Impact of criticality level on safety of accelerator driven systems with conventional Mox and advanced fertile free fuels

Full text follows: Two types of Accelerator Driven Systems are currently investigated in Europe: with conventional MOX fuel (of e.g. Superphenix type) for demonstration and experimental purposes; and with Advanced Fertile Free Fuels for nuclear waste burning. Usually a similar subcriticality level is assumed acceptable in the both cases: with keff being around 0,95 - 0,97. In the paper, analyses of protected and unprotected (i.e. with and without accelerator shutdown) transients are performed for a wider range of initial (at the beginning of the transient) keff values to assess corresponding advantages and disadvantages (from the safety point of view) for the both types of ADS. (authors)