Experimental measurements of the effective thermal conductivity and interface thermal conductance of a lithium ceramic pebble bed

This PhD thesis presents the results of experimental studies and analysis of the thermal properties of a lithium metatitanate (Li2TiO 3) pebble bed. The helium-cooled pebble bed blanket is a promising concept for fusion blankets, where layers of lithium ceramic pebble beds are arranged between cooling plates. These pebble beds are subjected to nuclear heating caused by the fusion neutrons; therefore, heat is transferred from the hot pebble beds to the coolant. The thermal properties of the lithium ceramic pebble beds have a significant impact on the blanket's temperature profile and the heat extraction processes. The effective thermal conductivity keff and interface thermal conductance h are the main thermal properties of the lithium ceramic pebbles beds. The literature review showed that for Li2TiO3 pebble beds: (i) no study is available on measuring h, and (ii) published data on keff is insufficient. Therefore data on h and keff is needed for the Li 2TiO3 pebble beds to build a reliable database of these properties. The objectives of this study are: (i) measuring h and keff of a Li2TiO3 pebble bed as a function of the bed temperature, and (ii) studying the temperature profile of the ITER Quarter Port Submodule (QPS) under different operating conditions. The measuring technique is based on the principles of the steady state and axial heat flow techniques. The Li2TiO3 pebble bed is single size (1.7-2.0mm diameter pebbles) with a packing fraction of 61%. Helium, at the atmospheric pressure, was used as a cover gas. The present results showed that keff decreased from 1.45 to 0.78W/m.K with the increase of the average bed temperature from 24 to 780°C. The results of keff were compared with the theoretical predictions of three models. Predictions of Shapiro et al. model and Yagi-Kunii model showed a good agreement with the experimental values of k eff, however, Masamune-Smith model over-predicted the values of keff The obtained results showed that h increased from 1800 to 5300W/m2.K with the increase of the wall temperature from 24 to 570°C. The experimental values of h were compared with the theoretical values of three models. The theoretical and experimental values of h have similar trend with the increase of temperature and the best predictions were given by Gorbis et al. model. Uncertainty analysis was carried out for h and keff. The present results of h and keff can help to build a reliable database of these properties, which is needed for the R&D of the fusion blankets. Thermal analysis of the QPS was performed to study its temperature profile under transient and steady state conditions, and different values of h. The results showed that h has a significant impact on the QPS temperature profile.