Jefimenko's equations

A sample of thorium.

The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, Template:SimpleNuclide2, as the fertile material. In the reactor, Template:SimpleNuclide2 is transmuted into the fissile artificial uranium isotope Template:SimpleNuclide2 which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as Template:SimpleNuclide2), which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source are necessary to initiate the fuel cycle. In a thorium-fueled reactor, Template:SimpleNuclide2 absorbs neutrons eventually to produce Template:SimpleNuclide2. This parallels the process in uranium breeder reactors whereby fertile Template:SimpleNuclide2 absorbs neutrons to form fissile Template:SimpleNuclide2. Depending on the design of the reactor and fuel cycle, the generated Template:SimpleNuclide2 either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.

The thorium fuel cycle claims several potential advantages over a uranium fuel cycle, including thorium's greater abundance, superior physical and nuclear properties, better resistance to nuclear weapons proliferation and reduced plutonium and actinide production.

History

Concerns about the limits of worldwide uranium resources motivated initial interest in the thorium fuel cycle. It was envisioned that as uranium reserves were depleted, thorium would supplement uranium as a fertile material. However, for most countries uranium was relatively abundant and research in thorium fuel cycles waned. A notable exception was India's three stage nuclear power programme. In the twenty-first century thorium's potential for improving proliferation resistance and waste characteristics led to renewed interest in the thorium fuel cycle.

At Oak Ridge National Laboratory in the 1960s, the Molten-Salt Reactor Experiment used Template:SimpleNuclide2 as the fissile fuel as an experiment to demonstrate a part of the Molten Salt Breeder Reactor that was designed to operate on the thorium fuel cycle. Molten Salt Reactor (MSR) experiments assessed thorium's feasibility, using thorium(IV) fluoride dissolved in a molten salt fluid which eliminated the need to fabricate fuel elements. The MSR program was defunded in 1976 after its patron Alvin Weinberg was fired.

In 2006, Carlo Rubbia proposed the concept of an energy amplifier or "accelerator driven system" (ADS), which he saw as a novel and safe way to produce nuclear energy that exploited existing accelerator technologies. Rubbia's proposal offered the potential to incinerate high-activity nuclear waste and produce energy from natural thorium and depleted uranium.

Kirk Sorensen, former NASA scientist and Chief Nuclear Technologist at Teledyne Brown Engineering, has been a long time promoter of thorium fuel cycle and particularly liquid fluoride thorium reactors (LFTRs). He first researched thorium reactors while working at NASA, while evaluating power plant designs suitable for lunar colonies. In 2006 Sorensen started "energyfromthorium.com" to promote and make information available about this technology.

A 2011 MIT study concluded that, although there is little in the way of barriers to a thorium fuel cycle, with current or near term light-water reactor designs there is also little incentive for any significant market penetration to occur. As such they conclude there is little chance of thorium cycles replacing conventional uranium cycles in the current nuclear power market, despite the potential benefits.

Nuclear reactions with thorium

In the thorium cycle, fuel is formed when Template:SimpleNuclide2 captures a neutron (whether in a fast reactor or thermal reactor) to become Template:SimpleNuclide2. This normally emits an electron and an anti-neutrino (Template:SubatomicParticle) by [[beta decay|Template:SubatomicParticle decay]] to become Template:SimpleNuclide2. This then emits another electron and anti-neutrino by a second Template:SubatomicParticle decay to become Template:SimpleNuclide2, the fuel:

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{233}\mathrm {Th} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{233}\mathrm {Pa} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{233}\mathrm {U}$

Fission product wastes

Nuclear fission produces radioactive fission products which can have half-lives from days to greater than 200,000 years. According to some toxicity studies, the thorium cycle can fully recycle actinide wastes and only emit fission product wastes, and after a few hundred years, the waste from a thorium reactor can be less toxic than the uranium ore that would have been used to produce low enriched uranium fuel for a light water reactor of the same power. Other studies assume some actinide losses and find that actinide wastes dominate thorium cycle waste radioactivity at some future periods.

Actinide wastes

In a reactor, when a neutron hits a fissile atom (such as certain isotopes of uranium), it either splits the nucleus or is captured and transmutes the atom. In the case of Template:SimpleNuclide2, the transmutations tend to produce useful nuclear fuels rather than transuranic wastes. When Template:SimpleNuclide2 absorbs a neutron, it either fissions or becomes Template:SimpleNuclide2. The chance of fissioning on absorption of a thermal neutron is about 92%; the capture-to-fission ratio of Template:SimpleNuclide2, therefore, is about 1:10 — which is better than the corresponding capture vs. fission ratios of Template:SimpleNuclide2 (about 1:6), or Template:SimpleNuclide2 (about 1:2), or Template:SimpleNuclide2 (about 1:4). The result is less transuranic waste than in a reactor using the uranium-plutonium fuel cycle. Template:Thorium Cycle Transmutation Template:SimpleNuclide2, like most actinides with an even number of neutrons, is not fissile, but neutron capture produces fissile Template:SimpleNuclide2. If the fissile isotope fails to fission on neutron capture, it produces Template:SimpleNuclide2, Template:SimpleNuclide2, Template:SimpleNuclide2, and eventually fissile Template:SimpleNuclide2 and heavier isotopes of plutonium. The Template:SimpleNuclide2 can be removed and stored as waste or retained and transmuted to plutonium, where more of it fissions, while the remainder becomes Template:SimpleNuclide2, then americium and curium, which in turn can be removed as waste or returned to reactors for further transmutation and fission.

However, the Template:SimpleNuclide2 (with a half-life of Template:Val) formed via (n,2n) reactions with Template:SimpleNuclide2 (yielding Template:SimpleNuclide2 that decays to Template:SimpleNuclide2), while not a transuranic waste, is a major contributor to the long term radiotoxicity of spent nuclear fuel.

Uranium-232 contamination

Uranium-232 is also formed in this process, via (n,2n) reactions between fast neutrons and Template:SimpleNuclide2, Template:SimpleNuclide2, and Template:SimpleNuclide2:

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{233}\mathrm {Th} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{233}\mathrm {Pa} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{233}\mathrm {U} +\mathrm {n} \rightarrow {}_{\ 92}^{232}\mathrm {U} +2\mathrm {n}$

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{233}\mathrm {Th} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{233}\mathrm {Pa} +\mathrm {n} \rightarrow {}_{\ 91}^{232}\mathrm {Pa} +2\mathrm {n} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{232}\mathrm {U}$

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{231}\mathrm {Th} +2\mathrm {n} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{231}\mathrm {Pa} +\mathrm {n} \rightarrow {}_{\ 91}^{232}\mathrm {Pa} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{232}\mathrm {U}$

Uranium-232 has a relatively short half-life (Template:Val), and some decay products emit high energy gamma radiation, such as Template:SimpleNuclide2, Template:SimpleNuclide2 and particularly Template:SimpleNuclide2. The full decay chain, along with half-lives and relevant gamma energies, is:

Template:SimpleNuclide2 decays to Template:SimpleNuclide2 where it joins [[thorium series|decay chain of Template:SimpleNuclide2]]

${}_{\ 92}^{232}\mathrm {U} {\xrightarrow {\ \alpha \ }}{}_{\ 90}^{228}\mathrm {Th} \ \mathrm {(68.9\ a)}$

${}_{\ 90}^{228}\mathrm {Th} {\xrightarrow {\ \alpha \ }}{}_{\ 88}^{224}\mathrm {Ra} \ \mathrm {(1.9\ a)}$

${}_{\ 88}^{224}\mathrm {Ra} {\xrightarrow {\ \alpha \ }}{}_{\ 86}^{220}\mathrm {Rn} \ \mathrm {(3.6\ d,\ 0.24\ MeV)}$

${}_{\ 86}^{220}\mathrm {Rn} {\xrightarrow {\ \alpha \ }}{}_{\ 84}^{216}\mathrm {Po} \ \mathrm {(55\ s,\ 0.54\ MeV)}$

${}_{\ 84}^{216}\mathrm {Po} {\xrightarrow {\ \alpha \ }}{}_{\ 82}^{212}\mathrm {Pb} \ \mathrm {(0.15\ s)}$

${}_{\ 82}^{212}\mathrm {Pb} {\xrightarrow {\beta ^{-}\ }}{}_{\ 83}^{212}\mathrm {Bi} \ \mathrm {(10.64\ h)}$

${}_{\ 83}^{212}\mathrm {Bi} {\xrightarrow {\ \alpha \ }}{}_{\ 81}^{208}\mathrm {Tl} \ \mathrm {(61\ m,\ 0.78\ MeV)}$

${}_{\ 81}^{208}\mathrm {Tl} {\xrightarrow {\beta ^{-}\ }}{}_{\ 82}^{208}\mathrm {Pb} \ \mathrm {(3\ m,\ 2.6\ MeV)}$

Thorium-cycle fuels produce hard gamma emissions, which damage electronics, limiting their use in military bomb triggers. Template:SimpleNuclide2 cannot be chemically separated from Template:SimpleNuclide2 from used nuclear fuel; however, chemical separation of thorium from uranium removes the decay product Template:SimpleNuclide2 and the radiation from the rest of the decay chain, which gradually build up as Template:SimpleNuclide2 reaccumulates. The hard gamma emissions also create a radiological hazard which requires remote handling during reprocessing.

Advantages as a nuclear fuel

Thorium is estimated to be about three to four times more abundant than uranium in the Earth's crust, although present knowledge of reserves is limited. Current demand for thorium has been satisfied as a by-product of rare-earth extraction from monazite sands. Also, unlike uranium, mined thorium consists of a single isotope (Template:SimpleNuclide2). Consequently, it is useful in thermal reactors without the need for isotope separation.

Thorium-based fuels exhibit several attractive properties relative to uranium-based fuels. The thermal neutron absorption cross section (σ_a) and resonance integral (average of neutron cross sections over intermediate neutron energies) for Template:SimpleNuclide2 are about three times and one third of the respective values for Template:SimpleNuclide2; consequently, fertile conversion of thorium is more efficient in a thermal reactor. Also, although the thermal neutron fission cross section (σ_f) of the resulting Template:SimpleNuclide2 is comparable to Template:SimpleNuclide2 and Template:SimpleNuclide2, it has a much lower capture cross section (σ_γ) than the latter two fissile isotopes, providing fewer non-fissile neutron absorptions and improved neutron economy. Finally, the ratio of neutrons released per neutron absorbed (η) in Template:SimpleNuclide2 is greater than two over a wide range of energies, including the thermal spectrum; as a result, thorium-based fuels can be the basis for a thermal breeder reactor.

Thorium-based fuels also display favorable physical and chemical properties which improve reactor and repository performance. Compared to the predominant reactor fuel, uranium dioxide (Template:Chem), thorium dioxide (Template:Chem) has a higher melting point, higher thermal conductivity, and lower coefficient of thermal expansion. Thorium dioxide also exhibits greater chemical stability and, unlike uranium dioxide, does not further oxidize.

Because the Template:SimpleNuclide2 produced in thorium fuels is inevitably contaminated with Template:SimpleNuclide2, thorium-based used nuclear fuel possesses inherent proliferation resistance. Template:SimpleNuclide2 can not be chemically separated from Template:SimpleNuclide2 and has several decay products which emit high energy gamma radiation. These high energy photons are a radiological hazard that necessitate the use of remote handling of separated uranium and aid in the passive detection of such materials. Template:SimpleNuclide2 can be denatured by mixing it with natural or depleted uranium, requiring isotope separation before it could be used in nuclear weapons.

The long term (on the order of roughly Template:Val to Template:Val) radiological hazard of conventional uranium-based used nuclear fuel is dominated by plutonium and other minor actinides, after which long-lived fission products become significant contributors again. A single neutron capture in Template:SimpleNuclide2 is sufficient to produce transuranic elements, whereas six captures are generally necessary to do so from Template:SimpleNuclide2. 98–99% of thorium-cycle fuel nuclei would fission at either Template:SimpleNuclide2 or Template:SimpleNuclide2, so fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in mixed oxide (MOX) fuels to minimize the generation of transuranics and maximize the destruction of plutonium.

Disadvantages as nuclear fuel

There are several challenges to the application of thorium as a nuclear fuel, particularly for solid fuel reactors:

Unlike uranium, natural thorium contains no fissile isotopes; fissile material, generally Template:SimpleNuclide2, Template:SimpleNuclide2 or plutonium, must be added to achieve criticality. This, along with the high sintering temperature necessary to make thorium-dioxide fuel, complicates fuel fabrication. Oak Ridge National Laboratory experimented with thorium tetrafluoride as fuel in a molten salt reactor from 1964–1969, which was far easier to both process and separate from contaminants that slow or stop the chain reaction.

In an open fuel cycle (i.e. utilizing Template:SimpleNuclide2 in situ), higher burnup is necessary to achieve a favorable neutron economy. Although thorium dioxide performed well at burnups of 170,000 MWd/t and 150,000 MWd/t at Fort St. Vrain Generating Station and AVR respectively, challenges complicate achieving this in light water reactors (LWR), which compose the vast majority of existing power reactors.

In a once-through thorium fuel cycle the residual Template:SimpleNuclide2 is long lived radioactive waste.

Another challenge associated with the thorium fuel cycle is the comparatively long interval over which Template:SimpleNuclide2 breeds to Template:SimpleNuclide2. The half-life of Template:SimpleNuclide2 is about 27 days, which is an order of magnitude longer than the half-life of Template:SimpleNuclide2. As a result, substantial Template:SimpleNuclide2 develops in thorium-based fuels. Template:SimpleNuclide2 is a significant neutron absorber, and although it eventually breeds into fissile Template:SimpleNuclide2, this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.

Alternatively, if solid thorium is used in a closed fuel cycle in which Template:SimpleNuclide2 is recycled, remote handling is necessary for fuel fabrication because of the high radiation levels resulting from the decay products of Template:SimpleNuclide2. This is also true of recycled thorium because of the presence of Template:SimpleNuclide2, which is part of the Template:SimpleNuclide2 decay sequence. Further, unlike proven uranium fuel recycling technology (e.g. PUREX), recycling technology for thorium (e.g. THOREX) is only under development.

Although the presence of Template:SimpleNuclide2 complicates matters, there are public documents showing that Template:SimpleNuclide2 has been used once in a nuclear weapon test. The United States tested a composite Template:SimpleNuclide2-plutonium bomb core in the MET (Military Effects Test) blast during Operation Teapot in 1955, though with much lower yield than expected.

Though thorium-based fuels produce far less long-lived transuranics than uranium-based fuels, some long-lived actinide products constitute a long term radiological impact, especially Template:SimpleNuclide2.

Advocates for liquid core and molten salt reactors such as LFTR claim that these technologies negate thorium's disadvantages present in solid fueled reactors. Since only two liquid core fluoride salt reactors have been built (the ORNL ARE and MSRE) and neither used thorium, it is hard to validate the exact benefits.

Reactors

Thorium fuels have fueled several different reactor types, including light water reactors, heavy water reactors, high temperature gas reactors, sodium-cooled fast reactors, and molten salt reactors.

List of thorium-fueled reactors

From IAEA TECDOC-1450 "Thorium Fuel Cycle - Potential Benefits and Challenges", Table 1: Thorium utilization in different experimental and power reactors. Additionally, Dresden 1 in the USA used "thorium oxide corner rods".

Name	Country	Type	Power	Fuel	Operation period
AVR	Germany	HTGR, Experimental (Pebble bed reactor)	015000 15 MW(e)	Th+Template:SimpleNuclide2 Driver Fuel, Coated fuel particles, Oxide & dicarbides	1967–1988
THTR-300	Germany	HTGR, Power (Pebble Type)	300000 300 MW(e)	Th+Template:SimpleNuclide2, Driver Fuel, Coated fuel particles, Oxide & dicarbides	1985–1989
Lingen	Germany	BWR Irradiation-testing	060000 60 MW(e)	Test Fuel (Th,Pu)O2 pellets	1968-1973
Dragon (OECD-Euratom)	UK (also Sweden, Norway & Switzerland)	HTGR, Experimental (Pin-in-Block Design)	020000 20 MWt	Th+Template:SimpleNuclide2 Driver Fuel, Coated fuel particles, Oxide & Dicarbides	1966–1973
Peach Bottom	USA	HTGR, Experimental (Prismatic Block)	040000 40 MW(e)	Th+Template:SimpleNuclide2 Driver Fuel, Coated fuel particles, Oxide & dicarbides	1966–1972
Fort St Vrain	USA	HTGR, Power (Prismatic Block)	330000 330 MW(e)	Th+Template:SimpleNuclide2 Driver Fuel, Coated fuel particles, Dicarbide	1976–1989
MSRE ORNL	USA	MSBR	007500 7.5 MWt	Template:SimpleNuclide2 Molten Fluorides	1964–1969
BORAX-IV & Elk River Station	USA	BWR (Pin Assemblies)	002400 2.4 MW(e); 24 MW(e)	Th+235U Driver Fuel Oxide Pellets	1963 - 1968
Shippingport	USA	LWBR PWR, (Pin Assemblies)	100000 100 MW(e)	Th+Template:SimpleNuclide2 Driver Fuel, Oxide Pellets	1977–1982
Indian Point 1	USA	LWBR PWR, (Pin Assemblies)	285000 285 MW(e)	Th+Template:SimpleNuclide2 Driver Fuel, Oxide Pellets	1962–1980
SUSPOP/KSTR KEMA	Netherlands	Aqueous Homogenous Suspension (Pin Assemblies)	001000 1 MWt	Th+HEU, Oxide Pellets	1974–1977
NRX & NRU	Canada	MTR (Pin Assemblies)	020000 20MW; 200MW (see)	Th+Template:SimpleNuclide2, Test Fuel	1947 (NRX) + 1957 (NRU); Irradiation–testing of few fuel elements
CIRUS; DHRUVA; & KAMINI	India	MTR Thermal	040000 40 MWt; 100 MWt; 30 kWt (low power, research)	Al+Template:SimpleNuclide2 Driver Fuel, ‘J’ rod of Th & ThO2, ‘J’ rod of ThO2	1960-2010 (CIRUS); others in operation
KAPS 1 &2; KGS 1 & 2; RAPS 2, 3 & 4	India	PHWR, (Pin Assemblies)	220000 220 MW(e)	ThO2 Pellets (For neutron flux flattening of initial core after start-up)	1980 (RAPS 2) +; continuing in all new PHWRs
FBTR	India	LMFBR, (Pin Assemblies)	040000 40 MWt	ThO2 blanket	1985; in operation

See also

• Radioactive waste
• World energy resources and consumption
• Peak uranium
• Fuji MSR
• Alvin Radkowsky
• Weinberg Foundation
• Flibe Energy
• CANDU reactor
• Advanced heavy water reactor
• Thorium Energy Alliance

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

External links

• FactSheet on Thorium, World Nuclear Association.
• Annotated bibliography for the thorium fuel cycle from the Alsos Digital Library for Nuclear Issues
• International Thorium Energy Organisation - www.IThEO.org