Characteristic differences of thermal creep effects between homogeneous materials and pebble beds are:

• In pebble beds, the contact surfaces between the pebbles increase with time. Blanket relevant creep time periods of greatest interest are in the order of less than a day because stress relaxation effects are expected to be quite fast.104

• Pebble beds are loaded first with relatively small stress gradients ds/dt; therefore, it is not differen­tiated between instantaneous plastic deformations and conventional thermal creep effects (the ther­mal creep correlations given below include both effects).

Thermal creep strains are measured by UCTs by keeping the uniaxial stress s constant at a given temperature T. Figure 21110 shows creep strains for Li4SiO4 pebble beds for different values of s and T; the thermal creep strain occurring during the stress increase period has been taken into account in this representation.

The data are well fitted by a constant exponent n for the time dependence. A constant exponent n was also found for most of the Li2TiO3 pebble beds.95 For some batches, during the first several hours, the same exponent n was observed; however, there was a subsequent increase in the creep rates. The Li2TiO3 batches that showed this behavior were in general characterized by low sintering temperatures, large

porosities, and small grain sizes, (for details, see Reimann et a/.98). Eventually, impurities could play a role, too. The data are fairly well plotted in Arrhenius graphs by straight curves, as shown in Figure 22.95

For a selection of ceramic breeder material candidates, the influence of pebble-bed thermal conductivity as a function of, for example, packing factor and pebble-bed compression was studied by
means of UCTs. These were performed for the ceramics Li4SiO4, Li2TiO3, Li2ZrO3, and Li2O with temperatures up to 480 °C and pressures up to 8 MPa, with packing factors varying between 56% and 63.5%. Creep strains in the pebble beds are identified to be functions of temperature, stress, and time and are found to be of the form ecr = A(T)amtn where m and n are independent of temperature and need to be deduced experimentally.97 This research was extended for Li4SiO4 pebble beds with temperatures up to 850 °C and pressures up to 9 MPa and concludes that thermal creep effects are negligible at temperatures below 600 °Q96,97 Creep behavior is also determined by the pebble properties: as mentioned earlier, lower creep strains were found for Li2TiO3 with small grain sizes (<5 pm) and high

98

sintering temperatures.