An alternative observable to estimate keff in fast ADS

In this paper we examine an alternative method for determining the neutron multiplication factor keff in fast subcritical reactors, based on the observable obtained by calculating the ratio between the “fast” component of the neutron flux (in this case by fast we mean neutrons with kinetic energy above 0.5 MeV) and the whole flux at all energies. The assumption behind the present study was that the above mentioned observable would be only sensitive to keff, provided a peripheral fuel rod is considered (i.e. far from the neutron source in subcritical systems) and keff is sufficiently high. Indeed, our results show that, for keff>0.7, the ratio calculated for LFR or GFR models turns out to show a quite similar trend well approximated by a straight line, while a different slope is observed for a sodium reactor. In our opinion, the results obtained show that such observable, after appropriate calibrations on the real systems under consideration, may be used for determining keff in all operational conditions, in particular for subcritical systems with different types of neutron source and different values of the beam current.