Term Paper
Independent Modelica/OpenModelica model of a helium heat grid for a lunar ISRU base, verifying an existing MULE reference model and analyzing reactor heat distribution, ISRU heating, radiator behavior, Brayton-cycle power production, and recuperator performance.
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Completed term paper. To request the full work or related material, please contact the associated TUM chair.
This term paper developed and verified a Modelica heat grid model for a lunar ISRU base. The work was connected to the MULE project at TUM, which investigates nuclear reactor systems for future lunar surface infrastructure. The model represents a helium-based thermal-fluid network that distributes heat from a simplified fission reactor source to ISRU equipment and a Brayton-cycle power conversion loop.
Lunar surface infrastructure must survive and operate through long day and night cycles. During the lunar night, solar power is unavailable for roughly two weeks, while high-temperature ISRU processes still require continuous heat and power. Nuclear fission systems can provide this continuous energy supply, but their heat distribution network must be modeled carefully before such systems can be integrated into a lunar base architecture.
The heat grid is divided into three coupled loops. The reactor loop circulates helium through a simplified 100 kW fission heat source. The ISRU loop transfers thermal energy into a regolith/molten-salt batch, which is reset every eight hours to represent periodic batch replacement. The power loop uses a Brayton-cycle architecture with turbine, compressor, radiator, and recuperator to generate electrical power and reject excess heat.
The model was built in OpenModelica using Modelica Standard Library components. The system was reconstructed independently from a previous reference model and structured into modular submodels. Several improvements were introduced, including corrected radiator heat-flux handling, a more robust ISRU batch representation, and a counter-flow recuperator model instead of a simplified temperature-shift approximation.
Four cases were analyzed:
The model showed strong agreement with the reference model for pressure and temperature behavior. Lunar night conditions improved heat rejection and electrical power generation while keeping the ISRU batch and reactor outlet temperatures within acceptable ranges. In the no-ISRU daytime case, the power loop produced 35.6 kW gross electrical power and 32.6 kW net electrical power after accounting for circulator consumption. Removing the recuperator caused the system to shift from useful net power production to a net electrical loss, demonstrating that recuperative heat exchange is required for efficient Brayton-cycle operation.
The model is a system-level thermal-fluid simulation, not a detailed reactor-core analysis. Reactor physics, neutronics, shielding, mechanical design, and detailed CFD were outside the project scope. Future work should integrate more realistic component geometries, improved valve regulation, detailed pipe routing, transient start-up and shutdown behavior, and a more complete transition analysis between lunar day and lunar night.