Numerical Evaluation of Spacer Effect on Heat Transfer and Helium Flow in Innovative High Temperature Gas-cooled Reactors Design

To achieve higher power density operation in pin-in-block type high-temperature gas-cooled reactors (HTGRs), a sleeveless fuel design is currently under investigation. In this design, the cylindrical fuel element is efficiently cooled on both its inner and outer surfaces by the helium coolant. While spacers—commonly employed in nuclear reactors to maintain appropriate clearance between fuel rods—are utilized to structurally support and position the fuel elements, their detailed design has not been investigated. This study evaluates the thermal-hydraulic performance of rod type spacers, which refer to ribs attached to the rods inside the cylindrical fuel element, under HTGR core conditions.

In the case of 10.6 mm-high streamline shaped spacer, heat transfer coefficient improves around the spacer and downstream region, resulting in maximum temperature differences of approximately 20 °C between the inner and outer surfaces of the fuel elements. Meanwhile, single spacer causes pressure drops of approximately 100 Pa. Since more than ten spacers are installed per fuel rod, minimizing pressure loss is essential. Thus, the introduced spacers have two effects: enhancing heat removal efficiency, while causing unpreferable pressure losses. This highlights the need for further improvements in spacer geometry.