Fabrication and Characterization of Lithium Orthosiiicate Pebbles Using LiOH as a New Raw Material

摘要

For the European Helium Cooled Pebble Bed (HCPB) blanket slightly overstoichiometric lithium orthosilicate pebbles (Li_4SiO_4+SiO_2) have been chosen as one optional breeder material. This material is developed in collaboration between Research Centre Karlsruhe (FZK) and the Schott Glas, Mainz. The lithium orthosilicate (OSi) pebbles are fabricated by the melting and spraying method in a semi-industrial scale facility. In the past, the not enriched pebbles were produced from a mixture of Li_4SiO_4 and SiO_2 powders, but due to the fact that enriched Li_4SiO_4 is not available on the market, highly enriched carbonate powder was used that finally resulted in nonsatisfying pebble characteristics. Enriched LiOH powder is commercially available, therefore, a new production route was pursued based on the following, simplified reaction: 4 LiOH + SiO_2 => Li_4SiO_4 + 2 H_2O The melting process of LiOH and SiO_2 is less difficult to control than the melting of Li_2CO_3 in spite of the decomposition of water. The pebbles produced from LiOH and SiO_2 are similar to those produced from Li_4SiO_4 and SiO_2. They exhibit a distinctly dendritic structure and show only a small amount of pores and cracks. In addition to the main constituent Li_4SiO_4, the high temperature phase Li_6Si_2O_7 was detected due to the quenching process and the excess of SiO_2. This minor constituent, however, decomposes to Li_4SiO_4 and Li_2SiO_3 during annealing. In compressive crush load tests of single pebbles a crush load of about 9.5 N was measured for pebbles after drying at 300°C. The chemical analysis revealed a further advantage of the use of LiOH in the melting process. As LiOH is available in high-purity quality, the pebbles contain impurities to a lower degree than pebbles produced from Li_4SiO_4 or Li_2CO_3. In order to obtain characteristic pebble bed data, first Uniaxial Compression Tests (UCTs) were performed at temperatures between ambient and at 850°C. Compared to results obtained with pebbles produced from Li_4SiO_4 and SiO_2, the pebble beds with the new material show a softer behaviour, that is, a larger strain for a given uniaxial stress. Thermal creep strains do not differ remarkably.