Experimental investigation on effective thermal conductivity of powder-packed assembly gaps in heat pipe cooled reactor core

•A solution for contradiction of thermal resistance and contact stress of assembly gaps was put forward.•The effective thermal conductivity of stainless-steel powder packed beds was measured in both air and helium.•Zehner-Schlünder model was calibrated based on the experimental investigation.•Powder packed gaps increase the reactor safety margin. The heat pipe cooled reactor utilizes monolithic solid core inserted with fuel rods and heat pipes. There exist assembly gaps between fuel rods, heat pipes and the solid block to avoid contact stress resulted from heterogeneous thermal expansion. In order to reduce the additional thermal resistance caused by assembly gaps, thermal conductive powders can be filled into the gaps. For the sake of evaluation on the effects of powder packed gaps on the reactor safety margin of heat pipe cooled reactor, the effective thermal conductivity of stainless-steel powder packed beds with different particle sizes was measured using the one-dimensional steady-state hollow-cylinder method in both air and helium atmosphere under the heating temperature of 800–1000 K and the pressure of 1 bar. The Zehner-Schlünder model was calibrated by adjusting the particle deformation parameter and the flattened surface fraction based on the experimental data in air. The comparison of prediction results by the adjusted Zehner-Schlünder model with experimental results in helium and relevant experimental data from previous literature shows the relatively error within ± 20%. Finally, the performance of powder packed gaps on increasing the reactor safety margin was evaluated by analyzing the different heat transfer modes and initial assembly-gap sizes. Results show that the performance improves with the increase of initial assembly-gap size and the fuel temperature decreases by 19.19 K compared to the helium-filled gap even with a small initial assembly-gap size of 0.2 mm.