MHD Flows in the Lower Expansion Chamber of a Generic Outboard Blanket Segment for DEMO

The water-cooled lead lithium (WCLL) blanket has been chosen as the European reference concept for a DEMO reactor. In the proposed design, liquid metal velocities in the columnar-arranged breeder units (BUs) are very small. This weak purge flow is required to circulate lead lithium (PbLi) toward external ancillary facilities for purification and tritium extraction. When the electrically conducting PbLi moves through the strong plasma-confining magnetic field, magnetohydrodynamics (MHD) affects the flow distribution and creates an increased pressure drop compared to hydrodynamic conditions. Previous MHD analyses have been performed for a single column of breeder units, with focus on the pressure drop in the poloidal manifolds that distribute PbLi among BUs. Manifolds have been identified as key and possibly critical components for the performance of blanket modules, since the major fraction of pressure drop arises in these elements. Moreover, they determine the partitioning of the PbLi flow among breeding zones. While the International Thermonuclear Experimental Reactor (ITER) Test Blanket Module (TBM) is designed only with two columns of BUs, in a DEMO reactor design, the number of parallel poloidal feeding and draining manifolds is larger. As a consequence, liquid metal has to be distributed from a single supply line into those channels via complex 3D geometries. In the present design of a DEMO blanket outboard segment, this is achieved by a toroidal expansion chamber located in the lower part of the module. The present analysis considers the MHD flow in this chamber, which expands from a radial circular entrance pipe along the toroidal direction toward two circular holes in the back plate of the blanket. Through these openings, the liquid metal is then distributed into the poloidal manifolds. Pressure and electric potential distributions are calculated via an asymptotic numerical approach valid for intense magnetic fields when inertia effects are negligible. Pressure losses, which arise preferentially from 3D MHD effects in expansion and contraction regions, are quantified.