The cascading pebble divertor for the spherical tokamak power plant

The design of a power plant based on the spherical tokamak (ST) is being developed in order to explore its potential advantages. The plasma is operated in a double null configuration, forming both an upper and lower divertor. In order to accommodate the high erosion rates and heat fluxes developed in the divertors, a system based on a cascading flow of silicon carbide pebbles is being developed. The pebbles flow into the upper divertor where they fall as a toroidal curtain, which intercepts the divertor particle flux. The pebbles then flow under gravity through ducts to the lower divertor where they form a similar curtain. The bulk temperature of the pebbles rises to about 1150 °C although the outer surface is transiently heated to about 1800 °C. The pebbles pass out of the vacuum chamber into holding tanks and then into a fluidised bed heat exchanger. Here the pebbles are cooled down to about 340 °C and dust and damaged pebbles are removed. The pebbles are transferred to an upper tank by a pneumatic conveyor where the remaining gas is removed and the pebbles flow into the upper divertor again.