For designing basic and advanced safety features of nuclear, chemical, or other kinds of plants, boiling flow is one of the most crucial concepts. Several initial and boundary parameters, such as pressure, inlet subcooling temperature, mass flux, heat flux, and flow geometry, control the bubble dynamics and behavior. While a previous flow boiling experiment by the authors' laboratory has revealed valuable parametric effects in four-sensor conductivity probes, these conventional probes feature near-wall blind zones that limit their coverage of radial profiles in narrower channels. An innovative five-sensor design has been therefore proposed and validated in the current study for reducing such blind zones. The new design has two leading sensors sharing three trailing sensors, and it uses the same principle as conventional four-sensor probes to calculate different properties of bubbles. New datasets of upward flow boiling have been collected using five-sensor probes of different sensor patterns in a narrow annular test section. The results are compared with each other's to identify the design that gives better consistency. The boiling annular channel has an inner diameter of 25.4 mm and an outer diameter of 38.10 mm, with a heated inner rod length of 3 m. The conductivity probes are adopted to measure void fraction, gas velocity, and interfacial area concentration following a two-group description.

Liquid-fueled molten-salt reactors have dynamic features that distinguish them from solid-fueled reactors, such that conventional system-analysis codes are not directly applicable. In this study, a coupled dynamic model of the Molten-Salt Reactor Experiment (MSRE) is developed. The coupled model includes the neutronics and single-phase thermal-hydraulics modeling of the reactor and validated xenon-transport modeling from previous studies. The coupled dynamic model is validated against the frequency-response and transient-response data from the MSRE. The validated model is then applied to study the effects of xenon and void transport on the dynamic behaviors of the reactor. Plant responses during the unique initiating events such as off-gas system blockages and loss of circulating voids are investigated.

The Two-Fluid Model is the backbone of thermal-hydraulics and system-analysis codes for nuclear design. The Two-Fluid Model tracks the transfer of conserved quantities—mass, momentum, and energy—without the need for bubble interface tracking. However, two-phase flows are characterized by their different flow regimes which change as more vapor is present in the flow, from bubbly flow to cap/slug flow to annular flow. The two-group Two-Fluid Model can track the progression of boiling flows beyond the bubbly region without the need for flow-regime maps. A two-group wall-boiling model is implemented and coupled with the two-group Interfacial Area Transport Equation. The resulting model is compared against experimental data and predicts the growth in cap/slug bubbles through interaction models rather than through flow-regime maps. The coupled model can predict the growth in void fraction and interfacial area concentration beyond the capability of the one-group model, demonstrating the applicability of a coupled two-group approach to modeling boiling flow.

As one of the Gen-IV advanced reactors, liquid-fueled MSRs possess several distinctive features, one of which is the inclusion of a xenon removal system. The xenon removal system could enhance fuel utilization and remove a major barrier for load-following operation. In this study, a system level Simulink model of the Molten Salt Demonstration Reactor is developed. The model is used to perform a design analysis of the xenon removal system. Preliminary but quantitative requirements from fuel utilization and load following operation perspectives are applied. A reference system design that could satisfy the proposed requirements is obtained from the simulations. Moreover, a preliminary cost-benefit analysis is conducted using the reference design. It is found that the xenon removal system mostly benefits from load-following operation, and the major cost lies in the processing and storage of the fission-product containing gas in the off-gas system.

The potential deployment of microreactors as a zero-emission source for critical applications within integrated energy systems such as microgrids has been gaining interest in recent years owing to the microreactors' dispatchable nature, modular design, small site footprint, and carbon-free generation. A particularly high-value but challenging application with rapidly growing demand is in the deployment of high-performance computing (HPC) clusters within microgrids. In this work, a model of a HPC cluster in an energy-diverse microgrid is developed to determine the requirements of a technology-agnostic microreactor deployed for such a challenging application. The minute-resolution simulations revealed that the cluster's electrical load fluctuation of up to 4.1 MW/min required a fast and responsive load-following capability. When the load-following capability of the microreactor was perturbed, the required microgrid storage capacity associated with having a 0.1 MW/min dispatchable microreactor decreased by two orders of magnitude as compared with load-following solely by energy storage devices, indicating that load-following capability in microreactors is of great value in such applications. The analysis methods described in this work can be extended to other microgrids, other HPC clusters, or other types of challenging applications, and can help microgrid planners in determining the storage size, output capacity, and ramping capabilities of the storage devices required for a given microgrid configuration.

A major issue with the economic validity of nuclear power technologies is their rate of return on investment. Since these systems have high expected initial capital costs, it is difficult to instill investment confidence against the backdrop of untested construction and operation. Nonetheless, emerging nuclear technologies such as microreactors remain promising as their output is carbon-free and the high outlet temperature associated with many microreactor concepts enables them to generate process heat that can power industrial processes, thereby widening their versatility beyond electricity generation. The pairing of these systems with higher value commodities, such as hydrogen, could potentially improve the economic viability of microreactors. Hydrogen production has become a subject of great interest in recent years for numerous applications such as for transportation, metal refining, and fertilizers. With a profitable microreactor-powered hydrogen production system, the price dependency of these applications and downstream commodities on the volatile natural gas prices can be reduced. In this work, the pairing of a microreactor with natural gas reforming (NGR) and high-temperature electrolysis (HTE) was modeled and it was found that a 10 MWth to 20 MWth microreactor could become economically viable through revenue from hydrogen production. The technology-agnostic microreactor energy source paired with the NGR and HTE plants was able to generate achievable principal loan values that were above $4.5 M/MWth for a 15 MWth reactor over a 20-year period. The cost of a first-of-a-kind microreactor according to available estimates exceeded the average achievable loan values for the HTE system while the NGR system was able to achieve these estimates within 21 years. The pairing of HTE with autothermal reforming was also investigated and it was found to be uncompetitive as compared with NGR and HTE.

Low-pressure two-phase flow instabilities can potentially challenge start-up transients of water-cooled nuclear reactors. Predicting oscillations caused by flow instability is therefore paramount to reactor safety. While many thermal hydraulics system codes are developed as general-purpose tools, their performance is not guaranteed outside their optimized application ranges. The current study therefore performs validations of two thermal hydraulics system analysis codes, the Adaptive SYStem Thermal-hydraulics Version 3 (ASYST VER3) and Reactor Excursion and Leak Analysis Program MOD3.3 (RELAP5/MOD3.3), in reproducing oscillatory two-phase flow induced by low-pressure flashing instability. Benchmark conditions are selected from a novel dataset resolving void fraction with radial resolution and quantifying periodic behavior with ensemble averaging. Focusing on the prediction of transient two-phase phenomena rather than the determination of stability, validations are conducted by simulating a single channel under prescribed periodic boundary conditions. ASYST exhibits a systematic underprediction of void fraction in an adiabatic chimney downstream of a heated section. The prominent causal discrepancy is identified as a lost travelling void wave due to overpredicted condensation. RELAP5 noticeably overpredicts subcooled boiling and underpredicts flashing, which is consistent with existing validations against steady-state separate-effect tests under low pressure. This degraded code performance also suggests that in low-pressure transients prone to flashing instability, the confidence in RELAP5 derived from existing validations against integral-effect tests shall be carefully limited to validated capabilities in integral systems. In general, the current study fills the previous gap of knowledge about the performance of ASYST and RELAP5 in predicting detailed transient two-phase phenomena in low-pressure flashing-induced oscillations. The identified code defects reveal future directions for code improvements eventually towards reliable application of ASYST and RELAP5 under low pressure beyond their original calibration.

Detailed reviews of a past advanced nuclear reactor-based integrated energy system, as well as other nuclear reactor and fossil fuel-based integrated energy systems, have been performed for this work. A review of the utilization of heat from nuclear reactors for various applications and cogeneration has been done. The heat can be utilized by extracting the steam from the turbine while the steam is still at a desired temperature. While the use of nuclear process heat for district heating in countries like Finland, France, China, Poland, and elsewhere is discussed, more focus of the review has been given to nuclear desalination processes.Integrated energy systems (IESs), where distinct types of reactors like pressurized water reactors, boiling water reactors, sodium-cooled fast reactors, heavy water reactors, and other advanced reactors are coupled with various nuclear desalination processes, like multi-effect distillation (MED), multistage flashing, and reverse osmosis methods, are discussed. The nuclear desalination plant at Aktau is discussed in more detail due to its decades of successful operation. The IES of the Aktau plant coupled with a five-effect MED desalination plant was taken as a reference for modeling the Open Modelica (OM)-based IES in this work. The OM IES model shows good agreement with the MED plant output of Aktau and can be extended for future applications of IESs.

In the quest for a sustainable and climate-resilient future, significant interest is found in advanced reactor technologies in recent years including the liquid-fueled molten salt reactors (MSRs). Liquid-fueled MSRs stand out due to their unique characteristics, especially the potential online removal of fission products such as xenon-135. The removal of xenon can enhance fuel utilization and make the reactor more adaptable to load-following operations. However, the development of xenon removal system for MSRs requires improved understanding of xenon behavior in liquid-fueled MSRs. In this study, a system level Simulink model for liquid-fueled MSRs is developed and then adapted to study the xenon behavior in the Molten Salt Reactor Experiment (MSRE). The steady-state and transient xenon behavior in the MSRE is simulated and compared with available data and existing models in the literature. Good agreement is found between the simulation and the experiment under the same set of model parameters. The importance of xenon transfer between the core graphite, circulating bubbles, and fuel salt is highlighted. The Simulink model can be easily extended for future development of a xenon removal system in commercial scale liquid-fueled MSRs.

Nuclear microreactors offer reliable, low-carbon dispatchable power and heat for various end use applications. Although the direct electricity end uses are straightforward, the feasibility of microreactors' integration for thermal end use has not been analyzed in literature in sufficient detail. Delivering process heat generated by nuclear microreactors to supply the high temperatures essential for hydrogen production has been proposed as one cogeneration option that can further aid in the alleviation of climate change, since hydrogen can replace carbon-emitting fuels such as gasoline, diesel or natural gas. This review provides a novel perspective on the intersection of microreactors and process heat use by investigating hydrogen production technologies, microreactor designs and process heat integration options. A comprehensive overview of hydrogen production methods including electrolysis and thermochemical conversions of hydrocarbons and water is presented by classifying the methods based on process temperatures and maturity. Additionally, an in-depth summary detailing the reactor type, power output, and maximum operating temperature of many prospective microreactor designs has been created. Finally, heat transfer options for integrating microreactors to hydrogen production systems were evaluated. The intermediate heat exchanger (IHX) assessment considers IHX material, IHX type, and heat transfer media utilized within the apparatus.

Two thermal hydraulics system codes, ASYST VER3 and RELAP5 MOD3.3, are assessed for their prediction of stability and limit cycles of low-pressure natural circulation with potential flashing instability. The benchmark conditions are from a nearly five-meter-tall water loop whose experiments have generated a published dataset covering both stability and limit-cycle oscillations. These conditions are simulated by a model of the entire system, and the target asymptotic behaviors are approached through sufficiently long transients under prescribed operational settings. ASYST is found not conservative for stability prediction, in the sense that the unstable operational range is underpredicted with discrepancy mainly in the high-subcooling stability boundary. Qualitative flow changes across the island of instability are however simulated satisfactorily, and for conditions correctly predicted as unstable, reasonable prediction is achieved for the oscillation period, time-averaged flow rate, and peak flow rate. Further comparison against experimental void fraction confirms that ASYST simulates physical two-phase behaviors in the limit cycles. RELAP5 predicts a much more stable system whose island of instability is significantly smaller than that of reality. Its simulated flow oscillations also noticeably deviate from the typical patterns of flashing instability, resembling sinusoidal waves following periods different from the experimental measurement. Underprediction of flashing and overproduction of subcooled boiling are identified as potential major defects in RELAP5, which calls for future model calibration and further investigation. In general, the current assessment contributes to the awareness of potential uncertainties when adopting ASYST and RELAP5 for predicting flashing instability and its induced transients.

In single phase flows, benchmarks like the lid driven cavity have become recognized as fundamental tests for newly developed computational fluid dynamics, CFD, codes. For multiphase free surface flows with variable surface tension, the presently studied pool with isothermal sidewalls is suggested as it is the simplest domain where Marangoni effects can dominate. It was also chosen due to its strange sensitivity to the initial setup which is discussed at length from a chosen number of ‘scenarios’. It was found that the fluid interface can reverse deformation by a change in the top boundary condition, the liquid equation of state, and the gravity level. For the top boundary condition, this reversal is due to vapor expansion within the closed volume, creating an additional convection mechanism. Not only does the interface reverse, but the peak height changes by more than an order of magnitude at the same Marangoni number. When including gravity, the peak velocity can increase significantly, but it can also cause a decrease when done in combination with a change in the top wall boundary condition. Finally, thermal expansion of the liquid phase causes the peak velocity to be reduced, with additional reductions from the gravity and top wall condition. The differences in each scenario could lead to significant errors in analyzing a practical application of Marangoni flows. Therefore, it is important to demonstrate that a new CFD code can not only resolve Marangoni convection, but also has the capability to resolve the scenario most relevant to the application at hand.

The Two-Fluid Model (TFM) has long been the backbone of engineering-scale two-phase flow simulation in system-analysis codes and computational fluid dynamics codes. The classical TFM is limited in how it can capture the differences in the transport of small and large bubbles. The two-group TFM provides the ability to specify the unique transport characteristics of small and large bubbles separately. Expanding to two sets of conservation equations for the two bubble groups presents the additional challenge of bubble group accounting as bubbles can cross the group boundary. The three mass transfer terms in the two-group TFM are evaluated for flashing, condensing, and boiling flows using a partitioning method. The axial trends in the source terms are examined for these flow conditions with the available intergroup models. Two-group interphase models are implemented and evaluated against experimental data for flashing, condensing, and boiling flows with accurate two-group results. The capabilities of the two-group TFM are evaluated for these flow types, demonstrating the ability to predict two-group vapor properties without the need for flow regime transitions.

Microreactors present an opportunity to revolutionize the role of nuclear energy via the development of these technologies in a diverse and distributed energy network for a clean energy future. Because of the limited output of these novel systems, the deployment of microreactors should be focused on high-value applications in order to realize their full potential. This involves understanding the microreactor performance and how it interacts with the preexisting infrastructure. In this work, an energy-diverse embedded grid is modeled using OpenModelica in order to study the impact of microreactor integration under several distinct deployment approaches. The University of Illinois at Urbana-Champaign (UIUC) is used as a prototypic market due to its well-characterized energy ecosystem with available extensive real-time and historical data. The UIUC model recreates the existing chilled-water, steam, and electricity infrastructure, including wind, solar, and cogeneration sources. The infrastructure model simulates the interplay between the three utilities and how different microreactor integration approaches would impact UIUC's embedded grid. From this study, the deployment of a single microreactor under electric load-conditioning, steam production retrofit, or a hybrid of both is found to be the most appropriate in consideration of their unique advantages toward cost savings and grid resilience. Meanwhile, if grid resiliency is not a main objective, the greatest emissions reduction and cost-savings benefits can be obtained by operating the reactor in a base-loading configuration. This study employed historically low coal and gas prices and provided a conservatively low estimate for the benefits from a microreactor. Given the price volatility of fossil fuels, the benefits of the microreactor are expected to be greater than this estimate. Finally, the modular nature of the modeling framework allows for an extension of the analysis to other similar embedded grids.

Fluid-Structure Interaction (FSI) is a significant phenomenon in most nuclear reactors, causing effects such as Flow-Induced Vibration (FIV) and thermally-driven Core-Radial Expansion (CRE). We demonstrate that Cardinal, an open-source coupling of NekRS and OpenMC to MOOSE, can be used for modelling FSI by coupling the Tensor Mechanics Module from the Multiphysics Object Oriented Simulation Environment (MOOSE) to NekRS's Arbitrary Lagrangian-Eulerian (ALE) solver. The solid mechanics-thermal hydraulics coupling is implemented using efficient in-memory coupling and data transfers. We provide a preliminary demonstration of these capabilities with a 3-D FSI benchmark for an elastic block in crossflow.