A constitutive model for the thermo-mechanical behaviour of fusion-relevant pebble beds and its application to the simulation of HELICA mock-up experimental results

Within the framework of the R&D activities promoted by EFDA on the Helium-Cooled Pebble Bed Test Blanket Module to be irradiated in ITER, attention has been focused on the modelling of the thermo-mechanical behaviour of both beryllium and lithiated ceramic pebble beds that are envisaged to be used respectively as neutron multiplier and tritium breeder. This behaviour depends, mainly, on the reactor-relevant conditions, the pebble sizes and the breeder cell geometries and a general constitutive model has not yet been validated, especially for fusion-relevant applications. ENEA-Brasimone and the Department of Nuclear Engineering (DIN) of the University of Palermo have performed intense research activities on this topic by means of an integrated experimental and theoretical approach. In particular, HELICA mock-up has been set-up and tested at the HE-FUS3 facility of ENEA-Brasimone to investigate the behaviour of lithiated ceramic pebble beds in reactor-relevant geometries, providing useful data sets on which DIN has validated its constitutive model. The paper presents the main features of the DIN constitutive model and results of two test campaigns on HELICA-mockup. The comparison between experimental results and numerical predictions is reported too, showing a good agreement that encourages the adoption of the DIN constitutive model.