The development of horizontally oriented prismatic micronuclear reactors is progressing rapidly, with deployment anticipated within the next decade. A thorough understanding of gas flow behavior during natural circulation is crucial for ensuring reactor safety and preventing accidents. To investigate this phenomenon, a dual-channel plenum-to-plenum facility (P2PF), designed as a simplified model of the reactor core, was experimentally studied under natural circulation conditions. Advanced measurement techniques, including thermocouples, microfoil sensors, and thermal gas flow sensors, were employed to capture critical thermal-hydraulic parameters such as helium temperature profiles, local heat transfer coefficients, and flow velocities across various heating intensities.

The results revealed that local buoyancy forces significantly influence the radial temperature distribution of helium in the hot channel, leading to asymmetric profiles downstream despite uniform inlet conditions. Enhanced heat transfer was observed near the channel exit due to flow reversal and associated turbulence. Notably, comparisons between horizontal and vertical P2PF configurations showed that the vertical orientation supports more robust natural circulation, achieving higher convective heat transfer rates and lower hot channel surface temperatures.

Additionally, predictive models for the axial-averaged Nusselt number were developed and validated, demonstrating high accuracy with values exceeding 0.98. These findings provide essential benchmark data for validating computational fluid dynamics simulations and improving heat transfer modeling in gas-cooled reactor systems. The study highlights the need for continued research into the thermal-hydraulic behavior of helium in horizontally oriented prismatic micronuclear reactors to support their safe and reliable operation.