In the dual-coolant lead lithium (DCLL) blanket, the key element is the flow channel insert (FCI) made of a silicon carbide composite (SiCf /SiC), which serves as electric and thermal insulator. The most important magnetohydrodynamic (MHD) and thermal issues of the FCI, associated with MHD flows and heat transfer in the poloidal channel of the blanket, were studied with numerical simulations using the U.S. DEMO DCLL design as a prototype. The mathematical model includes the two-dimensional momentum and induction equations for a fully developed flow and the three-dimensional (3-D) energy equation. Two FCI modifications, one with no pressure equalization openings and one with a pressure equalization slot, have been considered. The computations were performed in a parametric form, using the electric and thermal conductivity of the SiCf /SiC as parameters. Under the DEMO reactor conditions, parameters of the FCI have been identified that result in low MHD pressure drop and low heat leakage from the breeder into the helium flows. This paper also discusses the role of the pressure equalization openings, 3-D flow effects, and the effect of SiCf /SiC anisotropy.