Stability of advanced breeder pebbles in water containing purge gas atmosphere

Solid breeder blankets are widely considered as attractive blanket concepts for a future DEMO reactor and will be tested in ITER by a number of parties. Ceramic breeder pebble beds are the major functional component of such breeder blankets. The principal task of the individual breeder pebbles is the generation of tritium by transmutation of lithium through neutron capture. Yet, the pebbles must not significantly degrade during use to allow for safe operation ofthe blanket. Currently the design of the outer tritium fuel cycle uses helium with 0.1% of hydrogen as purge gas for the generated tritium, which is also released as tritiated water besides hydrogen isotopologues. As there is a difference of orders of magnitude between the expected concentration of tritium and the hydrogen content, the separation of both is challenging and costly. A change to water replacing hydrogen might simplify the separation process, since the separation of water from helium is much easier. For fuelling of the reactor, however, pure tritium is required. At this time it is difficult to judge whether the processing of tritiated water to pure tritium predominates the initial cost savings. ... mehrIn order to examine the stability of breeder pebbles under this alternative purge gas composition, five current grades of advanced breeder pebbles from Europe and Japan were annealed at 900 °C under a constant purging by helium with 0.1% water for up to 32 days. The annealed pebbles are characterized with regard to lithium depletion, mechanical rigidity, impurity phase formation and microstructural development. ICP-OES, single pebble crush load tests and XRD as well as SEM investigations were carried out to address these topics. The results show that all tested grades exhibit similar or better performance compared to annealing in helium/hydrogen atmosphere. Therefore all grades are applicable under helium/water purge gas.