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Nuclear chemistry is a subfield of chemistry dealing with radioactivity, nuclear processes and nuclear properties. * It is the chemistry of radioactive elements such as the actinides, radium and radon together with the chemistry associated with equipment (such as nuclear reactors) which are designed to perform nuclear processes. This includes the corrosion of surfaces and the behaviour under conditions of both normal and abnormal operation (such as during an accident). An important area is the behaviour of objects and materials after being placed into a waste store or otherwise disposed of.
- the study of the chemical effects resulting from the absorption of
radiation within living animals, plants, and other materials. The radiation chemistry controls much of radiation biology
as radiation has an effect on living things at the molecular scale, to
explain it another way the radiation alters the biochemicals within an
organism, the alteration of the biomolecules then changes the chemistry
which occurs within the organism, this change in biochemistry
then can lead to a biological outcome. As a result nuclear chemistry
greatly assists the understanding of medical treatments (such as cancer radiotherapy) and has enabled these treatments to improve.
Early history
After the discovery of X-rays by Wilhelm Röntgen, many scientists began to work on ionizing radiation. One of these was Henri Becquerel, who investigated the relationship between phosphorescence and the blackening of photographic plates.
When Becquerel (working in France) discovered that, with no external
source of energy, the uranium generated rays which could blacken (or fog) the photographic plate, radioactivity was discovered. Marie Curie (working in Paris) and her husband Pierre Curie isolated two new radioactive elements from uranium ore. They used radiometric
methods to identify which stream the radioactivity was in after each
chemical separation; they separated the uranium ore into each of the
different chemical elements that were known at the time, and measured
the radioactivity of each fraction. They then attempted to separate
these radioactive fractions further, to isolate a smaller fraction with
a higher specific activity (radioactivity divided by mass). In this
way, they isolated polonium and radium.
It was noticed in about 1901 that high doses of radiation could cause
an injury in humans, Becquerel had carried a sample of radium in his
pocket and as a result he suffered a high localised dose which resulted
in a radiation burn[7]
this injury resulted in the biological properties of radiation being
investigated, which in time resulted in the development of medical
treatments. Marie Curie's daughter (Irène Joliot-Curie) and her husband were the first to 'create' radioactivity: they bombarded boron with alpha particles to make a proton-rich isotope of nitrogen; this isotope emitted positrons.[8] In addition, they bombarded aluminium and magnesium with neutrons to make new radioisotopes. Ernest Rutherford, working in Canada and England, showed that radioactivity decay can be described by a simple equation (a linear first degree derivative equation, now called first order kinetics), implying that a given radioactive substance has a characteristic "half life" (the time taken for the amount of radioactivity present in a source to diminish by half). He also coined the terms alpha, beta and gamma rays, he converted nitrogen into oxygen, and most importantly he supervised the students who did the Geiger-Marsden experiment (gold leaf experiment) which showed that the 'plum pudding model' of the atom was wrong. In the plum pudding model, proposed by J. J. Thomson
in 1904, the atom is composed of electrons surrounded by a 'cloud' of
positive charge to balance the electrons' negative charge. To
Rutherford, the gold foil experiment implied that the positive charge
was confined to a very small nucleus leading first to the Rutherford model, and eventually to the Bohr model of the atom, where the positive nucleus is surrounded by the negative electrons.
Main areas
Radiochemistry is the chemistry of radioactive materials, where radioactive isotopes of elements are used to study the properties and chemical reactions
of non-radioactive isotopes (often within radiochemistry the absence of
radioactivity leads to a substance being described as being inactive as the isotopes are stable).
For further details please see the page on radiochemistry.
Radiation chemistry
Radiation chemistry is the study of the chemical effects of radiation on matter; this is very different to radiochemistry
as no radioactivity needs to be present in the material which is being
chemically changed by the radiation. An example is the conversion of
water into hydrogen gas and hydrogen peroxide.
Study of nuclear reactions
see also nuclear physics and nuclear reactions for further details.
A combination of radiochemistry and radiation chemistry is used to study nuclear reactions such as fission and fusion. Some early evidence for nuclear fission was the formation of a shortlived radioisotope of barium which was isolated from neutron irradiated uranium (139Ba, with a half-life of 83 minutes and 140Ba, with a half-life of 12.8 days, are major fission products
of uranium). At the time, it was thought that this was a new radium
isotope, as it was then standard radiochemical practice to use a barium
sulphate carrier precipitate to assist in the isolation of radium.[9].
More recently, a combination of radiochemical methods and nuclear
physics has been used to try to make new 'superheavy' elements; it is
thought that islands of relative stability exist where the nuclides
have half-lives of years, thus enabling weighable amounts of the new
elements to be isolated. For more details of the original discovery of
nuclear fission see the work of Otto Hahn.[1]
The nuclear fuel cycle
The chemistry associated with any part of the nuclear fuel cycle, including nuclear reprocessing.
The fuel cycle includes all the operations involved in producing fuel,
from mining, ore processing and enrichment to fuel production (Front end of the cycle). It also includes the 'in-pile' behaviour (use of the fuel in a reactor) before the back end of the cycle. The back end includes the management of the used nuclear fuel in either a cooling pond or dry storage, before it is disposed of into an underground waste store or reprocessed.
Normal and abnormal conditions
The nuclear chemistry associated with the nuclear fuel cycle can be
divided into two main areas, one area is concerned with operation under
the intended conditions while the other area is concerned with
maloperation conditions where some alteration from the normal operating
conditions has occurred or (more rarely) an accident is occurring.
Reprocessing
Law
PUREX chemistry
The current method of choice is to use the PUREX liquid-liquid extraction process which uses a tributyl phosphate/hydrocarbon mixture to extract both uranium and plutonium from nitric acid. This extraction is of the nitrate salts and is classed as being of a solvation
mechanism. For example the extraction of plutonium by an extraction
agent (S) in a nitrate medium occurs by the following reaction.
Pu4+aq + 4NO3-aq + 2Sorganic --> [Pu(NO3)4S2]organic
A complex is formed between the metal cation, the nitrates and the
tributyl phosphate, and a model compound of a dioxouranium(VI) complex
with two nitrates and two triethyl phosphates has been characterised by
X-ray crystallography.[2]
When the nitric acid concentration is high the extraction into the
organic phase is favoured, and when the nitric acid concentration is
low the extraction is reversed (the organic phase is stripped
of the metal). It is normal to dissolve the used fuel in nitric acid,
after the removal of the insoluble matter the uranium and plutonium are
extracted from the highly active liquor. It is normal to then back
extract the loaded organic phase to create a medium active
liquor which contains mostly uranium and plutonium with only small
traces of fission products. This medium active aqueous mixture is then
extracted again by tributyl phosphate/hydrocarbon to form a new organic
phase, the metal bearing organic phase is then stripped of the metals
to form an aqueous mixture of only uranium and plutonium. The two
stages of extraction are used to improve the purity of the actinide
product, the organic phase used for the first extraction will suffer a
far greater dose of radiation. The radiation can degrade the tributyl
phosphate into dibutyl hydrogen phosphate. The dibutyl hydrogen
phosphate can act as an extraction agent for both the actinides and
other metals such as ruthenium. The dibutyl hydrogen phosphate can make the system behave in a more complex manner as it tends to extract metals by an ion exchange
mechanism (extraction favoured by low acid concentration), to reduce
the effect of the dibutyl hydrogen phosphate it is common for the used
organic phase to be washed with sodium carbonate solution to remove the acidic degradation products of the tributyl phosphate.
New methods being considered for future use
The PUREX process can be modified to make a UREX (URanium EXtraction) process which could be used to save space inside high level nuclear waste disposal sites, such as Yucca Mountain, by removing the uranium which makes up the vast majority of the mass and volume of used fuel and recycling it as reprocessed uranium.
The UREX process is a PUREX process which has been modified to
prevent the plutonium being extracted. This can be done by adding a
plutonium reductant before the first metal extraction step. In the UREX
process, ~99.9% of the Uranium and >95% of Technetium
are separated from each other and the other fission products and
actinides. The key is the addition of acetohydroxamic acid (AHA) to the
extraction and scrub sections of the process. The addition of AHA
greatly diminishes the extractability of Plutonium and Neptunium, providing greater proliferation resistance than with the plutonium extraction stage of the PUREX process.
Adding a second extraction agent, octyl(phenyl)-N, N-dibutyl
carbamoylmethyl phosphine oxide(CMPO) in combination with
tributylphosphate, (TBP), the PUREX process can be turned into the TRUEX (TRansUranic EXtraction)
process this is a process which was invented in the USA by Argonne
National Laboratory, and is designed to remove the transuranic metals
(Am/Cm) from waste. The idea is that by lowering the alpha activity of
the waste, the majority of the waste can then be disposed of with
greater ease. In common with PUREX this process operates by a solvation
mechanism.
As an alternative to TRUEX, an extraction process using a malondiamide has been devised. The DIAMEX (DIAMideEXtraction) process has the advantage of avoiding the formation of organic waste which contains elements other than Carbon, Hydrogen, Nitrogen, and Oxygen. Such an organic waste can be burned without the formation of acidic gases which could contribute to acid rain. The DIAMEX process is being worked on in Europe by the French CEA.
The process is sufficiently mature that an industrial plant could be
constructed with the existing knowledge of the process. In common with
PUREX this process operates by a solvation mechanism.[10][11]
Selective ActiNide EXtraction. As part of the management of minor actinides it has been proposed that the lanthanides and trivalent minor actinides should be removed from the PUREX raffinate
by a process such as DIAMEX or TRUEX. In order to allow the actinides
such as americium to be either reused in industrial sources or used as
fuel the lanthanides
must be removed. The lanthanides has large neutron cross sections and
hence they would poison a neutron driven nuclear reaction. To date the
extraction system for the SANEX process has not been defined, but
currently several different research groups are working towards a
process. For instance the French CEA is working on a bis-triaiznyl pyridine (BTP) based process.
Other systems such as the dithiophosphinic acids are being worked on by some other workers.
This is the UNiversal EXtraction process which was developed in Russia and the Czech Republic, it is a process designed to remove all of the most troublesome (Sr, Cs and minor actinides) radioisotopes from the raffinates left after the extraction of uranium and plutonium from used nuclear fuel. [12][13] The chemistry is based upon the interaction of caesium and strontium with poly ethylene oxide (poly ethylene glycol) [14] and a cobalt carborane anion (known as chlorinated cobalt dicarbollide) . The actinides are extracted by CMPO, and the diluent is a polar aromatic such as nitrobenzene. Other dilents such as meta-nitrobenzotrifluoride and phenyl trifluoromethyl sulfone [15]have been suggested as well.
Absorption of fission products on surfaces
Another important area of nuclear chemistry is the study of how
fission products interact with surfaces; this is thought to control the
rate of release and migration of fission products both from waste
containers under normal conditions and from power reactors under
accident conditions. It is interesting to note that, like chromate and molybdate, the 99TcO4 anion can react with steel surfaces to form a corrosion resistant layer. In this way, these metaloxo anions act as anodic corrosion inhibitors. The formation of 99TcO2 on steel surfaces is one effect which will retard the release of 99Tc from nuclear waste drums and nuclear equipment which has been lost before decontamination (eg submarine reactors lost at sea). This 99TcO2 layer renders the steel surface passive, inhibiting the anodic corrosion
reaction. The radioactive nature of technetium makes this corrosion
protection impractical in almost all situations. It has also been shown
that 99TcO4 anions react to form a layer on the surface of activated carbon (charcoal) or aluminium.[3][16]. A short review of the biochemical properties of a series of key long lived radioisotopes can be read on line.[17]
It is important to note that 99Tc in nuclear waste may exist in chemical forms other than the 99TcO4 anion, these other forms have different chemical properties.[18]
Similarly, the release of iodine-131 in a serious power reactor accident could be retarded by absorption on metal surfaces within the nuclear plant.[4]
Spinout areas
Some methods first developed within nuclear chemistry and physics
have become so widely used within chemistry and other physical sciences
that they may be best thought of as separate from normal
nuclear chemistry. For example, the isotope effect is used so
extensively to investigate chemical mechanisms and the use of
cosmogenic isotopes and long-lived unstable isotopes in geology that it is best to consider much of isotopic chemistry as separate from nuclear chemistry.
Kinetics (use within mechanistic chemistry)
The mechanisms of chemical reactions can be investigated by
observing how the kinetics of a reaction are changed by making an
isotopic modification of a substrate. This is now a standard method in organic chemistry. Briefly, replacing normal hydrogens (protons) by deuterium within a chemical compound causes the rate of molecular vibration (C-H, N-H and O-H bonds show this) to decrease[citation needed].
This then can lead to a decrease in the reaction rate if the
rate-determining step involves breaking a bond between hydrogen and
another atom. Thus, if the reaction changes in rate when protons are
replaced by deuteriums, it is reasonable to assume that the breaking of
the bond to hydrogen is part of the step which determines the rate.
Uses within geology, biology and forensic science
Cosmogenic isotopes are formed by the interaction of cosmic rays
with the nucleus of an atom. These can be used for dating purposes and
for use as natural tracers. In addition, by careful measurement of some
ratios of stable isotopes it is possible to obtain new insights into
the origin of bullets, ages of ice samples, ages of rocks, and the diet
of a person can be identified from a hair or other tissue sample. (See Isotope geochemistry and Isotopic signature for further details).
Biology
Within living things, isotopic labels (both radioactive and
nonradioactive) can be used to probe how the complex web of reactions
which makes up the metabolism of an organism converts one substance to another. For instance a green plant uses light energy to convert water and carbon dioxide into glucose by photosynthesis.
If the oxygen in the water is labeled, then the label appears in the
oxygen gas formed by the plant and not in the glucose formed in the chloroplasts within the plant cells.
For biochemical and physiological experiments and medical methods, a number of specific isotopes have important applications.
- Stable isotopes have the advantage of not delivering a
radiation dose to the system being studied; however, a significant
excess of them in the organ or organism might still interfere with its
functionality, and the availability of sufficient amounts for
whole-animal studies is limited for many isotopes. Measurement is also
difficult, and usually requires mass spectroscopy
to determine how much of the isotope is present in particular
compounds, and there is no means of localizing measurements within the
cell.
- H-2 (deuterium), the stable isotope of hydrogen, is a stable
tracer, the concentration of which can be measured by mass spectroscopy
or NMR. It is incorporated into all cellular structures. Specific
deuterated compound can also be produced.
- N-15, the stable isotope of nitrogen, has also been used. It is incorporated mainly into proteins.
- Radioactive isotopes have the advantages of being detectable in very low quantities, in being easily measured by scintillation counting or other radiochemical methods, and in being localizable to particular regions of a cell, and quantifiable by autoradiography.
Many compounds with the radioactive atoms in specific positions can be
prepared, and are widely available commercially. In high quantities
they require precautions to guard the workers from the effects of
radiation--and they can easily contaminate laboratory glassware and
other equipment. For some isotopes the half-life is so short that
preparation and measurement is difficult.
By organic synthesis it is possible to create a complex molecule
with a radioactive label that can be confined to a small area of the
molecule. For short-lived isotopes such as 11C, very rapid
synthetic methods have been developed to permit the rapid addition of
the radioactive isotope to the molecule. For instance a palladium catalysed carbonylation reaction in a microfluidic device has been used to rapidly form amides[5] and it might be possible to use this method to form radioactive imaging agents for PET imaging.[19]
- ³H, Tritium, the radioisotope of hydrogen, it available at very
high specific activities, and compounds with this isotope in particular
positions are easily prepared by standard chemical reactions such as
hydrogenation of unsaturated precursors. The isotope emits very soft
beta radiation, and can be detected by scintillation counting.
- 11C, Carbon-11 can be made using a cyclotron, boron in the form of boric oxide is reacted with protons in a (p,n) reaction. An alternative route is to react 10B with deuterons. By rapid organic synthesis, the 11C compound formed in the cyclotron is converted into the imaging agent which is then used for PET.
- 14C, Carbon-14 can be made (as above), and it is
possible to convert the target material into simple inorganic and
organic compounds. In most organic synthesis
work it is normal to try to create a product out of two approximately
equal sized fragments and to use a convergent route, but when a
radioactive label is added, it is normal to try to add the label late
in the synthesis in the form of a very small fragment to the molecule
to enable the radioactivity to be localised in a single group. Late
addition of the label also reduces the number of synthetic stages where
radioactive material is used.
- 18F, fluorine-18 can be made by the reaction of neon with deuterons, 20Ne reacts in a (d,4He) reaction. It is normal to use neon gas with a trace of stable fluorine (19F2). The 19F2
acts as a carrier which increases the yield of radioactivity from the
cyclotron target by reducing the amount of radioactivity lost by
absorption on surfaces. However, this reduction in loss is at the cost
of the specific activity of the final product.
Nuclear magnetic resonance (NMR)
NMR spectroscopy
uses the net spin of nuclei in a substances upon energy absorption to
identify molecules. This has now become a standard spectroscopic tool
within synthetic chemistry. One major use of NMR is to determine the bond connectivity within an organic molecule.
NMR imaging also uses the net spin of nuclei (commonly protons) for
imaging. This is widely used for diagnostic purposes in medicine, and
can provide detailed images of the inside of a person without
inflicting any radiation upon them. In a medical setting, NMR is often
known simply as "magnetic resonance" imaging, as the word 'nuclear' has
negative connotations for many people.
References
- ^ Meitner L, Frisch OR (1939) Disintegration of uranium by neutrons: a new type of nuclear reaction Nature 143:239-240 [1]
- ^ J.H.
Burns, "Solvent-extraction complexes of the uranyl ion. 2. Crystal and
molecular structures of catena-bis(.mu.-di-n-butyl
phosphato-O,O')dioxouranium(VI) and bis(.mu.-di-n-butyl
phosphato-O,O')bis[(nitrato)(tri-n-butylphosphine
oxide)dioxouranium(VI)]", Inorganic Chemistry, 1983, 22, 1174-1178
- ^ Decontamination of surfaces, George H. Goodalland Barry.E. Gillespie, United States Patent 4839100
- ^ Glänneskog H (2004) Interactions of I2 and CH3I with reactive metals under BWR severe-accident conditions Nuclear Engineering and Design 227:323-9
- Glänneskog H (2005) Iodine chemistry under severe accident
conditions in a nuclear power reactor, PhD thesis, Chalmers University
of Technology, Sweden
- For other work on the iodine chemistry which would occur during a bad accident, see[2][3][4]
- ^ Miller PW et al (2006) Chemical Communications 546-548
Text books
- Radioactivity Radionuclides Radiation
- Textbook by Magill, Galy. ISBN -3-540-21116-0, Springer, 2005.
- Radiochemistry and Nuclear Chemistry
- Comprehensive textbook by Choppin, Liljenenzin and Rydberg. ISBN -0750674636, Butterworth-Heinemann, 2001 [20].
- Radioactivity, Ionizing radiation and Nuclear Energy
- Basic textbook for undergraduates by Jiri Hála and James D Navratil. ISBN -807302053-X, Konvoj, Brno 2003 [21]
- The Radiochemical Manual
- Overview of the production and uses of both open and sealed
sources. Edited by BJ Wilson and written by RJ Bayly, JR Catch, JC
Charlton, CC Evans, TT Gorsuch, JC Maynard, LC Myerscough, GR Newbery,
H Sheard, CBG Taylor and BJ Wilson. The radiochemical centre (Amersham)
was sold via HMSO, 1966 (second edition)
See also Important publications in nuclear chemistry
Classical
from 