When compared with critical reactors, ADSRs have two specific charac­teristics.

1. If well designed, they prevent any possible criticality accidents. This has led their most vocal advocates [45] to propose that they should replace critical reactors altogether. In a less radical but also more common approach it has been proposed [38, 46] to take advantage of this sub­criticality in order to use certain types of fuel with poor neutronic properties: in particular, because of their very small delayed neutron fractions, homogeneous incineration of minor actinides appears to be feasible only with subcritical reactors.

2. Subcriticality provides additional neutrons which can be used for increased breeding of U or 9Pu. It is even possible to breed them in the absence of any fissile element. Another possible use of the additional neutrons is to transmute long-lived fission products.

Let us elaborate on these points.