Inlet thermal feedback and resistance-limited natural circulation in a compact MNSR

Abstract Miniature neutron source reactors (MNSRs) rely exclusively on buoyancy-driven natural circulation for core cooling, making steady-state behavior sensitive to heat rejection, inlet thermal conditions, and localized hydraulic resistance. In compact MNSR configurations, short flow paths and dominant minor losses create strong inlet-outlet thermal coupling, allowing outlet thermal energy to influence inlet conditions before complete heat removal and thereby reduce the effective buoyancy head. This work examines inlet thermal feedback and resistance-limited natural circulation in the Nigeria Research Reactor-1 (NIRR-1) using a mechanism-oriented, FSAR-consistent GOTHIC thermal-hydraulic framework. Geometry, dominant hydraulic resistance, nodalization, and numerical conditions are held fixed while external heat-rejection pathways are varied in a controlled manner. This design isolates the influence of loop-to-pool thermal coupling on circulation response without altering the hydraulic structure or introducing nonphysical coupling to the surrounding pool. The results show that strengthening external cooling progressively suppresses inlet thermal feedback. At low cooling strength, enhanced heat rejection reduces inlet temperature, increases the effective density difference, and amplifies circulation. Beyond a defined range of cooling conditions, further increases in heat rejection produce only marginal changes in mass flow rate and core temperature rise, indicating that the loop becomes constrained primarily by localized hydraulic resistance. Analysis of the chimney-to-pool temperature difference shows that this transition is governed by reduction of recirculated outlet thermal energy available for inlet reheating. These findings provide a physically interpretable basis for representing heat-rejection boundaries in system-level analyses of compact research reactors.