Nuclear chemistry

Early history

After the discovery of neutrons to make new radioisotopes. Bohr model of the atom, where the positive nucleus is surrounded by the negative electrons.

Main areas

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

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 Otto Hahn.^[1]

The nuclear fuel cycle

The chemistry associated with any part of the 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

In the USA it is normal to use fuel once in a power reactor before placing it in a waste store. The long term plan is currently to place the civil used power reactor fuel in a deep store. This policy of not reprocessing was started in March 1977 for nuclear weapons proliferation reasons. The President Jimmy Carter issued a Presidential directive which indefinitely suspended the commercial reprocessing and recycling of plutonium in the USA. This Presidential directive is likely to have been an attempt by the USA to lead other countries by example, but many other nations continue to reprocess spent nuclear fuels. It is noteworthy that the government under Putin (President of Russia) repealed a law which had banned the import of used nuclear fuel into Russia, this change in Russian law now permits the Russians to offer a reprocessing service for clients outside Russia (In a similar way to that offered by BNFL).

PUREX chemistry

The current method of choice is to use the 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 + 2S_organic --> [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 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 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 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) 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 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 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

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 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 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, 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 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)

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

1. ^ Meitner L, Frisch OR (1939) Disintegration of uranium by neutrons: a new type of nuclear reaction Nature 143:239-240 [1]
2. ^ 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
3. ^ Decontamination of surfaces, George H. Goodalland Barry.E. Gillespie, United States Patent 4839100
4. ^ Glänneskog H (2004) Interactions of 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]
5. ^ 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