22.08.2015   Comprehensive nuclear materials

Base Properties

Table 2 summarizes lithium compounds considered as breeding material and lists some key properties.

Lithium oxide was favored in early blanket concepts, in particular because of its very high lithium density and good thermal conductivity. Its biggest disadvantage is its strong sensitivity to moisture and the evidence it shows of significant swelling under irradiation.

The silicates studied most widely are Li2SiO3 and Li4SiO4. In practice, traces of Li2SiO5 or Li6Si2O7 can be found as well.

A variety of lithium zirconates have also been stud­ied, such as metazirconate Li2ZrO3 and Li8ZrO6 octa — zirconate by Roux and coworkers.26,33,35,52,53,62,63,156 Because of activation issues for Zr in the fusion blanket spectra, these compounds became less attractive. Zirconates have been shaped in pellets as well as pebbles, and their irradiation performance at high burnups was considered promising in terms of pellet integrity and tritium release characteristics.33

Table 2 Overview of most relevant basic ceramic lithium compounds and their major characteristics

Material

Lithium-

Composition

Li

density

(gcc1)

Theoretical

density

(gcc1)

Melting

temp

(°C)

Specific heat at 400°C (JgK1)

Thermal conductivity at 400°C (Wm1K1)

Linear expansion at 400 °C, relat. to 25 °C(%)

Oxide

U2O

0.93

2.01

1432

2.5

6

1

Octo-zirconate

Li8ZrO6

0.69

3.01

1295

1.5

1.5

0.7

Ortho-silicate

Li4SiO4

0.54

2.39

1255

1.9

2.5

0.9

Ortho-tantalate

Li3TaO4

0.46

5.87

1679

Meta-titanate

Li2TiO3

0.44

3.55

1270

1.4

1.7

0.5

Hexa-zirconate

Li6Zr2O7

0.43

3.43

1535

1.3

1.7

0.6

Meta-silicate

Li2SiO3

0.39

2.53

1201

1.6

2.4

0.6

Meta-zirconate

Li2ZrO3

0.38

4.15

1600

1

1.4

0.4

g-Aluminate

LiAlO2

0.27

2.55

1610

1.3

2.6

0.4

Octo-plumbate

Li8PbO6

0.66

4.24

Lithium titanates were initially introduced at AECL by Kopasz and coworkers,64’65 and early tri­tium release experiments have shown promising results. Li2TiO3 has been studied by Japanese and EU parties. Hoshino et a/66-70,179-181,190 found that not only oxygen-deficient but also lithium oxide-deficient defects form on changing the atmo­sphere from hydrogen to oxygen. Thus, the doubly nonstoichiometric composition, Li2-xTiO3-j, has been confirmed. Further, it has been shown by ther­mal diffusivity measurement that 95Li2TiO3 has a higher thermal conductivity than 100Li2TiO3.66

Lithium aluminates have been studied mostly in the form of g-LiAlO2. Data accumulated have been summarized by Billone and Grayhack.71 Due to its low lithium density and modest tritium release per­formance, the material has gradually begun to receive less consideration.62

More recently, Zhu et a/.56 in China started inves­tigations of lithium tantalates such as Li3TaO4, which has a reasonably high lithium density.

Sedano and coworkers57 report on the develop­ment of lithium plumbates such as Li8PbO6, revisit­ing earlier work of Hayashi et a/.72