The thorium fuel cycle is a
nuclear fuel cycle
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the ''front end'', which are the preparation of the fuel, steps in the ''service period'' in w ...
that uses an
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
of
thorium
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...
, , as the
fertile material
Fertile material is a material that, although not itself fissionable by thermal neutrons, can be converted into a fissile material by neutron absorption and subsequent nuclei conversions.
Naturally occurring fertile materials
Naturally occurring ...
. In the reactor, is
transmuted into the
fissile
In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...
artificial
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
isotope which is the
nuclear fuel
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...
. Unlike
natural uranium
Natural uranium (NU or Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes fr ...
, natural thorium contains only trace amounts of fissile material (such as ), which are insufficient to initiate a
nuclear chain reaction
In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...
. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, absorbs
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s to produce . This parallels the process in uranium
breeder reactor
A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile mate ...
s whereby fertile absorbs neutrons to form fissile . Depending on the design of the reactor and fuel cycle, the generated either fissions
in situ
''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
or is chemically separated from the
used nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...
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
Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
and
actinide
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
production,
and better resistance to
nuclear weapons proliferation when used in a traditional
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
though not in a
molten salt reactor
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
.
[ ]
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
India's three-stage nuclear power programme was formulated by Homi Bhabha, the well-known physicist, in the 1950s to secure the country's long term energy independence, through the use of uranium and thorium reserves found in the monazite sa ...
.
In the twenty-first century thorium's claimed potential for improving proliferation resistance and
waste
Waste (or wastes) are unwanted or unusable materials. Waste is any substance discarded after primary use, or is worthless, defective and of no use. A by-product, by contrast is a joint product of relatively minor economic value. A waste prod ...
characteristics led to renewed interest in the thorium fuel cycle. While thorium is more abundant in the continental crust than uranium and easily extracted from
monazite
Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is monazite-(Ce), that is, the cerium- ...
as a side product of
rare earth element
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silv ...
mining, it is much less abundant in
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
than uranium.
At
Oak Ridge National Laboratory
Oak Ridge National Laboratory (ORNL) is a U.S. multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT–Battelle as a federally funded research and ...
in the 1960s, the
Molten-Salt Reactor Experiment
The Molten-Salt Reactor Experiment (MSRE) was an experimental molten salt reactor research reactor at the Oak Ridge National Laboratory (ORNL). This technology was researched through the 1960s, the reactor was constructed by 1964, it went critica ...
used 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
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
(MSR) experiments assessed thorium's feasibility, using
thorium(IV) fluoride
Thorium(IV) fluoride ( Th F4) is an inorganic chemical compound. It is a white hygroscopic powder which can be produced by reacting thorium with fluorine gas. At temperatures above 500 °C, it reacts with atmospheric moisture to produce Th ...
dissolved in a
molten salt
Molten salt is salt which is solid at standard temperature and pressure but enters the liquid phase due to elevated temperature. Regular table salt has a melting point of 801 °C (1474°F) and a heat of fusion of 520 J/g.Journal of Chemical T ...
fluid that eliminated the need to fabricate fuel elements. The MSR program was defunded in 1976 after its patron
Alvin Weinberg was fired.
In 1993,
Carlo Rubbia
Carlo Rubbia (born 31 March 1934) is an Italian particle physicist and inventor who shared the Nobel Prize in Physics in 1984 with Simon van der Meer for work leading to the discovery of the W and Z particles at CERN.
Early life and education
...
proposed the concept of an
energy amplifier In nuclear physics, an energy amplifier is a novel type of nuclear power reactor, a subcritical reactor, in which an energetic particle beam is used to stimulate a reaction, which in turn releases enough energy to power the particle accelerator and ...
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
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...
and depleted
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
.
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 reactor
The liquid fluoride thorium reactor (LFTR; often pronounced ''lifter'') is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based, molten, liquid salt for fuel. In a typical design, the liquid is pumped between a ...
s (LFTRs). He first researched thorium reactors while working at
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
, 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
captures a
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
(whether in a
fast reactor
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as opposed t ...
or
thermal reactor
A thermal-neutron reactor is a nuclear reactor that uses slow or thermal neutrons. ("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and struct ...
) to become . This normally emits an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
and an
anti-neutrino () by
decay to become . This then emits another electron and anti-neutrino by a second decay to become , the fuel:
:
\overset+ -> -> beta^- -> beta^-\overset
Fission product waste
Nuclear fission
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...
produces radioactive
fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
s which can have half-lives from
days
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two s ...
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
Uranium ore deposits are economically recoverable concentrations of uranium within the Earth's crust. Uranium is one of the more common elements in the Earth's crust, being 40 times more common than silver and 500 times more common than gold. It ...
that would have been used to produce
low enriched uranium
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 ...
fuel for a
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
of the same power.
Other studies assume some actinide losses and find that actinide wastes dominate thorium cycle waste radioactivity at some future periods.
Some fission products have been proposed for
nuclear transmutation
Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.
A transmutatio ...
, 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
A deep geological repository is a way of storing hazardous or radioactive waste within a stable geologic environment (typically 200–1000 m deep). It entails a combination of waste form, waste package, engineered seals and geology that is suited ...
or elsewhere). However, while the principal feasibility of some of those reactions has been demonstrated at laboratory scale, there is as of 2022 no large scale deliberate transmutation of fission products anywhere in the world and the upcoming
MYRRHA
Myrrha (Greek: , ''Mýrra''), also known as Smyrna (Greek: , ''Smýrna''), is the mother of Adonis in Greek mythology. She was transformed into a myrrh tree after having had intercourse with her father, and gave birth to Adonis in tree form. A ...
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 extremely expensive
isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
.
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 , the transmutations tend to produce useful nuclear fuels rather than
transuranic
The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. ...
waste. When absorbs a neutron, it either fissions or becomes . The chance of fissioning on absorption of a
thermal neutron
The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...
is about 92%; the capture-to-fission ratio of , therefore, is about 1:12 – which is better than the corresponding capture vs. fission ratios of (about 1:6), or or (both about 1:3).
The result is less
transuranic
The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. ...
waste than in a reactor using the uranium-plutonium fuel cycle.
, like most
actinide
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
s with an even number of neutrons, is not fissile, but neutron capture produces fissile . If the fissile isotope fails to fission on neutron capture, it produces , , , and eventually fissile and heavier
isotopes of plutonium
Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long ...
. The can be removed and stored as waste or retained and transmuted to plutonium, where more of it fissions, while the remainder becomes , then
americium
Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was na ...
and
curium
Curium is a transuranic, radioactive chemical element with the symbol Cm and atomic number 96. This actinide element was named after eminent scientists Marie and Pierre Curie, both known for their research on radioactivity. Curium was first inte ...
, which in turn can be removed as waste or returned to reactors for further transmutation and fission.
However, the (with a half-life of ) formed via (''n'',2''n'') reactions with (yielding that decays to ), while not a transuranic waste, is a major contributor to the long-term
radiotoxic
Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
ity of spent nuclear fuel. While can in principle be converted back to by
neutron absorption
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...
its neutron absorption cross section is relatively low, making this rather difficult and possibly uneconomic.
Uranium-232 contamination
is also formed in this process, via (''n'',2''n'') reactions between
fast neutron
The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...
s and , , and :
:
Unlike most even numbered heavy isotopes, is also a
fissile
In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...
fuel fissioning just over half the time when it absorbs a thermal neutron. has a relatively short half-life (), and some
decay products emit high energy
gamma radiation
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
, such as , and particularly . The
full decay chain, along with half-lives and relevant gamma energies, is:
decays to where it joins the
decay chain of
:
Thorium-cycle fuels produce hard
gamma emission
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
s, which damage electronics, limiting their use in bombs. cannot be chemically separated from from
used nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...
; however, chemical separation of thorium from uranium removes the decay product and the radiation from the rest of the decay chain, which gradually build up as reaccumulates. The contamination could also be avoided by using a molten-salt breeder reactor and separating the before it decays into .
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 , the major part of natural and depleted uranium. The thermal neutron absorption
cross section
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture & engineering 3D
*Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
**Abs ...
(σ
a) and
resonance integral
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...
(average of neutron cross sections over intermediate neutron energies) for are about three and one third times those of the respective values for .
Advantages
The primary physical advantage of thorium fuel is that it uniquely makes possible a
breeder reactor
A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile mate ...
that runs with
slow neutrons, otherwise known as a
thermal breeder reactor.
These reactors are often considered simpler than the more traditional fast-neutron breeders. Although the thermal neutron fission cross section (σ
f) of the resulting is comparable to and , 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
Neutron economy is defined as the ratio of an adjoint weighted average of the excess neutron production divided by an adjoint weighted average of the fission production.
The distribution of neutron energies in a nuclear reactor differs from the f ...
. The ratio of neutrons released per neutron absorbed (η) in 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 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,
although present knowledge of
reserves is limited. Current demand for thorium has been satisfied as a by-product of
rare-earth
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
extraction from
monazite
Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is monazite-(Ce), that is, the cerium- ...
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
Repository may refer to:
Archives and online databases
* Content repository, a database with an associated set of data management tools, allowing application-independent access to the content
* Disciplinary repository (or subject repository), an ...
performance. Compared to the predominant reactor fuel,
uranium dioxide
Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear rea ...
(),
thorium dioxide
Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the minera ...
() has a higher
melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...
, higher
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
, and lower
coefficient of thermal expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kineti ...
. Thorium dioxide also exhibits greater
chemical stability
In chemistry, chemical stability is the thermodynamic stability of a chemical system.
Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. This may be a dynamic equilibri ...
and, unlike uranium dioxide, does not further
oxidize
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
.
Because the produced in thorium fuels is significantly contaminated with in proposed power reactor designs, thorium-based
used nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...
possesses inherent
proliferation resistance. cannot be
chemically separated from and has several
decay products that emit high-energy
gamma radiation
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
. These high-energy photons are a
radiological hazard that necessitate the use of
remote handling
Telerobotics is the area of robotics concerned with the control of semi-autonomous robots from a distance, chiefly using television, wireless networks (like Wi-Fi, Bluetooth and the Deep Space Network) or tethered connections. It is a combinatio ...
of separated uranium and aid in the passive
detection {{Unreferenced, date=March 2018
In general, detection is the action of accessing information without specific cooperation from with the sender.
In the history of radio communications, the term " detector" was first used for a device that detected ...
of such materials.
The long-term (on the order of roughly to ) radiological hazard of conventional uranium-based used nuclear fuel is dominated by plutonium and other
minor actinide
The minor actinides are the actinide elements in used nuclear fuel other than uranium and plutonium, which are termed the major actinides. The minor actinides include neptunium (element 93), americium (element 95), curium (element 96), berkeliu ...
s, after which
long-lived fission products
Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity it is necessary to isolate them from man ...
become significant contributors again. A single neutron capture in is sufficient to produce
transuranic elements
The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. ...
, whereas five captures are generally necessary to do so from . 98–99% of thorium-cycle fuel nuclei would fission at either or , 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
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 , or plutonium, must be added to achieve
criticality. This, along with the high
sintering
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
temperature necessary to make thorium-dioxide fuel, complicates fuel fabrication.
Oak Ridge National Laboratory
Oak Ridge National Laboratory (ORNL) is a U.S. multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT–Battelle as a federally funded research and ...
experimented with
thorium tetrafluoride
Thorium(IV) fluoride ( Th F4) is an inorganic chemical compound. It is a white hygroscopic powder which can be produced by reacting thorium with fluorine gas. At temperatures above 500 °C, it reacts with atmospheric moisture to produce Th ...
as fuel in a
molten salt reactor
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
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 in situ), higher
burnup
In nuclear power technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per init ...
is necessary to achieve a favorable
neutron economy
Neutron economy is defined as the ratio of an adjoint weighted average of the excess neutron production divided by an adjoint weighted average of the fission production.
The distribution of neutron energies in a nuclear reactor differs from the f ...
. Although thorium dioxide performed well at burnups of 170,000 MWd/t and 150,000 MWd/t at
Fort St. Vrain Generating Station
The Fort St. Vrain Nuclear Power Plant was a commercial nuclear power station located near the town of Platteville in northern Colorado in the United States. It operated from 1979 until 1989. It had a 330 MWe High-temperature gas reactor (HTGR) ...
and
AVR respectively,
challenges complicate achieving this in
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
s (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
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
products constitute a long-term radiological impact, especially and .
On a closed cycle, and can be reprocessed. is also considered an excellent burnable poison absorber in light water reactors.
Another challenge associated with the thorium fuel cycle is the comparatively long interval over which breeds to . The
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of is about 27 days, which is an order of magnitude longer than the half-life of . As a result, substantial develops in thorium-based fuels. is a significant
neutron absorber
In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable eff ...
and, although it eventually
breeds
A breed is a specific group of domestic animals having homogeneous appearance (phenotype), homogeneous behavior, and/or other characteristics that distinguish it from other organisms of the same species. In literature, there exist several slight ...
into fissile , this requires two more neutron absorptions, which degrades
neutron economy
Neutron economy is defined as the ratio of an adjoint weighted average of the excess neutron production divided by an adjoint weighted average of the fission production.
The distribution of neutron energies in a nuclear reactor differs from the f ...
and increases the likelihood of
transuranic
The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. ...
production.
Alternatively, if solid thorium is used in a
closed fuel cycle
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the ''front end'', which are the preparation of the fuel, steps in the ''service period'' in ...
in which is
recycled
Recycling is the process of converting waste materials into new materials and objects. The recovery of energy from waste materials is often included in this concept. The recyclability of a material depends on its ability to reacquire the p ...
,
remote handling
Telerobotics is the area of robotics concerned with the control of semi-autonomous robots from a distance, chiefly using television, wireless networks (like Wi-Fi, Bluetooth and the Deep Space Network) or tethered connections. It is a combinatio ...
is necessary for fuel fabrication because of the high radiation levels resulting from the
decay products
In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps (de ...
of . This is also true of recycled thorium because of the presence of , which is part of the decay sequence. Further, unlike proven uranium fuel recycling technology (e.g.
PUREX
PUREX (plutonium uranium reduction extraction) is a chemical method used to purify fuel for nuclear reactors or nuclear weapons. PUREX is the ''de facto'' standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium ...
), recycling technology for thorium (e.g. THOREX) is only under development.
Although the presence of complicates matters, there are public documents showing that has been used once in a
nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
test. The United States tested a composite -plutonium bomb core in the MET (Military Effects Test) blast during
Operation Teapot
Operation Teapot was a series of 14 nuclear test explosions conducted at the Nevada Test Site in the first half of 1955. It was preceded by ''Operation Castle'', and followed by ''Operation Wigwam''. ''Wigwam'' was, administratively, a part of ...
in 1955, though with much lower yield than expected.
Advocates for liquid core and
molten salt reactor
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
s 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.
Thorium-fueled reactors
Thorium fuels have fueled several different reactor types, including
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
s,
heavy water reactor
A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water ( deuterium oxide D2O) as its coolant and neutron moderator. PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. T ...
s,
high temperature gas reactors,
sodium-cooled fast reactor
A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.
The initials SFR in particular refer to two Generation IV reactor proposals, one based on existing liquid metal cooled reactor (LMFR) technology using mixed oxide fue ...
s, and
molten salt reactor
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
s.
[
]
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 from Energy Information Administration, "Spent Nuclear Fuel Discharges from U. S. Reactors", Table B4: Dresden 1 Assembly Class.
[ They were manufactured by ]General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
(assembly code XDR07G) and later sent to the Savannah River Site
The Savannah River Site (SRS) is a U.S. Department of Energy (DOE) reservation in the United States in the state of South Carolina, located on land in Aiken, Allendale, and Barnwell counties adjacent to the Savannah River, southeast of August ...
for reprocessing.
See also
*
Thorium
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...
*
Thorium-232
Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay ...
*
Occurrence of thorium
Thorium is found in small amounts in most rocks and soils. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO4), thorianite (ThO2 + UO2) and monazite. Tho ...
*
Thorium-based nuclear power
Thorium-based nuclear power generation is fueled primarily by the nuclear fission of the isotope uranium-233 produced from the fertile element thorium. A thorium fuel cycle can offer several potential advantages over a uranium fuel cycleA nuclea ...
*
List of countries by thorium resources Thorium resources are a potential source for low-carbon energy. Thorium has been demonstrated to perform as a nuclear fuel in several reactor designs. It is present with a higher abundance than uranium in the crust of the earth. Thorium resources ...
*
List of countries by uranium reserves
Uranium reserves are reserves of recoverable uranium, regardless of isotope, based on a set market price. The list given here is based on ''Uranium 2020: Resources, Production and Demand'', a joint report by the OECD Nuclear Energy Agency and th ...
*
Advanced heavy-water reactor
The advanced heavy-water reactor (AHWR) or AHWR-300 is the latest Indian design for a next-generation nuclear reactor that burns thorium in its fuel core. It is slated to form the third stage in India's three-stage fuel-cycle plan. This phase ...
*
Alvin Radkowsky Alvin Radkowsky (30 June 1915 – 17 February 2002) was an American nuclear physicist and chief scientist at U.S. Navy nuclear propulsion division. His work in the 1950s led to major advances in nuclear-ship technology and civilian use of nuclear po ...
*
CANDU reactor
The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...
*
Fuji MSR
The FUJI molten salt reactor is a proposed molten-salt-fueled thorium fuel cycle thermal breeder reactor, using technology similar to the Oak Ridge National Laboratory's Molten Salt Reactor Experiment – liquid fluoride thorium reactor. It was bei ...
*
Peak uranium
Uranium mining is the process of extraction of uranium ore from the ground. Over 50 thousand tons of uranium were produced in 2019. Kazakhstan, Canada, and Australia were the top three uranium producers, respectively, and together account f ...
*
Radioactive waste
Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons r ...
*
Thorium Energy Alliance
*
Weinberg Foundation Weinberg Foundation may refer to:
*Harry and Jeanette Weinberg Foundation
*The Alvin Weinberg Foundation
{{disambiguation ...
*
World energy resources and consumption
World energy supply and consumption is global production and preparation of fuel, generation of electricity, energy transport, and energy consumption. It is a basic part of economic activity. It includes heat, but not energy from food.
This art ...
References
Further reading
* Kasten, P. R. (1998).
Review of the Radkowsky Thorium reactor concept Science & Global Security, 7(3), 237–269.
* Duncan Clark (9 September 2011),
Thorium advocates launch pressure group. Huge optimism for thorium nuclear energy at the launch of the Weinberg Foundation, The Guardian
*
* B.D. Kuz'minov, V.N. Manokhin, (1998
"Status of nuclear data for the thorium fuel cycle" IAEA translation from the Russian journal Yadernye Konstanty (Nuclear Constants) Issue No. 3–4, 1997
Thorium and uranium fuel cyclescomparison by the UK
National Nuclear Laboratory
The National Nuclear Laboratory (informally NNL, formerly Nexia Solutions) is a UK government owned and operated nuclear services technology provider covering the whole of the nuclear fuel cycle. It is fully customer-funded and operates at six ...
Fact sheet on thorium{{Webarchive, url=https://web.archive.org/web/20130216102005/http://www.world-nuclear.org/info/inf62.html , date=2013-02-16 at the
World Nuclear Association
World Nuclear Association is the international organization that promotes nuclear power and supports the companies that comprise the global nuclear industry. Its members come from all parts of the nuclear fuel cycle, including uranium mining, ur ...
.
Annotated bibliography for the thorium fuel cyclefrom the Alsos Digital Library for Nuclear Issues
External links
International Thorium Energy Committee
Nuclear chemistry
Nuclear fuels
Nuclear reprocessing
Nuclear technology
Actinides
Thorium