Tidal Current Energy can be eligible as one of the most interesting renewable and clean energy resources with the promise of a regular and predictable production of electricity. In the framework of the "MWATFORCE" Project from GUINARD Energies, a ducted tidal turbine has been developed. This paper presents the design of the runner based on the Blade Element Momentum theory and optimization of the shape minimizing cavitation development and stresses. The assessment of the turbine performance is made by presenting the unsteady turbulent flow numerical simulations of the full ducted tidal turbine in open flow conditions. A power coefficient as much as 55% is found for a tip speed ratio of 7. As expected, the pressure drop and the bulk velocity through the rotor as well are enhanced by the shaped duct compared to the free actuator disk, demonstrating the advantage of this design.