Liquid-Metal-Bonded Gap for Light Water Reactor Fuel Rods

As a means of improving the gap-thermal conductance, a liquid metal (LM) is proposed as the gap-filling material replacing helium gas in the conventional light water reactor fuel rod. The potential application of the concept includes power reactor fuel rods, special-purpose test-reactor experimental rods, and mixed-oxide fuel rods. Novel fabrication methods to ensure a uniform LM-filled gap between the fuel and the cladding of minirods have been developed. The main concern was overcoming the large surface tension of an LM to eliminate LM-free spots in the gap. Compatibility tests of the LM with a Zircaloy tube have been conducted. Liquid gallium showed excessive reaction with Zircaloy at 350°C for a month. Liquid Bi-Sn-Pb alloy, on the other hand, showed a nearly negligible reaction with Zircaloy under the same conditions. Thermal superiority of the LM-bonded gap over a conventional helium-gas gap in a miniature fuel rod was confirmed through theoretical calculations and experimental measurements. The experiments involved water-quenching the element from 600°C and measuring the decrease of the fuel centerline temperature. The LM-bonded element reached 100°C in 10 s, while the gas-bonded element required nearly 100 s to attain this temperature.