Ceramic breeder materials offer a wide range of possibilities for the development of fusion energy based on the deuterium-tritium fuel cycle. Currently, most ceramic breeder R&D is focused on the lithium orthosilicate and metatitanate systems in the form of pebble beds.

This chapter started with blanket designs, material requirements, manufacturing routes, pebble and pebble-bed thermomechanics, tritium production and release properties, neutron-irradiation behavior, chemistry, and modeling.

One of the most important missions of ITER is to provide a test bed for breeding blanket modules, the so-called test blanket modules (TBMs). However, because ITER testing is a cost-intensive exercise and is most likely the only opportunity to test a fusion

blanket component prior to a DEMO, the questions of the choice of materials for the ITER TBM and the definition of a set of requirements (and the related qualification program) to ensure safety, reliability, and test performances become particularly impor­tant. Accordingly, Ying et a/.19 proposed a roadmap outlining the necessary development steps for quali­fying and accepting the pebbles for ITER and fusion applications (see Figure 65).

For each development step, a set of criteria is presented as a means for initial screening before proceeding to the next evaluation tests to reduce development costs. However, it is important to rec­ognize that ITER conditions (neutron fluence about

1. 5—2 orders of magnitude lower) are far from suffi­cient to qualify any specific breeder material to be used in DEMO. Thus, parallel with ITER and subsequent to ITER testing, tests such as HICU or in fusion relevant neutron sources such as International Fusion Materials Irradiation Facility (IFMIF) for any candidate ceramic breeders under typical reactor blanket conditions with relevant nuclear environment are necessary for this purpose.

Though a significant R&D effort on ceramic breeder development has already been made and a vast amount of data on material performance have been obtained, the knowledge to date on the limiting factors in blanket designs for long-term operation is still modest. These limitations are addressed here: