Numerical analysis of extreme magnetoconvective phenomena in the WCLL blanket

The Water-Cooled Lead-Lithium (WCLL) is one of the breeding blanket concepts currently being developed for implementation in the fusion reactor demonstrator DEMO. The liquid breeder, lead-lithium, is circulated at velocities lower than 1 mm/s to minimize magnetohydrodynamic (MHD) pressure losses, whereas the power extraction is delegated to water flowing in double-walled pipes immersed in the breeder and square channels drilled in the first wall. Intense and spatially varying volumetric heating combines with a strong magnetic field and complex geometry in generating significant temperature gradients that, in turn, cause the onset of a magnetoconvective flow regime which effects on blanket performances have only been partially explored so far. The aim of this study is to contribute to the numerical characterization of the magnetoconvective effects present in the WCLL blanket. The attention is focused on the elementary cell placed on the outboard equatorial plane. A realistic representation of the blanket geometry, derived by the most recent design iteration, is adopted featuring bounding walls of uneven thickness. A toroidal-poloidal magnetic field is imposed on the computational domain to investigate the effect of a skewed magnetic field on the magnetoconvective regime. Heat removal by the cooling system is simulated directly to improve the prediction of the thermal field. Time-dependent distribution of the temperature and velocity field is obtained by a transient analysis and analyzed to identify potential harmful fluctuations. Particular attention is given to the temperature distribution in the Eurofer and at the structure/breeder interface to demonstrate that a safe operative temperature (< 550 °C) is maintained during normal operation with consistent safety margins.