Critical core design of MYRRHA-FASTEF: refinement of shutdown systems and neutronic characterisation of the equilibrium sub-cycle core

This work was carried out within the framework of the CDT/FASTEF EU FP7 project: it deals with the 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. Starting from the preliminary FA/core design defined by August 2010 in CDT-REP003-2010 (reference: ENEA Technical Report UTFISSM-P9P0-007), some remarkable modifications were introduced, as the MOX fuel composition, the Scram Rods (SR) concept and the refinement of Control Rods (CR) design. The impact of such modifications on the criticai layout was analysed in the present report dealing with: - the neutronic characterisation at Beginning of Life (BoL, par. 2): by adopting seven 'centrai' positions for samples irradiation, the criticality is reached by a 57 FA layout (~ 82.6 as max core power); - the study of a 100 MW-69 FA theoretical core (par. 3), necessary to estimate the needed Pu mass content for the criticality during the equilibrium sub-cycle; - the characterisation of the 100 MW-69 FA equilibrium sub-cycle core (par. 4). The Beginning of Cycle (BoC) and the End of Cycle (EoC) reference layouts were defined by adopting a 450 days fuel cycle (5 steps of 90 days) and an ln-to-Out shuffle strategy. The neutronic analysis, performed with the ERANOS deterministic code, is focused on power/flux distributions, peaking factors and the performances of shutdown systems. The results at BoL, BoC and EoC (summarised in par. 5) permitted to evaluate the impact of shuffling strategy (and CR partial insertion at BoC) on the main core features. Even if the ERANOS code can not supply results at the level of pin details, a preliminary value of the peak clad temperature (one of the most restricting constraints) is also indicated.