Fabrication and characterization of lithium orthosilicate pebbles using LiOH as a new raw material

For the European Helium Cooled Pebble Bed (HCPB) blanket slightly overstoichiometric lithium orthosilicate pebbles (Li{sub 4}SiO{sub 4}+SiO{sub 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{sub 4}SiO{sub 4} and SiO{sub 2} powders, but due to the fact that enriched Li{sub 4}SiO{sub 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{sub 2} {yields} Li{sub 4}SiO{sub 4} + 2 H{sub 2}O. The melting process of LiOH and SiO{sub 2} is less difficult to control than the melting of Li{sub 2}CO{sub 3} in spite of the decomposition of water. The pebbles produced from LiOH and SiO{sub 2} are similar to those produced from Li{sub 4}SiO{sub 4} and SiO{sub 2}. They exhibit a distinctly dendritic structure and show only a small amount of pores and cracks. In addition to the main constituent Li{sub 4}SiO{sub 4}, the high temperature phase Li{sub 6}Si{sub 2}O{sub 7} was detected due to the quenching process and the excess of SiO{sub 2}. This minor constituent, however, decomposes to Li{sub 4}SiO{sub 4} and Li{sub 2}SiO{sub 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 300degC. 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{sub 4}SiO{sub 4} or Li{sub 2}CO{sub 3}. In order to obtain characteristic pebble bed data, first Uniaxial Compression Tests (UCTs) were performed at temperatures between ambient and at 850degC. Compared to results obtained with pebbles produced from Li{sub 4}SiO{sub 4} and SiO{sub 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. (author)