Study of lithium germanate additions to advanced ceramic breeder pebbles

1. Abstract Worldwide, lithium rich ceramic pebbles are being developed to be used as tritium breeders inside future fusion reactors. Upon irradiation, the pebbles will produce tritium which will be used alongside deuterium to fuel the fusion reaction. It is essential that these pebbles are of sufficient quality to withstand the various forces experienced within the wall of the reactor. Currently, a melt-based process is used at the Karlsruhe Institute of Technology for the production of pebbles composed of lithium orthosilicate with a secondary strengthening phase of lithium metatitanate. Recent studies have focused on incorporating lithium orthogermanate, which forms a solid solution with lithium orthosilicate. Various compositions were used to perform fundamental studies to determine phase transitions as well as melting points for processing. The melt-based KALOS process was then used to produce pebbles with selected compositions. Thereafter the pebbles underwent a series of characterisation tests including microstructure and surface analysis using REM as well as elemental mappings using EDX. XRD was used for determining the crystal structures present in various compositions and the unit cell volumes. The measurements showed that the unit cell volume of each structure linearly depends on the composition. Additionally, pebbles were held at reactor relevant temperatures to determine any changes to the mechanical strength. It was found that pebbles with 30 mol% lithium metatitanate and a lithium orthosilicate/orthogermanate ratio in the range of 1.5–4 showed an increased strength as well as good thermal stability and may have the potential to be used as tritium breeders.