Heat conductivity between UO2 core and casing in fast reactor fuel elements

Abstract

1. The coefficient of heat conductivity between the fuel and OKh16N15M3B steel casing in fuel elements are determined from the changes of the structure of UO2 core in conditions of operation of fuel elements in fast reactors. 2. For the temperature of the casing equal to 500°C and linear thermal power equal to 550 W/cm and higher (medium-helium) the heat conductivity coefficient is ∼0.5 W/cm2·°C for relative diametral gap δ/D=0.008; with the increase of the diametral gap to 0.052 α decreases by a factor of 2. For δ/D≥0.05 the local value of α may be 1.5–2 times smaller than the average value because of the eccentric location of the fuel pellet in the casing. 3. In fuel elements filled with argon during assembly α is 1.2–2 times lower than in fuel elements with helium filling for identical δ/D and other conditions remaining unchanged. This difference is more significant for larger values of δ/D, which indicates a mixed mechanism of heat transfer from the fuel to the casing, i. e. through the solid contact and the gaseous layer. 4. It is found from the size of the melting zone that after attaining its maximum value the temperature at the center of the core decreases in the process of single emergence into power and during irradiation for 3–30 hr at constant power. The decrease is apparently due to the increase of heat conductivity of the fuel as a result of condensation and due to an increase in α. 5. It is found that the diametral gap, which had a relative value of 0.052–0.087 before irradiation, was filled by melted UO2 in some segments of the fuel elements. The casing was not damaged during its contact with UO2.