Only a few types of microstructures have been studied until today, preventing clear conclusions regarding their suitability for reactor application. Both for pebble-bed and pellet/block type designs, significant opportunities still exist to tailor stoi­chiometric composition, grain size, and porosity shape and size as well as surface treatments, coatings, and other pre-conditioning treatments.

4.15.8.1 High Burnup

Though some high lithium burnup samples have been achieved in experimental programs, the find­ings are not yet conclusive as to whether the
ceramic still functions adequately under normal operation, and whether interaction with the blanket structure satisfies, for example, future power plant safety requirements.

4.15.8.2 High Fluence

Few data were obtained in fast breeder experiments; the influence of fast neutron damage on the integrity and functionality of the breeder ceramics is yet to be addressed, in particular for the newly developed peb­ble types and compositions.

4.15.8.3 High Temperature

The effect of long-term exposure in a high radiation environment, causing damage, induced swelling, with increasing burnup, may also affect the vaporization behavior and even induce lithium transport within the breeder area.

4.15.8.4 Effects of Transients and Off-Normal Conditions

Though work has been done already on thermal cycling of pebble beds, there is uncertainty with regard to the effects of disruption and electromag­netic (EM) loads on, for example, pebble reloca­tion, fragment relocation, and ratcheting in case of inclined or vertical beds. Specific attention is required on the change of heat transfer properties and free flow of purge gas.