Magnetohydrodynamic investigations of a self-cooled Pb-17Li blanket with poloidal-radial-toroidal ducts

For self-cooled liquid metal blankets, the magnetohydrodynamic (MHD) pressure drop and velocity distributions are considered as critical issues. This paper summarizes MHD work performed for a DEMO-related Pb-17Li blanket, where the coolant flows downwards in rear poloidal ducts; turned around by 180 at the blanket bottom; is diverted from poloidal ducts into short radial channels which feed to toroidal First wall coolant ducts; flows through the subsequent radial channels; is collected again in poloidal channels and leaves the blanket segment at the blanket top. To reduce the pressure drop and to decouple electrically parallel channels, flow channel inserts are used for all the ducts except the first wall ducts. A previous pressure drop assessment resulted in significant values for duct geometries with flow distribution or collection, and multichannel effects for the system of U-bends. As a result of the uncertainty of these assessments, corresponding investigations were carried out recently. Characteristic results are presented in this paper. It is shown that, for both geometries, the pressure drops are considerably lower than those previously assessed. First results from experiments on the velocity distribution in a radial-toroidal-radial U-bend are also presented. Here, it is shown that, with an increasing interaction parameter, the liquid preferentially flows close to the First wall. Additionally, a pair of strong vortices was observed in a toroidal duct. Both effects are supposed very favourable for heat transfer. (orig.)