Thorium is an abundant element in nature with multiple advantages as a nuclear fuel for future reactors of all types. Thorium ore, or monazite, exists in vast amounts in the dark beach sand of India, Australia, and Brazil. It is also found in large amounts in Norway, the United States, Canada, and South Africa. Thorium-based fuel cycles have been studied for about 30 years, but on a much smaller scale than uranium or uranium/plutonium cycles. Germany, India, Japan, Russia, the UK, and the USA have conducted research and development, including irradiating thorium fuel in test reactors to high burn-ups. Several reactors have used thorium-based fuel, as discussed below. |
India is by far the nation most committed to the study and use of thorium fuel. |
India is by far the nation most committed to the study and use of thorium fuel; no other country has done as much neutron physics work on thorium. The positive results obtained in this neutron physics work have motivated the Indian nuclear engineers to use thorium-based fuels in their current plans for the more advanced reactors that are now under construction. |
India decided on a three-stage nuclear programme back in the 1950s, when its nuclear power generation programme was set up. In the first stage, natural uranium (U-238) was used in pressurised heavy water reactors (PHWRs), of which there are now 12. In the second stage, the plutonium extracted from the spent fuel of the PHWRs was scheduled to be used to run fast breeder reactors. The fast breeders would burn a 70 per cent mixed oxide (MOX) fuel to breed fissile uranium-233 (U-233) in a thorium-232 (Th-232) blanket around the core. In the final stage, the fast breeders would use Th-232 and produce U-233 for use in new reactors. |
One main advantage of using a combination of thorium and uranium is related to the proliferation question: There is a significant reduction in the plutonium content of the spent fuel, compared with what comes out of a conventional uranium-fuelled reactor. |
Just how much less plutonium is made? The answer depends on exactly how the uranium and thorium are combined. For example, uranium and thorium can be mixed homogeneously within each fuel rod, and in this case the amount of plutonium produced is roughly halved. But mixing them uniformly is not the only way to combine the two elements, and the mix determines the plutonium production. |
Top Indian Initiatives The Indian authorities have approved the Department of Atomic Energy's proposal to set up a 500 MW prototype of the next-generation fast breeder nuclear power reactor at Kalpakkam, thereby setting the stage for the commercial exploitation of thorium as a fuel source. |
To a certain extent, India has completed the first stage of its nuclear programme, putting on line a dozen nuclear power plants so far, with a few more plants now in the construction process. The second stage is as yet realised only by a small experimental fast breeder reactor (13 megawatts), at Kalpakkam near Chennai in South India. Meanwhile, the Indian authorities have approved the Department of Atomic Energy's proposal to set up a 500 MW prototype of the next-generation fast breeder nuclear power reactor at Kalpakkam, thereby setting the stage for the commercial exploitation of thorium as a fuel source. |
India's commitment to switchover to thorium stems, in part, from its large indigenous thorium supply. India's estimated thorium reserves are 290,000 tonnes, second only to Australia's but the nation's choice of thorium, which helps bring it independence from overseas uranium sources, came about for a reason that has nothing to do with its balance of trade. |
India is not a signatory to the Nuclear Non-Proliferation Treaty (NPT). Hence, India foresaw that it would be constrained in the long-term by the provisions laid down by the commercial uranium suppliers, which would jeopardise India's nuclear power generation programme. The 44-member nuclear suppliers group requires that purchasers sign the NPT, and thereby allow enough oversight to ensure that the fuel (or the plutonium spawned from it) is not used for making nuclear weapons. |
India began the construction on the facility for reactor physics of the Advanced Heavy Water Reactor (AHWR) last year. The AHWR will use thorium, the ‘fuel of the future’, to generate 300 megawatts of electricity, up from its original design output of 235 megawatts. The reactor will have a lifetime of 100 years, and is scheduled to be built on the campus of India's main nuclear research and development centre, the Bhabha Atomic Research Centre (BARC) at Trombay. |
The construction of the AHWR will mark the beginning of the third phase of India's nuclear electricity generation programme. The fuel for the AHWR will be a hybrid core, partly thorium/U-233, and partly thorium-plutonium. The reactor will be a technology demonstrator for thorium utilisation. According to B. Bhattacharjee, Director of the Bhabha Atomic Research Centre, “At the international level, the AHWR has been selected for a case study at the International Atomic Energy Agency (IAEA) for acceptance as per international standards for next generation reactors.” |
Top Abundance of Thorium The thorium fuel cycle itself has many attractive features. To begin with, thorium is more abundant in nature than uranium. Soil commonly contains an average of around 6 parts per million (ppm) of thorium, three times as much as uranium. |
Although India's embrace of thorium as its future nuclear fuel is based mostly on necessity, the thorium fuel cycle itself has many attractive features. To begin with, thorium is more abundant in nature than uranium. Soil commonly contains an average of around 6 parts per million (ppm) of thorium, three times as much as uranium. Thorium occurs in several minerals, the most common being the rare earth thorium-phosphate mineral, monazite, which usually contains from 3 to 9 per cent, and sometimes up to 12 per cent thorium oxide. In India, the monazite is found in the southern beach sands. |
Th-232 decays very slowly (its half life is about three times the age of the Earth). Most other thorium isotopes are short-lived and thus more radioactive than Th-232, but of negligible quantity. |
In addition to thorium's abundance, all of the mined thorium is potentially usable in a reactor, compared with only 0.7 per cent of natural uranium. In other words, thorium has some 40 times the amount of energy per unit mass that could be made available, compared with uranium. |
From the technological angle, one reason that thorium is preferred over enriched uranium is that the breeding of U-233 from thorium is more efficient than the breeding of plutonium from U-238. |
From the technological angle, one reason that thorium is preferred over enriched uranium is that the breeding of U-233 from thorium is more efficient than the breeding of plutonium from U-238. This is so because the thorium fuel creates fewer non-fissile isotopes. Fuel-cycle designers can take advantage of this efficiency to decrease the amount of spent fuel per unit of energy generated, which reduces the amount of waste to be disposed of. |
There are some other benefits. For example, thorium oxide, the form of thorium used for nuclear power, is a highly stable compound—more so than the uranium dioxide that is usually employed in today's conventional nuclear fuel. Also, the thermal conductivity of thorium oxide is 10–15 per cent higher than that of uranium dioxide, making it easier for heat to flow out of the fuel rods used inside a reactor.
WORLD THORIUM RESOURCES (economically extractable) | | Country Reserves | (Tonnes) |
| | Australia | 300,000 | | India | 290,000 | | Norway | 170,000 | | USA | 160,000 | | Canada | 100,000 | | South Africa | 35,000 | | Brazil | 16,000 | | Other countries | 95,000 | | World Total | 1,200,000 |
| | Source: US Geological Survey, Mineral Commodity Summaries, January 1999. |
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In addition, the melting point of thorium oxide is about 500 degrees Celsius higher than that of uranium dioxide, which gives the reactor an additional safety margin, if there is a temporary loss of coolant. |
The one challenge in using thorium as a fuel is that it requires neutrons to start off its fission process. |
The one challenge in using thorium as a fuel is that it requires neutrons to start off its fission process. These neutrons can be provided by the conventional fissioning of uranium or plutonium fuel mixed into the thorium, or by a particle accelerator. Most of the past thorium research has involved combining thorium with conventional nuclear fuels to provide the neutrons to trigger the fission process. |
The approach under investigation now is a combination that keeps a uranium rich ‘seed’ in the core, separate from a thorium-rich ‘blanket’. The chief proponent of this concept was the late Alvin Radkowsky, a nuclear pioneer who, under the direction of Admiral Hyman Rickover, helped launch America's Nuclear Navy during the 1950s, when he was chief scientist of the US Naval Reactors Programme. Radkowsky, who died in 2002 at 86, headed up the design team that built the first US civilian nuclear reactor at Shippingport, Pennsylvania, and made significant contributions to the commercial nuclear industry during the 1960s and 1970s. |
Although thorium is not fissile like U-235, Th-232 absorbs slow neutrons to produce U-233, which is fissile. In other words, Th-232 is fertile, like U238. The Th-232 absorbs a neutron to become Th-233, which decays to protactinium-233 (Pa-233) and then to fissionable U-233. When the irradiated fuel is unloaded from the reactor, the U-233 can be separated from the thorium, and then used as fuel in another nuclear reactor. U-233 is superior to the conventional nuclear fuels, U-235 and Pu-239, because it has a higher neutron yield per neutron absorbed. This means that once it is activated by neutrons from fissile U-235 or Pu-239, thorium's breeding cycle is more efficient than that using U-238 and plutonium.
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Top The Russian-US Programme Since the early 1990s, Russia has had a programme based at Moscow's Kurchatov Institute to develop a thorium-uranium fuel. The Russian programme involves the US company Thorium Power, Inc. (founded by Radkowsky), which has US government and private funding to design fuel for the conventional Russian VVER-1000 reactors. Unlike the usual nuclear fuel, which uses enriched uranium oxide, the new fuel assembly design has the plutonium in the centre as the ‘seed’, in a demountable arrangement, with the thorium and uranium around it as a ‘blanket’. |
A normal VVER-1000 fuel assembly has 331 fuel rods, each of 9-millimeter diameter, forming a hexagonal assembly 235-mm wide. The centre portion of each assembly is 155-mm across and holds the seed material, consisting of metallic plutonium-zirconium alloy (about 10 per cent of the alloy is plutonium, of which more than 90 per cent is the isotope Pu-239) in the form of 108 twisted three-section rods, which are 12.75-mm wide, with cladding of zirconium alloy. |
The blanket consists of uranium-thorium oxide fuel pellets (in a ratio of uranium to thorium of 1:9, with the uranium enriched up to almost 20 per cent) in 228 cladding tubes of zirconium alloy, 8.4-mm diameter. These pellets are in four layers around the centre portion. The blanket material achieves 100 gigawatt-days burn-up. Together as one fuel assembly, the seed and blanket have the same geometry as a normal VVER-100 fuel assembly. |
Thorium fuel offers a promising means to dispose of excess weapons grade plutonium in Russian VVER-1000 reactors. Using the thorium fuel technology, plutonium can be disposed of up to three times as fast as MOX at a significantly lower cost. |
As reported by Grae et al., thorium fuel burns 75 per cent of the originally loaded weapons-grade plutonium, compared with a 31 per cent burn for mixed oxide (MOX) fuel, which is made of a mixture of uranium and plutonium. But unlike MOX, thorium fuel does not produce more plutonium and has cost advantages over MOX. Grae et al. conclude: |
“Thorium fuel offers a promising means to dispose of excess weapons grade plutonium in Russian VVER-1000 reactors. Using the thorium fuel technology, plutonium can be disposed of up to three times as fast as MOX at a significantly lower cost. Spent thorium fuel would be more proliferation-resistant than spent MOX fuel… [The thorium fuel technology] will not require significant and costly reactor modifications. Thorium fuel also offers additional benefits in terms of reduced weight and volume of spent fuel and therefore lower disposal costs.” |
Top Four Decades of R&D Concepts for advanced reactors based on thorium fuel cycles include: |
Light Water Reactors Fuels based on plutonium oxide (PuO2), thorium oxide (ThO2), and/or uranium oxide (UO2) particles are arranged in fuel rods. High-Temperature Gas-cooled Reactors: (HTGR) These are of two kinds: the pebble bed and the prismatic fuel design. The Pebble Bed Modular Reactor (PBMR) originated in Germany, and is now being developed in South Africa and China. It can potentially use thorium in its fuel pebbles. The Gas Turbine-Modular Helium Reactor (GT-MHR) was developed in the US by General Atomics using a prismatic fuel. The use of helium as a coolant at high temperature, and the relatively small power output per module (600 megawatts thermal), permit direct coupling of the reactor to a gas turbine (a Brayton cycle), resulting in power generation at 48 per cent thermal efficiency (which is 50 per cent more efficient than the conventional nuclear reactors in use today). The GT-MHR core can accommodate a wide range of fuel options, including highly enriched uranium/thorium, U-233/Th, and Pu/Th. The use of highly enriched uranium/thorium fuel was demonstrated in General Atomics’ Fort St. Vrain reactor in Colorado. Molten salt reactors This advanced breeder concept circulates the fuel in molten salt, without any external coolant in the core. The primary circuit runs through a heat exchanger, which transfers the heat from fission to a secondary salt circuit for steam generation. It was studied in depth in the 1960s, and is now being revived because of the availability of advanced technology for the materials and components. Advanced Heavy Water Reactors (AHWR) India is working on this, and like the Canadian CANDU-NG, this 250-megawatt-electric (MWe) design is lightwater cooled. The main part of the core is subcritical, with Th/U-233 oxide, mixed so that the system is self-sustaining in U-233. A few seed regions with conventional MOX fuel will drive the reaction and give it a negative void coefficient overall. In other words, as the reactor heats up, the fission process slows down. Accelerator Driven Systems (ADS) In accelerator driven systems, high-energy neutrons are produced through the spallation reaction of high-energy protons from an accelerator striking target heavy nuclei (lead, lead-bismuth, or other materials). These neutrons can be directed to a subcritical reactor containing thorium, where the neutrons breed U-233 and promote its fission. There is therefore the possibility of sustaining a fission reaction which can readily be turned off, and used either for power generation or destruction of actinides resulting from the uranium/plutonium fuel cycle. The use of thorium instead of uranium means that fewer new actinides are produced in the acceleratordriven system itself. The difficulties, as of now, in developing the thorium fuel cycle include the high cost of fuel fabrication. This is partly because of the high radioactivity of U-233. |
The difficulties, as of now, in developing the thorium fuel cycle include the high cost of fuel fabrication. This is partly because of the high radioactivity of U-233, which is always contaminated with traces of U-232; there are similar problems in recycling thorium because of the highly radioactive Th-228. Some weapons proliferation risk of U-233; and the technical problems (not yet satisfactorily solved) in reprocessing are also to be considered.Thorium Fuel Operating Experience Between 1967 and 1988, the AVR experimental pebble-bed reactor at Jülich, Germany, operated for more than 750 weeks at 15 megawatts electric, about 95 per cent of the time with thorium-based fuel. The fuel used consisted of about 100,000 billiard ball-size fuel elements. Overall, a total of 1,360 kilograms of thorium was used, mixed with highly enriched uranium (HEU). Maximum burn-ups of 150,000 megawatt-days were achieved. Thorium fuel elements with a 10:1 ratio of thorium to highly enriched uranium were irradiated in the 20-megawatts-thermal (MWt) Dragon reactor at Winfrith, United Kingdom, for 741 full-power days. Dragon was run as a cooperative project of the OECD and Euratom, involving Austria, Denmark, Sweden, Norway, and Switzerland, in addition to the United Kingdom, from 1964 to 1973. The thorium uranium fuel was used to ‘breed and feed’, so that the U-233 that was formed, replaced the U-235 at about the same rate, and fuel could be left in the reactor for about six years. The General Atomics Peach Bottom high-temperature, graphite-moderated, helium-cooled reactor (HTGR) in the United States operated between 1967 and 1974 at 110-MWt, using highly enriched uranium with thorium. In India, the Kamini 30-KWt experimental neutron-source research reactor started up in 1996 near Kalpakkam, using U-233 which was recovered from thorium-dioxide fuel that had been irradiated in another reactor. The Kamini reactor is adjacent to the 40-MWt Fast Breeder Test Reactor, in which the thorium-dioxide is irradiated. In the Netherlands, an aqueous homogenous suspension reactor has operated at one megawatt-thermal for three years. The highly enriched uranium/thorium fuel is circulated in solution, and reprocessing occurs continuously to remove fission products, resulting in a high conversion rate to U-233. Thorium In Power Reactors The 300-MWe THTR reactor in Germany was developed from the AVR, and operated between 1983 and 1989 with 674,000 pebbles, over half of them containing thorium/highly enriched uranium fuel (the rest of the pebbles were graphite moderators and some neutron absorbers). These pebbles were continuously recycled on load, and on an average the fuel passed six times through the core. Fuel fabrication was on an industrial scale. The Fort St. Vrain reactor in Colorado was the only commercial thoriumfuelled nuclear plant in the United States. Developed from the AVR in Germany, it operated from 1976 to 1989. It was a high-temperature (700°C), graphite moderated, helium-cooled reactor with a thorium/highly enriched uranium fuel, which was designed to operate at 842 megawatts-thermal (330 MWe). The fuel was contained in microspheres of thorium carbide and Th/U-235 carbide, coated with silicon oxide and pyrolytic carbon to retain fission products. Unlike the pebble bed-design, the fuel was arranged in hexagonal columns (‘prisms’) in an annular configuration. Almost 25 tonnes of thorium were used in the reactor fuel, achieving a 170,000-megawatt-days burn-up. Thorium-based fuel for Pressurised Water Reactors (PWRs) was investigated at the Shippingport reactor in the United States (the first US commercial reactor, started up in 1957), 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 at Shippingport, from 1977 to 1982, with thorium and U-233 fuel clad with zircaloy, using the ‘seed/blanket’ concept. Another reactor type, the 60-Mwe Lingen Boiling Water Reactor (BWR) in Germany also utilised fuel test elements that were thorium-plutonium-based. Proliferation Issues In the early days of the civilian nuclear programme, the Acheson-Lilienthal Report in 1946 warned of the connection between civilian nuclear power and nuclear weapons, and concluded that the world could not rely on safeguards alone “to protect complying states against the hazards of violations and evasions” i.e., illicit nuclear weapons. Acheson-Lilienthal proposed international controls over nuclear power, but also considered possible technical innovations that would make it harder to divert nuclear materials into bomb making. The thorium fuel cycle is one such technical innovation—as yet untapped. A 1998 paper by Radkowsky and Galparin describes the most advanced work in developing a practical nuclear power system that could be made more ‘proliferation resistant’ than conventional reactors and fuel cycles. Based on a thorium fuel cycle, it has the potential to reduce the amount of plutonium generated per gigawatt-year by a factor of five, compared to conventional uraniumfuelled reactors. It would also make the generated plutonium and U-233 more difficult to use for producing bomb material. Heightened current concerns about preventing the spread of bomb-making materials, have led to an increase in interest in developing thorium-based fuels. |
Heightened current concerns about preventing the spread of bomb-making materials, have led to an increase in interest in developing thorium-based fuels. The US Department of Energy has funded Radkowsky's company (Thorium Power) and its partners in their tests with Russian reactors, as well as three other efforts (two national laboratories, two fuel fabrication companies, and a consortium of three universities). This research is geared to designing a thorium fuel system that will fit with conventional reactors. There is also a new company, Novastar Resources that is buying up thorium mines in anticipation of thoriumfuelled reactors in the future.The proliferation potential of the light water reactor fuel cycle may be significantly reduced by using thorium as a fertile component of the nuclear fuel, as noted above. The main challenge of thorium utilisation is to design a core and a fuel cycle that would be proliferation resistant and economically feasible. This challenge is met by the Radkowsky Thorium Reactor concept. So far, the concept has been applied to a Russian design of a 1,000 MWe pressurised water reactor VVER, designated as VVERT. The main results of the preliminary reference design are as follows: The amount of plutonium contained in the Radkowsky Thorium Reactor spent fuel stockpile is reduced by 80 per cent, in comparison with a VVER of conventional design. The isotopic composition of the reactor's plutonium greatly increases the probability of pre-initiation and yield degradation of a nuclear explosion. An extremely large Pu-238 content causes correspondingly large heat emission, which would complicate the design of an explosive device based on plutonium from this reactor. The economic incentive to reprocess and reuse the fissile component of the Radkowsky Thorium Reactor spent fuel is also decreased. The once-through cycle is economically optimal for its core and cycle. The replacement of a standard (uranium-based) fuel for nuclear reactors of current generation by the Radkowsky Thorium Reactor fuel will provide a strong barrier for nuclear weapon proliferation. |
To reiterate the proliferation difficulties, the replacement of a standard (uranium-based) fuel for nuclear reactors of current generation by the Radkowsky Thorium Reactor fuel will provide a strong barrier for nuclear weapon proliferation. This barrier, in combination with existing safeguard measures and procedures, is adequate to unambiguously disassociate civilian nuclear power from military nuclear power.Other scientists point out that even if a terrorist group wanted to use the blanket plutonium for making a bomb, the process of extracting it from thorium fuel would be more difficult than removing it from conventional spent fuel. This is because the spent blanket fuel from a thorium fuel cycle would contain U-232, which over time decays into isotopes that emit high-energy gamma rays. To extract the plutonium from this spent fuel would require significantly more radiation shielding plus additional remotely operated equipment in order to reprocess it for weapons use, making a daunting task even more difficult. It would also be more complicated to separate the fissionable U-233 from U-238, because of the highly radioactive products present. Overall, the development of thorium fuel cycles makes sense for the future, for advancing the efficiency and economy of nuclear power plants, and for easier recycling, while making it more difficult to divert radioactive materials for weapons. Top |