Can Marathon Fusion make gold from mercury in a fusion reactor?

The alchemist’s ambition has always been to turn ordinary metals into gold - but is it actually possible?

The physics behind converting one element into another is already well understood. It has been applied for decades in accelerators and colliders, where subatomic particles are driven into collisions.

A leading modern example is Cern’s Large Hadron Collider in Geneva. Yet producing gold in this manner is enormously expensive, while the amount created is extremely small.

Cern’s Alice experiment, for instance, calculated that it generated just 29 picogrammes of gold during four years of operation. At that pace, producing one troy ounce of gold would require hundreds of times the age of the universe.

Marathon Fusion’s proposal to make gold

Californian start-up Marathon Fusion has suggested a markedly different method. It would use the radioactivity of neutron particles inside a nuclear fusion reactor to convert one type of mercury into another isotope, mercury-197.

Mercury-197 then decays into a stable gold isotope, gold-197. Particle decay is the process through which one subatomic particle spontaneously changes into two or more lighter particles.

Marathon Fusion’s team estimates that a fusion power station could create several tonnes of gold for every gigawatt of thermal power over a single year of operation.

When the isotope mercury-198 is bombarded with neutrons, it produces the radioactive isotope mercury-197, which then decays into gold’s only stable isotope.

The crucial requirement is neutrons energetic enough to initiate mercury’s decay chain. If the method can be made to work, it is an intriguing prospect. Whether it could deliver a worthwhile profit, however, is a separate question.

Neutron energy and fusion reactor requirements

Achieving this would require a substantial neutron flux, which measures the intensity of neutron radiation. A conventional fusion-reactor fuel combination of deuterium and tritium - both forms of hydrogen - could generate this flux while producing energy in a reactor’s plasma.

Neutrons readily pass through matter and scatter from atomic nuclei, or cores, losing energy in the process. To convert mercury-198 into gold, neutrons must have energies greater than 6 million electron volts.

Marathon Fusion developed its projections using a fusion reactor’s "digital twin": a computer simulation of fusion-reaction physics and the radioactive processes that follow. The weakness of this approach is that a digital twin must be tested against a working commercial fusion reactor, and no such reactor currently exists.

Scientists still face numerous obstacles before a commercial fusion reactor can become reality. They include developing new construction materials, mastering the science needed to run the system while extracting power continuously, and creating AI systems capable of helping to sustain the plasma fusion reaction.

Even highly developed fusion projects, including the UK’s JET (Joint European Torus), have been able to produce only comparatively limited quantities of energy.

Researchers in the UK have nevertheless developed a new approach to reducing fusion reactor size by altering how exhaust plasma is managed. A prototype based on this new concept, the Spherical Tokomak for Energy Production (Step), is intended to be ready by 2040.

Radioactive waste

In principle, a fusion reactor can make gold from mercury. But until commercial fusion reactors exist, the assumptions Marathon Fusion has used in its digital-twin research cannot be verified.

In addition, any gold made in a fusion reactor would initially be radioactive. It would therefore be treated as radioactive waste and would have to be managed for a considerable period after it was produced.

Nuclear and particle physicists are well aware that significant physical effects and crucial details can easily be overlooked when building an experiment’s digital twin.

Processing this waste into usable pure gold would present another problem. Even so, that challenge may not discourage investors with a long-term outlook.

For the moment, the idea is appealing in theory - but a new Californian gold rush remains some distance away.

Adrian Bevan, Professor of Physics, School of Physical and Chemical Sciences, Queen Mary University of London

This article is republished from The Conversation under a Creative Commons licence. Read the original article.