IFMIF (International Fusion Materials Irradiation Facility) is an accelerator based deuteron-lithium (D-Li) neutron source to simulate the neutron irradiation field in a fusion reactor. The target assembly of the IFMIF consists of the flow conditioners and the nozzle, which has to form a stable lithium jet. This work focuses on a numerical study of the flow conditioner efficiency, in which two different types of flow conditioners are compared by means of a detailed numerical analysis with respect to specific hydraulic effects in the pipe elbow and the inflow conditioners. The adequateness of two modelling approaches - Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) - to simulate an unstable flow through a 90° bend of circular cross section has been examined. Both methods investigated exhibit a reasonable agreement with the experimental data, but the DES approach does not require a very fine grid resolution and is less time consuming. The further conducted numerical analysis of the flow conditioner uses a DES approach. The calculations show that a honeycomb-screen combination is not capable to suppress effectively large scale swirl motions emerging from the bend. A frequency analysis of the static pressure fluctuation reveals instabilities in the shear layer between the separation zone and the accelerated outer region, which additionally increase the inhomogeneity of the axial velocity distribution.