Conference Agenda

Heat pipes are highly efficient self-contained two-phase passive cooling devices. They are used in a wide range of applications and have recently been investigated as cooling systems for new Micro Modular Reactor (MMR) concepts. The Alkali Metal Heat Pipe Assembly Testing (AHPAT) rig in the single heat pipe configuration has been used in the High Temperature Fuel Channel (HTFC) laboratory of the Canadian Nuclear Laboratories (CNL) to investigate the behaviour of sodium heat pipes near their operational limits. Power is delivered to the AHPAT rig through a heating bank attached to the evaporator of the heat pipe to simulate heat provided by a reactor core. Power output is measured using a gas-cooled stainless-steel block attached to the condenser of the heat pipe. After reaching steady state near the operational limits of the heat pipe, the heaters were pulsed to enable the characterization of transient behaviour. The results of these tests show the temperature distribution of the heat pipe at steady state and near its operational limit. Pulsing the power shows its effect on the temperature distribution as well as the recovery behavior and return to steady state once pulsed heating has stopped. The results of this work will be used for the development and benchmarking of numerical codes that simulate the behaviour of alkali metal heat pipes.

Heat pipe-cooled Micro Modular Reactors (HP-MMR) are mobile systems with a low power output of below 10 MW_el. Terrestrial and space applications are envisaged such as supplying energy to a remote settlement or to a space probe. A heat pipe is a passively working, two-phase heat transfer device that exploits phase change and capillary pumping of the liquid in a wick for the efficient and reliable cooling, e.g. of a reactor core. The high‑temperature heat pipes integrated in HP-MMRs are typically filled with an alkali metal such as sodium or potassium. To enable the safety analysis of a HP-MMR, the thermal hydraulic system code AC²/ATHLET is currently in development for the simulation of high-temperature heat pipes within the MISHA project. A potassium material property package has recently been implemented for the purpose of heat pipe simulations, so that the latest code version provides the fluid properties of sodium and potassium. In addition, many relevant phenomena occurring in a heat pipe have been modelled such as capillary pumping, phase change, radial heat transfer through the wick, friction in each of the phases, and pooling. A verification case will be presented and discussed. The future validation of the module is planed based on upcoming experiments with potassium heat pipes at the IKE Stuttgart which cooperates within the MISHA project.

Heat pipes are essential components in space nuclear reactors which play a key role in facilitating deep space exploration. The temperature difference between the evaporator and condenser, along with the heat transfer limit, are critical performance metrics that govern the thermal efficiency and operational capacity of heat pipes. This study presents an optimization framework that integrates the Genetic Algorithm (GA) with COMSOL Multiphysics simulations to minimize the heat pipe temperature difference while maximizing its heat transfer limit. Key design parameters, including wick thickness, porosity, and vapour core diameter, are systematically optimized using GA to enhance overall thermal performance. Simulation results demonstrate the varying influence of these parameters on heat pipe efficiency, providing valuable insights for optimizing the design and operation of heat pipes in space reactor applications.

The simulation of heat pipe cooled microreactors is a significant challenge, involving tight coupling between specialized codes and solvers. A wide array of governing equations, discretization schemes, and numerical methods may be employed in modelling the involved physics at different degrees of resolution. A flexible and high-performance coupling framework is needed to incorporate such a variety of components in a sustainable way. To this end, this work seeks to assess the usability and performance of the preCICE coupling library for microreactor simulations, with an eye towards nuclear electric propulsion systems. Until now, the use of preCICE in the field of nuclear energy has been limited to surface coupling or “high-low” applications, since the mesh mapping capabilities needed for overlapping 3-D domains have only recently become available in the library. At present, these and other attractive features of preCICE are leveraged, including pre-existing “code adapters” which facilitate the coupling of simulation codes that employ popular PDE libraries such as OpenFOAM, deal.II, and FEniCS. The adapter for OpenFOAM is modified to allow the transfer of arbitrary scalar fields; thereby preCICE is used to couple a custom heat conduction solver with neutron diffusion, as well as point-kinetics. A simplified version of the KRUSTY reactor is modelled under steady-state and transient conditions. The coupling scheme is compared with a standalone-OpenFOAM approach, with good agreement observed. Finally, the viability of the preCICE-based framework for more advanced simulations of space nuclear reactors is discussed.

GRS cooperates with the University of Stuttgart in the MISHA project to establish a calculation chain for innovative MMR designs. Simulation of these new designs comes with unique challenges like rotatable control drums with absorber crescents, solid monolithic cores, and heat pipe cooling. To validate the coupled system of the GRS-codes ATHLET and FENNECS for such simulations, reference calculations based on the Special Purpose Reactor design by the Los Alamos National Laboratory for a heat pipe cooled fast micro reactor will be performed. This design was chosen as reference because some thermal and neutronic data as well as specific reactor parameters are publicly available.

To this end, we present the results of Monte Carlo simulations of the core with Serpent for different absorber configurations, performed to obtain a macroscopic cross-section library and data on delayed neutrons. Core reactivities agree with published values and the operational state is reached with a similar control drum configuration.

Utilizing data from the Serpent results, a first model of the core was created in the neutron diffusion and SP3 code FENNECS and initial stand-alone calculations were performed. Additionally, we show the results for normal operational state with the thermal-hydraulics code ATHLET and point kinetics utilizing the power distributions and delayed neutron data from Serpent. This model adequately reproduces the limited amount of publicly available thermal-hydraulic data for the reference design. In the future, models in both GRS codes will be further refined and eventually coupled to simulate the reactor during operation and in transient conditions.

Heat pipe microreactors provide a unique opportunity for decarbonization as a source of industrial process heat. For example, in the cement production industry, the carbon-heavy kiln flue gas used to decompose calcium carbonate feed meal might be partially or fully replaced by air heated by a heat pipe microreactor. To evaluate the feasibility of such a change, an experiment is being planned to link a single representative heat pipe to a lab scale fluidized bed calcination reactor. The present work summarizes the experimental system design process used in the facility planning. Analytical and empirical correlations from the fluidized bed literature were employed to calculate the air flow rate, temperature, and pressure needed to operate the calcination reactor. This information was then used to inform the design of the rest of the system, which will function as a heated wind tunnel. Special attention was paid to the filtering which occurs directly upstream of the calcination reactor to ensure a predicable velocity profile entering the test section. CFD methods were applied to investigate the calcium carbonate particle distribution in the calcination reactor considering the flow preconditioning under different Reynolds numbers for the determined system design.