Nuclear transmutation is theconversion of onechemical element or anisotope into another chemical element.[1] Nuclear transmutation occurs in any process where the number ofprotons orneutrons in thenucleus of an atom is changed.
A transmutation can be achieved either bynuclear reactions (in which an outside particle reacts with a nucleus) or byradioactive decay, where no outside cause is needed.
Natural transmutation bystellar nucleosynthesis in the past created most of the heavier chemical elements in the known existing universe, and continues to take place to this day, creating the vast majority of the most common elements in the universe, includinghelium,oxygen andcarbon. Most stars carry out transmutation throughfusion reactions involvinghydrogen and helium, while much larger stars are also capable of fusing heavier elements up toiron late in their evolution.
Elements heavier than iron, such asgold orlead, are created through elemental transmutations that can naturally occur insupernovae. One goal of alchemy, the transmutation of base substances into gold, is now known to be impossible by chemical means but possible by physical means. As stars begin to fuse heavier elements, substantially less energy is released from each fusion reaction. This continues until it reaches iron which is produced by anendothermic reaction consuming energy. No heavier element can be produced in such conditions.
One type of natural transmutation observable in the present occurs when certainradioactive elements present in nature spontaneously decay by a process that causes transmutation, such asalpha orbeta decay. An example is the natural decay ofpotassium-40 toargon-40, which forms most of theargon in the air. Also on Earth, natural transmutations from the different mechanisms ofnaturalnuclear reactions occur, due tocosmic ray bombardment of elements (for example, to formcarbon-14), and also occasionally from natural neutron bombardment (for example, seenatural nuclear fission reactor).
Artificial transmutation may occur in machinery that has enough energy to cause changes in the nuclear structure of the elements. Such machines includeparticle accelerators andtokamak reactors. Conventionalfission power reactors also cause artificial transmutation, not from the power of the machine, but by exposing elements toneutrons produced by fission from an artificially producednuclear chain reaction. For instance, when a uranium atom is bombarded with slow neutrons, fission takes place. This releases, on average, three neutrons and a large amount of energy. The released neutrons then cause fission of other uranium atoms, until all of the available uranium is exhausted. This is called achain reaction.
Artificial nuclear transmutation has been considered as a possible mechanism for reducing the volume and hazard ofradioactive waste.[2]
The termtransmutation dates back toalchemy. Alchemists pursued thephilosopher's stone, capable ofchrysopoeia – the transformation ofbase metals into gold.[3] While alchemists often understood chrysopoeia as a metaphor for a mystical or religious process, some practitioners adopted a literal interpretation and tried to make gold through physical experimentation. The impossibility of the metallic transmutation had been debated amongst alchemists, philosophers and scientists since the Middle Ages. Pseudo-alchemical transmutation was outlawed[4] and publicly mocked beginning in the fourteenth century. Alchemists likeMichael Maier andHeinrich Khunrath wrote tracts exposing fraudulent claims of gold making. By the 1720s, there were no longer any respectable figures pursuing the physical transmutation of substances into gold.[5]Antoine Lavoisier, in the 18th century, replaced thealchemical theory of elements with the modern theory of chemical elements, andJohn Dalton further developed the notion of atoms (from the alchemical theory ofcorpuscles) 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 byFrederick Soddy when he, along withErnest Rutherford in 1901, discovered that radioactivethorium was converting itself intoradium. 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 ittransmutation. They'll have our heads off as alchemists."[6] Rutherford and Soddy were observing natural transmutation as a part ofradioactive decay of thealpha decay type.
The first artificial transmutation was accomplished in 1925 byPatrick Blackett, a research fellow working under Rutherford, with the transmutation of nitrogen intooxygen, usingalpha particles directed at nitrogen14N + α →17O + p.[7] Rutherford had shown in 1919 that a proton (he called it a hydrogen atom) was emitted from alpha bombardment experiments, but he had no information about the residual nucleus. Blackett's 1921–1924 experiments provided the first experimental evidence of an artificial nuclear transmutation reaction. Blackett correctly identified the underlying integration process and the identity of the residual nucleus.
In 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleaguesJohn Cockcroft andErnest 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 modernnuclear fission reaction discovered in 1938 byOtto Hahn,Lise Meitner and their assistantFritz Strassmann in heavy elements.[8] In 1941,Rubby Sherr,Kenneth Bainbridge andHerbert Lawrence Anderson reported the nuclear transmutation ofmercury intogold.[9]
Later in the twentieth century the transmutation of elements within stars was elaborated, accounting for the relative abundance of heavier elements in the universe. Save for the first five elements, which were produced in the Big Bang and othercosmic ray processes, stellar nucleosynthesis accounted for the abundance of all elements heavier thanboron. In their 1957 paperSynthesis of the Elements in Stars,[10]William Alfred Fowler,Margaret Burbidge,Geoffrey Burbidge, andFred Hoyle explained how the abundances of essentially all but the lightest chemical elements could be explained by the process ofnucleosynthesis in stars.
The alchemical tradition sought to turn the "base metal", lead, into gold. As a nuclear transmutation, it requires far less energy to turn gold into lead; for example, this would occur vianeutron capture andbeta decay if gold were left in a nuclear reactor for a sufficiently long period of time.[citation needed] In 1980,Glenn Seaborg, K. Aleklett, and a team atLawrence Berkeley National Laboratory'sBevatron succeeded in producing a minuscule amount of gold from bismuth, at a net energy loss.[11][12]
In 2002 and 2004,CERN scientists at theSuper Proton Synchrotron reported producing a minuscule amount of gold nuclei from induced photon emissions within deliberate near-miss collisions of lead nuclei.[13][14] In 2022, CERN'sISOLDE team reported producing 18 gold nuclei from proton bombardment of a uranium target.[15] In 2025, CERN'sALICE experiment team announced that over the previous decade, they had used theLarge Hadron Collider to replicate the 2002 SPS mechanisms at higher energies. A total of roughly 260 billion gold nuclei were created over three experimental runs, a minuscule amount massing about 90 picograms.[16][17]
TheBig Bang is thought to be the origin of the hydrogen (including alldeuterium) and helium in the universe. Hydrogen and helium together account for 98% of the mass of ordinary matter in the universe, while the other 2% makes up everything else. The Big Bang also produced small amounts oflithium,beryllium and perhapsboron. More lithium, beryllium and boron were produced later, in a natural nuclear reaction,cosmic ray spallation.
Stellar nucleosynthesis is responsible for all of the other elements occurring naturally in the universe asstable isotopes andprimordial nuclide, fromcarbon touranium. These occurred after the Big Bang, during star formation. Some lighter elements from carbon to iron were formed in stars and released into space byasymptotic giant branch (AGB) stars. These are a type of red giant that "puffs" off its outer atmosphere, containing some elements from carbon to nickel and iron. Nuclides withmass number greater than 64 are predominantly produced byneutron capture processes—thes-process andr-process–insupernova explosions andneutron star mergers.
TheSolar System is thought to have condensed approximately 4.6 billion years before the present, from a cloud of hydrogen and helium containing heavier elements in dust grains formed previously by a large number of such stars. These grains contained the heavier elements formed by transmutation earlier in the history of the universe.
All of these natural processes of transmutation in stars are continuing today, in our own galaxy and in others. Stars fuse hydrogen and helium into heavier and heavier elements (up to iron), producing energy. For example, the observed light curves of supernova stars such asSN 1987A show them blasting large amounts (comparable to the mass of Earth) of radioactive nickel and cobalt into space. However, little of this material reaches Earth. Most natural transmutation on the Earth today is mediated bycosmic rays (such as production ofcarbon-14) and by the radioactive decay of radioactiveprimordial nuclides left over from the initial formation of the Solar System (such aspotassium-40, uranium and thorium), plus thedecay chain of products of these nuclides (radium, radon, polonium, etc.).
Transmutation oftransuranium elements (i.e.actinides minusactinium touranium) such as theisotopes ofplutonium (about 1wt% in thelight water reactors' usednuclear fuel or theminor actinides (MAs, i.e.neptunium,americium, andcurium), about 0.1wt% each in light water reactors' used nuclear fuel) has the potential to help solve some problems posed by the management ofradioactive waste by reducing the proportion of long-lived isotopes it contains. (This does not rule out the need for adeep geological repository forhigh level radioactive waste.)[citation needed] When irradiated withfast neutrons in anuclear reactor, these isotopes can undergonuclear fission, destroying the originalactinide isotope and producing a spectrum of radioactive and nonradioactivefission products.
Ceramic targets containing actinides can be bombarded with neutrons to induce transmutation reactions to remove the most difficult long-lived species. These can consist of actinide-containing solid solutions such as(Am,Zr)N,(Am,Y)N,(Zr,Cm)O2,(Zr,Cm,Am)O2,(Zr,Am,Y)O2 or just actinide phases such asAmO2,NpO2,NpN,AmN mixed with some inert phases such asMgO,MgAl2O4,(Zr,Y)O2,TiN andZrN. The role of non-radioactive inert phases is mainly to provide stable mechanical behaviour to the target under neutron irradiation.[18]
There are issues with this P&T (partitioning and transmutation) strategy however:
The new study led by Satoshi Chiba at Tokyo Tech (called "Method to Reduce Long-lived Fission Products by Nuclear Transmutations with Fast Spectrum Reactors"[19]) shows that effective transmutation of long-lived fission products can be achieved in fast spectrum reactors without the need for isotope separation. This can be achieved by adding ayttrium deuteride moderator.[20]
For instance, plutonium can be reprocessed intomixed oxide fuels and transmuted in standard reactors. However, this is limited by the accumulation ofplutonium-240 in spent MOX fuel, which is neither particularly fertile (transmutation to fissileplutonium-241 does occur, but at lower rates than production of more plutonium-240 from neutron capture byplutonium-239) nor fissile with thermal neutrons. Even countries like France which practicenuclear reprocessing extensively, usually do not reuse the Plutonium content of used MOX-fuel. The heavier elements could be transmuted infast reactors, but probably more effectively in asubcritical reactor which is sometimes known as anenergy amplifier and which was devised byCarlo Rubbia.Fusionneutron sources have also been proposed as well suited.[21][22][23]
There are several fuels that can incorporate plutonium in their initial composition at their beginning of cycle and have a smaller amount of this element at the end of cycle. 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 surplusweapons grade plutonium from the weapons program and plutonium resulting ofreprocessing used nuclear fuel.
Mixed oxide fuel is one of these. Its blend of oxides of plutonium and uranium constitutes an alternative to the low enriched uranium fuel predominantly used in light water reactors. Since uranium is present in mixed oxide, although plutonium will be burnt, second generation plutonium will be produced through the radiative capture ofuranium-238 and the two subsequent beta minus decays.
Fuels with plutonium andthorium are also an option. In these, the neutrons released in the fission of plutonium are captured bythorium-232. After this radiative capture, thorium-232 becomes thorium-233, which undergoes two beta minus decays resulting in the production of the fissile isotopeuranium-233. The radiative capture cross section for thorium-232 is more than three times that of uranium-238, yielding a higher conversion to fissile fuel than that from uranium-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 mixed oxide fuels. However, uranium-233, which is fissile, will be present in the used nuclear fuel. Weapons-grade andreactor-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 plutonium-239.
| Nuclide | t1⁄2 | Yield | Q[a 1] | βγ |
|---|---|---|---|---|
| (Ma) | (%)[a 2] | (keV) | ||
| 99Tc | 0.211 | 6.1385 | 294 | β |
| 126Sn | 0.23 | 0.1084 | 4050[a 3] | βγ |
| 79Se | 0.33 | 0.0447 | 151 | β |
| 135Cs | 1.33 | 6.9110[a 4] | 269 | β |
| 93Zr | 1.61 | 5.4575 | 91 | βγ |
| 107Pd | 6.5 | 1.2499 | 33 | β |
| 129I | 16.1 | 0.8410 | 194 | βγ |
Some radioactive fission products can be converted into shorter-lived radioisotopes by transmutation. Transmutation of all fission products with half-life greater than one year is studied in Grenoble,[24] with varying results.
Strontium-90 and caesium-137, with half-lives of about 30 years, are the largest radiation (including heat) emitters in used nuclear fuel on a scale of decades to ~305 years (tin-121m is insignificant because of the low yield), and are not easily transmuted because they have lowneutron absorptioncross sections. Instead, they should simply be stored until they decay. Given that this length of storage is necessary, the fission products with shorter half-lives can also be stored until they decay.
The next longer-lived fission product issamarium-151, which has a half-life 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 remaining151
Sm in nuclear waste would require separation from other isotopes ofsamarium. Given the smaller quantities and its low-energy radioactivity,151
Sm is less dangerous than90
Sr and137
Cs and can also be left to decay for ~970 years.
Finally, there are sevenlong-lived fission products. They have much longer half-lives in the range 211,000 years to 15.7 million years. Two of them,technetium-99 andiodine-129, are mobile enough in the environment to be potential dangers, are free (Technetium has no known stable isotopes) 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.Additionally,99
Tc can substitute for uranium-238 in supplyingDoppler broadening for negative feedback for reactor stability.[25]Most studies of proposed transmutation schemes have assumed99
Tc,129
I, and transuranium elements as the targets for transmutation, with other fission products,activation products, and possiblyreprocessed uranium remaining as waste.[26] Technetium-99 is also produced as a waste product innuclear medicine fromTechnetium-99m, anuclear isomer that decays to its ground state which has no further use. Due to the decay product of100
Tc (the result of99
Tc capturing a neutron) decaying with a relatively short half-life to a stable isotope ofruthenium, aprecious metal, there might also be some economic incentive to transmutation, if costs can be brought low enough.
Of the remaining five long-lived fission products,selenium-79,tin-126 andpalladium-107 are produced only in small quantities (at least in today'sthermal neutron,235
U-burninglight water reactors) and the last two should be relatively inert. The other two,zirconium-93 andcaesium-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. Zirconium is used as cladding in fuel rods due to being virtually "transparent" to neutrons, but a small amount of93
Zr is produced by neutron absorption from the regularzircalloy without much ill effect. Whether93
Zr could be reused for new cladding material has not been subject of much study thus far.
{{cite journal}}: CS1 maint: numeric names: authors list (link)
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||
