Optimization of the filling for the improvement of the performance of reference ITER/DEMO ceramic and beryllium pebble beds

Both the EU Long Term Programmes for DEMO and the ITER R&D foresee the ther mal-mechanical qualification of the beryllium, as neutron multiplier, and l ithium orthosilicate or lithium metatitanate as breeder ceramics pebble bed s. FZK has performed measurements on the pebble bed thermal-mechanical prop erties using cylindrical test sections. Using an alternative approach, ENEA , has launched similar testing on the SMARTS mock-up, reproducing on a smal l scale the reactor reference plane geometry(1) instead. The tests have sho wn that the pebble bed thermal behaviour is strongly affected by the initia l filling Packing Factor (PF). In fact, the higher the PF, the higher the t hermal conductivity of the bed. Therefore, if the neutron multiplication ne eds an increase in the pebble PF, the only possibility is to adopt binary p ebble beds (small pebbles infiltrating between larger ones) as an alternati ve to the mono-sized lattice. Using binary pebble beds, the filling quality should be guaranteed against the occurrence of de-mixing or swimming of th e larger pebbles over the smaller ones during the thermal transients. A pos sible solution is to optimise the filling procedure, to improve the PF and its relevant thermal performance, and also to achieve a stable bed lattice during the cycling loads. In this case, the mechanical characteristics of t he pebble beds would also be heavily affected, thus requiring a new tests c ampaign to determine the actual mechanical properties of an "optimised" peb ble bed. This paper presents a new filling optimisation method and the expe rimental results from the compression tests of optimised pebble beds.