Development and verification of transport and tally capabilities based on unstructured mesh in Reactor Monte Carlo code RMC

The transport and tally capabilities based on unstructured mesh (UM) offer significant advantages in representing complex geometry and producing precise tally results over Constructive Solid Geometry(CSG) and Direct Accelerated Geometry Monte Carlo (DAGMC), which can enhance Monte Carlo applications in high-fidelity multi-physics coupling. In this paper, two new capabilities of tally and particle transport based on UM have been studied and implemented in the Reactor Monte Carlo code (RMC). First, a tally capability based on UM under CSG or DAGMC transport framework has been researched, implemented, and verified in RMC, demonstrating considerable tally efficiency in the KRUSTY application. Next, particle transport and tally directly on UM have also been implemented and verified in RMC. In comparison to DAGMC for the irregular Stanford Bunny model, the calculation speed is 8.63 times faster. Additionally, a practical Molten Salt Fast Reactor (MSFR) model has been calculated with RMC's new transport and tally capabilities, showcasing RMC's ability to handle complex geometries and produce precise tally results in real irregular reactor application.