Towards Control Drums Homogenization in Microreactors: A Study on Neutron Absorption

In nuclear reactor design, particularly in the context of compact nuclear reactors, control drums play a key role in managing reactivity and ensuring operational safety and efficiency. This is exemplified in the Holos Gen-2+ design, which incorporates 20 control drums segmented into quadrants and positioned with azimuthal symmetry around the core's periphery. Each drum features a thin sector spanning 120°, composed of boron carbide (B4C), a neutron-absorbing material. The remaining portion of the drum is constructed from beryllium oxide (BeO). Given this design, the array of potential control drum configurations is virtually limitless. This study employs Monte Carlo simulations to investigate the characteristics of these diverse configurations, particularly when multiple control drums are involved. The findings of this study highlight the significant influence that interactions between adjacent control drums exert on their neutron-absorbing capacities, particularly through their relative and absolute rotational states. A significant aspect of this investigation is examining approaches to homogenize these configurations for employing multigroup nodal diffusion solvers. This is critical for simplifying the analysis and enabling more exhaustive searches in parameter space during a design process. It is shown that the homogenized absorption cross sections show highly irregular behavior as a function of the rotational state of adjacent drums. Moreover, it is shown that it is possible to obtain high accuracy in calculating the control drums configurations using multigroup nodal diffusion criticality calculations and homogenized cross sections.