Nuclear transmutation is the process of changing one chemical element or isotope into another chemical element. This happens when the number of protons or neutrons in the nucleus of an atom is altered.

Transmutation can occur through nuclear reactions, where an outside particle interacts with a nucleus, or through radioactive decay, which does not require an outside cause.

Natural transmutation, which happened in stars long ago, created most of the heavier elements in the universe and still occurs today, forming many of the most common elements, such as helium, oxygen, and carbon. Most stars create elements through fusion reactions involving hydrogen and helium, while larger stars can fuse heavier elements up to iron during their later stages.

Elements heavier than iron, like gold or lead, are formed in supernovae explosions. Alchemy, an ancient practice, aimed to change common materials into gold, but this is not possible with chemical methods. However, it can be achieved through physical methods. As stars fuse heavier elements, less energy is released with each reaction. This continues until iron is formed, which requires energy instead of releasing it. No heavier elements can be made under these conditions.

A type of natural transmutation that happens today involves radioactive elements decaying on their own, such as through alpha or beta decay. For example, potassium-40 decays into argon-40, which is a major source of argon in the air. On Earth, natural transmutations also occur due to cosmic rays hitting elements (like forming carbon-14) or from natural neutron bombardment (such as in natural nuclear fission reactors).

Artificial transmutation happens in machines that have enough energy to change the structure of atomic nuclei. These machines include particle accelerators and tokamak reactors. Conventional fission power reactors also cause artificial transmutation by exposing elements to neutrons created during nuclear chain reactions. For example, when uranium atoms are struck by slow neutrons, fission occurs, releasing three neutrons and a large amount of energy. These neutrons trigger more fission in other uranium atoms until all available uranium is used. This is called a chain reaction.

Artificial nuclear transmutation has been studied as a way to reduce the amount and danger of radioactive waste.

History

The term transmutation comes from alchemy, an ancient practice. Alchemists tried to create the philosopher's stone, which could change base metals into gold. Some saw this as a spiritual goal, while others tried physical experiments. For centuries, people debated whether changing metals into gold was possible. By the 1300s, fake claims about making gold were banned and mocked. Scientists like Michael Maier and Heinrich Khunrath wrote about these false claims. By the 1720s, respected scientists no longer tried to make gold physically. In the 1700s, Antoine Lavoisier replaced alchemy’s ideas with the modern theory of chemical elements. John Dalton later explained how atoms work, building on earlier alchemical ideas. Breaking apart atoms requires much more energy than alchemists could achieve.

In 1901, Frederick Soddy and Ernest Rutherford discovered that radioactive thorium changed into radium. Soddy excitedly called this "transmutation," but Rutherford warned against using the term, fearing it would make them seem like alchemists. Their work showed that natural transmutation happens during radioactive decay.

In 1925, Patrick Blackett, working under Rutherford, made the first artificial transmutation by turning nitrogen into oxygen using alpha particles. Rutherford had earlier found that protons were released during similar experiments, but he did not know what remained. Blackett’s experiments proved that artificial nuclear reactions could change elements.

In 1932, John Cockcroft and Ernest Walton created the first fully artificial nuclear reaction by splitting lithium-7 with protons. This was called "splitting the atom," though it was not the same as the nuclear fission discovered later in 1938. In 1941, scientists reported turning mercury into gold through nuclear transmutation.

Later in the 20th century, scientists learned how elements are created in stars. Most heavy elements in the universe were formed through stellar processes, not the Big Bang. In 1957, scientists explained how stars create elements through a process called nucleosynthesis.

Alchemists aimed to turn lead into gold, but nuclear transmutation requires less energy to turn gold into lead. For example, gold could become lead in a nuclear reactor over time. In 1980, scientists made a tiny amount of gold from bismuth, but it used more energy than it produced.

In 2002 and 2004, CERN scientists created a small amount of gold nuclei by colliding lead nuclei. In 2022, they made 18 gold nuclei by bombarding uranium with protons. By 2025, CERN’s ALICE team had created about 260 billion gold nuclei using the Large Hadron Collider, but the total mass was only 90 picograms.

Startup Marathon Fusion suggests a way to make gold from mercury using a nuclear fusion reactor. High-energy neutrons would change mercury into unstable gold, which would then decay into stable gold over about 64 hours.

Transmutation in the universe

The Big Bang is believed to be the source of hydrogen (including deuterium) and helium in the universe. Together, hydrogen and helium make up 98% of the mass of ordinary matter in the universe, while the remaining 2% includes all other elements. The Big Bang also created small amounts of lithium, beryllium, and possibly boron. Additional lithium, beryllium, and boron were later formed through natural nuclear reactions called cosmic ray spallation.

Stellar nucleosynthesis is responsible for creating all other naturally occurring elements in the universe, from carbon to uranium. These elements formed after the Big Bang, during the creation of stars. Some lighter elements, such as those from carbon to iron, were produced in stars and released into space by asymptotic giant branch (AGB) stars. These are a type of red giant that releases its outer atmosphere, which contains elements from carbon to nickel and iron. Elements with a mass number greater than 64 are mostly created through neutron capture processes—the s-process and r-process—during supernova explosions and neutron star mergers.

The Solar System is believed to have formed about 4.6 billion years ago from a cloud of hydrogen and helium that contained heavier elements in dust grains. These grains were created earlier by many stars and included heavier elements formed through transmutation in the universe’s history.

These natural processes of transmutation in stars continue today in our galaxy and others. Stars combine hydrogen and helium to form heavier elements (up to iron), producing energy. For example, light patterns observed from supernova stars like SN 1987A show them sending large amounts of radioactive nickel and cobalt into space, similar in mass to Earth. However, little of this material reaches Earth. Most natural transmutation on Earth today occurs through cosmic rays (such as the creation of carbon-14) and the radioactive decay of primordial nuclides left over from the Solar System’s formation (such as potassium-40, uranium, and thorium). This also includes the decay of products from these nuclides, such as radium, radon, and polonium.

Artificial transmutation of nuclear waste

Transmutation of transuranium elements (elements heavier than uranium), such as plutonium isotopes (about 1 weight percent in used nuclear fuel from light water reactors) and minor actinides (like neptunium, americium, and curium, each about 0.1 weight percent in used nuclear fuel), can help reduce the amount of long-lived radioactive waste. This does not eliminate the need for deep geological storage for high-level waste. When these isotopes are exposed to fast neutrons in a nuclear reactor, they can undergo fission, breaking down the original actinide and creating a mix of radioactive and nonradioactive products.

Ceramic materials containing actinides can be bombarded with neutrons to change long-lived radioactive elements into less harmful forms. These materials may include solid solutions like (Am,Zr)N or (Zr,Cm)O₂, or actinide phases like AmO₂ mixed with nonradioactive materials such as MgO or MgAl₂O₄. These nonradioactive parts help keep the material strong and stable during neutron exposure.

However, there are challenges with this strategy. A study by Satoshi Chiba at Tokyo Tech shows that long-lived fission products can be changed in fast reactors without separating isotopes, using a yttrium deuteride moderator. For example, plutonium can be reprocessed into mixed oxide fuel and used in standard reactors, but this is limited by the buildup of plutonium-240, which is not easily used in thermal reactors. Countries like France often do not reuse plutonium from spent MOX fuel. Heavier elements might be better transmuted in fast reactors or in subcritical reactors like energy amplifiers, designed by Carlo Rubbia. Fusion neutron sources are also being considered.

Some fuels can incorporate plutonium and reduce its amount over time. For instance, mixed oxide fuel (MOX) combines plutonium and uranium oxides, offering an alternative to low-enriched uranium fuel. While plutonium is consumed, new plutonium may form from uranium-238. Fuels with plutonium and thorium also work by converting thorium-232 into uranium-233, which can be used as fuel. These fuels avoid producing second-generation plutonium and burn more plutonium than MOX. Weapons-grade plutonium can be used in these fuels, reducing plutonium-239 more effectively.

Some radioactive fission products can be changed into shorter-lived isotopes through transmutation. Studies in Grenoble explore transmuting all fission products with half-lives longer than one year, with mixed results. Strontium-90 and caesium-137, which emit significant radiation for decades, are hard to transmute due to low neutron absorption. These should be stored until they decay. Samarium-151, with a 90-year half-life, is more easily transmuted during fuel use but requires separation from other samarium isotopes for full transmutation.

Seven long-lived fission products have half-lives from 211,000 years to 15.7 million years. Technetium-99 and iodine-129 are mobile in the environment and can be transmuted because they absorb neutrons. Technetium-99 is also produced in nuclear medicine and may have economic value if transmuted into stable ruthenium. Other long-lived products like selenium-79 and tin-126 are produced in small amounts and are less harmful. Zirconium-93 and caesium-135 are produced in larger amounts but are not highly mobile. Zirconium is used in reactor fuel cladding, and its production from neutron absorption is not a major issue. Reusing zirconium for new cladding has not been widely studied.