Studies of the thorium cycle are ongoing in several countries, such as the USA, Russia, China, Canada, Sweden, Norway, Japan, France and, above all, India. The European Union is also active in fostering R&D actions for the thorium cycle. Finally, it must be mentioned that the IAEA (International Atomic Energy Agency) regularly publishes synthesis documents on this topic. Nevertheless, most of these programmes were limited till today to academic studies. The exception is India and we will, therefore, describe the Indian programme in more detail.

India has limited indigenous uranium resources (1% of the world’s uranium resources) and has difficulties in importing uranium (presently) because of political reasons. With about six times more thorium than uranium, India has made utilization of thorium for large-scale energy production a major goal in its nuclear power programme, envisaging a three-stage approach:

1 Water-cooled thermal reactors, namely, Pressurized Heavy Water Reactors (PHWRs), elsewhere known as CANDUs (CANada Deuterium uranium) fuelled by natural uranium and Light Water Reactors (LWRs) of the pressurized (PWR), boiling water (BWR) or VVER types. In this stage plutonium is produced.

2 Fast Neutron Reactors (FNRs) using this plutonium-based fuel to breed U-233 from thorium. The blanket around the core contains uranium as well as thorium, so that further plutonium (ideally plutonium of high fissile quality) is produced as well as U-233.

3 Advanced Heavy Water Reactors (AHWRs) that burn U-233 and plutonium with thorium, getting about 75% of their power from thorium.

Thorium oxide pellets have been irradiated in Indian research reactors and reprocessed via a simplified THOREX process to recover U-233 (see Section 3.3). Recovered U-233 has also been utilized in research reactor programs. India manufactures ThO2 pellets, which are irradiated as stainless steel clad blanket assemblies in a fast breeder test reactor. Some are also irradiated as Zircaloy clad pin assemblies for neutron flux flattening of the initial core during start-up in pressurized heavy water reactors. The Kakrapar-1 and -2 units are loaded with 500 kg of thorium fuel in order to improve their operation when newly started. Kakrapar-1 was the first reactor in the world to use thorium, rather than depleted uranium, to achieve power flattening across the reactor core. In 1995, Kakrapar-1 achieved about 300 days of full power operation and Kakrapar-2 about 100 days using thorium fuel. The use of thorium-based fuel is planned in the Kaiga-1 and -2 and Rajasthan-3 and -4 reactors, which are today in commercial operation. India is currently building a 500 MWe sodium-cooled fast neutron reactor with, according to the Indian three-phase programme, the possibility, later, of using thorium in the blanket to breed U-233.18

A 300 MWe advanced heavy water reactor (AHWR 300) is now undergoing design and development. The driver fuel will be thorium/plutonium oxide and thorium/U-233. The AHWRs will obtain about 75% of their power from thorium.8 Spent fuel will then be reprocessed to recover fissile materials for recycling.

Based on this overview of past and present developments of the thorium fuel cycle, the next part will summarize the main findings and will give an industrial view of the advantages and weaknesses of thorium as a fertile material for nuclear (fission) energy.