Li₄SiO₄ based breeder ceramics with Li₂TiO₃ and other strengthening phase additions

Ceramic breeder pebbles, applied as pebble beds, are the major functional components of the solid breeder blankets developed for ITER and DEMO. The primary task of the breeder pebbles is the generation of tritium by neutron capture and transmutation of lithium. To achieve tritium self-sufficiency, these ceramic materials must carry high amounts of lithium. Yet, the pebbles must be able to withstand the conditions within the blanket during the anticipated time of use of several years. As the pebbles are going to endure significant mechanical forces during operation from differences in thermal expansion, it is important to increase the strength of the pebbles, while maintaining a suitable lithium density and low activation properties. Currently Li4Si04 and LizTi03 are considered as reference class breeder compounds. Lately the routine fabrication of two-phase pebbles consisting of Li4 Si04 and LizTi03 was established. The rigidity of such pebbles is significantly improved over reference grade Li4Si04 pebbles. However, the applied melt-based fabrication process is limited to a maximum LizTi03 content of 40 mol% as the processing temperatures exceed manageable limits. ... mehrWet-chemical fabrication processes are highly adaptable to a wide range of raw materials and naturally require lower processing temperatures. Therefore they are well suited for evaluating new material compositions. In this work, novel two-phase Li4Si04 based pebbles of 1 mm diameter with higher Li2Ti03 contents as well as pebbles containing other strengthening compounds, i.e. LiA102 and LixLayTi03, are fabricated by modifying the established emulsion method at JAEA. As the lithium density of the latter two compounds is relatively low, only moderate contents were added. The LizTi03 additions, however, cover the full compositional range. In order to evaluate the effect of these additions, the produced pebbles were characterized with regard to their mechanical rigidity, microstructure, phase content and porosity by crush load tests, SEM, XRD and helium pycnometry, respectively. This work shows that the additions of the second phases to Li4 Si04 considerably improve the mechanical strength. Depending on the sinteractivity of the compounds, the grain sizes slightly change. To evaluate the activation properties of the pebbles, FISPACT calculations with a DEMO relevant neutron source are applied as well.