FEM Modeling of Pebble Bed/Structural Wall Separation

This work has developed FEM models of ceramic breeder pebble beds and applied them to two categories of blanket design (edge-on and layer configurations) to predict the thermomechanical behavior of a pebble bed under ITER pulsed operating condition. To explore the pebble bed/structural wall separation phenomenon, a thermomechanical contact is considered using contact elements meshed along pebble/structure interface. The pebble bed/wall dynamic contact/separation process has been simulated, and the gap distance distribution and variation have been analyzed and presented. Pebble bed/wall separation occurs during the plasma-off period and varies with both location and time. A maximal radial gap of 0.64mm is found for an edge-on configuration after the 1st ITER cycle within the range of studied parameters. For the layer configuration, a poloidal gap of 1.99mm, larger than the pebble diameter, is found. The generated gap can cause the even large rearrangement of pebbles and result in a disturbed packing during further cycling. Consequently, a design solution is suggested to mitigate this situation.