Nuclear transmutation
Encyclopedia
Nuclear transmutation is the conversion of one chemical element
or isotope
into another. In other words, atom
s of one element can be changed into atoms of other element by 'transmutation'. This occurs either through nuclear reaction
s (in which an outside particle reacts with a nucleus), or through radioactive decay
(where no outside particle is needed).
One type of natural transmutation occurs when certain radioactive elements spontaneously decay over a period of time, transforming into other elements. An example is the natural decay of potassium-40
to the argon-40 which forms most of the argon
in air. Also on Earth, natural transmutations from the different mechanism of natural nuclear reactions occur, due to cosmic ray
bombardment of elements (for example, to form carbon-14
), and also occasionally from natural neutron bombardment (for example, see natural nuclear fission reactor
).
Artificial transmutation may occur in machinery that has enough energy to cause changes in the nuclear structure of the elements. Machines that can cause artificial transmutation include particle accelerator
s and tokamak
reactors. Conventional fission power reactors also cause artificial transmutation, not from the power of the machine, but by exposing elements to neutron
s produced by a fission from an artificially produced nuclear chain reaction
.
Artificial nuclear transmutation has been considered as a possible mechanism for reducing the volume and hazard of radioactive waste
.
. In alchemy
, it was believed that the transformation of base metals into gold could be accomplished in table-top experiments. The alchemical belief in transmutation was based on a thoroughly wrong understanding of the underlying processes. The belief in transmutation continued to be widespread in Europe until Antoine Lavoisier
, in the 18th century, replaced the alchemical theory of elements with the modern theory of chemical element
s and when John Dalton
further developed the notion of atom
s (from the alchemical theory of corpuscles
) to explain various chemical processes. The disintegration of atoms is a distinct process involving much greater energies than could be achieved by alchemists.
It was first consciously applied to modern physics by Frederick Soddy
when he, along with Ernest Rutherford
, discovered that radioactive thorium
was converting itself into radium
in 1901. At the moment of realization, Soddy later recalled, he shouted out: "Rutherford, this is transmutation!" Rutherford snapped back, "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."
Rutherford and Soddy were observing natural transmutation as a part of radioactive decay
of the alpha decay
type. However in 1917, Rutherford was able to accomplish transmutation of nitrogen into oxygen
, using alpha particles directed at nitrogen 14N + α → 17O
+ p. This was the first observation of a nuclear reaction
, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleagues John Cockcroft
and Ernest Walton
, who used artificially accelerated protons against lithium-7 to split the nucleus into two alpha particles. The feat was popularly known as "splitting the atom," although it was not the modern nuclear fission
reaction later discovered in heavy elements.
Later in the twentieth century the transmutation of elements within stars was elaborated, accounting for the relative abundance of elements in the universe. In their 1957 paper Synthesis of the Elements in Stars, William Alfred Fowler
, Margaret Burbidge
, Geoffrey Burbidge
, and Fred Hoyle
explained how the abundances of essentially all but the lightest chemical elements could be explained by the process of nucleosynthesis
in stars.
Author Ken Croswell
summarised their discoveries thus:
It transpired that, under true nuclear transmutation, it is far easier to turn gold into lead than the reverse reaction, which was the one the alchemists had ardently pursued. Nuclear experiments have successfully transmuted lead into gold, but the expense far exceeds any gain. It would be easier to convert gold into lead via neutron capture
and beta decay
by leaving gold in a nuclear reactor for a long period of time.
More information on gold synthesis, see Synthesis of precious metals.
197 + n
→ 198Au (halflife 2.7 days) → 198 + n → 199Hg + n → 200Hg + n → 201Hg + n → 202Hg + n → 203Hg (halflife 47 days) → 203 + n → 204Tl (halflife 3.8 years) → 204Pb
(halflife 1.4x1017 years)
s of plutonium
, neptunium
, americium
, and curium
has the potential to help solve the problems posed by the management of radioactive waste
, by reducing the proportion of long-lived isotopes it contains. When irradiated with fast neutrons in a nuclear reactor
, these isotopes can be made to undergo nuclear fission
, destroying the original actinide
isotope and producing a spectrum of radioactive and nonradioactive fission products.
s and transmuted in standard reactors. The heavier elements could be transmuted in fast reactors, but probably more effectively in a subcritical reactor which is sometimes known as an energy amplifier
and which was devised by Carlo Rubbia
. Fusion
neutron source
s have also been proposed as well suited.
Mixed Oxide fuel (MOX)
is one of these. Its blend of oxides of plutonium and uranium constitutes an alternative to the Low Enriched Uranium (LEU) fuel predominantly used in Light Water Reactors (LWR). Since uranium is present in MOX, although plutonium will be burnt, second generation plutonium will be produced through the radiative capture of U-238 and the two subsequent beta minus decays.
Fuels with plutonium and thorium are also an option. In these, the neutrons released in the fission of plutonium are captured by Th-232. After this radiative capture, Th-232 becomes Th-233, which undergoes two beta minus decays resulting in the production of the fissile isotope U-233. The radiative capture cross section for Th-232 is more than three times that of U-238, yielding a higher conversion to fissile fuel than that from U-238. Due to the absence of uranium in the fuel, there is no second generation plutonium produced, and the amount of plutonium burnt will be higher than in MOX fuels. However, U-233, which is fissile, will be present in the SNF. Weapons-grade and reactor-grade plutonium can be used in plutonium-thorium fuels, with weapons-grade plutonium being the one that shows a bigger reduction in the amount of Pu-239.
of many thousands of years, whereas radioactive fission products tend to be shorter-lived (most with half-lives of 30 years or less). From a waste management viewpoint, transmutation of actinides eliminates a very long-term radioactive hazard and replaces it with a much shorter-term one.
It is important to understand that the threat posed by a radioisotope is influenced by many factors including the chemical and biological
properties of the element. For instance caesium
has a relatively short biological halflife (1 to 4 months) while strontium
and radium
both have very long biological half-lives. As a result strontium-90
and radium are much more able to cause harm than caesium-137
when a given activity is ingested.
Many of the actinides are very radiotoxic because they have long biological half-lives and are alpha
emitters. In transmutation the intention is to convert the actinides into fission products. The fission products are very radioactive, but the majority of the activity will decay away within a short time. The most worrying short-lived fission products are those that accumulate in the body, such as iodine
-131 which accumulates in the thyroid gland, but it is hoped that by good design of the nuclear fuel
and transmutation plant that such fission products can be isolated from humans and their environment and allowed to decay. In the medium term the fission products of highest concern are strontium-90 and caesium-137; both have a half-life of about 30 years. The caesium-137 is responsible for the majority of the external gamma dose experienced by workers in nuclear reprocessing
plants and, in 2005, to workers at the Chernobyl
site. When these medium-lived isotopes have decayed the remaining isotopes will pose a much smaller threat.
Sr-90 and Cs-137, with halflives of about 30 years, are the largest radiation emitters in used nuclear fuel on a scale of decades to a few hundreds of years, and are not easily transmuted because they have low neutron absorption cross section
s. Instead, they should simply be stored until they decay. Given that this length of storage is necessary, the fission products with shorter halflives can also be stored until they decay.
The next longer-lived fission product is Sm-151, which has a halflife of 90 years, and is such a good neutron absorber that most of it is transmuted while the nuclear fuel is still being used; however, effectively transmuting the remaining Sm-151 in nuclear waste would require separation from other isotopes of samarium
. Given the smaller quantities and its low-energy radioactivity, Sm-151 is less dangerous than Sr-90 and Cs-137 and can also be left to decay.
Finally, there are 7 long-lived fission product
s. They have much longer halflives in the range 211,000 years to 16 million years. Two of them, Tc-99 and I-129
, are mobile enough in the environment to be potential dangers, are free or mostly free of mixture with stable isotopes of the same element, and have neutron cross sections that are small but adequate to support transmutation.
Also, Tc-99 can substitute for U-238
in supplying Doppler broadening
for negative feedback for reactor stability.
Most studies of proposed transmutation schemes have assumed 99Tc
, 129I
, and transuranics as the targets for transmutation, with other fission products, activation products, and possibly reprocessed uranium
remaining as waste. http://www.ieer.org/reports/transm/report.pdf
Of the remaining 5 long-lived fission products, Se-79, Sn-126 and Pd-107 are produced only in small quantities (at least in today's thermal neutron, U-235
-burning light water reactors) and the last two should be relatively inert. The other two, Zr-93 and Cs-135, are produced in larger quantities, but also not highly mobile in the environment. They are also mixed with larger quantities of other isotopes of the same element.
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
or isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
into another. In other words, atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of one element can be changed into atoms of other element by 'transmutation'. This occurs either through nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
s (in which an outside particle reacts with a nucleus), or through radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
(where no outside particle is needed).
One type of natural transmutation occurs when certain radioactive elements spontaneously decay over a period of time, transforming into other elements. An example is the natural decay of potassium-40
Potassium-40
Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1.248 years, or about 39.38 seconds.Potassium-40 is a rare example of an isotope which undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 with emission of a beta particle...
to the argon-40 which forms most of the argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
in air. Also on Earth, natural transmutations from the different mechanism of natural nuclear reactions occur, due to cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
bombardment of elements (for example, to form carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...
), and also occasionally from natural neutron bombardment (for example, see natural nuclear fission reactor
Natural nuclear fission reactor
A natural nuclear fission reactor is a uranium deposit where analysis of isotope ratios has shown that self-sustaining nuclear chain reactions have occurred. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under...
).
Artificial transmutation may occur in machinery that has enough energy to cause changes in the nuclear structure of the elements. Machines that can cause artificial transmutation include particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s and tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
reactors. Conventional fission power reactors also cause artificial transmutation, not from the power of the machine, but by exposing elements to neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s produced by a fission from an artificially produced nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
.
Artificial nuclear transmutation has been considered as a possible mechanism for reducing the volume and hazard of radioactive waste
Radioactive waste
Radioactive wastes are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine...
.
History
The term transmutation dates back to the search for the philosopher's stonePhilosopher's stone
The philosopher's stone is a legendary alchemical substance said to be capable of turning base metals into gold or silver. It was also sometimes believed to be an elixir of life, useful for rejuvenation and possibly for achieving immortality. For many centuries, it was the most sought-after goal...
. In alchemy
Alchemy
Alchemy is an influential philosophical tradition whose early practitioners’ claims to profound powers were known from antiquity. The defining objectives of alchemy are varied; these include the creation of the fabled philosopher's stone possessing powers including the capability of turning base...
, it was believed that the transformation of base metals into gold could be accomplished in table-top experiments. The alchemical belief in transmutation was based on a thoroughly wrong understanding of the underlying processes. The belief in transmutation continued to be widespread in Europe until Antoine Lavoisier
Antoine Lavoisier
Antoine-Laurent de Lavoisier , the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology...
, in the 18th century, replaced the alchemical theory of elements with the modern theory of chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s and when John Dalton
John Dalton
John Dalton FRS was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness .-Early life:John Dalton was born into a Quaker family at Eaglesfield, near Cockermouth, Cumberland,...
further developed the notion of atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s (from the alchemical theory of corpuscles
Corpuscularianism
Corpuscularianism is a physical theory that supposed all matter to be composed of minute particles, which became important in the Seventeenth century. Among the leading corpuscularians were Rene Descartes, Robert Boyle, and John Locke....
) to explain various chemical processes. The disintegration of atoms is a distinct process involving much greater energies than could be achieved by alchemists.
It was first consciously applied to modern physics by Frederick Soddy
Frederick Soddy
Frederick Soddy was an English radiochemist who explained, with Ernest Rutherford, that radioactivity is due to the transmutation of elements, now known to involve nuclear reactions. He also proved the existence of isotopes of certain radioactive elements...
when he, along with Ernest Rutherford
Ernest Rutherford
Ernest Rutherford, 1st Baron Rutherford of Nelson OM, FRS was a New Zealand-born British chemist and physicist who became known as the father of nuclear physics...
, discovered that radioactive thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
was converting itself into radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
in 1901. At the moment of realization, Soddy later recalled, he shouted out: "Rutherford, this is transmutation!" Rutherford snapped back, "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."
Rutherford and Soddy were observing natural transmutation as a part of radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
of the alpha decay
Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
type. However in 1917, Rutherford was able to accomplish transmutation of nitrogen into oxygen
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...
, using alpha particles directed at nitrogen 14N + α → 17O
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...
+ p. This was the first observation of a nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleagues John Cockcroft
John Cockcroft
Sir John Douglas Cockcroft OM KCB CBE FRS was a British physicist. He shared the Nobel Prize in Physics for splitting the atomic nucleus with Ernest Walton, and was instrumental in the development of nuclear power....
and Ernest Walton
Ernest Walton
Ernest Thomas Sinton Walton was an Irish physicist and Nobel laureate for his work with John Cockcroft with "atom-smashing" experiments done at Cambridge University in the early 1930s, and so became the first person in history to artificially split the atom, thus ushering the nuclear age...
, who used artificially accelerated protons against lithium-7 to split the nucleus into two alpha particles. The feat was popularly known as "splitting the atom," although it was not the modern nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
reaction later discovered in heavy elements.
Later in the twentieth century the transmutation of elements within stars was elaborated, accounting for the relative abundance of elements in the universe. In their 1957 paper Synthesis of the Elements in Stars, William Alfred Fowler
William Alfred Fowler
William Alfred "Willy" Fowler was an American astrophysicist and winner of the Nobel Prize for Physics in 1983. He should not be confused with the British astronomer Alfred Fowler....
, Margaret Burbidge
Margaret Burbidge
Eleanor Margaret Burbidge, née Peachey, FRS is a British-born American astrophysicist, noted for original research and holding many administrative posts, including director of the Royal Greenwich Observatory....
, Geoffrey Burbidge
Geoffrey Burbidge
Geoffrey Ronald Burbidge FRS was an English astronomy professor, most recently at the University of California, San Diego. He was married to astrophysicist Dr. Margaret Burbidge.-Education:...
, and Fred Hoyle
Fred Hoyle
Sir Fred Hoyle FRS was an English astronomer and mathematician noted primarily for his contribution to the theory of stellar nucleosynthesis and his often controversial stance on other cosmological and scientific matters—in particular his rejection of the "Big Bang" theory, a term originally...
explained how the abundances of essentially all but the lightest chemical elements could be explained by the process of nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
in stars.
Author Ken Croswell
Ken Croswell
Ken Croswell is an astronomer and author living in Berkeley, California. His first degree mixed science and wider interests, majoring in physics and minoring in English literature. He also got a PhD in astronomy for studying the Milky Way's halo....
summarised their discoveries thus:
It transpired that, under true nuclear transmutation, it is far easier to turn gold into lead than the reverse reaction, which was the one the alchemists had ardently pursued. Nuclear experiments have successfully transmuted lead into gold, but the expense far exceeds any gain. It would be easier to convert gold into lead via neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
and beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
by leaving gold in a nuclear reactor for a long period of time.
More information on gold synthesis, see Synthesis of precious metals.
197 + n
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
→ 198Au (halflife 2.7 days) → 198 + n → 199Hg + n → 200Hg + n → 201Hg + n → 202Hg + n → 203Hg (halflife 47 days) → 203 + n → 204Tl (halflife 3.8 years) → 204Pb
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
(halflife 1.4x1017 years)
Overview
Transmutation of transuranium elements (actinides) such as the isotopeIsotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s of plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
, neptunium
Neptunium
Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
, americium
Americium
Americium is a synthetic element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide element europium, and thus by analogy was named after another continent, America.Americium was first produced in 1944...
, and curium
Curium
Curium is a synthetic chemical element with the symbol Cm and atomic number 96. This radioactive transuranic element of the actinide series was named after Marie Skłodowska-Curie and her husband Pierre Curie. Curium was first intentionally produced and identified in summer 1944 by the group of...
has the potential to help solve the problems posed by the management of radioactive waste
Radioactive waste
Radioactive wastes are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine...
, by reducing the proportion of long-lived isotopes it contains. When irradiated with fast neutrons in a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
, these isotopes can be made to undergo nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
, destroying the original actinide
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 group 3 element actinium...
isotope and producing a spectrum of radioactive and nonradioactive fission products.
Reactor types
For instance, plutonium can be reprocessed into MOX fuelMOX fuel
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material. MOX fuel contains plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the...
s and transmuted in standard reactors. The heavier elements could be transmuted in fast reactors, but probably more effectively in a subcritical reactor which is sometimes known as an energy amplifier
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 leave an energy profit for power generation...
and which was devised by Carlo Rubbia
Carlo Rubbia
Carlo Rubbia Knight Grand Cross 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.-Biography:...
. Fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
neutron source
Neutron source
A Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
s have also been proposed as well suited.
Fuel types
There are several fuels that can incorporate plutonium in their initial composition at Beginning of Cycle (BOC) and have a smaller amount of this element at the End of Cycle (EOC). During the cycle, plutonium can be burnt in a power reactor, generating electricity. This process is not only interesting from a power generation standpoint, but also due to its capability of consuming the surplus weapons grade plutonium from the weapons program and plutonium resulting of reprocessing Spent Nuclear Fuel (SNF).Mixed Oxide fuel (MOX)
MOX fuel
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material. MOX fuel contains plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the...
is one of these. Its blend of oxides of plutonium and uranium constitutes an alternative to the Low Enriched Uranium (LEU) fuel predominantly used in Light Water Reactors (LWR). Since uranium is present in MOX, although plutonium will be burnt, second generation plutonium will be produced through the radiative capture of U-238 and the two subsequent beta minus decays.
Fuels with plutonium and thorium are also an option. In these, the neutrons released in the fission of plutonium are captured by Th-232. After this radiative capture, Th-232 becomes Th-233, which undergoes two beta minus decays resulting in the production of the fissile isotope U-233. The radiative capture cross section for Th-232 is more than three times that of U-238, yielding a higher conversion to fissile fuel than that from U-238. Due to the absence of uranium in the fuel, there is no second generation plutonium produced, and the amount of plutonium burnt will be higher than in MOX fuels. However, U-233, which is fissile, will be present in the SNF. Weapons-grade and reactor-grade plutonium can be used in plutonium-thorium fuels, with weapons-grade plutonium being the one that shows a bigger reduction in the amount of Pu-239.
Reasoning behind transmutation
Isotopes of plutonium and other actinides tend to be long-lived with half-livesHalf-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of many thousands of years, whereas radioactive fission products tend to be shorter-lived (most with half-lives of 30 years or less). From a waste management viewpoint, transmutation of actinides eliminates a very long-term radioactive hazard and replaces it with a much shorter-term one.
It is important to understand that the threat posed by a radioisotope is influenced by many factors including the chemical and biological
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
properties of the element. For instance caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
has a relatively short biological halflife (1 to 4 months) while strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
and radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
both have very long biological half-lives. As a result strontium-90
Strontium-90
Strontium-90 is a radioactive isotope of strontium, with a half-life of 28.8 years.-Radioactivity:Natural strontium is nonradioactive and nontoxic, but 90Sr is a radioactivity hazard...
and radium are much more able to cause harm than caesium-137
Caesium-137
Caesium-137 is a radioactive isotope of caesium which is formed as a fission product by nuclear fission.It has a half-life of about 30.17 years, and decays by beta emission to a metastable nuclear isomer of barium-137: barium-137m . Caesium-137 is a radioactive isotope of caesium which is formed...
when a given activity is ingested.
Many of the actinides are very radiotoxic because they have long biological half-lives and are alpha
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
emitters. In transmutation the intention is to convert the actinides into fission products. The fission products are very radioactive, but the majority of the activity will decay away within a short time. The most worrying short-lived fission products are those that accumulate in the body, such as iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
-131 which accumulates in the thyroid gland, but it is hoped that by good design of the nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
and transmutation plant that such fission products can be isolated from humans and their environment and allowed to decay. In the medium term the fission products of highest concern are strontium-90 and caesium-137; both have a half-life of about 30 years. The caesium-137 is responsible for the majority of the external gamma dose experienced by workers in nuclear reprocessing
Nuclear reprocessing
Nuclear reprocessing technology was developed to chemically separate and recover fissionable plutonium from irradiated nuclear fuel. Reprocessing serves multiple purposes, whose relative importance has changed over time. Originally reprocessing was used solely to extract plutonium for producing...
plants and, in 2005, to workers at the Chernobyl
Chernobyl
Chernobyl or Chornobyl is an abandoned city in northern Ukraine, in Kiev Oblast, near the border with Belarus. The city had been the administrative centre of the Chernobyl Raion since 1932....
site. When these medium-lived isotopes have decayed the remaining isotopes will pose a much smaller threat.
Long-lived fission products
Some radioactive fission products can be converted into shorter-lived radioisotopes by transmutation. Transmutation of all fission products with halflife greater than one year is studied in http://www.patentstorm.us/patents/4721596-description.html, with varying results.Sr-90 and Cs-137, with halflives of about 30 years, are the largest radiation emitters in used nuclear fuel on a scale of decades to a few hundreds of years, and are not easily transmuted because they have low neutron absorption cross section
Neutron cross-section
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power...
s. Instead, they should simply be stored until they decay. Given that this length of storage is necessary, the fission products with shorter halflives can also be stored until they decay.
The next longer-lived fission product is Sm-151, which has a halflife of 90 years, and is such a good neutron absorber that most of it is transmuted while the nuclear fuel is still being used; however, effectively transmuting the remaining Sm-151 in nuclear waste would require separation from other isotopes of samarium
Samarium
Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...
. Given the smaller quantities and its low-energy radioactivity, Sm-151 is less dangerous than Sr-90 and Cs-137 and can also be left to decay.
Finally, there are 7 long-lived fission product
Long-lived fission product
Long-lived fission products are radioactive materials with a long half-life produced by nuclear fission.-Evolution of radioactivity in nuclear waste:...
s. They have much longer halflives in the range 211,000 years to 16 million years. Two of them, Tc-99 and I-129
Iodine-129
Iodine-129 is long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant....
, are mobile enough in the environment to be potential dangers, are free or mostly free of mixture with stable isotopes of the same element, and have neutron cross sections that are small but adequate to support transmutation.
Also, Tc-99 can substitute for U-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
in supplying Doppler broadening
Doppler broadening
In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different shifts, the cumulative effect of which is the line broadening.The...
for negative feedback for reactor stability.
Most studies of proposed transmutation schemes have assumed 99Tc
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
, 129I
Iodine-129
Iodine-129 is long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant....
, and transuranics as the targets for transmutation, with other fission products, activation products, and possibly reprocessed uranium
Reprocessed uranium
Reprocessed uranium is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium actually makes up the bulk of the material separated during reprocessing...
remaining as waste. http://www.ieer.org/reports/transm/report.pdf
Of the remaining 5 long-lived fission products, Se-79, Sn-126 and Pd-107 are produced only in small quantities (at least in today's thermal neutron, U-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
-burning light water reactors) and the last two should be relatively inert. The other two, Zr-93 and Cs-135, are produced in larger quantities, but also not highly mobile in the environment. They are also mixed with larger quantities of other isotopes of the same element.