Balance of power in ADS operation and safety

Subcritical, accelerator driven systems (ADS) are viewed in recent years with increasing interest for their possible role in perspective future nuclear energy scenarios, as actinide incinerators, and/or claimed energy plants with potential enhanced safety characteristics. Important research programs and in particular extensive studies on their behavior under incidental conditions are made, for verifying their claimed advantage with respect to the corresponding fast critical reactors. An extensive, effective safety analysis of these latter systems was made in the late 80ʹs at the Argonne National Laboratory, specifically with respect to the Integrated Fast Reactor (IFR). A synthetic, quite effective deterministic method was used based on a "balance of reactivity" approach. In the present paper an attempt is made to redefine a similar methodology in relation to the ADS, making rather use of a "balance of power" approach. This would allow to estimate the peculiarities of the ADS systems, in particular with respect to the corresponding critical ones. A number of important incidental events have been considered, such as TOC (transient of power following a current increase, in particular the current reserve for compensating burn up), LOF (loss of coolant flow), HTWS (heat transfer without sink), CIT (chilled inlet temperature) events. As an example of application of this methodology, the Russian lead cooled BREST fast reactor project has been considered. The results obtained for this system, and for the corresponding ADS at various levels of subcriticality, are given.