Uranium Fuel Substitute Potential

Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by a Swedish chemist, Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. The silvery white metal is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. Typical garden variety soil commonly contains an average of around 6 parts per million (ppm) of thorium.

Applications

Thorium oxide, also called thoria, has one of the highest melting points of all oxides at 3300°C. When this oxide is heated in air, thorium metal turnings ignite and burn brilliantly with a white light. Because of these properties, thorium has found applications in welding electrodes, heat-resistant ceramics, light bulb elements, lantern mantles and arc-light lamps. Glass containing thorium oxide has a high refractive index and dispersion and is used in high quality lenses for cameras and scientific instruments.

Sources and geographical distribution

The most common source of thorium is the rare earth phosphate mineral, monazite, which may contain up to about 12 percent thorium phosphate; however, the average is closer to a 6-7 percent range. 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. Australia is estimated by the USGS to host approximately 24 percent of the world’s thorium reserves.  A large vein deposit of thorium and rare earth metals have been discovered in the Lemhi Pass region of Idaho and Montana.

Going nuclear

Although not fissile itself, thorium has started to reemerge as a tempting prospect to employ as fuel in nuclear power reactors. Thorium 232 will absorb slow neutrons to produce uranium 233, which is fissile (and long-lived). The irradiated fuel can then be unloaded from the reactor, the uranium 233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle. Alternatively, uranium 233 can be bred from thorium in a blanket, the uranium 233 separated, and then fed into the core.

The use of thorium-based fuel cycles has been studied for about 40 years, but on a much smaller scale than uranium or uranium/plutonium cycles. Basic research and development has been conducted in Germany, India, Japan, Russia, the UK and the USA.  China and India have been among primary catalysts in research efforts to use it. Test reactor irradiation of thorium fuel to high burn-ups has also been conducted and several test reactors have either been partially or completely loaded with thorium-based fuel.

Thorium can be used in Generation IV and other advanced nuclear fuel cycle systems. China has been working on developing the technology for sodium cooled fast reactors which are a type of liquid fluoride thorium reactors (LFTRs). The advanced breeder concept features a molten salt as the coolant, usually a fluoride salt mixture. This is hot, but not under pressure, and does not boil below about 1400°C. Much research has focused on lithium and beryllium additions to the salt mixture. In mid-2009, AECL signed agreements with three Chinese entities to develop and demonstrate the use of thorium fuel in the Candu reactors at Qinshan in China.

India is working on adapting heavy water reactors in order to effectively secure domestic long term energy requirements and make use of their abundant supply of thorium for prospective commercial international energy solutions. The technological development would harness external innovation in both equipment and fuel that would allow India to use its ample indigenous supply of thorium.

Areva Group (EPA:CEI) and Lightbridge Corporation (NASDAQ:LTBR), agreed in 2009 to collaborate on earlier research efforts to assess the use of thorium fuel in Areva’s Pressurized Water Reactor (EPR). Other endeavours include the development of the Radkowsky Thorium Reactor concept being carried out as a joint venture involving Lightbridge aligned with a Russian collaboration.

Well-known mining and exploration analyst and geologist Mickey Fulp summarized some disadvantages of the challenges that the potential of thorium could possess in substitution for uranium as a fuel for the nuclear industry:  U233 can be used in nuclear weapons; some long-lived radionuclides; and complicated fuel fabrication. As a matter of course, he qualified the initial consideration regarding U233, “although it is not commonly used and it has never been used to any great extent, it can be used to make an atomic bomb.”  In a recently published paper he summarizes, “Perhaps the most promising niche for thorium-fueled electrical power is in small modular reactors designed for remote locations. In my opinion, thorium will supplement base load electrical generation within the next decade or so but it will not replace uranium-fueled nuclear power in our lifetimes.”

Sense of urgency

With the world progressively cognizant and sensitive to global warming, the news that 2010 was a record year for greenhouse gases levels was yet another undesirable milestone. The earth’s population is estimated to hit nine billion by 2050, which underscores the increasing urgency of delivering safer, cleaner, reliable and renewable sources of energy. In reconciling competing economic, social, environmental and political agendas the future of nuclear technology will be of significant interest in the coming decades. Much development work appears to be required before the thorium fuel cycle can be commercialised, and the effort required may be unlikely while (or where) abundant uranium is available. 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 and should be a consideration in the sustainability of nuclear energy.