This work was carried out within the framework of 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 present neutronic analysis is focused on the criticai option: the main reactivity coefficients were evaluated by means of the ERANOS ver. 2.1 deterministic code. The reference core, with seven centrai In-Pile-Sections (IPS), was defined in CDT-REP007-2011 (reference ENEA Technical Report UTFISSM-P9P0-011). By adopting: the 34.5 wt. % Pu mass content (with 1.65 wt. % of Am in the Pu vector), a 450 days fuel cycle (5 steps of 90 days) and an ln-To-Out shuffle strategy, the core should be criticai at Beginning of Life (BoL) with 57 FA and in the equilibrium sub-cycle with 69 FA (100 MW). Besides the study of the main reactivity coefficients (related to thermal effects) at different time steps of the fuel cycle, other important safety features were investigated as: the anti-reactivity requirements of shutdown systems, the delayed neutron fraction, the void and core compaction effects.