Investigation of Burn-up Dependent Thermo-mechanical Behavior of Heat-Pipe Cooled Microreactors

When graphite is exposed to neutron irradiation, its microstructure undergoes significant changes that impact dimensional stability and thermo-mechanical properties. These changes can alter the material's mechanical strength and capacity to dissipate heat, subjecting the component to additional stresses beyond those induced by temperature gradients. To ensure the long-term structural integrity of graphite within the reactor core, such effects must be accurately modeled. This work proposes a high-fidelity coupling methodology to investigate the thermo-mechanical behavior of the graphite moderator block in a Heat-Pipe Cooled Microreactor (HPM) as burnup progresses. The methodology is applied to a reduced-scale core design inspired by the eVinci® microreactor concept. The thermo-mechanical response is analyzed using OFFBEAT, an OpenFOAM-based fuel performance code, while neutron transport and burnup calculations are carried out with the Serpent2 Monte Carlo code. Coupling between the two codes is achieved through a detailed cell-by-cell exchange of temperature, neutron fast flux, and power density. Models are implemented to capture irradiation-induced changes in graphite properties, including dimensional evolution, creep, and degradation of mechanical and thermal performance. The results provide insights into stress evolution, mechanical interactions between components, and the formation of thermal hotspots during early depletion stages.