One of the key components regarding heat transfer and tritium inventories in deuterium-tritium nuclear fusion reactors is the (tritium) Breeding Blanket, called Test Blanket Module or TBM in ITER experiment. Several designs are going to be tested in ITER, one of those is the HCLL (Helium Cooled Lithium Lead) design. Before being tested, it is of major interest to predict in detail several ow parameters such as pressure drop, tritium inventories and tritium permeation rates through walls. The goal of the present study is to analyze the ow near the gap region (close to the rst wall) in the HCLL-TBM so as to quantify tritium inventories and permeation uxes. To do so, simpli ed C-shaped channels are simulated under ITER speci cations. The ow appears to be very complex and, in order to get the origin of this complexity, the phe- nomenon physics are decoupled. First, the pure hydrodynamic case is simulated; obtaining that the critical Reynolds number is around TBM/ITER speci cations. Second, the MHD ow with perfectly insulating walls is studied and, as expected due to the high Hartmann number, hydrodynamic instabilities disappear. Finally, when heat transfer is introduced, vorticity is generated due to Rayleigh-B enard instabilities at the channel inlet and, as the ow travels through the channel, faster vortices appear in the gap region and in the outlet channel. These vortices originate high tritium concentration zones. Hence, the existence of vortices is of crucial interest for tritium inventories prediction and HCLL design.