On the allowed sub-criticality level of lead (-bismuth) cooled ADS: the EU FP6 EFIT and FP7 FASTEF cases

Abstract

The Accelerator Driven System (ADS) devoted to the transmutation of Minor Actinides (MAs: Am, Np, Cm nuclides) might play a key role for the sustainability of nuclear energy. While the MAs content in nuclear reactors must be limited for the worsening of some safety related parameters (e.g., delayed neutron fraction and doppler feedback), the sub-criticality of the ADS allows operating with rather high MA loadings. On the other hand, the allowed margin from criticality (1−keff) is core-dependent and, as usual in engineering processes, it results to be defined only when the ADS is actually designed and all the operational and accidental conditions have been analysed with detailed safety studies. But, at the same time, an initial indication of this margin is necessary in the early design phase of such innovative systems.This paper proposes a simple – and at some extent general – methodology to estimate a priori the allowed sub-criticality level in operation of lead (-bismuth) cooled ADSs with steady states neutronic analyses only. The allowed sub-criticality was evaluated for two different ADS concepts, designed within international collaborative projects co-funded by the EURATOM 6th and 7th Framework Programmes (FPs):•the long-term large-scale (∼400MW) ADS named EFIT (EUROTRANS FP6 project), cooled by pure lead and specifically designed as MAs burner without neither Pu burning nor Pu breeding;•the short-term short-scale (∼70MW) ADS named FASTEF (CDT FP7 project), conceived as an intermediary step (e.g., MOX fuel, lead-bismuth coolant) to demonstrate the ADS technology and to provide an irradiation test-bed for transmutation experiments.The steady state neutronic analyses of the EFIT and FASTEF sub-critical cores, here considered as reference examples, were carried out with the ERANOS ver. 2.2 deterministic code and JEFF3.1 nuclear data. The allowed sub-criticality in operation and the margins from criticality in accidental conditions were conservatively estimated, by confirming that such keff levels are core (-size) and plant configuration dependent. Finally, the possible application of the methodology to other lead (-bismuth) cooled ADS designs is briefly discussed.