This work was carried out in the COT/FASTEF EU FP7 project: it deals with the prelirninary core design of MYRRHA-FASTEF, a 100 MW LBE cooled reactor (working in both criticai and sub-critical rnodes) that should be able to dernonstrate transrnutation and associated technology. The report surnrnarizes the presentation done by the author in the COT/WP2 rneeting on January 18-20th, 2012, at the SCK-CEN research centre in Moi (B). By working with the 30 wt% Pu (+ 1.65% Am) rnass content in the MOX pellet, the reactivity feedbacks and coefficient of a 58 FA BoL core were investigated by rneans of the ERANOS deterrninistic code. By considering also three main incidental situations, and analysing their effect on the keff value, it resulted possible to establish (at the first glance) the allowed level of sub-criticality at BoL. Finally, the steps of the rernaining T2.2 work plan and the timing of the activities (to be perforrned before the end of the project by the COT T2.2 partners) is indicated.