The ultimate energy output from U and hence indirectly thorium depends on numerous fuel design parameters, including: fuel burn-up attained, fuel arrangement, neutron energy spectrum and neutron flux affecting the intermediate product protactinium, which is a neutron absorber. The fission of a U nucleus releases about the same amount of energy MeV as that of U An important principle in the design of thorium fuel systems is that of heterogeneous fuel arrangement in which a high fissile and therefore higher power fuel zone called the seed region is physically separated from the fertile low or zero power thorium part of the fuel — often called the blanket.
Such an arrangement is far better for supplying surplus neutrons to thorium nuclei so they can convert to fissile U, in fact all thermal breeding fuel designs are heterogeneous.
This principle applies to all the thorium-capable reactor systems. Th is fissionable with fast neutrons of over 1 MeV energy. It could therefore be used in fast molten salt and other Gen IV reactors with uranium or plutonium fuel to initiate fission. However, Th fast fissions only one tenth as well as U, so there is no particular reason for using thorium in fast reactors, given the huge amount of depleted uranium awaiting use.
In Norway, Thor Energy is developing and testing a thorium-bearing fuel for use in existing nuclear power plants. Fuel rods containing thorium additive Th-Add and also thorium MOX with Pu fuel rods were tested in a five-year irradiation trial that started in April at the Halden test reactor. The company is working towards obtaining regulatory approval for the commercial production and use of Th-Add fuel. This fuel is promoted as a means to improve power profiles within commercial reactors. There are seven types of reactor into which thorium can be introduced as a nuclear fuel.
The first five of these have all entered into operational service at some point. The last two are still conceptual:. Heavy Water Reactors PHWRs : These are well suited for thorium fuels due to their combination of: i excellent neutron economy their low parasitic neutron absorption means more neutrons can be absorbed by thorium to produce useful U , ii slightly faster average neutron energy which favours conversion to U, iii flexible on-line refueling capability.
Furthermore, heavy water reactors especially CANDU are well established and widely-deployed commercial technology for which there is extensive licensing experience. Fleets of PHWRs with near-self-sufficient equilibrium thorium fuel cycles could be supported by a few fast breeder reactors to provide plutonium. 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 their original fissile charge. Boiling Light Water Reactors BWRs : BWR fuel assemblies can be flexibly designed in terms of rods with varying compositions fissile content , and structural features enabling the fuel to experience more or less moderation eg, half-length fuel rods.
This design flexibility is very good for being able to come up with suitable heterogeneous arrangements and create well-optimised thorium fuels. And importantly, BWRs are a well-understood and licensed reactor type. Fuel needs to be in heterogeneous arrangements in order to achieve satisfactory fuel burn-up.
It is not possible to design viable thorium-based PWR fuels that convert significant amounts of U Even though PWRs are not the perfect reactor in which to use thorium, they are the industry workhorse and there is a lot of PWR licensing experience. They are a viable early-entry thorium platform. Fast Neutron Reactors FNRs : Thorium can serve as a fuel component for reactors operating with a fast neutron spectrum — in which a wider range of heavy nuclides are fissionable and may potentially drive a thorium fuel.
There is, however, no relative advantage in using thorium instead of depleted uranium DU as a fertile fuel matrix in these reactor systems due to a higher fast-fission rate for U and the fission contribution from residual U in this material. Also, there is a huge amount of surplus DU available for use when more FNRs are commercially available, so thorium has little or no competitive edge in these systems.
Molten Salt Reactors MSRs : These reactors are still at the design stage but are likely to be very well suited for using thorium as a fuel. The level of moderation is given by the amount of graphite built into the core. Certain MSR designs c will be designed specifically for thorium fuels to produce useful amounts of U Spallation neutrons are produced d when high-energy protons from an accelerator strike a heavy target like lead.
These neutrons are directed at a region containing a thorium fuel, eg, Th-plutonium which reacts to produce heat as in a conventional reactor. The system remains subcritical ie, unable to sustain a chain reaction without the proton beam.
Difficulties lie with the reliability of high-energy accelerators and also with economics due to their high power consumption. See also information page on Accelerator-Driven Nuclear Energy. With regard to proliferation significance, thorium-based power reactor fuels would be a poor source for fissile material usable in the illicit manufacture of an explosive device.
U contained in spent thorium fuel contains U which decays to produce very radioactive daughter nuclides and these create a strong gamma radiation field. This confers proliferation resistance by creating significant handling problems and by greatly boosting the detectability traceability and ability to safeguard this material.
There have been several significant demonstrations of the use of thorium-based fuels to generate electricity in several reactor types. Over half of its , pebbles contained Th-HEU fuel particles the rest comprised graphite moderator and some neutron absorbers. These were continuously moved through the reactor as it operated, and on average each fuel pebble passed six times through the core. These were embedded in annular graphite segments not pebbles.
It also used thorium-HEU fuel in the form of microspheres of mixed thorium-uranium carbide coated with silicon oxide and pyrolytic carbon to retain fission products. The reactor core was housed in a reconfigured early PWR. Post-operation inspections revealed that 1. A NRC report quotes a breeding ratio of 1. Chemically reprocessing the fuel was not attempted. Indian heavy water reactors PHWRs have for a long time used thorium-bearing fuel bundles for power flattening in some fuel channels — especially in initial cores when special reactivity control measures are needed.
Research into the use of thorium as a nuclear fuel has been taking place for over 50 years, though with much less intensity than that for uranium or uranium-plutonium fuels.
Test irradiations have been conducted on a number of different thorium-based fuel forms. These two "new" ingredients were not chosen by accident by Beijing: molten-salt reactors are among the most promising technologies for power plants, according to the Generation IV forum — a US initiative to push for international cooperation on civil nuclear power.
Theoretically, this process would make the installations safer. There's another advantage for China: this type of reactor does not need to be built near watercourses, since the molten salts themselves "serve as a coolant, unlike conventional uranium power plants that need huge amounts of water to cool their reactors", French newspaper Les Echos noted. As a result, the reactors can be installed in isolated and arid regions… like the Gobi Desert.
Beijing has also opted to use thorium rather than uranium in its new molten-salt reactor, a combination that has drawn attention from experts for years. In addition, thorium belongs to a famous family of rare-earth metals that are much more abundant in China than elsewhere ; this is the icing on the cake for Chinese authorities, who could increase its energy independence from major uranium exporting countries, such as Canada and Australia, two countries whose diplomatic relations with China have collapsed in recent years.
But if the number of reactors increases, we could reach a situation where supply would no longer keep up, and using thorium can drastically reduce the need for uranium. That makes it a potentially more sustainable option," Sylvain David explained.
According to supporters of thorium, it would also a "greener" solution. It might sound like this kind of reactor is almost the stuff of science fiction, but just such a reactor was operated in the United States in the s and is currently being built in the Gobi Desert in China. Thorium was discovered by Jons Jakob Berzelius in , who named it after Thor, the Norse god of thunder.
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Nuclear Science. When Th absorbs a neutron it becomes Th, which is unstable and decays into protactinium and then into U That's the same uranium isotope we use in reactors now as a nuclear fuel, the one that is fissile all on its own.
Thankfully, it is also relatively long lived, which means at this point in the cycle the irradiated fuel can be unloaded from the reactor and the U separated from the remaining thorium.
The uranium is then fed into another reactor all on its own, to generate energy. The U does its thing, splitting apart and releasing high-energy neutrons. But there isn't a pile of U sitting by. Remember, with uranium reactors it's the U, turned into U by absorbing some of those high-flying neutrons, that produces all the highly radioactive waste products.
With thorium, the U is isolated and the result is far fewer highly radioactive, long-lived byproducts. Thorium nuclear waste only stays radioactive for years, instead of 10,, and there is 1, to 10, times less of it to start with.
Researchers have studied thorium-based fuel cycles for 50 years, but India leads the pack when it comes to commercialization. In , India's nuclear regulatory agency issued approval to start construction of a megawatts electric prototype fast breeder reactor, which should be completed this year.
In the next decade, construction will begin on six more of these fast breeder reactors, which "breed" U and plutonium from thorium and uranium. Design work is also largely complete for India's first Advanced Heavy Water Reactor AHWR , which will involve a reactor fueled primarily by thorium that has gone through a series of tests in full-scale replica. The biggest holdup at present is finding a suitable location for the plant, which will generate MW of electricity.
Indian officials say they are aiming to have the plant operational by the end of the decade. China is the other nation with a firm commitment to develop thorium power. This molten salt blanket becomes less dense as temperatures rise, slowing the reaction down in a sort of built-in safety catch. This kind of thorium reactor gets the most attention in the thorium world; China's research program is in a race with similar though smaller programs in Japan, Russia, France, and the U.
There are at least seven types of reactors that can use thorium as a nuclear fuel, five of which have entered into operation at some point. Several were abandoned not for technical reasons but because of a lack of interest or research funding blame the Cold War again. So proven designs for thorium-based reactors exist and need but for some support.
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