Thorium fuel cycle

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Template:Short description

File:Thorium sample 0.1g.jpg
A sample of thorium

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

The thorium fuel cycle has several potential advantages over a uranium fuel cycle, including thorium's greater abundance, superior physical and nuclear properties, reduced plutonium and actinide production,[1] and better resistance to nuclear weapons proliferation when used in a traditional light water reactor[1][2] though not in a molten salt reactor.[3][4][5]

History

Script error: No such module "Labelled list hatnote". Concerns about the limits of worldwide uranium resources motivated initial interest in the thorium fuel cycle.[6] 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.[7] In the twenty-first century thorium's claimed potential for improving proliferation resistance and waste characteristics led to renewed interest in the thorium fuel cycle.[8][9][10] While thorium is more abundant in the continental crust than uranium and easily extracted from monazite as a side product of rare earth element mining, it is much less abundant in seawater than uranium.[11]

At Oak Ridge National Laboratory in the 1960s, the Molten-Salt Reactor Experiment used Template:SimpleNuclide as the fissile fuel in 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 that eliminated the need to fabricate fuel elements. The MSR program was defunded in 1976 after its patron Alvin Weinberg was fired.[12]

In 1993, 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.[13][14]

Kirk Sorensen, former NASA scientist and Chief Technologist at Flibe Energy, 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.[15]

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.[16]

Nuclear reactions with thorium

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

nneutron+23290Th23390ThβA23391PaβAA92233A2922233Ufuel

Fission product waste

Nuclear fission produces radioactive fission products which can have half-lives from days to greater than 200,000 years. According to some toxicity studies,[17] 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.[18] Some fission products have been proposed for nuclear transmutation, which would further reduce the amount of nuclear waste and the duration during which it would have to be stored (whether in a deep geological repository or elsewhere). However, while the principal feasibility of some of those reactions has been demonstrated at laboratory scale, there is, as of 2024, no large scale deliberate transmutation of fission products anywhere in the world, and the upcoming MYRRHA research project into transmutation is mostly focused on transuranic waste. Furthermore, the cross section of some fission products is relatively low and others - such as caesium - are present as a mixture of stable, short lived and long lived isotopes in nuclear waste, making transmutation dependent on expensive isotope separation.

Actinide waste

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:SimpleNuclide, the transmutations tend to produce useful nuclear fuels rather than transuranic waste. When Template:SimpleNuclide absorbs a neutron, it either fissions or becomes Template:SimpleNuclide. The chance of fissioning on absorption of a thermal neutron is about 92%; the capture-to-fission ratio of Template:SimpleNuclide, therefore, is about 1:12 – which is better than the corresponding capture vs. fission ratios of Template:SimpleNuclide (about 1:6), or Template:SimpleNuclide or Template:SimpleNuclide (both about 1:3).[6][19] The result is less transuranic waste than in a reactor using the uranium-plutonium fuel cycle. Template:Thorium Cycle Transmutation Template:SimpleNuclide, like most actinides with an even number of neutrons, is not fissile, but neutron capture produces fissile Template:SimpleNuclide. If the fissile isotope fails to fission on neutron capture, it produces Template:SimpleNuclide, Template:SimpleNuclide, Template:SimpleNuclide, and eventually fissile Template:SimpleNuclide and heavier isotopes of plutonium. The Template:SimpleNuclide can be removed and stored as waste or retained and transmuted to plutonium, where more of it fissions, while the remainder becomes Template:SimpleNuclide, then americium and curium, which in turn can be removed as waste or returned to reactors for further transmutation and fission.

However, the Template:SimpleNuclide (with a half-life of Template:Val) formed via (n,2n) reactions with Template:SimpleNuclide (yielding Template:SimpleNuclide that decays to Template:SimpleNuclide), while not a transuranic waste, is a major contributor to the long-term radiotoxicity of spent nuclear fuel. While Template:Chem can in principle be converted back to Template:Chem by neutron absorption, its neutron absorption cross section is relatively low, making this rather difficult and possibly uneconomic.

Uranium-232 contamination

Template:SimpleNuclide is also formed in this process, via (n,2n) reactions between fast neutrons and Template:SimpleNuclide, Template:SimpleNuclide, and Template:SimpleNuclide:

23290Th+n23390ThβA23391Pa βA23392U+n-2n23292U23290Th+n23390ThβA23391Pa +n-2n23291PaβA23292U23290Th+n-2n23190ThβA23191Pa +n23291PaβA23292U

Unlike most even numbered heavy isotopes, Template:SimpleNuclide is also a fissile fuel fissioning just over half the time when it absorbs a thermal neutron.[20] Template:SimpleNuclide has a relatively short half-life (Template:Val), and some decay products emit high energy gamma radiation, such as Template:SimpleNuclide, Template:SimpleNuclide and particularly Template:SimpleNuclide. The full decay chain, along with half-lives and relevant gamma energies, is:

File:Decay chain(4n,Thorium series).PNG
The 4n decay chain of 232Th, commonly called the "thorium series"

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

A92232A2922232UαA90228A2902228Th (68.9 years)A90228A2902228ThαA88224A2882224Ra (1.9 year)A88224A2882224RaαA86220A2862220Rn (3.6 day, 0.24 MeV)A86220A2862220RnαA84216A2842216Po (55 s, 0.54 MeV)A84216A2842216PoαA82212A2822212Pb (0.15 s)A82212A2822212PbβAA83212A2832212Bi (10.64 h)A83212A2832212BiαA81208A2812208Tl (61 m, 0.78 MeV)A81208A2812208TlβAA82208A2822208Pb (3 m, 2.6 MeV)

Thorium-cycle fuels produce hard gamma emissions, which damage electronics, limiting their use in bombs. Template:SimpleNuclide cannot be chemically separated from Template:SimpleNuclide from used nuclear fuel; however, chemical separation of thorium from uranium removes the decay product Template:SimpleNuclide and the radiation from the rest of the decay chain, which gradually build up as Template:SimpleNuclide reaccumulates. The contamination could also be avoided by using a molten-salt breeder reactor and separating the Template:SimpleNuclide before it decays into Template:SimpleNuclide.[3] The hard gamma emissions also create a radiological hazard which requires remote handling during reprocessing.

Nuclear fuel

As a fertile material thorium is similar to Template:SimpleNuclide, the major part of natural and depleted uranium. The thermal neutron absorption cross sectiona) and resonance integral (average of neutron cross sections over intermediate neutron energies) for Template:SimpleNuclide are about three and one third times those of the respective values for Template:SimpleNuclide.

Advantages

The primary physical advantage of thorium fuel is that it uniquely makes possible a breeder reactor that runs with slow neutrons, otherwise known as a thermal breeder reactor.[6] These reactors are often considered simpler than the more traditional fast-neutron breeders. Although the thermal neutron fission cross section (σf) of the resulting Template:SimpleNuclide is comparable to Template:SimpleNuclide and Template:SimpleNuclide, it has a much lower capture cross section (σγ) than the latter two fissile isotopes, providing fewer non-fissile neutron absorptions and improved neutron economy. The ratio of neutrons released per neutron absorbed (η) in Template:SimpleNuclide is greater than two over a wide range of energies, including the thermal spectrum. A breeding reactor in the uranium–plutonium cycle needs to use fast neutrons, because in the thermal spectrum one neutron absorbed by Template:SimpleNuclide on average leads to less than two neutrons.

Thorium is estimated to be about three to four times more abundant than uranium in Earth's crust,[21] 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. Notably, there is very little thorium dissolved in seawater, so seawater extraction is not viable, as it is with uranium. Using breeder reactors, known thorium and uranium resources can both generate world-scale energy for thousands of years.

Thorium-based fuels also display favorable physical and chemical properties that 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.[6]

Because the Template:SimpleNuclide produced in thorium fuels is significantly contaminated with Template:SimpleNuclide in proposed power reactor designs, thorium-based used nuclear fuel possesses inherent proliferation resistance. Template:SimpleNuclide cannot be chemically separated from Template:SimpleNuclide and has several decay products that 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.

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:SimpleNuclide is sufficient to produce transuranic elements, whereas five captures are generally necessary to do so from Template:SimpleNuclide. 98–99% of thorium-cycle fuel nuclei would fission at either Template:SimpleNuclide or Template:SimpleNuclide, 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.[22]

Disadvantages

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

In contrast to uranium, naturally occurring thorium is effectively mononuclidic and contains no fissile isotopes; fissile material, generally Template:SimpleNuclide, Template:SimpleNuclide 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 to 1969, which was expected to be easier to process and separate from contaminants that slow or stop the chain reaction.

In an open fuel cycle (i.e. utilizing Template:SimpleNuclide 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,[6] 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, 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:SimpleNuclide and Template:SimpleNuclide.[17] On a closed cycle,Template:SimpleNuclide and Template:SimpleNuclide can be reprocessed. Template:SimpleNuclide is also considered an excellent burnable poison absorber in light water reactors.[23]

Another challenge associated with the thorium fuel cycle is the comparatively long interval over which Template:SimpleNuclide breeds to Template:SimpleNuclide. The half-life of Template:SimpleNuclide is about 27 days, which is an order of magnitude longer than the half-life of Template:SimpleNuclide. As a result, substantial Template:SimpleNuclide develops in thorium-based fuels. Template:SimpleNuclide is a significant neutron absorber and, although it eventually breeds into fissile Template:SimpleNuclide, 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:SimpleNuclide is recycled, remote handling is necessary for fuel fabrication because of the high radiation levels resulting from the decay products of Template:SimpleNuclide. This is also true of recycled thorium because of the presence of Template:SimpleNuclide, which is part of the Template:SimpleNuclide 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:SimpleNuclide complicates matters, there are public documents showing that Template:SimpleNuclide has been used once in a nuclear weapon test. The United States tested a composite Template:SimpleNuclide-plutonium bomb core in the MET (Military Effects Test) blast during Operation Teapot in 1955, though with much lower yield than expected.[24]

Advocates for liquid core and molten salt reactors such as LFTRs claim that these technologies negate thorium's disadvantages present in solid fuelled reactors. As only two liquid-core fluoride salt reactors have been built (the ORNL ARE and MSRE) and neither have used thorium, it is hard to validate the exact benefits.[6]

Thorium-fueled 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.[25]

List of thorium-fueled reactors

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From IAEA TECDOC-1450 "Thorium Fuel Cycle – Potential Benefits and Challenges", Table 1: Thorium utilization in different experimental and power reactors.[6] Additionally from Energy Information Administration, "Spent Nuclear Fuel Discharges from U. S. Reactors", Table B4: Dresden 1 Assembly Class.[26]

Name Operation period Country Reactor type Power Fuel
NRX & NRU 1947 (NRX) + 1957 (NRU); Irradiation–testing of few fuel elements Template:Flagicon Canada MTR (pin assemblies) 020000 20 MW; 200 MW (see) Th+Template:SimpleNuclide, Test Fuel
Dresden Unit 1 1960–1978 Template:Flagicon United States BWR 300000 197 MW(e) ThO2 corner rods, UO2 clad in Zircaloy-2 tube
CIRUS; DHRUVA; & KAMINI 1960–2010 (CIRUS); others in operation Template:Flagicon India MTR thermal 040000 40 MWt; 100 MWt; 30 kWt (low power, research) Al+Template:SimpleNuclide Driver fuel, ‘J’ rod of Th & ThO2, ‘J’ rod of ThO2
Indian Point Unit 1 1962–1965[27] Template:Flagicon United States LWBR, PWR, (pin assemblies) 285000 285 MW(e) Th+Template:SimpleNuclide Driver fuel, oxide pellets
BORAX-IV & Elk River Station 1963–1968 Template:Flagicon United States BWR (pin assemblies) 002400 2.4 MW(e); 24 MW(e) Th+Template:SimpleNuclide Driver fuel oxide pellets
MSRE ORNL 1964–1969 Template:Flagicon United States MSR 007500 7.5 MWt Template:SimpleNuclide molten fluorides
Peach Bottom Unit 1 1966–1972 Template:Flagicon United States HTGR, Experimental (prismatic block) 040000 40 MW(e) Th+Template:SimpleNuclide Driver fuel, coated fuel particles, oxide & dicarbides
Dragon (OECD-Euratom) 1966–1973 Template:Flagicon UK (also Template:Flagicon Sweden, Template:Flagicon Norway and Template:Flagicon Switzerland) HTGR, Experimental (pin-in-block design) 020000 20 MWt Th+Template:SimpleNuclide Driver fuel, coated fuel particles, oxide & dicarbides
AVR 1967–1988 Template:Flagicon Germany (West) HTGR, experimental (pebble bed reactor) 015000 15 MW(e) Th+Template:SimpleNuclide Driver fuel, coated fuel particles, oxide & dicarbides
Lingen 1968–1973 Template:Flagicon Germany (West) BWR irradiation-testing 060000 60 MW(e) Test fuel (Th,Pu)O2 pellets
SUSPOP/KSTR KEMA 1974–1977 Template:Flagicon Netherlands Aqueous homogeneous suspension (pin assemblies) 001000 1 MWt Th+HEU, oxide pellets
Fort St Vrain 1976–1989 Template:Flagicon United States HTGR, Power (prismatic block) 330000 330 MW(e) Th+Template:SimpleNuclide Driver fuel, coated fuel particles, Dicarbide
Shippingport 1977–1982 Template:Flagicon United States LWBR, PWR, (pin assemblies) 100000 100 MW(e) Th+Template:SimpleNuclide Driver fuel, oxide pellets
KAPS 1 &2; KGS 1 & 2; RAPS 2, 3 & 4 1980 (RAPS 2) +; continuing in all new PHWRs Template:Flagicon India PHWR, (pin assemblies) 220000 220 MW(e) ThO2 pellets (for neutron flux flattening of initial core after start-up)
FBTR 1985; in operation Template:Flagicon India LMFBR, (pin assemblies) 040000 40 MWt ThO2 blanket
THTR-300 1985–1989 Template:Flagicon Germany (West) HTGR, power (pebble type) 300000 300 MW(e) Th+Template:SimpleNuclide Driver fuel, coated fuel particles, oxide & dicarbides
TMSR-LF1 2023; operating license issued Template:Flagicon China Liquid fuel thorium-based molten salt experimental reactor 002000 2 MWt Thorium-based molten salt
Petten 2024; planned Template:Flagicon Netherlands High Flux Reactor thorium molten salt experiment 060000 45 MW(e) ?
SINAP 2030; planned[28] Template:Flagicon China thorium-based molten-salt reactor 060000 10 MWt Thorium-based molten salt

See also

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References

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Further reading

External links

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