Die wärmeleitfähigkeit von urandioxid bei sehr hohen temperaturgradienten

Abstract

Despite the fact that in practice, fuel materials are subject to very large temperature gradients (> 10 4 ° C/ cm) no attempt has been made to determine the effect of such gradients on their thermal conductivities. Consequently a method was developed that permitted the measurement of the thermal conductivity of UO 2 over a large temperature range (300 ° C < T < 2000 ° C) with variable, controlled temperature gradients (30 ° C/ cm < dT/ dx < 5000 ° C/ cm). The accuracy was a function of the temperature and temperature gradient varying between ± 3 to ± 8 %. The measurements show that the thermal conductivity of UO 2, K, is a function of the temperature and the temperature gradient. The dependence is reversible as long as a specimen develops no cracks. The change in elastic properties which result from the thermal stresses produced in a thermal gradient may be responsible for this behavior. Large thermal gradients can also be expected to disturb the uniformity of lattice vibrations. The thermal conductivity exhibits a maximum at 1050 °C. Large temperature gradients can also be responsible for this phenomenon. It is possible to correlate this result with the zero point of the Seebeck coefficient, as measured by other experimenters, since the electronic contribution to the thermal conductivity can exhibit a maximum in this temperature range. Measurements made in the high-temperature region (1200 ° C < T < 2000 ° C) with large temperature gradients (5000 °C/cm) show agreement with conductivity values measured in-pile. In this connection, the specimens employed exhibit columnar grains similar to those which appear in irradiated samples. This indicates that it is possible to simulate certain in-pile thermal conditions by this technique.