An accelerator-driven subcritical reactor (ADSR) is a nuclear reactor design formed by coupling a substantially subcritical nuclear reactor core with a high-energy proton or electron accelerator. It could use thorium as a fuel, which is more abundant than uranium.[1]
The neutrons needed for sustaining the fission process would be provided by a particle accelerator producing neutrons by spallation or photo-neutron production. These neutrons activate the thorium, enabling fission without needing to make the reactor critical. One benefit of such reactors is the relatively short half-lives of their waste products. For proton accelerators, the high-energy proton beam impacts a molten lead target inside the core, chipping or "spalling" neutrons from the lead nuclei. These spallation neutrons convert fertile thorium to protactinium-233 and after 27 days into fissile uranium-233 and drive the fission reaction in the uranium.[1]
Thorium reactors can generate power from the plutonium residue left by uranium reactors. Thorium does not require significant refining, unlike uranium, and has a higher neutron yield per neutron absorbed.