This publication presents the main results and achievements of an undertaking (part of a wider IAEA coordinated research project) devoted to the benchmark analyses of two mixed oxide (MOX) fuelled BN-600 core designs. The studies conducted contributed to the progress in development, verification and validation of new codes and data libraries for fast reactor analyses; promoted deeper understanding of the influence of reactivity coefficients and their uncertainties on the results of experience and transient analyses in the initial phase of transients, such as unprotected loss of flow (ULOF); and facilitated exchange of opinions between specialists.