This work was carried out in the CDT/FASTEF EU FP7 project: it deals with the preliminary core design of MYRRHA-FASTEF, a 100 MW LBE cooled reactor (working in both criticai and sub-critical modes) that should be able to demonstrate transmutation and associated technology. The report summarizes the first investigations carried out for MYRRHA-FASTEF working in sub-critical mode. Differently from the criticai core layout, the Pu mass content in MOX (with 1.65 wt.% of Am) was decreased from the 35 wt.% down to 30 wt% for manufacturing / licensing reasons. Furthermore the FA was slightly modified by adopting 127 fuel pins, instead of 126 used in the FA defined for the criticai mode (in which the central one was an empty clad devoted to instrumentation). Basing on the first neutronic evaluations carried out with the MCNPX code (by A. Stankowskij, SCK-CEN) and the preliminary T/H analysis needed to evaluate the max clad T (by D. Castelliti, SCK-CEN), an envelope for the sub-critical core layout is proposed. While the preliminary MCNPX analysis considers as a boundary constraint a max clad T of 466°C (due to the O2 control issue), the envelope layout proposed foresee a max clad T of 550°C (that should be feasible with a coated clad). By adopting a 540 days fuel cycle (6 steps of 90 days) and an In-to-Out shuffle strategy, the equilibrium sub-cycle core (with six centrai IPS for irradiation rigs) should exploit 72 FA and a max proton current of about 3.5 mA. Such 72 FA equilibrium core will be investigated by the CDT WP2/T2.2 partners in the near future. If some room will be available (i.e. max proton current < 3.5 mA) a mixed shuffle layout (also proposed in the report) will be implemented in order to further increase the flux performances.