In India, both 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 nuclear 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 utilizing thorium fuel. The use of thorium-based fuel is planned in Kaiga-1 and -2 and Rajasthan-3 and -4 reactors.
In India, the Kamini 30 kWth experimental neutron-source research reactor using 233U, recovered from ThO2 fuel irradiated in another reactor, started up in 1996 near Kalpakkam. The reactor was built adjacent to the 40 MWt Fast Breeder Test Reactor, in which the ThO2 is irradiated.
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 program, utilizing a three-stage concept:
Concepts for advanced nuclear power reactors based on thorium-fuel cycles include:
* Light Water Reactors (LWRs) - With fuel based on plutonium oxide (PuO2), thorium oxide (ThO2) and/or uranium oxide (UO2) particles arranged in fuel rods.
* High-Temperature Gas-cooled Reactors (HTGRs) of two kinds: pebble bed and with prismatic fuel elements.
* Gas Turbine-Modular Helium Reactors (GT-MHRs) - Research on HTGRs in the USA led to a concept using a prismatic fuel. The use of helium as a coolant at high temperature, and the relatively small power output per module (600 MWth), permit direct coupling of the MHR to a gas turbine (a Brayton cycle), resulting in generation at almost 50% thermal efficiency. The GT-MHR core can accommodate a wide range of fuel options, including HEU/Th, 233U/Th and Pu/Th. The use of HEU/Th fuel was demonstrated in the Fort St Vrain reacto.
* Pebble-Bed Modular Reactors (PBMRs) - Arising from German work, the PBMR was conceived in South Africa and is now being developed by a multinational consortium. It can potentially use thorium in its fuel pebbles.
* Molten Salt Reactors (MSRs) - This is an advanced breeder concept, in which the fuel is circulated in molten salt, without any external coolant in the core. The primary circuit runs through a heat exchanger, which transfers the heat from fission to a secondary salt circuit for steam generation. It was studied in depth in the 1960s, and is now being revived because of the availability of advanced technology for the materials and components.
* There is now renewed interest in the MSR concept in Japan, Russia, France and the USA, and one of the six generation IV designs selected for further development is the MSR. In 2002 a Thorium MSR was designed in France with a fissile zone where most power would be produced and a surrounding fertile zone where most conversion of Th-232 to U-233 would occur.
* Advanced Heavy Water Reactors (AHWRs) - India is working on this design, and like the Canadian CANDU-NG, the 250 MWe design is light water-cooled. The main part of the core is subcritical with Th/233U oxide, mixed so that the system is self-sustaining in 233U. A few seed regions with conventional MOX fuel will drive the reaction and give a negative void coefficient overall.
* Plutonium disposition - Today, MOX (U,Pu) fuels are used in some conventional nuclear reactors, with 239Pu providing the main fissile ingredient. An alternative is to use Th/Pu fuel, with plutonium being consumed and fissile 233U bred. The remaining 233U after separation could be used in a Th/U fuel cycle.
Use of thorium in Accelerator Driven Systems (ADS)
In an Accelerator Driven System (ADS), high-energy neutrons are produced through the spallation reaction of high-energy protons from an accelerator striking heavy target nuclei (lead, lead-bismuth or other material). These neutrons can be directed to a subcritical reactor containing thorium, where the neutrons breed 233U and promote the fission of it. There is therefore the possibility of sustaining a fission reaction which can readily be turned off, and used either for power generation or destruction of actinides resulting from the U/Pu fuel cycle. The use of thorium instead of uranium means that less actinides are produced in the ADS itself.
Developing a thorium-based fuel cycle
Despite the thorium fuel cycle having a number of attractive features, development even on the scale of India's has always run into difficulties.
The main attractive features are:
• the possibility of utilising a very abundant resource which has hitherto been of so little interest that it has never been quantified properly,
• the production of power with few long-lived transuranic elements in the waste,
• reduced radioactive wastes generally.
Problems include
• the high cost of fuel fabrication due partly to the high radioactivity of 233U chemically separated from the irradiated thorium fuel. Separated U-233 is always contaminated with traces of 232U (69 year half life but whose daughter products such as thallium-208 are strong gamma emitters with very short half lives);
• the similar problems in recycling thorium due to highly radioactive 228Th (an alpha emitter with two-year half life) present;
• the technical problems (not yet satisfactorily solved) in reprocessing solid fuels.
Much development work is still required before the thorium fuel cycle can be commercialized, and the effort required seems unlikely while (or where) abundant uranium is available. In this respect international moves to bring India into the ambit of international trade are critical. If India has ready access to traded uranium and conventional reactor designs, it may not persist with the thorium cycle.
Nevertheless, the thorium fuel cycle, with its potential for breeding fuel without the need for fast neutron reactors, holds considerable potential in the long-term. It is a key factor in the sustainability of nuclear energy.
* Pressurized Heavy Water Reactors (PHWRs), elsewhere known as CANDUs fuelled by natural uranium, plus light water reactors, produce plutonium;
* Fast Breeder Reactors (FBRs) use this plutonium-based fuel to breed 233U from thorium. The blanket around the core will have uranium as well as thorium, so that further plutonium (ideally high-fissile Pu) is produced as well as the 233U; and then
* Advanced Heavy Water Reactors (AHWRs) burn the 233U and this plutonium with thorium, getting about 75% of their power from the thorium.
The used fuel will then be reprocessed to recover fissile materials for recycling.
This Indian program has moved from aiming to be sustained simply with thorium to one "driven" with the addition of further fissile uranium and plutonium, to give greater efficiency.
Another option for the third stage, while continuing with the PHWR and FBR programs, is the subcritical Accelerator-Driven Systems (ADS) (see below).
Estimated World thorium resources
(RAR + IR to USD 80/kg Th)
Country tonnes % of world
Australia 425,000 18
USA 400,000 16
Turkey 344,000 13
India 319,000 12
Brazil 302,000 12
Venezuela 300,000 12
Norway 132,000 5
Egypt 100,000 4
Russia 75,000 3
Greenland 54,000 2
Canada 44,000 2
South Africa 18,000 1
Other countries 95,000 1
World total 2,573,000
Source: OECD/NEA Uranium 2007: Resources, Production and Demand (Red Book) 2008
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