Multilayer pebbles for application to new divertor systems

A 'pebble drop' divertor concept is proposed for future commercial fusion reactors. The marked feature of this system is the use of multilayer pebbles that consist of a central kernel and some coating layers. By using multilayer pebbles the pebble drop divertor has advantages such as steady state wall pumping with low bulk tritium retention. The performance of the whole divertor system depends on the characteristics of the multilayer pebble. In particular, the maximum heat load of the system is determined by the dimensions, the layer structure and the material of the kernel. The kernel also has an important role in determining surface temperature, which affects the wall pumping efficiency. Numerical results for the maximum allowable heat load and the surface temperature of the divertor pebble are presented. From the numerical estimation of thermal stress and surface temperature, it is found that the radius of a divertor pebble with a ceramic kernel should be 0.5-1 mm. (author)