Unless you’re really interested in knowing about gas lanterns and the coats 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 humble metal could one day rival uranium as the nuclear fuel of choice.
What is Thorium
Thorium was discovered by the Swedish chemist Jons Jakob Berzelius in 1828 and is named after Thor, the Nordic god of thunder. It is a slightly radioactive metal found in traces in rocks and soils around the world and is particularly abundant in India and the state of Idaho.
Thorium has only one major isotope – 232Th – and its others exist only in tiny traces. This isotope eventually breaks down into the lead isotope 208Pb. But what makes thorium interesting is that 232Th can easily absorb passing neutrons and convert it to 233Th. This new isotope emits an electron and an antineutrino in a matter of minutes to become 233Pa, an isotope of palladium. With a half-life of 27 days, this then transforms into a uranium isotope called 233U.
In other words, nuclear fuel.
The challenge is to develop fuels and reactors that can produce more 233 U than the reactor consumes. If this can be achieved, thorium will have an advantage over uranium, which can no longer produce or “breed” fuel in a conventional reactor. It is also possible to mix thorium and plutonium into a hybrid fuel that produces uranium when plutonium is consumed.
The trick is to find the optimal mix and arrangement of fuel to handle the neutrons and their absorption. Thorium also absorbs fast neutrons, so in fast molten salts and other Generation IV reactors that are now being built, they can be used with uranium or plutonium fuel to initiate fission – although 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 several research reactors are in operation today. Thorium is now viewed by some as a millennial solution to energy and environmental problems, but it is offset by high start-up costs and a number of technical hurdles.
One of the reasons for the slow development is that uranium-based reactors and the infrastructure to support them had a long head start after World War II. The development of the 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.
At the beginning of the 21st century, many engineers in the field did not even know about thorium reactors. Today, a number of different thorium reactor concepts are being developed, especially in India and China. Here’s a look at some of the thorium reactors that are operating, being built, or still on the drawing board.
Advanced Heavy Water Reactor (AHWR)
These are reactors in which 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 additional neutron (2H). Cooling is provided by light water that naturally circulates in a gravity driven pool.
Since thorium absorbs neutrons, it is a very good fuel for AHWRs. In addition, the technology has been used in heavy water reactors such as CANDU for decades. After replacing the driver’s fuel with recycled 233U, 80 percent of the energy generated comes from the thorium cycle.
The latest Indian design, the AHWR-300 reactor, will use a thorium core when it goes online at the Bhabha Atomic Research Center (BARC) in Mumbai.
Aqueous homogeneous reactor (AHR)
Aqueous homogeneous reactors (AHR) differ from other reactors in that they have nuclear salts such as uranium sulfate or uranium nitrate dissolved in either light or heavy water, which acts as a fuel source, coolant and moderator. By using heavy water it is possible to incorporate a soluble thorium salt into the mixture.
Boiling water reactor (BWR)
As the name suggests, boiling water reactors work by boiling the cooling water to create steam for turning turbines. They have the advantage of a flexible design with fuel rods of different lengths and compositions, which can be arranged in the core to match thorium-plutonium fuels. In these reactors it is possible to configure the thorium elements so that the BWR becomes a breeder reactor that produces more fuel than it consumes, which is normally not 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 is possible to manufacture thorium fuel assemblies for these reactors, their design is not very flexible and cannot produce appreciable quantities of 233U.
Melting Salt Reactor (MSR)
Molten Salt Reactors (MSR) use a salt mixture heated 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 is mixed into the salts instead of contained in fuel rods. This not only makes the reactor more efficient, but also eliminates the need for heavy structures to enclose the reactor, as it operates at atmospheric pressure and enables passive safety systems in the event of a shutdown. In addition, the reactor can be regularly refueled and cleaned of by-products via a chemical cycle, and it has the potential to become a breeder reactor.
Gas-cooled high temperature reactor (HTR)
Gas-cooled high-temperature reactors (HTR) are reactors of the IV protects the fuel from high temperatures. These pebble bed reactors are charged with fuel at the top and the used pebbles are removed from below. The cooling takes place 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 type. This type of reactor does not need a moderator to function and it can burn thorium, but it can also use depleted uranium, which is available in large quantities and is relatively cheap.
Accelerator Powered 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 required 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 reliable enough for such a reactor remains a major obstacle.
advantages disadvantages
As a future nuclear fuel, thorium offers a number of advantages and disadvantages compared to uranium. Last but not least, a different fuel source would enormously increase the available energy resources. Thorium is as abundant in the earth’s crust as lead, and supplies in the United States could meet the country’s energy needs for a thousand years without the extensive enrichment required for uranium fuels. In addition, some thorium reactor designs could produce less nuclear waste than current pressure 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 plant 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 fuel. In addition, uranium-based fuels would still be needed to power the nuclear reaction, which means that both the thorium and uranium infrastructure must be preserved.
Then there is the 233U, which is difficult to use due to radiation issues as it contains traces of 232U, which is a very active gamma emitter.
misunderstandings
The idea of using thorium to produce energy has drawn a number of misunderstandings and even open conspiracy theories. This is in part because many thorium reactor designs are advanced Generation IV and breeder reactors.
This seems to have led people to think that all thorium reactors are a bit more advanced than uranium reactors, and that thorium and breeder reactors are synonymous. In some circles this has elevated thorium to a miracle technology that is supposedly uselessly suppressed by dark forces.
A persistent misconception is that thorium cannot be used to make nuclear weapons and that is why the technology has been abandoned. This is true when talking about thorium itself, but the 233U it produces can and has been used in a bomb, despite being too radioactive to be handled by anyone other than experts and if the design is not accurate is correct, the 233U will pre-detonate and the gun will not function properly.
Some have argued that thorium was suppressed by the Nixon government because it could not be used to make plutonium, which is used in nuclear weapons. That doesn’t last, because the US has always kept its civil and military nuclear programs strictly separate. In addition, civil reactors are in any case unsuitable for producing weapons-grade plutonium.
In fact, thorium was largely avoided for economic reasons – the fuel was expensive to produce and uranium was still needed in a mixture.
Another misconception is that there is more thorium than uranium. While it is true that the earth’s crust contains three times as much thorium as uranium, thorium, unlike uranium, is not water-soluble. That means the oceans contain about five billion tons of uranium, compared to 6.4 million tons of thorium in the earth’s crust, and more is leached out of the crust into the sea during extraction.
Long story short, while thorium could power our civilization for thousands of years, uranium could power humanity if extraction from the ocean becomes possible until we have to move to another star because the sun has gotten too old.
However, thorium is abundant and readily available in places like India, using its native supplies to build thorium reactors. In any case, since most advanced nuclear reactors are breeding grounds, the fuel issue could quickly become debatable.
This last point is particularly important because, while thorium reactors produce much less long-term transuranic nuclear waste than uranium reactors, rapid neutron breeder reactors combined with reprocessing promise the same thing.
The future
Thorium is currently experiencing a revival, with molten salt thorium technology experiments in the Netherlands and reactors being built not only in India but also in China and elsewhere. In a world that is increasingly concerned about CO2 emissions, calls for an increase in the world market share of carbon-free nuclear power are growing louder. It may well be that our energy for the commissioning of Generation IV reactor technology comes from a network with uranium and thorium in the mixture.
That is, if fusion energy is not practical by then. If so, all bets are disabled.
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