Thorium has been considered as a complement to uranium-based fuel since the earliest days of the nuclear industry, initially based on considerations of resource utilization, and more recently also as a result of concerns about proliferation and waste management. These last two points will be further developed later in this chapter as well as the primary incentive for considering the thorium cycle in the past: potential uranium savings.

In summary, reasons for considering the introduction of a thorium-based fuel cycle include:

• increasing fissile resources by breeding U-233 from thorium

• improving fissile fuel utilization in thermal reactors

• significantly reducing U-235 enrichment requirements

• decreasing production of plutonium and other transuranic (TRU) elements compared to the uranium fuel cycle

• advantageous neutronic and physical properties of thorium-based fuel (e. g., higher thermal conductivity, higher melting point, better behaviour under irradiation, higher burn-up achievable)

Possible disadvantages are associated with the handling of U-233 fuel and the reprocessing of thorium-based fuel/blankets next to the need to deploy a parallel nuclear fuel cycle to the U/Pu fuel cycle when deploying a full Th/U-233 fuel cycle. These topics will be further developed in this chapter.