Two of the experiment vehicles being developed for the Versatile Test Reactor (VTR) are presented here. The first is a rabbit system that will enable rapid insertion of small test capsules into the high fast flux of the VTR core for relatively short durations. The rabbit concept development includes the construction/demonstration of a near-full-scale system in a deep-water pool to demonstrate functionality, development of a concept of operations and initial procedures, and validation of thermal-hydraulic modeling. In addition, modeling efforts are underway to simulate the thermal and neutronic environment of a rabbit capsule. The second type of experiment vehicle presented here is a driver fuel test assembly for inserting fuel and materials tests into the core by replacing a driver fuel assembly. A novel design for dismountable test assemblies is proposed for the VTR.

To further the effort of coal-to-nuclear (C2N) research, the pros and cons of integrating small modular, high temperature gas cooled, and molten salt nuclear reactor technologies with existing coal plant infrastructure at the Limestone coal plant near Jewett, Texas, USA, were analyzed. Reactor operating characteristics were compared against the Limestone coal plant. The technical requirements and feasibility of various integration schemes were assessed to determine what, if any, reactor technology is optimal for C2N at Limestone. In this analysis large light water reactors were excluded as the size and capacity of these reactors adds regulatory, licensing, and technical complexities that likely make most C2N integration schemes infeasible for these designs.

Previous work has shown in-core printed circuit heat exchange can provide substantial benefits for molten salt fueled reactors. Reductions in overall salt volume and corrosion/deposition cycles due to loop temperature gradients can be achieved. Excore delayed neutron losses are drastically reduced or eliminated, power density is substantially increased, and hot leg/cold leg corrosion is eliminated along with gaining a nearly flat axial temperature profile across the core. In this analysis, a gas cooled, molten salt fueled core is directly coupled to a Brayton cycle and peak core temperature is tracked over the course of a pump failure transient to determine if safe temperature levels are maintained. An NTU-effectiveness calculation is coupled with a point kinetics model as well as feedback to coolant flow rate and heat exchange to model the overall behavior of the system over time. These feedbacks are used to estimate the power and temperature response for a molten salt reactor at steady state and during a primary flow loss transient. Peak temperature conditions are of particular interest in the transient case to determine if the temperature feedback response is sufficient to prevent the system from achieving high temperatures that are incompatible with standard structural materials such as SS-316H or nickel/molybdenum alloys such as Hastelloy-N. While the strongly negative temperature reactivity coefficients inherent to molten salt fuels aid in preventing high temperatures, the passive nature of the coolant flow through the power cycle allows for excellent safety performance in the event of loss of off-site power accidents or pump failures. Safe temperatures can be maintained in an accident scenario, and as much as a third of nameplate electrical power can continue to be produced during a primary pump failure or loss of offsite power accident. The concept promises not only to be “walk-away-safe” but to be a resilient source of power production in the face of accident conditions. This may offer advantages in terms of grid stability in unforeseen circumstances.

Nuclear energy has played an important role in the last fifty years. It currently accounts for about 11% of the world’s electricity generation (IEA, 2014). For nuclear energy to continue to play a prominent role and to help in the transition to a carbon-thrifty economy, it needs to become a more widely accepted energy source. This will require more efficient, safer, and cheaper advanced reactor designs. Molten salt reactors have been identified by multiple parties, and most noticeably by numerous private entities, as one of the most likely advanced reactor concept to be able to achieve all objectives described above in a relatively short time frame. The scope of this project is to tackle some of the recognized challenges that remain towards the deployment of molten salt reactors. This Integrated Research Project is another step in that advancement. NuSTEM The project will contribute to the molten salt fast reactor concept with its combination of very attractive safety characteristics and the unique ability to reduce the inventory of long-lived transuranic wastes. The integrating nature of the project cuts across many dimensions. These include the technical diversity of the research and collaboration across international boundaries. Also as implied by the title of the project, NuSTEM will not only contribute this advanced technology, but it will be critical to the development of young professionals and the human capacity to successfully and safely utilize nuclear technology over the next several decades and it will be used to excite and stimulate young people to pursue STEM fields while gaining a knowledge and understanding of the potential and contributions of nuclear energy to meet

A novel methodology is developed to predict the neutron permeation through media, enabling easier selection of shields and attenuators. The methodology relies on generalized metrics parametrized from MCNP & REG;6.2 simulations where the average neutron energy and relative intensity are tallied through various media. This work builds on previously developed neutron energy attenuation coefficients by discussing accompanying neutron permeation coefficients. Both metrics are used to accurately predict the intensity of neutrons penetrating any medium and the average penetrating neutron energy, allowing for a simplified and generalized approach to predicting neutron penetration. Benchmarks with increasing complexity are used to demonstrate the applicability of the metrics to any geometry and medium. The benchmarks revealed 1.29-3.25% deviation from results in higher fidelity simulations. The data-driven methodology enables streamlined approaches to analyze complex neutronattenuating media and presents a computationally efficient alternative to iterative neutronics simulations for optimizing neutron shielding and moderation setups.

Molten salt reactor safety studies currently rely on correlations originally created for water, which requires further validation to establish uncertainty estimates when using such correlations. This work represents an assessment of low fidelity turbulence modeling in molten salt flows by considering a range of Reynolds Averaged Navier Stokes models for FLiNaK flow in a circular pipe. Star CCM+ is adopted to run forced convection simulations using five different turbulence models. Nusselt number trends show good agreement with Sieder-Tate and Dittus-Boelter correlations in the fully developed region, but the maximum spread between these five models nearly doubles in the entry region. Since fully developed flow conditions are unrealistic in an actual reactor scenario, quantifying the hydrodynamic and thermal entry length is important. This research suggests further studies be conducted for reactor specific flow geometries. This will help establish a more trusted framework upon which molten salt thermal-hydraulic predictions can be based.

The multiphysics object-oriented simulation environment (moose) is a code package that couples a variety of physics modules, allowing for highly accessible multiphysics simulations. The physics modules include a finite element Navier-Stokes (N-S) module that is designed to solve laminar fluid dynamics problems. The usage of this module in multiple recent studies coupled with the growing interest in moose for usage in nonlight water reactor safety studies by the Nuclear Regulatory Commission (NRC) prompted the authors to investigate the computational fluid dynamics capabilities of moose. A two-dimensional laminar flow past a circular cylinder scenario is simulated in the moose framework to investigate the effectiveness of the N-S module. Simulations assumed an unsteady laminar flow with a Reynolds number of 200. To verify the results from moose, similar simulations were conducted using the well-utilized simulation of turbulent flow in arbitrary regions-computational continuum mechanics C++ (star-ccm+) finite volume code. Results from both codes are also compared to some results from literature. Velocity and pressure profiles of both transient simulations were compared. The numerical and input errors in moose are also visualized with contour plots to qualitatively understand the evolution of the errors across time and space. The comparisons between moose and star-ccm+ showed nearly perfect agreement between the codes for velocity and pressure, especially after the development of the vortex street in later time-steps. The force coefficients showed excellent agreement after the development of the vortex street, but demonstrated notable discrepancies prior to the vortex street development, which is likely due to how each code simulated the approach to the vortex street in earlier time-steps.

A low-order neutronics model is developed to carry out hundreds of simulations efficiently and investigate the neutronics behavior of samples being irradiated in a test reactor setting under different geometrical constraints. The low-order model allowed for simulations that yield the expected neutronics behavior of any irradiated sample in any environment and allows for the calculation of highly accurate spatially averaged statistics and idealized spatial distributions in the neutron flux. Several benchmarks are performed to evaluate the performance and limitations of the low-order model revealing many important findings. The low-order model predicted the LHGR in the EBR-II driver fuel to within 2.34% by only simulating the fuel rod by itself, which served as a validation for the model. Sensitivity studies investigated 3% enriched UO2 and U-10Zr being irradiated in the Versatile Test Reactor rabbit system. The analyses investigated a range of combinations of 15 radii and 5 heights for each sample in the rabbit system. Similar data sets are also provided for irradiations in the Advanced Test Reactor’s B-10 irradiation position, which is a thermal neutron spectrum environment. Generalized fits and fit coefficients are obtained for sample heating, reaction rate densities, and local multiplication rate characteristics, allowing the predictions of the neutronics behavior of the samples based on their geometrical constraints. The analyses and fits laid the groundwork for developing a user-end Multiphysics analysis framework to assist and accelerate irradiation experiment design and optimization.

In recent years, Molten Salt Reactor (MSR) technology has received significant attention. Its low pressure makes the MSR a low-risk option which, when coupled with its high temperature, makes it a highly beneficial choice for power generation and suggests the ease of integration with other systems such as desalination. It is worthwhile with the growing interest in MSRs, to develop a simulation tool that can predict performance in a multi-faceted MSR-based flow loop. This study focuses on MSR applications, namely clean water production utilizing a specific optimized combination of FLiNaK and supercritical carbon dioxide (SCO2) cycles. It addresses the growing worldwide need to decarbonize our energy production and simultaneously provide clean water. The results of optimized MSR-desalination configurations exhibit promising implications for the production of abundant amounts of clean water while also maintaining ample power supply to the electrical grid. They show that a design for a coupled MSR and desalination plant are not only possible but that the components and design are scalable for 50 - 250 MWth power input.

The multiphysics object-oriented simulation environment (moose) is a code package that couples a variety of physics modules, allowing for highly accessible multiphysics simulations. The physics modules include a finite element Navier–Stokes (N–S) module that is designed to solve laminar fluid dynamics problems. The usage of this module in multiple recent studies coupled with the growing interest in moose for usage in nonlight water reactor safety studies by the Nuclear Regulatory Commission (NRC) prompted the authors to investigate the computational fluid dynamics capabilities of moose. A two-dimensional laminar flow past a circular cylinder scenario is simulated in the moose framework to investigate the effectiveness of the N–S module. Simulations assumed an unsteady laminar flow with a Reynolds number of 200. To verify the results from moose, similar simulations were conducted using the well-utilized simulation of turbulent flow in arbitrary regions—computational continuum mechanics C++ (star-ccm+) finite volume code. Results from both codes are also compared to some results from literature. Velocity and pressure profiles of both transient simulations were compared. The numerical and input errors in moose are also visualized with contour plots to qualitatively understand the evolution of the errors across time and space. The comparisons between moose and star-ccm+ showed nearly perfect agreement between the codes for velocity and pressure, especially after the development of the vortex street in later time-steps. The force coefficients showed excellent agreement after the development of the vortex street, but demonstrated notable discrepancies prior to the vortex street development, which is likely due to how each code simulated the approach to the vortex street in earlier time-steps.

Printed circuit heat exchange has been found beneficial in various chemical applications to allow for high rates of reaction per unit volume of reactor space.This is especially true where large amounts of heat exchange are necessary to provide either energy input to the reaction or remove heat produced from an exothermic reaction.Given that fission is a highly exothermic chemical reaction, the same concept may be applied to a fluid phase fission reaction like in a molten salt reactor.Calculations performed both with the NTU-efficiency method as well as in RELAP5-3D suggest an order of magnitude increase in power density is possible relative to a base case thermal spectrum, graphite moderated, molten salt reactor core.Along with the benefit of increased power density, other benefits are identified: elimination of hot leg/cold leg corrosion and deposition mechanisms, nearly flat axial temperature profile, having a maximum axial temperature gradient of 6.5 °C or less in the core at full power, elimination of the primary heat exchanger from the fuel loop, since primary heat exchange occurs in-core; minimal reduction in delayed neutron fraction due to a lower ex-core salt fraction, and facilitation of chemical separations of radionuclides due to smaller fuel salt volume and higher radionuclide concentration.Increasing the rate of heat transfer in the core with a novel heat exchange design drastically improves the output of a small footprint reactor, allowing for the benefits of small modular manufacturing while exceeding the thermal and electrical output of traditional large scale nuclear construction projects on a volumetric basis.

The hydraulic single-phase mixing of three parallel rectangular channels is experimentally investigated at various Reynolds numbers (Re) and flow regime combinations. Particle Image Velocimetry results for seven mixing cases are presented and discussed with varying Re combinations ranging from 1,824 to 20,844. While all cases result in the same Re ratio of ∼0.69 between the inner and outer flows, two cases represent multi-regime mixing with the inner-outer regime pair of laminar-transitional and transitional-turbulent, while the other 5 cases are all characteristic of turbulent mixing with varying levels of turbulence. The outer channels initially share characteristics with a backward facing step. The center channel is found to initially behave like a slot jet, but then sees a significant increase in velocity decay. This inner flow velocity decay increased dramatically in the laminar-transitional mixing case, whose centerline velocity decay was ∼6 times larger than the decay in the turbulent mixing cases. Second order statistics revealed a consistent mixing layer thickness of ∼0.1 hydraulic diameters for all the cases but showed more intense shearing in the multi-regime mixing cases. The combined point and thereby the mixing layer length is determined using centerline velocity decay profiles, which show a much more aggressive mixing in multi-regime flows. Multi-regime mixing demonstrated superior characteristics relative to turbulent mixing due to a more dramatic velocity decay in the inner flow and a shorter mixing length. The contributions of this work include communicating the benefits of multi-regime mixing and providing detailed characterization efforts that can serve future efforts for validating computational models. This research also lays the groundwork for future studies aimed at achieving high levels of mixing without a severe penalty in pressure drop.

The near field mixing phenomenon created by a round jet with three slot lobes exhausting into a crossflow are investigated at a velocity ratio of 0.5. Time-resolved particle image velocimetry measurements provide instantaneous velocity fields of the slotted jet in crossflow, allowing for evaluation of the first and second order turbulent statistics in two perpendicular planes of interest. The independently controlled jet exit and crossflow inlet are first characterized extensively to confirm the velocity ratio and anticipated momentum exchanges. Spanwise and transverse mean velocity profiles reveal that the interaction of the three slot lobes and the center round jet primarily occur in the immediate jet exit region, though residual effects are also found in the wake. Evaluation of the Reynold stresses aims to quantify the near region mixing between the jets collated geometric features and their interaction with the crossflow. Frequency analysis reveals that low-frequency harmonics in the wake region provide greater energy contributions than that of the higher-frequency harmonics found along the leading edge shear layer. This behavior is attributed to the low velocity ratio, where the freestream velocity is twice as large as the jet exit velocity. The experimental data and observations herein serve analogous computational modeling efforts for the slotted jet in crossflow at low velocity ratios, with ample information to inform necessary boundary conditions, fluid properties, and flow fields for validation.

The reactor cavity cooling system (RCCS) is a common reactor safety system in high-temperature gas-cooled Reactors (HTGR) that removes heat from the reactor pressure vessel (RPV) by radiation (similar to 80%) and natural convection (similar to 20%). For the simulation of accident scenarios of HTGRs, intermediate fidelity and system codes models must be employed to limit the models' execution time. While an accurate quantification of the radiative heat transfer is available in these models, the quantification of natural convection must rely on correlations of questionable accuracy for the Nusselt number. Commonly used correlations are based in experiments performed at low Rayleigh numbers and/or using isothermal walls in simplified geometries. This work improves on the accuracy of natural convection heat transfer correlations to support HTGR designs.These correlations include both local and average Nusselt numbers as a function of the global Rayleigh number, the local Rayleigh number, and the temperature profile at the hot wall of the RCCS. In the absence of dedicated experiments and the difficulty of performing high-fidelity simulations at realistic Rayleigh numbers, the data to fit the correlations are generated with computational fluid dynamics (CFD) using Reynolds Averaged Navier-Stokes (RANS) models. First, a careful selection of the RANS turbulence model is performed by comparing the results obtained with different RANS turbulence models against high-fidelity simulations of natural convection at Ra 1 x 10(11) in a rectangular cavity. Next, the selected model is used to perform simulations of an HTGR cavity at different high Rayleigh numbers is an element of [6.1 x 10(11) ,2.9 x 10(13)] to encompass several HTGR designs, assuming an isothermal RPV wall. The results obtained are used to fit a correlation for the average and space-varying Nusselt number as a function of the global and local Rayleigh numbers via a sparsity-promoting, least-squares method. The selected RANS model is then used to perform simulations of a PBMR-400 (Pebble Bed Modular Reactor) HTGR cavity with the temperature profiles at the RPV wall obtained during a PLOFC (Pressurized loss of forced cooling) transient. We use the results obtained to fit a temperature-dependent correction to the space-varying Nusselt number with the sparsity-promoting, least-squares method. The results obtained in this work enable system-level codes, such as Pronghorn, to perform higher-fidelity simulations of the heat exchange process in the RCCS while still maintaining a low computational cost. Published by Elsevier Ltd.

Many complex turbulent flows in nature and engineering can be qualitatively regarded as being constituted of multiple simpler unit flows. The objective of this work is to characterize the coherent structures in such complex flows as a combination of constituent unitary flow structures for the purpose of reduced-order representation. While turbulence is clearly a non-linear phenomenon, we aim to establish the degree to which the optimally weighted superposition of unitary flow structures can represent the complex flow structures. The rationale for investigating such superposition stems from the fact that the large-scale coherent structures are generated by underlying flow instabilities that may be reasonably described using linear analysis. Clearly, the degree of validity of superposition will depend on the flow under consideration. In this work, we take the first step toward establishing a procedure for investigating superposition. Experimental data of single and triple tandem jets in crossflow are used to demonstrate the procedure. A composite triple tandem jet flow field is generated from optimal superposition of single jet data and compared against ‘true’ triple jet data. Direct comparisons between the true and composite fields are made for spatial, temporal, and kinetic energy content. The large-scale features (obtained from proper orthogonal decomposition or POD) of true and composite tandem jet wakes exhibit nearly 70% agreement in terms of modal eigenvector correlation. Corresponding eigenvalues reveal that the kinetic energy of the flow is also emulated with only a slight overprediction. Temporal frequency features are also examined in an effort to completely characterize POD modes. The proposed method serves as a foundation for more rigorous and robust dimensional reduction in complex flows based on unit flow modes.

Turbulent mixing in the near region of a round jet with three slot lobes is examined via mean velocity and turbulent statistics and structures at a Reynolds number of 15,000. The design utilizes separate flow motivations upstream of each geometric feature, deviating from conventional nozzles or orifice plates. Immediate outlet velocity profiles are heavily influenced by opposing pressure gradients between the neighboring round and slot streams. Spanwise mean velocity profiles reveal the majority of the convective exchange between a given slot and the round center occurs in the immediate near field, but has lasting effects on the axial centerline profiles downstream. This is also reflected by the velocity half-widths, exhibiting asymmetry across the entirety of available measurements. Centerline turbulence intensities exhibit strong and short-lived isotropy. The increasingly anisotropic intensities found downstream are lower than similar geometries from the literature, implying that mixing development is inhibited. Reynolds stresses at the round-slot interface are significantly smaller than the round-stagnant exchange, but achieve a symmetric condition at x/D congruent to 4. Two-point spatial correlations of the fluctuating streamwise velocity exhibit stronger dependence toward the axial centerline at the round-slot interface in comparison to the nominal round radius. In contrast, spanwise velocity fluctuations exhibit nearly identical, localized behaviors on each side of the jet. Corresponding differences in streamwise integral length scale peak in the range 1.0 <= x/D <= 1.5, and so too do the turbulent structures in this area, as a result of the collated jet geometry.

A new methodology for designing neutron shields and selecting moderators using sets of generalized metrics is presented. The metrics were derived from many MCNP®6.2 simulations. Neutron and photon statistics were tracked at the outer surface of a sphere for each of the materials in each simulation to derive neutron energy attenuation coefficients that can accurately track the exponential energy decrement of neutrons in media. Neutron penetration probabilities as well as a normalized measure of the number of collisions were also obtained as a measure of the number of neutrons that penetrate the media and the number of collisions the neutrons undergo, respectively. Finally, the penetrative photon generation probability and their average energy were also obtained, allowing for photon dose rate computations. Attempts at generalizing each of the derived metrics are also discussed, showing the limitations of such simplifications. The methodology allows more streamlined approaches for analyzing complex neutron attenuating media.

Investigation of the near field dynamics of a single and tandem array of three jets are provided by 2-D time-resolved particle image velocimetry (TR-PIV) measurements. Instantaneous velocity fields are examined in the transverse and spanwise planes with jet to cross flow velocity ratios in the range from 0.9 to 1.7. Previous studies have shown that for high ratios (≥2), the leading jet provides sufficient shielding to ensure that all jets downstream exhibit nearly identical flow characteristics. The current transverse plane measurements exhibit more unique and localized features as a result of the competing effects of pressure gradients and vortex mechanisms assessed via the jet exit profiles, first and second order turbulent statistics, streamline trajectories, recirculation areas, and penetrations depths. Proper orthogonal decomposition (POD) is applied to the spanwise plane instantaneous velocity fields to determine the statistically dominant features of the single and tandem jet configurations at equivalent velocity ratios. The velocity fields are then reconstructed using the truncated POD modes to provide further insight into the shear layer and wake vortices that drive these configurations. Vortex identification algorithms are applied to the reconstructed velocity fields to determine the statistical characteristics of the vortices, including their centroids, populations, areas, and strengths, each of which exhibit largely different dependencies on jet configuration and velocity ratio. Several of the investigated metrics are found to exhibit different behaviors below and above a velocity ratio of unity, and also as a function of increasing velocity ratio between 1 and 2, implying that several transitions mechanisms are present in the low velocity ratio regime investigated herein.