PUREX
Encyclopedia
PUREX is an acronym standing for Plutonium - URanium EXtraction — de facto
standard aqueous nuclear reprocessing
method for the recovery of uranium
and plutonium
from used nuclear fuel
. It is based on liquid-liquid extraction
ion-exchange.
The PUREX process was invented by Herbert H. Anderson and Larned B. Asprey
at the Metallurgical Laboratory
at the University of Chicago
, as part of the Manhattan Project
under Glenn T. Seaborg
; their patent "Solvent Extraction Process for Plutonium" filed in 1947, mentions tributyl phosphate
as the major reactant which accomplishes the bulk of the chemical extraction.
to which this process is applied consists primarily of certain very high atomic-weight
(actinoid or "actinide") elements
(e.g., uranium
) along with smaller amounts of material composed of lighter atoms, notably the so-called fission products.
The actinoid elements in this case consist primarily of the largely unconsumed remains of the original fuel (typically U-238
and other isotopes of uranium). In addition there are smaller quantities of other actinoids, created when one isotope is transmuted into another by a reaction involving neutron capture
. Plutonium-239
is the leading example. Another term sometimes seen in relation to this secondary material (and other material produced similarly) is activation products.
In response to the PUREX process' ability to extract nuclear weapons materials from the spent fuel, trade in the relevant chemicals is monitored.
In brief, the PUREX process is a liquid-liquid extraction
ion-exchange method used to reprocess spent nuclear fuel, in order to extract primarily uranium and plutonium, independent of each other, from the other constituents.
The irradiated fuel is first dissolved in nitric acid
at a concentration of ca. 7 mol dm-3. After the dissolution step it is normal to remove the fine insoluble solids, because otherwise they will disturb the solvent extraction process by altering the liquid-liquid interface. It is known that the presence of a fine solid can stabilize an emulsion
. Emulsions are often referred to as third phase
s in the solvent extraction community.
An organic solvent composed of 30% tributyl phosphate
(TBP) in a hydrocarbon
solvent, such as kerosene
, is used to extract the uranium and plutonium as MO2((NO3)2.2TBP complexes from other fission products, which remain in the aqueous phase. The transuranium element
s americium
and curium
also remain in the aqueous phase. The nature of the organic soluble uranium complex
has been the subject of some research
. A series of complexes of uranium with nitrate and trialkyl phosphates and phosphine oxide
s have been characterized.
Plutonium is separated from uranium by treating the kerosene solution with aqueous ferrous
sulphamate, which selectively reduces the plutonium to the +3 oxidation state. The plutonium passes into the aqueous phase. The uranium is stripped from the kerosene solution by back-extraction into nitric acid at a concentration of ca. 0.2 mol dm-3.
Dialkyl hydrogen phosphates are able to form complexes with many metals. These include some polymer
ic metal complexes. Formation of these coordination polymers
is one way in which fine solids can be formed in the process. While the cadmium concentration in both the fuel dissolution liquor and the PUREX raffinate
is very low, the polymeric complex of cadmium
of diethyl phosphate is shown in the left image. The right one is the structure of a lanthanide
complex of diethyl phosphate. Unlike cadmium the concentration of neodymium
in these mixtures formed from fuel is very high.
Below is a mixed tributyl phosphate dibutyl phosphate complex of uranium. Because the dibutyl phosphate ligands are acidic, it will now be possible to extract uranium by an ion exchange
liquid-liquid extraction mechanism rather than only by a solvation mechanism. This will potentially make the stripping of uranium with dilute nitric acid less effective.
as pertechnetate
through an ion pair extraction mechanism. Here is an example of a rhenium
version of a uranium / technetium complex which is thought to be responsible for the extraction of technetium into the organic phase. Here are two pictures of actinyl complexes of triphenylphosphine oxide
which have been crystallised with perrhenate
. With the less highly charged neptunyl ion it is also possible to form a complex.
was responsible for producing 'copious volumes of liquid wastes', resulting in the radioactive contamination of groundwater. A U.S. government report released in 1992 estimated that 685000 Ci of radioactive iodine-131 had been released into the river and air from the Hanford site between 1944 and 1947. Clean up costs are an estimated $2 billion a year.
De facto
De facto is a Latin expression that means "concerning fact." In law, it often means "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established." It is commonly used in contrast to de jure when referring to matters of law, governance, or...
standard aqueous 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...
method for the recovery of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and 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...
from used 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...
. It is based on liquid-liquid extraction
Liquid-liquid extraction
Liquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
ion-exchange.
The PUREX process was invented by Herbert H. Anderson and Larned B. Asprey
Larned B. Asprey
Larned "Larry" Brown Asprey was an American chemist noted for his work on actinide and rare earth and fluoride chemistry, and for his contributions to nuclear chemistry on the Manhattan project and later....
at the Metallurgical Laboratory
Metallurgical Laboratory
The Metallurgical Laboratory or "Met Lab" at the University of Chicago was part of the World War II–era Manhattan Project, created by the United States to develop an atomic bomb...
at the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
, as part of the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
under Glenn T. Seaborg
Glenn T. Seaborg
Glenn Theodore Seaborg was an American scientist who won the 1951 Nobel Prize in Chemistry for "discoveries in the chemistry of the transuranium elements", contributed to the discovery and isolation of ten elements, and developed the actinide concept, which led to the current arrangement of the...
; their patent "Solvent Extraction Process for Plutonium" filed in 1947, mentions tributyl phosphate
Tributyl phosphate
Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the formula 3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of orthophosphoric acid with n-butanol.- Production :Tributyl phosphate is manufactured by...
as the major reactant which accomplishes the bulk of the chemical extraction.
Overview
The spent nuclear fuelSpent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor...
to which this process is applied consists primarily of certain very high atomic-weight
Atomic weight
Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
(actinoid or "actinide") elements
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...
(e.g., uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
) along with smaller amounts of material composed of lighter atoms, notably the so-called fission products.
The actinoid elements in this case consist primarily of the largely unconsumed remains of the original fuel (typically 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...
and other isotopes of uranium). In addition there are smaller quantities of other actinoids, created when one isotope is transmuted into another by a reaction involving 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...
. Plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
is the leading example. Another term sometimes seen in relation to this secondary material (and other material produced similarly) is activation products.
In response to the PUREX process' ability to extract nuclear weapons materials from the spent fuel, trade in the relevant chemicals is monitored.
In brief, the PUREX process is a liquid-liquid extraction
Liquid-liquid extraction
Liquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
ion-exchange method used to reprocess spent nuclear fuel, in order to extract primarily uranium and plutonium, independent of each other, from the other constituents.
Chemical process
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
at a concentration of ca. 7 mol dm-3. After the dissolution step it is normal to remove the fine insoluble solids, because otherwise they will disturb the solvent extraction process by altering the liquid-liquid interface. It is known that the presence of a fine solid can stabilize an emulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
. Emulsions are often referred to as third phase
Third phase
Third phase is the term for a stable emulsion which forms in a solvent extraction system when the original two phases are mixed.The third phase can be caused by a detergent or a fine solid...
s in the solvent extraction community.
An organic solvent composed of 30% tributyl phosphate
Tributyl phosphate
Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the formula 3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of orthophosphoric acid with n-butanol.- Production :Tributyl phosphate is manufactured by...
(TBP) in a hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
solvent, such as kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
, is used to extract the uranium and plutonium as MO2((NO3)2.2TBP complexes from other fission products, which remain in the aqueous phase. The transuranium element
Transuranium element
In chemistry, transuranium elements are the chemical elements with atomic numbers greater than 92...
s 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...
also remain in the aqueous phase. The nature of the organic soluble uranium complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
has been the subject of some research
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
. A series of complexes of uranium with nitrate and trialkyl phosphates and phosphine oxide
Phosphine oxide
Phosphine oxides are either inorganic phosphorus compounds such as phosphoryl trichloride or organophosphorus compounds with the formula OPR3, where R = alkyl or aryl...
s have been characterized.
Plutonium is separated from uranium by treating the kerosene solution with aqueous ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
sulphamate, which selectively reduces the plutonium to the +3 oxidation state. The plutonium passes into the aqueous phase. The uranium is stripped from the kerosene solution by back-extraction into nitric acid at a concentration of ca. 0.2 mol dm-3.
Degradation products of TBP
It is normal to extract both the uranium and plutonium from the majority of the fission products, but it is not possible to get an acceptable separation of the fission products from the actinide products with a single extraction cycle. The unavoidable irradiation (by the material being processed) of the tributyl phosphate / hydrocarbon mixture produces dibutyl hydrogen phosphate. This degradation product is able to act as an extraction agent for many metals, hence leading to the contamination of the product by fission products. Hence it is normal to use more than one extraction cycle. The first cycle lowers the radioactivity of the mixture, allowing the later extraction cycles to be kept cleaner in terms of degradation products.Dialkyl hydrogen phosphates are able to form complexes with many metals. These include some polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
ic metal complexes. Formation of these coordination polymers
Coordination polymers
A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands, extending in an array. It can also be described as a polymer whose repeat units are coordination complexes...
is one way in which fine solids can be formed in the process. While the cadmium concentration in both the fuel dissolution liquor and the PUREX raffinate
PUREX raffinate
The term PUREX raffinate is a better term for the mixture of metals in nitric acid which are left behind when the uranium and plutonium have been removed by the PUREX process from a nuclear fuel dissolution liquor. This mixture is often known as high level nuclear waste.Two PUREX raffinates exist...
is very low, the polymeric complex of cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
of diethyl phosphate is shown in the left image. The right one is the structure of a lanthanide
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
complex of diethyl phosphate. Unlike cadmium the concentration of neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
in these mixtures formed from fuel is very high.
Below is a mixed tributyl phosphate dibutyl phosphate complex of uranium. Because the dibutyl phosphate ligands are acidic, it will now be possible to extract uranium by an ion exchange
Ion exchange
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
liquid-liquid extraction mechanism rather than only by a solvation mechanism. This will potentially make the stripping of uranium with dilute nitric acid less effective.
Extraction of technetium
In addition, the uranium(VI) tributyl phosphate system is able to extract technetiumTechnetium
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...
as pertechnetate
Pertechnetate
The pertechnetate ion is an oxoanion with the chemical formula TcO4−. It is often used as a convenient water-soluble source of isotopes of the radioactive element technetium...
through an ion pair extraction mechanism. Here is an example of a rhenium
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
version of a uranium / technetium complex which is thought to be responsible for the extraction of technetium into the organic phase. Here are two pictures of actinyl complexes of triphenylphosphine oxide
Triphenylphosphine oxide
Triphenylphosphine oxide is the chemical compound with the formula OP3. Often chemists abbreviate the formula by writing Ph3PO or PPh3O . This white crystalline compound is a common side product in reactions involving triphenylphosphine...
which have been crystallised with perrhenate
Perrhenate
The perrhenate ion is ReO4− and a perrhenate is a compound containing this ion. Salts of perrhenic acid are known as perrhenates. The perrhenate anion is tetrahedral, as shown by IR and Raman spectroscopy...
. With the less highly charged neptunyl ion it is also possible to form a complex.
Pollution
The PUREX Plant at the Hanford SiteHanford Site
The Hanford Site is a mostly decommissioned nuclear production complex on the Columbia River in the U.S. state of Washington, operated by the United States federal government. The site has been known by many names, including Hanford Works, Hanford Engineer Works or HEW, Hanford Nuclear Reservation...
was responsible for producing 'copious volumes of liquid wastes', resulting in the radioactive contamination of groundwater. A U.S. government report released in 1992 estimated that 685000 Ci of radioactive iodine-131 had been released into the river and air from the Hanford site between 1944 and 1947. Clean up costs are an estimated $2 billion a year.
List of nuclear reprocessing sites
- COGEMA La Hague siteCOGEMA La Hague siteThe AREVA NC La Hague site is a nuclear fuel reprocessing plant of AREVA in La Hague on the French Cotentin Peninsula that currently has nearly half of the world's light water reactor spent nuclear fuel reprocessing capacity. It has been in operation since 1976, and has a capacity of about 1700...
- MayakMayakMayak Production Association refers to an industrial complex that is one of the biggest nuclear facilities in the Russian Federation. It housed plutonium production reactors and a reprocessing plant...
- Thermal Oxide Reprocessing Plant and B205B205B205 is the name of the Magnox nuclear reprocessing plant at Sellafield in northern England. This plant uses PUREX chemistry to extract plutonium and uranium from used nuclear fuel....
at SellafieldSellafieldSellafield is a nuclear reprocessing site, close to the village of Seascale on the coast of the Irish Sea in Cumbria, England. The site is served by Sellafield railway station. Sellafield is an off-shoot from the original nuclear reactor site at Windscale which is currently undergoing... - Tokai, IbarakiTokai, Ibarakiis a village located in Naka District, Ibaraki, Japan. It is approximately 120 km north of Tokyo, Japan on the Pacific coast.As of 1 January 2005, the village has an estimated population of 35,467 and a population density of 946.29 persons per km²...
- West Valley Reprocessing PlantWest Valley Reprocessing PlantWest Valley Reprocessing Plant was a formerly operational plant for the reprocessing of used nuclear fuel at West Valley, New York, USA. It was operated from 1966-72. During this time period, 600,000 gallons of highly radioactive waste accumulated in an underground waste tank...
- Savannah River SiteSavannah River SiteThe Savannah River Site is a nuclear 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 Augusta, Georgia. The site was built during the 1950s to refine nuclear materials for...
- Idaho Chemical Processing Plant, (now Idaho National LaboratoryIdaho National LaboratoryIdaho National Laboratory is an complex located in the high desert of eastern Idaho, between the town of Arco to the west and the cities of Idaho Falls and Blackfoot to the east. It lies within Butte, Bingham, Bonneville and Jefferson counties...
) - Radiochemical Engineering Development Center, Oak Ridge National LaboratoryOak Ridge National LaboratoryOak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
See also
- Nuclear fuel cycleNuclear fuel cycleThe 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 which the fuel is used during reactor operation, and steps in...
- Nuclear breeder reactor
- Spent nuclear fuel shipping caskSpent nuclear fuel shipping caskSpent nuclear fuel shipping casks are used to transport spent nuclear fuel used in nuclear power plants and research reactors to disposal sites such as the nuclear reprocessing center at COGEMA La Hague site...
- Global Nuclear Energy PartnershipGlobal Nuclear Energy PartnershipThe International Framework for Nuclear Energy Cooperation formerly the Global Nuclear Energy Partnership began as a U.S. proposal, announced by United States Secretary of Energy Samuel Bodman on February 6, 2006, to form an international partnership to promote the use of nuclear power and close...
announced February, 2006
Further reading
- OECD Nuclear Energy Agency, The Economics of the Nuclear Fuel Cycle, Paris, 1994
- I. Hensing and W Schultz, Economic Comparison of Nuclear Fuel Cycle Options, Energiewirtschaftlichen Instituts, Cologne, 1995.
- Cogema, Reprocessing-Recycling: the Industrial Stakes, presentation to the Konrad-Adenauer-Stiftung, Bonn, 9 May 1995.
- OECD Nuclear Energy Agency, Plutonium Fuel: An Assessment, Paris, 1989.
- National Research Council, "Nuclear Wastes: Technologies for Separation and Transmutation", National Academy Press, Washington D.C. 1996.
External links
- Processing of Used Nuclear Fuel, World Nuclear Association
- Reactor-Grade Plutonium and Development of Nuclear Weapons, Analytical Center for Non-proliferation
- PUREX Process, European Nuclear Society
- Mixed Oxide Fuel (MOX) - World Nuclear Association
- Disposal Options for Surplus Weapons-Usable Plutonium - Congressional Research Service Report for Congress
- Brief History of Fuel Reprocessing