Neutron efficiency and multiplication factor in spallation neutron-driven subcritical systems: The case of a fast neutron system

The fission chain reaction in accelerator-driven subcritical systems (ADS) is sustained by multiplying spallation neutrons, which are generated when a high-energy ion beam (such as protons) interacts with a suitable high-density target material. The presence of these neutrons, along with other neutrons of non-fission origin, is expected to affect the efficiency of neutrons ( E* ) in inducing fission in fissionable nuclei within the system. Consequently, the neutron multiplication factor ( k s ) will differ from that ( k eff ) observed when fission is solely induced by fission neutrons. This paper examines the theoretical aspects of the neutronics in spallation-neutron-driven systems, explores several factors influencing the values of E* and k s , and analyses their variations in different neutron fields using the Monte Carlo approach. It is shown that in ADS in general k s is greater than k eff and E* > 1. The neutron field conditions in which k s < k eff and E* < 1 are given and discussed. This study focuses on a deeply subcritical system with a fast neutron field.