General Atomics Fast Modular Reactor (FMR) based on an axi-symmetric (2D-RZ) geometry. Three fuel performance model sets that fit the FMR fuel specifications best, i.e., a BISON baseline model set, a BISON diffusion enhancement model set, and a BISON-FASTGRASS model set, were identified and evaluated against a series of relevant experimental cases featuring high burnup and low irradiation temperature conditions. The three BISON-based model sets were then utilized to conduct a comprehensive fuel performance analysis of the FMR fuel under normal operation including the shutdown/restarting periods for refueling. The evaluation of the fuel performance parameters, represented by temperature, internal pressure, stress, and strain, shows that the FMR fuel maintains its thermal and mechanical integrity during normal operation. Technology gaps and limitations are also discussed to guide future efforts for extending the performance analysis to transient scenarios as well as improving the fuel performance evaluation through experiments.

An analytical study of a power conversion unit consisting of a dual compressor and a turbine operating on a closed Brayton cycle with helium is presented. The turbomachinery was a constant hub design with two compressor modules and a turbine module. The design code included the capability to perform sweeps of input parameters such as the polytropic efficiency or loss coefficient, degree of reaction, etc. It is required that the pressure rise of each of the two compressor modules be approximately equal. An optimization procedure for the low-and high-pressure compressors resulted in both of them having seven stages. The performance for each of the compressor stages is almost identical. Another optimization procedure for the turbine yielded six stages. Likewise, the performance for each of the turbine stages is almost identical. Performance maps were developed individually for each of the compressor modules and the turbine. Mathematical constraints based on problem physics were used in conjunction with the component maps to trace turbomachinery operating points. Results show that the low-pressure compressor operates close to the surge line. It is recommended to implement a recirculation path to avoid possible surge in compressor.

Optically active spin defects in wide band-gap semiconductors serve as a local sensor of multiple degrees of freedom in a variety of "hard" and "soft" condensed matter systems. Taking advantage of the recent progress on quantum sensing using van der Waals (vdW) quantum materials, here we report direct measurements of spin waves excited in magnetic insulator Y3Fe5O12 (YIG) by boron vacancy $V_B^-$ spin defects contained in few-layer thick hexagonal boron nitride nanoflakes. We show that the ferromagnetic resonance and parametric spin excitations can be effectively detected by $V_B^-$ spin defects under various experimental conditions through optically detected magnetic resonance measurements. The off-resonant dipole interaction between YIG magnons and $V_B^-$ spin defects is mediated by multi-magnon scattering processes, which may find relevant applications in a range of emerging quantum sensing, computing, and metrology technologies. Our results also highlight the opportunities offered by quantum spin defects in layered two-dimensional vdW materials for investigating local spin dynamic behaviors in magnetic solid-state matters.

The Fast Modular Reactor (FMR) is a 100-MW(thermal) gas-cooled fast reactor being developed by General Atomics Electromagnetic System with the goal of developing a FMR for flexible and dispatchable power to the U.S. electricity market in the mid-2030s. The conceptual design aims to develop and verify simplified design features. These include an inert helium gas coolant, pellet-loaded fuel rods, installations with air cooling as ultimate heat sink, and small and passive heat removal systems. The goal is to ensure the development of a safe, maintainable, cost-effective, and distributed nuclear energy-generating station. The baseline technologies selected to achieve this goal are a helium coolant that is an inert gas with no chemical reaction with structural components, not activated, single phase, enabling high-temperature operation and a high thermal efficiency Brayton cycle; conventional uranium dioxide (UO2) fuel, which is the most widely used and well-known fuel material, capable of high burnup (100 MWd/kg) and a long fuel life; and silicon carbide composite (SiGA (R)) cladding and internal structures that are chemically inert in the helium environment, exceptionally radiation tolerant, and being derisked by accident tolerant fuel technology development. The reactor was specifically designed with passive safety features, including high-temperature in-core materials and a reactor vessel cooling system consisting of cooling panels of naturally circulating water. The passive safety of the core was confirmed for the depressurized loss-of-forced cooling accident, which showed the peak cladding temperature at similar to 1600 degrees C during the transient, which is below the current design limit of 1800 degrees C. The conceptual design of the FMR has been conducted for the reactor system, vessel system, generator and turbomachine, instrumentation and control, residual heat removal system, plant service system, and containment, as well as pre-application licensing documents.

The discovery of topological Hall effect (THE) has important implications for next-generation high-density nonvolatile memories, energy-efficient nanoelectronics, and spintronic devices. Both real-space topological spin configurations and two anomalous Hall effects (AHE) with opposite polarity due to two magnetic phases have been proposed for THE-like feature in SrRuO3 (SRO) films. In this work, SRO thin films with and without THE-like features are systematically Investigated to decipher the origin of the THE feature. Magnetic measurement reveals the coexistence of two magnetic phases of different coercivity (H-c) in both the films, but the hump feature cannot be explained by the two channel AHE model based on these two magnetic phases. In fact, the AHE is mainly governed by the magnetic phase with higher H-c. A diffusive Berry phase transition model is proposed to explain the THE feature. The coexistence of two Berry phases with opposite signs over a narrow temperature range in the high Hc magnetic phase can explain the THE like feature. Such a coexistence of two Berry phases is due to the strong local structural tilt and microstructure variation in the thinner films. This work provides an insight between structure/micro structure and THE like features in SRO epitaxial thin films.

General Atomics is developing a new 100-MW(thermal) fast modular reactor (FMR) that provides safe, carbon-free electricity and is capable of incremental capacity additions. The modular design allows it to be factory built and assembled onsite to keep the capital cost low, while the use of dry cooling facilitates siting to complement renewables in nearly any location.The FMR uses high-assay low-enriched uranium-dioxide fuel encapsulated by recognized irradiation-resistant silicon carbide composite (SiGA (R)) cladding that is derisked in the current accident-tolerant fuel program. The FMR fuel assembly is a hexagonal fuel bundle of 120 fuel rods. The total length of the fuel assembly is less than 4 m, with an active fuel length of 1.8 m. The fuel assemblies are configured in an annular core that is located and supported by the reactor internals. The coolant material is helium at a normal operating pressure of 7 MPa. The core is surrounded by zirconium silicide (Zr3Si2) and graphite reflector blocks. The fuel, coolant, internals, and reflectors are contained within a reactor pressure vessel.The preliminary nuclear design and analysis established the arrangement of the active core and reflector blocks. The nuclear design analyses of the FMR defined the design parameters, such as fuel enrichments, excess reactivity, fueling scheme, fuel cycle, power distribution, and control rod worth. The preliminary conceptual design determined the three-batch fueling scheme with the allowable total power peaking factor of 1.5. The average discharge burnup is 100 GW days per ton of uranium.