There are a number of issues impacting the choice of ADS neutronic parameters, in particular the ADS reactivity, keff. The degree of sub-criticality (keff) must be a balance between safety and acceptable economics. Here keff represents the sum of the initial reactivity and all other possible effects, e. g. burn-up reactivity swing including Np or Pa effects, power and void reactivity, etc.

ADS can be used for minimising the sources of long-term radiotoxicity, e. g. reactor fuel inventories, fuel wastes from reprocessing, and long-lived radioactive fission products (Slessarev, 1997). According to Salvatores et al. (1995), the latter two of these sources are the most important in terms of the accumulation of radiotoxicity.

For example, consider the neutronic potential of a representative ADS within a uranium fuel cycle complex (Slessarev, 1997) in the following system. A slightly sub-critical lead — cooled fast breeder reactor with nitride fuel and proton beam source with a keff of 0.98 would exhibit a neutron surplus of about 0.4 neutrons per fission (zero breeding gain in the fuel) plus 0.05 neutrons/fission due to spallation in the lead target. The lead is used as a liquid and target. This gives a total neutron surplus of 0.45 neutrons/fission, sufficient to burnout all dangerous fission products and/or reproduce new fuels for further nuclear power utilisation.

In this system, there is no need for control rods; it is a dual circuit, and a relatively inert coolant from the point of view of safety, e. g. fire hazard. The neutronics are sub-critical plus a stabilised reactivity increment. This system provides an apparently good balance

with regard to economics, reduction of fuel waste potential and safety for the uranium fuel cycle.

The thorium fuel cycle has a much lower waste toxicity level for both thermal and fast reactors than does the uranium fuel cycle. This is because of its smaller production of trans-plutonium (Carminati et al., 1994; Rubbia et al., 1995) and, therefore lower minor actinide concentrations (at least for about 1000 years before some build up of long-term toxic U, U, Pa). From a neutronic perspective, however, every fission of Th produces fewer neutrons than does 238U. There are other disadvantages in relation to achieving sub-criticality at economic cost and a protactinium effect, which implies a low keff value. Thus for the thorium cycle, it is necessary to have a compromise between the economics, sub-criticality level and safety margin. This is difficult because a low keff can only be achieved at more expense; reduced cost would be at the expense of higher keff and less safety margin.