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The energy potential of the thorium fuel cycle

Thorium is a tantalising power source for future decades and generations. It is far more plentiful in nature then uranium. This would ensure a sustainable fuel cycle for thousands of years on a planet where a population of over nine billion is anticipated by 2050. It is fertile – not fissile – and can only be used in conjunction with fissile materials as a nuclear fuel. Like uranium, because of its energy density, it is far more cost effective than the hydro carbons and the so called “renewables”.

However a lot more basic physics and economic analysis is needed before commercial thorium based reactors become available. The lead in this area is currently held by India, China and Japan although the U.S.A., Canada, U.K. and Germany have experimented with thorium fuel over the past fifty years. I well remember my early visits to India and Bangladesh in the 1970’s to lecture on nuclear power and to discuss the technology of the potential thorium cycle at a time when that resource was being discovered in the monazite beach sands, of The Bay of Bengal (see Table 1).

Thorium (Th-232) is not of itself fissile and so is not directly usable in thermal neutron reactors – in this regard it is very similar to uranium-238. However, it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 (U-233), which is an excellent fissile fuel material. Thorium fuel concepts therefore require that Th-232 is first irradiated in a reactor to provide the necessary neutron dosing. The U-233 that is produced can either be chemically separated from the parent thorium fuel and recycled into new fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form.

The most common source of thorium is the rare earth phosphate mineral, monazite, which contains up to about 12% thorium phosphate, but only 6-7% on average. Monazite is found in igneous and other rocks but the richest concentrations are in placer deposits, concentrated by wave and current action with other heavy minerals. World monazite resources are estimated to be about 12 million tonnes, two-thirds of which are in heavy mineral sands deposits on the south and east coasts of India. There are substantial deposits in several other countries (see table below). Thorium recovery from monazite usually involves leaching with sodium hydroxide at 140oC followed by a complex process to precipitate pure ThO2.

Thorite (ThSiO4) is another common mineral. A large vein deposit of thorium and rare earth metals is to be found in Idaho. For Australian rare earths miners, thorium, at the present, time remain mainly a nuisance. Because of its intimate association with the rare earths and its mild radioactivity, rare-earths transport and extraction can become yet another focal point and issue for green pseudo-science and political activism for special interest groups! This author has long specialised in uranium, thorium and rare earths recovery from many sources ranging from sea-water to monazite. He has patented plant designs which can effect high purity separations based on ion-exchange chromatography, in parallel continuous streams.

Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium exists in nature in a single isotopic form – Th-232 – which decays very slowly (its half-life is about three times the age of the earth). The decay chains of natural thorium and uranium gives rise to minute traces of Th-228, Th-230 and Th-234, but the presence of these in mass terms is negligible.

When pure, thorium is a silvery white metal that retains its lustre for several months. However, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium oxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300oC). When heated in air, thorium metal turnings ignite and burn brilliantly with white light. Because of these properties, thorium has found applications in light bulb elements, lantern mantles, arc-light lamps, welding electrodes and heat-resistant ceramics. Glass containing thorium oxide has a high refractive index and dispersion and is used in high quality lenses for cameras and scientific instruments.

There are at least seven reactor types under development for the commercial utilisation of thorium fuel. The undersigned has been most closely connected with the two most advanced concepts – the Pressurised Heavy Water Reactor (PHWR) and the High Temperature Gas Cooled Reactor (HTGCR).

Heavy Water Reactors (PHWRs) are very well suited for thorium fuels due to their physical and mechanical characteristics. They have an excellent neutron economy (their low parasitic neutron absorption means more neutrons can be absorbed by thorium to produce useful U-233). They have a slightly faster average fission neutron energy which favours conversion to U-233. Finally they have a flexible on-line refuelling capability. Furthermore, heavy water reactors (especially the Canadian Candu) are well established and have a widely-deployed commercial technology for which there is extensive licensing experience.

High-Temperature Gas-Cooled Reactors (H.T.G.C.) are well suited for thorium- based fuels in the form of robust coated particles of thorium mixed with plutonium or enriched uranium, coated with pyrolytic carbon and silicon carbide layers which retain fission gases. The fuel particles are embedded in a graphite matrix that is very stable at high temperatures. Such fuels can be irradiated for very long periods and thus deeply burn to exploit their original fissile charge. Thorium fuels can be designed for both ‘pebble bed’ and ‘prismatic’ HTR fuel varieties.

The other reactor types which may one day utilise thorium as fuel may ultimately prove uneconomic and are still speculative. They include molten salt systems, fast breeder reactors and accelerator driven reactors. We conclude this review by studying the reaction of the two countries to thorium’s as yet untapped energy potential.

With huge resources of easily-accessible thorium and relatively little uranium, India has made utilisation of thorium for large-scale energy production a major goal in its nuclear power programme, utilising a three-stage concept:

1. Pressurised heavy water reactors (PHWRs) fuelled by natural uranium, plus light water reactors, producing plutonium.

2. Fast breeder reactors (FBRs) using plutonium-based fuel to breed U-233 from thorium. The blanket around the core will have uranium as well as thorium, so that further plutonium (particularly Pu-239) is produced as well as the U-233.

3. Advanced heavy water reactors (AHWRs) burn the U-233 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 programme has moved from aiming to be sustained simply with thorium to one ‘driven’ with the addition of further fissile plutonium from the FBR fleet, to give greater efficiency. In 2009, despite the relaxation of trade restrictions on uranium, India reaffirmed its intention to proceed with developing the thorium cycle.

A 500 MWe prototype FBR under construction in Kalpakkam is designed to produce plutonium to enable AHWRs to breed U-233 from thorium. India is focusing and prioritising the construction and commissioning of its sodium-cooled fast reactor fleet in which it will breed the required plutonium. This will take another 15-20 years and so it will still be some time before India is using thorium energy to a significant extent.

A different attitude prevails in, countries with a well established nuclear power programmes and with a society reasonably aware of its inherent safety, energy security and ability to offset greenhouse gas production in a cost-effective manner. These will proceed with caution into developing thorium systems. Consider the advice of the United Kingdom’s National Nuclear Laboratory (NNL) published in 2010.

“NNL believes that the thorium fuel cycle does not currently have a role to play in the UK context, other than its potential application for plutonium management in medium to long term. Depending on the indigenous thorium reserves, thorium fuel is likely to have only a limited role internationally for some years ahead. The technology is innovative, although technically immature and currently not of interest to the utilities. Representing significant financial investment and risk without notable benefits. In many cases, the benefits of the thorium fuel cycle have been over-stated.”

Professor Leslie G Kemeny is the Australian Foundation member of the International Nuclear Energy Academy. He is an internationally acknowledged expert in nuclear power plant design, safety and siting. Kemeny is a Fellow of Engineers Australia and the Australian Institute of Energy. He is also a Founding Member of the Australian Nuclear Association.

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