Thorium is destined to eclipse Uranium as the nuclear fuel of choice. Thorium cannot melt down due to its fertile nature. Due to Thorium’s high burn-up rate nuclear waste is dramatically reduced by a factor of 90%. Thorium reactors are nuclear weapons proliferation resistant. It is virtually impossible to make nuclear weapons from thorium reactors as demonstrated by the US in the 1950’s – ‘60’s. Due to Thorium’s nature we can now make smaller more efficient reactors in a modular design, dramatically reducing manufacturing and operating cost.
Ideas for using thorium have been around since the 1960s, and by 1973 there were proposals for serious, concerted research in the US. But that program fizzled to a halt only a few years later. Why? The answer is nuclear weapons. The 1960s and ’70s were the height of the Cold War and weaponization was the driving force for all nuclear research. Any nuclear research that did not support the US nuclear arsenal was simply not given priority.
Conventional nuclear power using a fuel cycle involving uranium-235 and/or plutonium-239 was seen as killing two birds with one stone: reducing America’s dependence on foreign oil, and creating the fuel needed for nuclear bombs. Thorium power, on the other hand, didn’t have military potential. And by decreasing the need for conventional nuclear power, a potentially successful thorium program would have actually been seen as threatening to U.S. interests in the Cold War environment.
Today, however, the situation is very different. Rather than wanting to make weapons, many global leaders are worried about proliferating nuclear technology. And that has led several nations to take a closer look at thorium power generation.
- Thorium is more abundant in nature than uranium.
- It is fertile rather than fissile, and can only be used as a fuel in conjunction with a fissile material such as recycled plutonium.
- Thorium fuels can breed fissile uranium-233 to be used in various kinds of nuclear reactors.
- Molten salt reactors are well suited to thorium fuel, as normal fuel fabrication is avoided.
The use of thorium as a new primary energy source has been a tantalizing prospect for many years. Extracting its latent energy value in a cost-effective manner remains a challenge, and will require considerable R&D investment. This is occurring preeminently in China, with modest US support.
Nature and sources of thorium
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 contains an average of around 6 parts per million (ppm) of thorium. Thorium is very insoluble, which is why it is plentiful in sands but not in seawater, in contrast to uranium.
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 give rise to minute traces of Th-228, Th-230 and Th-234, but the presence of these in mass terms is negligible. It decays eventually to lead-208.
When pure, thorium is a silvery white metal that retains its lustre for several months. However, when it is contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and eventually black. When heated in air, thorium metal ignites and burns brilliantly with a white light. Thorium oxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300°C) and so it has found applications in light bulb elements, lantern mantles, arc-light lamps, welding electrodes and heat-resistant ceramics. Glass containing thorium oxide has both a high refractive index and wavelength dispersion, and is used in high quality lenses for cameras and scientific instruments.
Thorium oxide (ThO2) is relatively inert and does not oxidise further, unlike UO2. It has higher thermal conductivity and lower thermal expansion than UO2, as well as a much higher melting point. In nuclear fuel, fission gas release is much lower than in UO2.
The most common source of thorium is the rare earth phosphate mineral, monazite, which contains up to about 12% thorium phosphate, but 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 16 million tonnes, 12 Mt 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 140°C followed by a complex process to precipitate pure ThO2. Thorite (ThSiO4) is another common thorium mineral. A large vein deposit of thorium and rare earth metals is in Idaho.
The IAEA-NEA publication Uranium 2014: Resources, Production and Demand (often referred to as the Red Book) gives a figure of 6.2 million tonnes of total known and estimated resources. Data for reasonably assured and inferred resources recoverable at a cost of $80/kg Th or less are given in the table below, excluding some less-certain Asian figures. Some of the figures are based on assumptions and surrogate data for mineral sands (monazite x assumed Th content), not direct geological data in the same way as most mineral resources.
Thorium: Cleaner Nuclear Power?
Decades ago, many countries abandoned the idea of using thorium as a replacement for uranium. But long-term proponents have always believed the thorium fuel cycle could make nuclear energy as safe and sustainable as possible.
But now there are new concerns pushing the thorium debate that revolve around secure uranium supplies and nuclear proliferation – these are encouraging research and development around the world. And then there are nations like India, which has said it aims to base its future nuclear industry on the fuel source.
The attraction for the likes of India are the several major advantages that thorium can claim over uranium. Thorium is seen by some as the nuclear fuel of the future. For a start, there is much more thorium than uranium in the Earth’s crust, and all the thorium mined can be used in a reactor (compared to below 1% of natural uranium). Thorium fuel cycles also produce much less plutonium and other radioactive transuranic elements than uranium fuel cycles.
Uranium-based reactors can be retrofitted, bringing three major benefits – improving security, allaying environmental concerns and improving economics. The fuel cycle can also be proliferation resistant, stopping a reactor from producing nuclear weapons-usable plutonium. And with the spent fuel having significantly reduced volume, weight and long-term radio-toxicity, safety margins are increased and operating costs reduced.
Thorium could power the next generation of nuclear reactors
A Dutch nuclear research institute has just fired up the first experiment in nearly half a century on next-generation molten-salt nuclear reactors based on thorium.
Thorium has long held promise for “safer” nuclear power. A slightly radioactive element, it transforms into fissionable U-233 when hit by high-energy neutrons. But after use, U-233 creates fewer long-lived radioactive waste products than the conventional U-235 now used in nuclear power plants.
But because nuclear power was traditionally tied up with nuclear weapons research into uranium and plutonium, thorium was mostly abandoned. Except for one test reactor that has been under construction at Kalpakkam since 2004, thorium reactor research has been moribund.
But now, NRG, a nuclear research facility in Petten, on the North Sea coast of the Netherlands, has launched the Salt Irradiation Experiment (SALIENT) in collaboration with the EU Commission. The researchers want to use thorium as a fuel for a molten salt reactor, one of the next-generation designs for nuclear power in which both the reactor coolant, and the fuel itself, are a mixture of hot, molten salt.
Many believe that molten salt reactors are well suited for using thorium as a fuel. Their unique working fluid can achieve very high temperatures, significantly boosting the efficiency of the power generation process.
The Petten team will use the facility’s reactor to melt a sample of thorium fuel and then bombard it with neutrons to transmute the thorium into U-233, which can sustain the chain reaction needed to generate energy.
A later step is to study tough, temperature-resistant metal alloys and other materials that can survive the high heat and corrosive conditions inside a molten-salt reactor. Eventually, they’ll need to examine how to deal with the waste from a molten salt thorium reactor. While largely considered safer than the long-lived products from a standard nuke, these will still need special disposal.