An ITER TBM will experience not a constant heat load but a cyclic heat load behavior due to burn pulses of the plasma. Dalle Donne eta/.11 per­formed exploratory thermal cycling experiments with densified Li4SiO4 pebble beds in a horizontal tube (D = 20mm, L = 110mm). After about 500 cycles (350-600°C), fractions of 1.8% and 3.5 wt% of broken pebbles in two measurement campaigns were found. The pressure drop in the helium purge flow was found to saturate after some hundred cycles, indicating that no further pebble fragmentation occurred. Thermal shock is not considered an issue for ceramic breeder pebbles; the experiments showed that pebbles only fail for dT/dt>60°Cs~ that is, values much larger than expected in the blanket.

Cycles of UCTs up to 4 MPa at ambient tempera­tures lead to negligible plastic deformations in the Li4SiO4 and Li2TiO3 pebble beds. The residual com­paction is likely to be due to pebble relocation or pebble cracking. Multiple 6 MPa compression cycles at 750 °C led to irreversible strains of ^3.5% and 4% for Li2TiO3 and Li4SiO4 pebble beds, respectively. When 4 MPa compression cycles are performed on an Li4SiO4 pebble bed at 840 °C, the creep strains reach values of 6%. In these last tests, the influence of thermal creep is clearly visible.110

Similar cyclic compression tests on Japanese Li2TiO3 pebble beds reveal a compression ‘equilib­rium’ of ~1.5% only after several cycles of 10 MPa at 400 “C.1 2 Performing these tests at 600 °C gives lower strain values in the pebble bed, likely due to the increased compaction.1 This is in agreement

with the above-mentioned results.110

Several HCPB mock-up experiments have been per­formed in the HE-FUS3 facility at ENEA, Brasimone by Dell’Orco et a/.108,109,114 In the HELICHETTA and HELICA experiments, the thermomechanical behavior of Li4SiO4 or Li2TiO3 pebble beds under thermal cycling loads was investigated. The following larger HEXCALIBUR type experiments were char­acterized by two beryllium pebble beds and one ceramic breeder pebble bed in between. Nuclear heating was simulated by using electrically heated plates within the pebble beds.

In the HELICA mock-up tests, the thermal cycling of the pebble beds showed a saturation of the pebble-bed strains after ^30 cycles. The bed heights were then reduced by «4% and «1.5% for the Li4SiO4 and Li2TiO3 pebble beds, respec­tively. For constant power levels, the system did not change thermally with increasing cycle number. During demounting, submicron pulverized material was released, especially from the Li4SiO4 pebble beds, and pebble fragmentation was observed. An important objective of these experiments was the validation of pebble-bed thermomechanical models as outlined below.