Hybrid reactors

A hybrid reactor or accelerator-driven subcritical reactor (ADS [Accelerator Driven System] or ADSR [Accelerator Driven Subcritical Reactor]) comprises three main components: a gas pedal, usually of protons, a target, usually of heavy elements, and a subcritical assembly of fissile nuclei. The interaction of protons with the target produces a large number of neutrons, providing a source for the subcritical assembly. The Canadians (Lewis 1952) were the first to come up with the idea of using such a device to generate energy: lacking an isotope separation plant, they wanted to produce uranium 233 from thorium. As the amount of uranium 233 in the thorium assembly grew, the number of fissions would increase, and the resulting energy could be used to power the gas pedal. Given the success of water-cooled critical reactors, this avenue was abandoned in the 1950s until the early 1990s. It was the teams around K. Furukawa, C. Bowman and C. Rubbia who brought the hybrid gas pedal concept back into fashion, for a variety of reasons. K. Furukawa was the first to envisage combining a high-intensity gas pedal with a molten-salt subcritical reactor, with the motivation of rapidly implementing a uranium 233 subregeneration system in place of conventional subregenerator reactors using the uranium-plutonium cycle. C. Bowman added the constraint of incinerating transuranic waste and long-lived fission products as completely as possible. Finally, C. Rubbia pursued the same goal as C. Bowman, but using a sub-critical lead-cooled fast-neutron reactor.

In recent years, various hybrid gas pedal concepts have been studied around the world as part of international collaborations. The consensus emerging from these studies is that the safety advantages associated with subcriticality will probably not be sufficient to justify the deployment of a complete power generation system based on hybrid reactors. Hybrid reactors could, however, occupy an interesting niche for the transmutation of minor actinides (MA) and the development of new reactor technologies.