Ilyas Yilgor

This report continues the body of work led by the National Reactor Innovation Center at Idaho National Laboratory following the development of the Road Map for the Development of Commercial Maritime Applications of Advanced Nuclear Technology. Previous efforts examined candidate nuclear-maritime configurations, deployment pathways, feasibility considerations, and the readiness of existing U.S. testing capabilities. The present work addresses the technical research and development needs associated with nuclear systems in maritime environments, with emphasis on the effects of ocean-induced motion. An initial assessment of motion-sensitive physical phenomena is performed to identify the mechanisms through which vessel translation, rotation, inclination, and acceleration may influence reactor-system behavior. A common non-inertial reference-frame formulation is developed in which the reactor geometry remains fixed while transformed gravity and apparent accelerations simulate the effects of ship motion. The formulation is implemented using complementary modeling approaches. RELAP5-3D provides a one-dimensional system-level representation of integrated reactor and balance-of-plant behavior, while the open-source MOOSE capability provides a multidimensional finite-volume representation of local flow and temperature fields. The maritime-motion models are assessed using analytical verification problems, benchmark calculations, and comparisons with available experimental data. The developed capabilities are subsequently applied to selected reactor systems, including light water reactor models and a Generation IV liquid-fueled molten salt reactor. The analyses examine the influence of ocean-induced motion on natural circulation and reactor response during postulated operating and accident conditions. The results are used to identify motion-sensitive parameters, evaluate the applicability of existing modeling approaches, and determine where additional experimental evidence is needed. The report concludes with a discussion of modeling limitations, validation gaps, and data needs for maritime nuclear applications. Simulation and experiment are treated as an iterative process: numerical models guide the selection of test conditions and measurements, while experimental observations support model validation, refinement, and uncertainty reduction. This simulation-to-experiment framework provides a technical basis for future component-level and system-level experimental campaigns supporting the development and deployment of commercial maritime nuclear technologies.

Micro-scaled high-temperature gas-cooled reactors (micro-HTGRs) offer a promising option for reliable power in remote or off-grid locations. While the safety characteristics of modular HTGRs have been widely studied, a micro-HTGR configuration alters several key thermal-fluid phenomena that govern both normal operation and passive decay-heat removal. In many proposed concepts, the reactor vessel is oriented horizontally and integrated into an ISO shipping container to enhance transportability and modular deployment. This report documents a Phenomena Identification and Ranking Table (PIRT) exercise focused on the thermal hydraulic safety phenomena relevant to all micro-HTGRs. The objective is to systematically identify, describe, and rank the importance, uncertainty, and modeling complexity of the key phenomena that control core and vessel temperatures during normal operation, pressurized conduction cooldown (PCC), and depressurized conduction cooldown (with air ingress) conditions.

Steam generators are integral components of both conventional and advanced reactors utilizing steam power cycles, as they influence the power conversion performance. Steam generator designs are highly influenced by the type of reactor coolant used, which mainly governs the different control, monitoring, and safety system actuation requirements. Therefore, the present work reviews the thermal hydraulics instrumentation within steam generators designed for pressurized water reactors (PWRs), high-temperature gas-cooled reactors (HTGRs), sodium fast reactors (SFRs), and molten salt cooled reactors (MSRs). Emphasis is given to measurement techniques that are either unique or particularly important to specific reactor types. Lastly, a discussion is provided on the instrumentation and measured parameters. The work identifies standard parameters of interest and is expected to aid the development of test beds and novel measurements techniques.

Effective gas-gap calorimeter sizing and rating tools are required to design calorimeters that enable well-defined operating conditions, high cooling powers, and accurate calorimetry. The present report describes two tools utilizing Microsoft Excel and MathWorks MATLAB that enable the sizing and rating of gas-gap calorimeters with a He-Ar binary gas mixture in the gap. The Excel tools are two easy-to-use spreadsheets for calculating heat pipe operating temperature or He-Ar mole fractions given the required inputs. The MATLAB tool includes similar models with more robust solution algorithms along with sub-options for full-vacuum, static gas-gap, and forced circulation in the gas-gap. The accuracy of the developed tools were demonstrated by comparing with existing work in literature. The MATLAB tool was used for a parametric study to determine the gas-gap thickness, coolant gap thickness, coolant flow rate, and coolant inlet temperatures for the testing of a 3/4 in outer diameter heat pipe up to powers of 10 kW and temperatures of 1,000°C. The effects of heat pipe and calorimeter inner shell surface emissivities were investigated, considering the inability to control or accurately measure surface emissivities in most applications. It was found that controlling the He-Ar mole fractions provides a wide operating range for gas-gap thicknesses around ~ 0.042--0.090 in (~ 1.07--2.29 mm) for a surface emissivity range of 0.4--0.8, since conduction heat transfer is a significant fraction of the heat transfer across the gas-gap. In addition, it was found that turbulent flow in the coolant gap is needed at high input powers to prevent shell temperatures from approaching the boiling temperature of the water coolant. The parametric study resulted in choosing stainless steel tubes with an outer diameter and thickness of 1 x 0.065 in as the inner shell, and a 1-1/2 x 0.156 in as the outer shell of the calorimeter. Overall, this report presents the necessary information for the sizing and rating of gas-gap calorimeters with He-Ar mixtures for the testing of high-temperature heat pipes (~ 500--1,000°C).

Interest in heat pipe microreactors (HPMRs) has recently grown due to several unique advantages compared with other reactor types. These compact and mobile reactors are expected to find applications in a variety of fields to provide carbon-free power in remote or off-grid locations. Experimental work is needed to aid and expedite the design and licensing of future HPMRs, especially on the validation of heat pipe performance as key heat transfer components. A Low-Temperature Heat Pipe Test Facility (LTHPF) was designed and constructed according to previously developed scaling laws to bypass the difficulties of experimenting with liquid-metal working fluids by using surrogate fluids. The design, instrumentation, and experimental capabilities of the facility are described. The testing conditions, including various operating limits and the ranges of the nondimensional parameters used for scaling analysis, are reported. It is found that certain nondimensional parameters could yield a wide range over the operating conditions, whereas some showed minimal variation when water was used as the working fluid. Last, the performance of several types of wicks, including the annulus-screen, groove-screen, and wrapped-screen designs, were investigated for applications in the LTHPF. It is observed that the groove-screen wick structure provided slight improvement in capillary limits at higher temperatures and that the wrapped-screen wick yielded lower capillary limits due to the absence of a low-resistance flow path for the liquid.