A Fusion Startup Says It Turned Mercury Into Gold. The Numbers Are Missing

October 24 — According to the outlet Interesting Engineering, the San Francisco fusion startup Marathon Fusion claims to have achieved “turning mercury into gold” using the high-energy neutrons produced by nuclear fusion.

The company announced on the 21st local time that its gold production rate had reached 10 times CERN’s 2025 record. However, the experimental results have not been independently verified in public, and Marathon has not revealed how much gold it actually made.

According to Marathon’s account of the mechanism, 14 MeV neutrons produced by deuterium-tritium fusion strike mercury-198 and convert it into radioactive mercury-197. Mercury-197 is said to have a half-life of about 64 hours and then decays into gold-197, the only stable isotope of gold.

Marathon plans to place mercury in the blanket of a fusion reactor so that the blanket breeds tritium while producing gold, supplying fuel for the continuing fusion reaction. The company proposed the scheme in a preprint paper back in 2025, but that paper has not been peer-reviewed.

In 2025, CERN published gold-production results from its ALICE experiment. Over four years of running the Large Hadron Collider, the gold nuclei produced by lead-ion collisions had a total mass of about 29 picograms. Marathon claims its gold production rate is 10 times that record, but does not explain what basis of comparison it used.

The company gives no clear answer on total gold mass, on the number of nuclei produced per unit time, or on irradiation duration.

Marathon also released a gamma-ray spectrum, claiming that the characteristic signals of mercury-197 and gold-197 can be seen in it, but it did not publish the actual output of gold, its purity, recovery efficiency or production cost.

Marathon’s existing reactor has a fusion power of about 1 kW — one millionth of a gigawatt. Its model-based estimate of at most 2 tonnes of gold a year would require gigawatt-scale fusion facilities that have not yet been built.

Even if gold could be produced, it could not be sold immediately. Freshly made material still contains radioactive isotopes and must be stored, left to decay and then processed. Marathon’s model is said to account for long-term storage; under different assumptions the estimated storage time is about 7 to 18 years.

Besides gold, Marathon also claims to have made copper-64 and copper-67 by irradiating zinc with fusion neutrons. Confirming this fully would still require direct first-hand data. Copper-64 and copper-67 can be used for cancer imaging and targeted radiotherapy; there is real demand in medicine, so even small quantities can fetch high market prices.

Founded in 2023, Marathon completed a $5.9 million seed round the following year (about 39.4 million yuan at current exchange rates). It is now building a compact fusion reactor in San Francisco, aiming for a power-density record. Beyond fusion hardware, the company lists neutron utilisation and isotope production as business lines, with large-scale fusion power generation and supplying the grid as its long-term goal.