This work has been carried out in the framework of the CDT/FASTEF EU FP7 project: it deals with the preliminary FA/core design for MYRRHA-FASTEF, a 100 MW LBE cooled reactor (working in both critical and sub-critical modes) that should be able to demonstrate transmutation and associated technology. This analysis is focused on the critical option, even if the main features of the sub-critical one (having the same FA structure) have to be foreseen since the beginning. Starting from the fuel pellet/pin manufacturing constraints given by AREVA, the present report sketches two possible layouts of the critical care by adopting either a hollow pellet or a solid pellet (both having 6.55 mm as pin diameter). A third solution is studied by considering the 'Rapsodie Fortissimo' pin (used in the Rapsodie French experience). The three proposals exploit the common FA architecture of SFR with helical wire spacers, with a enlarged wire diameter for the LBE coolant (having a lower specific heat than sodium). The three cores are compared in terms of the obtained flux level, but also some safety aspects (as void and Doppler effects) are investigated. The second part of the presentation (done at the WP2 meeting, on 27-28 ApriI 2010 in Mol, Belgium) defines in details the work plan far the beginning of WP2 - Task 2.2 (in which ENEA is responsible).