Abstract
Enhancing heat transfer in the pebble bed reactor core could reduce the surface temperatures of the fuel elements and lower the possibility of forming hot-spots. The effectiveness of inserting a smaller sphere into a structured pebble bed on optimizing the heat transfer has been verified, and yet, the mechanism of heat transfer enhancement is still not fully understood. By keeping the packing ratio constant and respectively adding 1, 2, 3, and 4 small spheres in the structured pebble bed, the impact of the quantity and size of the small spheres on the heat transfer characteristics has been investigated in this study and the mechanism of enhancement was analyzed. It was found that: (1) adding more spheres strengthened the heat transfer of the pebble bed more; compared to the case without any small sphere, the overall heat transfer coefficient was enhanced by 6.7% and 8.6% for the cases with 1 sphere and 4 small spheres, respectively; (2) from 0.7% to 2% of heat power generated by the fuel elements was transferred through the pebble-sphere contact areas as the number of small spheres increased, and more small spheres led to a higher transferred heat power percentage, which explained the above-mentioned phenomenon. Such findings may provide a reference for the future design of a more reliable pebble bed reactor core.
