Unless you're really into trivia about gas lanterns and the mantles that make their light so bright, you've probably never heard of thorium, but you may hear a lot more about it in the future. This unassuming metal could one day rival uranium as the nuclear fuel of choice.
What is thorium?
Discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, thorium is named after Thor, the Norse god of thunder. It is a slightly radioactive metal found in trace in rocks and soils all over the world and is particularly abundant in India and the state of Idaho.
Thorium has only one major isotope – 232Th – and its others only exist in minute traces. This isotope eventually decays into the lead isotope 208Pb. But what makes thorium interesting is that 232Th can easily absorb passing neutrons, turning it into 233Th. This new isotope, in a matter of minutes, emits an electron and an antineutrino to become 233Pa, an isotope of protactinium. With a half-life of 27 days, this then converts into a uranium isotope called 233U.
In other words, nuclear fuel.
The challenge is to design fuels and reactors that can produce more 233U than the reactor consumes. If this can be achieved, then thorium has an advantage over uranium, which cannot produce more fuel or "breed" in a conventional reactor. It's also possible to mix thorium and plutonium into a hybrid fuel, where uranium is produced as the plutonium is consumed.
The trick is to find the optimum mix and arrangement of the fuel to handle the neutrons and their absorption. Thorium also absorbs fast neutrons, so they can be used in fast molten salt and other Generation IV reactors that are now emerging, with uranium or plutonium fuel to initiate fission – though it doesn't work as well as 238U.
Thorium reactors
A number of thorium reactors have been built since 1960, starting with the thorium-based nuclear reactor at Oak Ridge National Laboratory and a few research reactors are in operation today. Today, thorium is seen by some as a thousand year solution to energy and environmental problems, but one that is offset by high start-up costs and a number of technical hurdles.
Part of the reason why development has been so slow is that uranium-based reactors and the infrastructure to support them had a long head start after the Second World War. The development of liquid-metal fast-breeder reactor (LMFBR) in the 1970s seemed much more promising than thorium for commercial applications and the US government largely abandoned thorium research after 1973.
By the early 21st century, many engineers in the field weren't even aware of thorium reactors. Today, there are a number of different thorium reactor designs under development, especially in India and China. Here's a look at some of the thorium reactors that are operating, being built, or are still on the drawing board.
Advanced Heavy Water Reactor (AHWR)
These are reactors where the neutrons are slowed down or moderated by heavy water, which is chemically identical to ordinary light water, but the hydrogen atoms are replaced by deuterium, which is hydrogen with an extra neutron (2H). Cooling is by light water naturally circulating in a pool driven by gravity.
Because thorium absorbs neutrons, it makes a very good fuel for AHWRs. In addition, the technology has already been used for decades in heavy water reactors like CANDU. Once the driver fuel has been replaced with recycled 233U, 80 percent of the energy produced is from the thorium cycle.
The latest Indian design, the AHWR-300 reactor, will use a thorium core when it comes on line at the Bhabha Atomic Research Centre (BARC), in Mumbai.
Aqueous Homogeneous Reactor (AHR)
Aqueous homogeneous reactors (AHR) differ from other reactors in that they have nuclear salts like uranium sulfate or uranium nitrate dissolved in either light or heavy water, which acts as fuel source, coolant, and moderator. By using heavy water, it's possible to introduce a soluble thorium salt into the mix.
Boiling Water Reactor (BWR)
As the name implies, boiling water reactors work by boiling the coolant water to produce steam to spin turbines. They have the advantage of having a flexible design with fuel rods of different lengths and compositions that can be arranged in the core to suit thorium-plutonium fuels. In these reactors, it's possible to configure the thorium elements to turn the BWR into a breeder reactor that produces more fuel than it consumes, which isn't normally possible with thermal neutron cores.
Pressurized Water Reactor (PWR)
Pressurized Water Reactors (PWR) are one of the most common nuclear reactors and use a core set in a pressure vessel to raise the water temperature. While it's possible to produce thorium fuel elements for these reactors, their design isn't very flexible and can't produce significant amounts of 233U.
Molten Salt Reactor (MSR)
Molten Salt Reactors (MSR) use a mixture of salts heated to up to 700 °C (1,292 °F) as both a coolant and a container for the nuclear fuel. In this case, a mixture of thorium fluoride and uranium fluoride mixed into the salts instead of contained in fuel rods. This not only makes the reactor more efficient, but removes the need for heavy structures to contain the reactor because it operates at atmospheric pressure and allows for passive safety systems in the event of a shutdown. In addition, the reactor can be regularly refueled and cleansed of byproducts through a chemical loop, and it has the potential to be a breeder reactor.
High-Temperature Gas-Cooled Reactor (HTR)
High-Temperature Gas-Cooled Reactors (HTR) are Generation IV reactors that use thorium-based fuels in the form of pebbles coated with pyrolytic carbon and silicon carbide layers, which retain fission gases, and then coated with graphite that acts as a moderator and protects the fuel from high temperatures. These pebble bed reactors are fed with fuel at the top and the spent pebbles are removed from the bottom. Cooling is through the circulation of inert helium gas.
Fast Neutron Reactor (FNR)
Fast Neutron Reactors (FNR) use fast neutrons instead of slow or thermal neutrons used in reactors of the conventional variety. This type of reactor doesn't need a moderator to function and it can burn thorium, but it can also use depleted uranium, which is in large supply and relatively cheap.
Accelerator Driven Reactor (ADS)
The Accelerator Driven Reactor (ADS) is a concept reactor that could use thorium mixed with plutonium. In this design, the fuel is kept at a lower density than would be needed to sustain a nuclear reaction. Instead, the fuel is bombarded with neutrons generated by a particle accelerator. This makes it very safe and produces very short-lived nuclear waste, but building an accelerator that's reliable enough for such a reactor remains a major obstacle.
Advantages & Disadvantages
Thorium as a future nuclear fuel offers a number of advantages and disadvantages compared to uranium. Not the least of these is that another fuel source would vastly increase available energy resources. Thorium is as abundant as lead in the Earth's crust and the supply in the United States could meet the country's energy needs for a thousand years, without the extensive enrichment needed for uranium fuels. In addition, some thorium reactor designs could produce less nuclear waste than current pressurized reactors, and the waste produced decays much faster than the isotopes from conventional fuels.
On the other side of the coin, developing a thorium nuclear power system would require expensive development and testing, which is difficult to justify, since uranium is relatively cheap and very little of the cost of building a reactor is in the fuel. In addition, uranium-based fuels would still be needed as a driver to start the nuclear reaction, which means that both the thorium and uranium infrastructures need to be preserved.
Then there is the matter of 233U, which is difficult to handle because of radiation issues because it contains traces of 232U, which is a very active gamma ray emitter.
Misconceptions
The idea of using thorium to produce energy has attracted a number of misconceptions and even outright conspiracy theories. Part of this is because many designs for thorium reactors are advanced Generation IV and breeder reactors.
This seems to have confused people into thinking all thorium reactors are something more advanced than uranium reactors, and that thorium and breeder reactors are synonymous. In some circles, this has elevated thorium into a wonder technology that's supposedly being suppressed by dark forces up to no good.
One persistent misconception is that thorium can't be used to make nuclear weapons and this is why the technology was abandoned. This is true if one is talking about thorium itself, but the 233U it produces can and has been used in a bomb, though it's too radioactive to be handled by anyone but experts and if the design isn't just right, the 233U will pre-detonate and the weapon won't function correctly.
Some have argued that thorium was suppressed by the Nixon administration because it couldn't be used to produce plutonium, which is used in nuclear weapons. This doesn't hold up, because the US has always kept its civilian and military nuclear programs strictly separate. Also, civilian reactors aren't suited to producing weapon-grade plutonium anyway.
In fact, thorium was largely given up on for economic reasons – the fuel was expensive to fabricate and uranium was still needed in the mix.
Another misconception is that there is more thorium than uranium. While it is true that there is three times as much thorium in the Earth's crust compared to uranium, thorium isn't soluble in water, while uranium is. This means that the oceans hold roughly five billion tonnes of uranium, as opposed to 6.4 million tons of thorium in the Earth's crust, and more will leach out of the crust into the sea as it is extracted.
Long story short, while thorium could power our civilization for thousands of years, if sea extraction becomes practical, uranium could power humanity until we have to move to another star because the Sun has grown too old.
However, thorium is abundant and readily accessible in places like India, which is taking advantage of its native supplies to build thorium reactors. At any rate, since most advanced nuclear reactors are breeders, the fuel question could quickly become moot.
This last bit is particularly important because, while thorium reactors produce much less long-term transuranic nuclear wastes than uranium reactors, fast neutron breeder reactors combined with reprocessing hold the same promise.
The future
Currently, thorium is enjoying a revival, with experiments on molten salt thorium technology in the Netherlands and reactors being built not only in India, but also in China and elsewhere. In a world becoming increasingly concerned about carbon emissions, calls to expand carbon-zero nuclear power's share of the world market are becoming stronger. It may well be that as Generation IV reactor technology comes on line, our energy will come from a grid with both uranium and thorium in the mix.
That is, if fusion power isn't made practical by then. If it is, all bets are off.
Update (Jan 4, 2022): This article originally stated Pa was the chemical symbol for palladium when it is the symbol for protactinium. We apologize for the error, which has now been corrected, and thank the readers who brought this to our attention.
