Long-term thermal stability of biphasic Li4SiO4-Li2TiO3 EU reference tritium breeder ceramics enriched in Li-6

• Experiment on long-term thermal stability (up to 60 d) of tritium breeder pebbles. • Advanced Ceramic Breeder (ACB) pebbles with 60–90 at% lithium-6 enrichment. • Biphasic pebbles remain chemically and mechanically stable at 900 °C. The long-term thermal stability of Advanced Ceramic Breeder (ACB) pebbles was investigated in an annealing experiment at 700 and 900 °C in He + 0.1 vol% H 2 gas atmosphere for up to 60 days. Four batches of biphasic ACB pebbles, consisting of Li 4 SiO 4 with additions of 30 and 35 mol% Li 2 TiO 3 and different lithium-6 contents (7.6, 60 and 90 at%) were selected for the experiment. Material properties such as the chemical composition, phase content, microstructure, porosity, and mechanical properties were determined after certain time intervals and at the end of the experiment. The ACB pebbles show good performance under high temperatures as no lithium loss nor secondary phases occur. Furthermore, the microstructure, porosities, and mechanical stabilities did not exhibit significant changes over time.