A duct-dimension-regulation approach for eliminating MHD PbLi flow reversal in buoyancy-opposed poloidal duct relevant for typical liquid breeder blanket designs

Abstract

Lithium-lead (PbLi) flow plays the major roles: removing nuclear heating, breeding tritium and multiplying neutrons, in the appeared typical liquid breeder blanket designs for magnetically confined fusion reactors. Under the buoyancy effect caused by the volumetric nuclear heating with large gradient in the radial direction, flow reversals can be generated in the magnetohydrodynamic (MHD) PbLi flow in the poloidal downward-flow duct(s) of such blanket designs, which may import negative consequences to the functionality of the breeder blanket. Based on a theory regarding mixed convection in fully developed MHD duct flows developed by Smolentsev et al. (Smolentsev, Moreau & Abdou, Characterization of key magnetohydrodynamic phenomena in PbLi flows for the US DCLL blanket, Fusion Engineering and Design 83, 771-783, 2008), this study attempts to further quantitatively analyze the above-mentioned flow-reversal issues, and in the meantime to develop an approach to eliminate the reverse flow regime by regulation of the duct dimensions. Numerical simulations are performed for buoyancy-opposed MHD duct flows of PbLi in order to demonstrate the effectiveness of the proposed duct-dimension-regulation approach on eliminating the occurrence of reverse flows. It is found that, in the reverse flow regime, the reversal structures travel to the downstream from their onset locations at a speed essentially equal to the mean flow velocity in the bulk; the reverse flow regime can be effectively eliminated by regulating the duct dimensions. Comprehensive discussions about the reverse flow related issues in the DCLL type blanket are then stressed.