[en] The new criticality experiments facility STACY will be able to contribute to the validation of criticality calculations related to the fuel debris. The experimental core design is in progress in the frame of JAEA/IRSN collaboration. This paper presents the method applied to optimize the design of core configurations of the new STACY to measure the criticality characteristics of pseudo fuel debris focused on Molten Core Concrete Interaction (MCCI) debris. To ensure that a core configuration is relevant for code validation, it is important to evaluate the reactivity worth of the main isotopes and the k_eff sensitivity to their cross sections. If the sensitivities profiles are similar to those of the configuration to be validated, it is potentially feasible to provide relevant feedback on its nuclear data. In the case of MCCI debris described in this study, silicon is the nucleus that has the highest k_eff sensitivity in the concrete. Therefore, some parameters of the core configuration, as for example the lattice pitch or the core dimensions, were adjusted using optimization algorithm to research efficiently the optimal core configurations to obtain high sensitivity of silicon capture cross section. This method allows exploring a large space of possibilities by limiting the number of calculations. Two examples of designs tested using this approach are presented in this paper. The first study was performed on a simple square core configuration with ideal conditions. In the second study, the core was divided in two zones to investigate the interest of having both an experimental zone and a driver zone. Based on these results, realistic series of experiments for fuel debris in the new STACY could be defined to obtain an interesting feedback for the MCCI. (authors)