Thermal conductivity of compressed ceramic breeder pebble beds

Abstract

The effective thermal conductivity of pebble beds is an important parameter for the thermomechanical design of solid breeder blankets operating at temperatures of up to 900 °C in breeder pebble beds and up to 700 °C in beryllium pebble beds. Compressive stresses and creep might cause significant pebble deformations. The knowledge of the thermal conductivity as a function of bed deformation, therefore, is of prime importance. For strongly deformed pebble beds, only results for beryllium pebbles existed where the conductivity increased by a factor of about 5 for bed deformations of about 1%. For ceramic breeder beds, the increase of the bed conductivity with increasing bed deformation is expected to be much smaller. Quantitative results were missing. This paper presents results on the thermal conductivity of lithium orthosilicate and different types of lithium metatitanate pebble beds (monsized and binary beds) for bed deformations up to 4.5% and temperatures up to 800 °C using the pulsed hot wire technique. Most of the measurements at high temperatures were performed in air; at ambient temperature, helium and argon were also used. A distinct increase of the thermal conductivity with bed deformation was found: however, this effect is quite small compared to deformed beryllium beds and might be neglected at high temperatures. The results for zero bed deformation agree well with correlations from literature.