Micro high-temperature gas-cooled reactors (micro-HTGRs) are emerging as compact, transportable nuclear systems designed to fit within a 40-foot ISO container, offering flexible deployment for remote or off-grid energy applications. Many advanced designs—including GA Micro, Holos Quad, Xe-Mobile, and NuGen Engine—adopt the graphite-moderated, TRISO-fueled prismatic core concept, derived from vertical-core HTGRs such as Fort St. Vrain and MHTGR-350. Natural convection aids passive cooling in vertical HTGRs, but its behavior in horizontally oriented microreactors remains uncertain due to altered flow paths and gravity alignment. While vertical HTGRs have been extensively studied under both steady-state and accident conditions, the thermal-hydraulic behavior of horizontally oriented cores represents a newly proposed design concept, and no studies have yet been reported in the open literature. To address this gap, a scaled-down, high-pressure, high-temperature multilayer, multi-block prismatic experimental facility was designed, manufactured, and deployed to investigate helium thermal hydraulics under PCC accident scenarios. The reactor core model consists of a structured assembly of stacked hexagonal graphite fuel blocks, graphite reflector blocks, and hot and cold plena, all housed within a Reactor Pressure Vessel (RPV). Each graphite block is a scaled representation of the MHTGR-350 reference design and contains seven coolant channels (16 mm diameter) and twelve blind heater rod channels (12 mm diameter). The system is heated using embedded heater rods to simulate decay heat generation within fuel rods. Advanced measurement techniques—including thermocouples, heat flux sensors, and hot wire anemometry—are integrated into the core to measure local solid and gas temperatures, convective heat transfer, and helium velocities within the coolant channels. Experiments were conducted at various power levels and system pressures to characterize the onset of natural convection, quantify heat transfer coefficients, track the location and evolution of peak temperatures, and resolve local flow structures across channel diameters and inter-block gaps. Complementary computational fluid dynamics (CFD) simulations were performed under the same experimental conditions, showing reasonable agreement with the measured data. The combined experimental and numerical results provide new insights into the development of natural circulation, the shifting location of peak temperatures, and the overall thermal-hydraulic behavior in horizontally oriented micro-HTGRs. These findings offer a foundational benchmark for validating CFD models and enhance the understanding of passive heat removal in the proposed horizontally oriented micro-HTGRs.
