Nuclear transmutation

Nuclear transmutation is the conversion of one chemical element or isotope into another. In other words, atoms of one element can be changed into atoms of other element by 'transmutation'. This occurs either through nuclear reactions (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 accelerators and tokamak reactors. Conventional fission power reactors also cause artificial transmutation, not from the power of the machine, but by exposing elements to neutrons 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.

History

The term transmutation dates back to the search for the philosopher's stone. 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 elements and when John Dalton further developed the notion of atoms (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."^[1]

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.^[2]

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,^[3] 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:

Burbidge, Burbidge, Fowler, Hoyle

Took the stars and made them toil:
Carbon, copper, gold, and lead
Formed in stars, is what they said^[4]

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.^[5] 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.

¹⁹⁷Au + n → ¹⁹⁸Au (halflife 2.7 days) → ¹⁹⁸Hg + n → ¹⁹⁹Hg + n → ²⁰⁰Hg + n → ²⁰¹Hg + n → ²⁰²Hg + n → ²⁰³Hg (halflife 47 days) → ²⁰³Tl + n → ²⁰⁴Tl (halflife 3.8 years) → ²⁰⁴Pb (halflife 1.4x10¹⁷ years)

Transmutation of nuclear wastes

Overview

Transmutation of transuranium elements (actinides) such as the isotopes 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.

Reactor types

For instance, plutonium can be reprocessed into MOX fuels 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 sources have also been proposed as well suited.^{[6][7] [8]}

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) 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-lives 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.

Long-lived fission products

Medium-lived
fission products

Prop: Unit:	t½ a	Yield %	Q * keV	βγ *
155Eu	4.76	.0803	252	βγ
85Kr	10.76	.2180	687	βγ
113mCd	14.1	.0008	316	β
90Sr	28.9	4.505	2826	β
137Cs	30.23	6.337	1176	βγ
121mSn	43.9	.00005	390	βγ
151Sm	90	.5314	77	β

Long-lived
fission products

Prop: Unit:	t½ Ma	Yield %	Q * KeV	βγ *
99Tc	0.211	6.1385	294	β
126Sn	0.230	0.1084	4050	βγ
79Se	0.327	0.0447	151	β
93Zr	1.53	5.4575	91	βγ
135Cs	2.3	6.9110	269	β
107Pd	6.5	1.2499	33	β
129I	15.7	0.8410	194	βγ

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 [1], 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 sections. 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 products. 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.^[9] 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. [2]

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.

See also

• Nuclear power
• List of nuclear waste treatment technologies

References

1. ^ Muriel Howorth,Pioneer Research on the Atom: The Life Story of Frederick Soddy, New World, London 1958, pp 83-84; Lawrence Badash, Radium, Radioactivity and the Popularity of Scientific Discovery, Proceedings of the American Philosophical Society 122,1978: 145-54; Thaddeus J. Trenn, The Self-Splitting Atom: The History of the Rutherford-Soddy Collaboration, Taylor & Francis, London, 1977, pp 42, 58-60, 111-17.
2. ^ Cockcroft and Walton split lithium with high energy protons April 1932.
3. ^ William Alfred Fowler, Margaret Burbidge, Geoffrey Burbidge, and Fred Hoyle, 'Synthesis of the Elements in Stars', Reviews of Modern Physics, vol. 29, Issue 4, pp. 547–650
4. ^ Ken Croswell, The Alchemy of the Heavens
5. ^ Anne Marie Helmenstine, Turning Lead into Gold: Is Alchemy Real?, About.com:Chemistry, retrieved January 2008
6. ^ Rita Plukiene, Evolution Of Transuranium Isotopic Composition In Power Reactors And Innovative Nuclear Systems For Transmutation, PhD Thesis, Vytautas Magnus University, 2003, retrieved January 2008
7. ^ Takibayev A., Saito M., Artisyuk V., and Sagara H., 'Fusion-driven transmutation of selected long-lived fission products', Progress in nuclear energy, Vol. 47, 2005, retrieved January 2008.
8. ^ http://aries.ucsd.edu/LIB/MEETINGS/0103-TRANSMUT/gohar/Gohar-present.pdf Transmutation of Transuranic Elements and Long Lived Fission Products in Fusion Devices, Y. Gohar, Argonne National Laboratory
9. ^ http://www.osti.gov/bridge/servlets/purl/10180577-PCAEyE/10180577.PDF Transmutation of Selected Fission Products in a Fast Reactor