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PowerPedia:Thorium Reactors

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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 commonly contains an average of around 6 parts per million (ppm) of thorium. World Nuclear Association

Thorium is used to make ceramics, gas lantern mantles, and metals used in the aerospace industry and in nuclear reactions. Thorium can also be used as a fuel for generating nuclear energy. Agency for Toxic Substances and Disease Registry

Thorium itself is a metal in the actinide series, which is a run of 15 heavy radioactive elements that occupy their own period in the periodic table between actinium and lawrencium. Thorium sits on the periodic table two spots to the left (making it lighter) of the only other naturally occurring actinide, uranium (which is two spots to the left of synthetic plutonium).

It can't sustain a nuclear reaction once it has been started. This means the U-233 produced at the end of the thorium fuel cycle doesn't pump out enough neutrons when it splits to keep the reaction self-sustaining: eventually the reaction fizzles out. It's why a reactor using thorium fuel is often called a 'sub-critical' reactor. Cosmos




Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide, but average 6-7%. There are substantial deposits in several countries (see table). Thorium-232 decays very slowly (its half-life is about three times the age of the earth) but other thorium isotopes occur in its and in uranium's decay chains. Most of these are short-lived and hence much more radioactive than Th-232, though on a mass basis they are negligible.

A thorium reactor, which operates at relatively low temperatures (<1000 K), has no magnets, vacuum chamber or high-pressure systems, and no huge superstructure holding it together. A liquid-fluoride thorium reactor is very power dense, compared to fusion, meaning that it physically has a smaller "footprint" and could conceivably be built small enough to fit in submarines or trailers.

Right now, thorium is so "worthless" that the US government buried 3200 metric tonnes of it in the Nevada desert due to lack of demand.

All of my research points me to the liquid-fluoride reactor as the machine that can make thorium useful. Fluoride reactor technology was developed and demonstrated in the United States at Oak Ridge National Lab. But because it threatened the AEC's commitment to sodium-cooled plutonium fast-breeder reactors, the AEC killed it in 1974.

Kirk Fredrick Sorensen


  • Thorium is combined with magnesium alloys and in Tungsten filaments for light bulbs and in electronic tubes
  • Ceramic items, such as lab crucibles, become more heat resistant by adding Thorium
  • Makes carbon arc-light lamps burn brighter
  • Added to bulb filaments, it helps sun lamps mimic the light emitted from the sun
  • Makes Tungsten welding electrodes burn hotter
  • Forms strong but less brittle alloys
  • Thoria - Thorium oxide (ThO2) is added to help gas mantles burn hotter and brighter
  • Added to make high refractive glass, camera and binocular lenses. See NCRP Report No. 95 "Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources," section (page 47)
  • A catalyst for the oxidation of ammonia to nitric acid and other industrial chemical reactions
  • One of the breeded reactor fuels (see the Thorium Fuel Cycle)
  • Used to date very old materials, e.g. seabeds or mountain ranges The Complete Thorium Website



  • THORIUM: CLEANER NUCLEAR POWER? - Uranium-based reactors can be retrofitted, bringing major benefits –
    • improving security
    • allaying environmental concerns
    • improving economics
    • The fuel cycle can also be proliferation resistant, stopping a reactor from producing nuclear weapons-usable plutonium
    • With the spent fuel having significantly reduced volume, weight and long-term radio-toxicity, safety margins are increased and operating costs reduced (Power Technology; Aug. 7, 2007)


  • Thorium mining produces waste products like mill tailings which can escape into the environment and as with uranium, thorium mining produces radioactive mining residues containing natural long-lived natural decay products.
  • Besides being relatively costly to produce, the thorium can be difficult to reprocess. The highly-radioactive U-233 has higher alpha activity than U-235 which makes it more dangerous, and separating it on its own still allows some weapons proliferation risk. U-233 can also be contaminated with U-232 traces, decaying to gamma emitters with very short half lives.
  • Thorium itself can be difficult to recycle because it is contaminated by highly radioactive Th-228 and despite the lower overall radioactivity, the waste from thorium plants will still take several hundred years to decay to safe levels. (Power Technology; Aug. 7, 2007)
  • Powdered thorium metal is often pyrophoric and should be handled carefully. Natural thorium decays very slowly compared to many other radioactive materials, and the alpha radiation emitted cannot penetrate human skin. Owning and handling small amounts of thorium, such as a gas mantle, is considered safe if care is taken not to ingest the thorium -- lungs and other internal organs can be penetrated by alpha radiation.
  • Exposure to aerosolized thorium can lead to increased risk of cancers of the lung, pancreas and blood. Exposure to thorium internally leads to increased risk of liver diseases. Thorium

Types of Reactors

Liquid Flouride Thorium Reactor

The liquid fluoride fuel form remains liquid even under extreme temperatures without boiling. This allows the reactor to run at atmospheric pressure, making it much safer than current high-pressure water cooled reactors.

I've never understood why Carlo Rubbia took a perfectly good liquid-fluoride reactor and complicated it so terribly by trying to add a particle accelerator, and then marketed it to the world as an "energy amplifier". All he did was reduce the energy "gain" by a few orders of magnitude and increased the costs. If safety was his argument, then he should have studied how strong the negative temperature coefficient of reactivity can be in a properly-designed liquid fluoride reactor and he would realize the accelerator was superfluous." Kirk Sorensen

Molten Salt Reactor

It is not true that Thorium can't sustain a chain reaction on its own, because the U-233 doesn't pump out enough neutrons to sustain the chain reaction. U-233 produces something like 2.4 neutrons on average in a low-speed (a.k.a. thermal) fission. 1 of those sustains the chain reaction, 1 is used to convert Th-232 to Th-233 and thence to U-233, leaving 0.4 to be wasted.

Thorium Power Inc is proposing to put solid-phase thorium oxide fuel into pressurized water reactors. In this configuration, more than 0.4 neutrons from every U-233 fission will end up absorbed somewhere unhelpful, and the Thorium fuel rod needs something else to boost the reactivity so that it sustains the chain reaction. Pu-239 works well.

In the molten salt reactor, there are fewer things to absorb the neutrons (less structure, no coolant water, and fewer fission products), so the reactor can break even without help from the Pu-239.

It is the ability of molten salt reactors to separate fission products from fuel while operating that makes it possible for them to burn all their actinides, and thus have shorter-term waste.

Advanced Heavy Water Reactor

The Advanced Heavy Water Reactor (AHWR) is a proposed heavy water moderated nuclear power reactor that will be the next generation design of the PHWR type. It is now being developed at Bhabha Atomic Research Centre (BARC) and aims to meet the objectives of using thorium fuel cycles for commercial power generation.

Liquid Flouride Thorium Reactor

The liquid fluoride fuel form remains liquid even under extreme temperatures without boiling. This allows the reactor to run at atmospheric pressure, making it much safer than current high-pressure water cooled reactors.

I've never understood why Carlo Rubbia took a perfectly good liquid-fluoride reactor and complicated it so terribly by trying to add a particle accelerator, and then marketed it to the world as an "energy amplifier". All he did was reduce the energy "gain" by a few orders of magnitude and increased the costs. If safety was his argument, then he should have studied how strong the negative temperature coefficient of reactivity can be in a properly-designed liquid fluoride reactor and he would realize the accelerator was superfluous." Kirk Sorensen

Subcritical Reactor

  • Subcritical reactor - A subcritical reactor is a nuclear fission reactor that produces fission without achieving criticality. Instead of a sustaining chain reaction, a subcritical reactor uses additional neutrons from an outside source. A reactor coupled to a particle accelerator to produce neutrons by spallation is called an Accelerator-Driven System (ADS). See Carlo Rubbia.

Pebble Bed Reactor

A pebble bed power plant combines a gas-cooled core and a novel packaging of the fuel that dramatically reduces complexity while improving safety. The uranium, thorium or plutonium nuclear fuels are in the form of a ceramic (usually oxides or carbides) contained within spherical pebbles made of pyrolytic graphite, which acts as the primary neutron moderator. Each sphere is effectively a complete "mini-reactor", containing all of the parts that would normally be separate components of a conventional reactor. Simply piling enough of the fuel spheres together will eventually reach criticality.



In AECL’s fuel-cycle vision, thorium fuel cycles ensure a long-term fuel supply using CANDU (CANadian Deuterium Uranium) reactors. Possible CANDU thorium fuel cycles include open cycles, such as Once-Through Thorium (OTT) fuel cycles, and closed cycles, which involve reprocessing the used fuel and recycling the separated 233U. For the longer term, AECL is proposing the Self-sufficient Equilibrium Thorium fuel cycle, which breeds enough 233U that – through its recycle – is sufficient to keep the fuel cycle running indefinitely, without the need for an additional, external supply of fissile material.


The 300 MWe THTR (Thorium High-Temperature Reactor) in Germany was developed from the AVR and operated between 1983 and 1989 with 674,000 pebbles, over half containing Th/HEU fuel (the rest graphite moderator and some neutron absorbers). These were continuously recycled on load and on average the fuel passed six times through the core. Fuel fabrication was on an industrial scale.

The 60 MWe Lingen Boiling Water Reactor (BWR) in Germany utilised Th/Pu-based fuel test elements.

Between 1967 and 1988, the AVR experimental pebble bed reactor (PBR) at Julich, Germany, operated for over 750 weeks at 15 MWe, about 95% of the time with thorium-based fuel. The fuel used consisted of about 100 000 billiard ball-sized fuel elements. Overall a total of 1360 kg of thorium was used, mixed with high-enriched uranium (HEU). Maximum burnups of 150,000 MWd/t were achieved.


  • AHWR critical facility commissioned at BARC - The advanced heavy water reactor (AHWR) would use thorium, “the fuel of the future�?. It would be powered by the naturally available thorium and the fissile material, uranium-233. (The Hindu; April 9, 2008)
  • Thorium Reactors Integral To Indian Energy Independence - President A.P.J. Abdul Kalam said that India is determined to achieve energy independence by the year 2030 and for this "India has to go for nuclear power generation in a big way using thorium based reactors." ([Energy Daily; May 8, 2007)

In India, both Kakrapar-1 and -2 units are loaded with 500 kg of thorium fuel in order to improve their operation when newly-started. Kakrapar-1 was the first reactor in the world to use thorium, rather than depleted uranium, to achieve power flattening across the reactor core. In 1995, Kakrapar-1 achieved about 300 days of full power operation and Kakrapar-2 about 100 days utilising thorium fuel. The use of thorium-based fuel was planned in Kaiga-1 and -2 and Rajasthan-3 and -4 (Rawatbhata) reactors.

In India, the Kamini 30 kWth experimental neutron-source research reactor using 233U, recovered from ThO2 fuel irradiated in another reactor, started up in 1996 near Kalpakkam. The reactor was built adjacent to the 40 MWt Fast Breeder Test Reactor, in which the ThO2 is irradiated.


In the Netherlands, an aqueous homogenous suspension reactor operated at 1 MWth for three years. The HEU/Th fuel is circulated in solution, and reprocessing occurs continuously to remove fission products, resulting in a high conversion rate to 233U.


Russia has a long running thorium-uranium fuel project based at Moscow's Kurchatov Institute. Thorium Power is involved in this, along with the US government which is providing funding. The program should produce fuel for Russian VVER-1000 reactors instead of the standard enriched uranium oxide.

A demountable centre portion has the plutonium, and the blanket arrangement surrounding it has the thorium / uranium. Taken together, the seed and blanket are the same size as a normal VVER-1000 fuel assembly. The central seed fuel rods are based on extensive experience of Russian navy reactor design and are burned for three years (as normal for VVERs). The blanket material stays in the reactor for nine years. The process produces about half the spent fuel of MOX (mixed oxide) fuel plants and contains less fissile plutonium.


Thorium fuel elements with a 10:1 Th/U (HEU) ratio were irradiated in the 20 MWth Dragon reactor at Winfrith, UK for 741 full power days. Dragon was run as an OECD/Euratom cooperation project, involving Austria, Denmark, Sweden, Norway and Switzerland in addition to the UK, from 1964 to 1973. The Th/U fuel was used to 'breed and feed', so that the 233U formed replaced the 235U at about the same rate, and fuel could be left in the reactor for about six years.


The Fort St Vrain reactor was the only commercial thorium-fuelled nuclear plant in the USA, also developed from the AVR in Germany, and operated 1976 - 1989. It was a high-temperature (700°C), graphite-moderated, helium-cooled reactor with a Th/HEU fuel designed to operate at 842 MWth (330 MWe). The fuel was in microspheres of thorium carbide and Th/U-235 carbide coated with silicon oxide and pyrolytic carbon to retain fission products. It was arranged in hexagonal columns ('prisms') rather than as pebbles. Almost 25 tonnes of thorium was used in fuel for the reactor, and this achieved 170,000 MWd/t burn-up.

Thorium-based fuel for Pressurised Water Reactors (PWRs) was investigated at the Shippingport reactor in the USA using both U-235 and plutonium as the initial fissile material. It was concluded that thorium would not significantly affect operating strategies or core margins. The light water breeder reactor (LWBR) concept was also successfully tested here from 1977 to 1982 with thorium and U-233 fuel clad with Zircaloy using the 'seed/blanket' concept.

Other Resources


See Discussion page

Green Atomic Energy?

On May 16, 2008, New Energy Congress member, Sepp Hasslberger wrote:

Through another list, I came across this site, which proposes to convert current atomic reactors to run on Thorium rather than Uranium.

Although atomic energy is not my favorite technology for the future, I find it intriguing that:

  1. such a conversion seems possible with little technical trouble
  2. reactors running on thorium would be much less polluting than current ones
  3. changing reactor technology would take wind out of the sails of wartime use of atoms - both bombs and DU munitions.
  4. such reactors could be downsized considerably and promise to be non-critical, i.e. they could not get out of control.

As such, I think we should at least be aware of this option.

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