Evaluation of gap heat transfer between boron carbide pellet and cladding in control rod of FBR

Heat transfer across a gas gap between a boron carbide pellet and a cladding in FBR control rod has been experimentally investigated. The main purpose of this investigation is to present a calculational method for the gap heat transfer in order to improve accuracy of thermal design for the control rod with 0.5 mm gap width at a beginning of reactor operation. Two types of tests have been carried out using simulated control rods. One is low temperature tests below 200degC. The test results indicated that the convective heat transfer has a negligible effect on the gap conductance when the Rayleigh number using the gap width as the characteristics length is below 0.1. The other is high temperature tests up to 600degC. The results showed +10 to -5 % variations in the gap conductance data due to eccentricity between the pellet and the cladding. The prediction based on conduction and radiation heat transfers considering a thermal expansion and an eccentricity gave better results of gap conductance having a maximum difference of only 17 % from the measured ones. Calculation in the radiation heat transfer used thermal emissivities, 0.85 for the boron carbide and 0.15 for the cladding, measured by infrared thermography. (author)