SAFETY ISSUES AND SAFETY INDICATORS FOR ACCELERATOR DRIVEN TRANSMUTERS WITH DEDICATED OXIDE FUELS

In order to exploit the full potential of accelerator driven systems (ADSs) for transmuting and incinerating Minor Actinides (MAs), innovative fuels have to be developed. These so-called dedicated fuels are characterized by a high MA content and by the lack of classical fertile materials as U238. Safety investigations of subcritical cores with these advanced dedicated fuels reveal some safety problems, which are discussed in this paper and put into a new perspective. Characteristically, the subcriticality of the core has to balance large positive void and clad reactivity worth potentials and a negligible prompt negative fuel temperature feedback (Doppler). A safety gain in a Pb/Bi cooled ADT (accelerator driven transmuter) is the high boiling point of the coolant. A conceptual problem for assessing the safety potential appears in analyses of large ADTs when the void and clad worths might be higher than the subcriticality margin. Many safety analyses of critical sodium-cooled fast reactors with a positive void worth were performed in the past, giving a guide-line on the size of void that could be handled under transient and accident conditions. Having in mind this experience, an approach is discussed to estimate which positive reactivity potentials might be acceptable in subcritical accelerator driven transmuters. The paper concentrates on the issue of severe transients, driven by internal reactivity potentials. Compliance with the criterion given in the paper does not exclude core-melt accidents, but should exclude rapid accident developments and potential cliff-edge effect behavior. The dependency of the void worth on the ADT size and the calculational uncertainties are discussed. In addition the influence of some kinetics parameters is discussed.