Thermo-mechanical behavior of titanium beryllide pebble beds at ambient temperature

The thermomechanical behavior of titanium beryllide pebble beds was investigated experimentally at ambient temperature in helium. The pebbles consist of a mixture of TiBe12 and Ti2Be17 titanate beryllide phases and a small residual amount of Be phase, denominated as Be-7.7Ti. For purposes of comparison, experiments in air and helium atmosphere were also conducted with Be, Al2O3 and orthosilicate pebble beds. The pebble beds were compressed uniaxially up to 4.5 MPa; the effective thermal conductivity was measured using the hot wire technique. The stress-strain curves do not differ significantly for the different granular materials in contrast to the effective thermal conductivity k. Whereas for Be pebble beds k increases strongly with increasing stress (about 600 % for the uniaxial stress of 4 MPa), the increase is much smaller for the other granular materials (about 60 % for Be-7.7Ti at 4 MPa). The main reason for the large differences is the much lower thermal conductivity of the solid materials. Another influence exists because the other materials are mechanically harder resulting in smaller contact surfaces. As long as the thermal conductivity of other beryllide materials is significantly lower than that of Be, comparable results are expected as for Be-7.7Ti. It is argued that similar results are expected for single phase TiBe12 pebble beds. Because of the harder behavior compared to Be-7.7Ti pebbles, the effective thermal conductivity might be slightly smaller. Although the above reasonings suggest that a phase mixture can be used instead of the single phase TiBe12, this issue has to be investigated in more detail