Estimation of multi-layer pebble flow in pebble divertor

Pebble divertor concept is proposed for high-power density steady state fusion reactor. In this system, the dropping pebbles receive the divertor plasma and the pebbles are circulated to be cooled down. In this report, the total storage amount of pebble in the circulation system is estimated. That total amount depends on the pebble cooling time, which is the duration that the temperature of the pebbles after irradiation is recovered to the initial temperature before irradiation. The pebble cooling time is changed by how the pebbles are cooled. In this study, the pebbles are considered to be laid in the pebble cooling bed for the radiation sink and to be cooled at a concerned temperature. From the calculation, the desired pebble flow rate has been found to be 30 kg/s/m in toroidal direction at the slit width of 2.5 cm. The pebble cooling time becomes shorter when the pebble cooling bed is thinner or the initial temperature of pebbles before the irradiation is higher so that the storage amount of the pebble in the circulation system will be suppressed to small value.