The first ceramic breeder blanket concepts used blocks or plates; later, pellet designs emerged. The larger the breeder component, the greater the threat to its integrity because of cracking or fragmentation due to thermal stresses and irradiation-induced swelling and embrittlement. This is the major reason for favoring pebble-bed concepts.

4.15.3.2.1 Pellets or blocks

Pellet or block fabrication makes use of proven technologies in the ceramic industry. Pressing and sintering of ceramic powders is a widely used and cost-effective industrial process. Pellets and rectan­gular blocks can be manufactured up to some centi­meters in size with excellent material homogeneity and controlled density. Thus, LiAlO2, Li2ZrO3, and Li2TiO3 pellets meeting dimensional, microstruc­tural, and purity characteristics were produced by Pechiney in collaboration with CEA.52 Similar results were obtained by ENEA, SCK/CEN, UKAEA- Springfields, and US laboratories.65,73-75

Kapychev et a/.30 fabricated pellets of Li4SiO4, metasilicate (Li2SiO3), and aluminate (LiAlO2), with a diameter of about 10 mm and heights of 5, 10, and 14 mm.

4.15.3.2.2 Pebbles

For TBR considerations, the density of the pebbles should be high and enable a dense packing to achieve a high lithium density. Further comments on pebble shapes are given in a later section. The presently used or developed processes are as follows:

1. A melting-spraying process was used at KIT (formerly FZK), in collaboration with Schott Glas — werke, for the production of 0.25-0.63 mm Li4SiO4 and Li4SiO4-SiO2 pebbles76 (see Figure 11). After annealing, spherical pebbles of 95-96% of theoretical density (TD) exhibiting satisfactory mechanical strength were obtained. Long-term annealing experiments on various candidates ceramic breeder materials were performed by Piazza et a/.77 An alternative route avoiding use of carbonate and using hydroxide was developed by Knitter et a/.,78 with slightly lower density. The reference composition is Li4SiO4 + 2.5 wt SiO2, resulting in a two-phase Li4SiO4 + Li6Si2O7 struc­ture in as-melted condition, and Li4SiO4 + Li2SiO3 after heat treatment (see Figure 12).

A melting-dropping process was investigated by Tsuchiya et a/.79 in collaboration with Mitsubishi to produce 1-mm Li2O spheres.

2. Sol-gel type processes were developed at Japan Atomic Energy Agency (JAEA), with Nuclear Fuel Industries, to produce 1 mm Li2O and 1.6 mm Li2TiO3 pebbles80 (see Figure 13). Similarly, Muis and coworkers81 at Energy Research Centre of The Netherlands (ECN) produced 0.5-1.0mm Li2TiO3

pebbles. In these cases, the pebble densities were <80% TD. Further work led to pebbles with Li2TiO3 + 5% TiO2 composition82 (see Figure 14).

Wu et a/.83 started the development of a sol-gel type process for Li4SiO4 and achieved 75% of TD for 1.2 mm diameter pebbles.

3. A process consisting of extrusion, spheronization, and sintering has, for several years, been used by AECL to produce 1.2 mm LiAlO2, Li2ZrO3, and Li2TiO3 pebbles in collaboration with Cera­mics Kingston.3 Material densities are in the 80-90% TD range.

Similar process trials were made by Lulewicz and Roux53 at CEA, with Pechiney, to produce 1 mm Li2ZrO3 pebbles, and later work concerned Li2TiO384,193 (Figure 15).

4. An agglomeration-sintering process has been used by JAEA, in collaboration with Kawasaki Industries,

Figure 13 Manufacturing process for Li2TiO3 by sol-gel method at Japan Atomic Energy Agency. Reproduced from Tsuchiya, K.; Kawamura, H.; Uchida, M.; Casadio, S.; Alvani, C.; Ito, Y. Fusion Eng. Des. 2003, 69, 449-453.

ECN SEI 15.0kV x50 100ptm WD36mm

Figure 15 Scanning electron micrographs of L12T1O3 pebbles produced by an extrusion-spheronization method (see text).

for producing 1 mm Li2O, Li4SiO4, and Li2ZrO3 pebbles. Pebble densities in the 90% TD range were obtained.85 This process has also been inves­tigated at CEA for producing 1 mm Li2TiO3 peb­bles. Pebble density of 90% TD and good mechanical strength were obtained.84

5. Zhu et al. developed a wet process for fabrication of Li3TaO4 ceramic pebbles. Typical pebble dia­meters are about 0.7-1.0mm, and the density achieved is over 90% TD, with crush loads more than 40 N.56 X-ray diffraction (XRD) patterns showed 99% of p-Li3TaO4 and traces of LiTaO3.

The necessity to recycle ceramic breeders after service imposes specific requirements on pebble manufacturing technologies. This reprocessing aspect may become a significant driver in fusion power economics on a longer term. See Sections 4.15.7 and 4.15.8.8 for further discussion.