Every time deuterium and tritium fuse in a fusion reactor, a fast neutron flies out. It can turn one substance into another — for example, create valuable isotopes for medicine.
Today such neutrons cost a fortune, because there are few experimental facilities and they rarely operate. But the authors of the study calculated that over ten years the price of a neutron will drop by roughly a factor of ten million. Most of the drop will come simply from having more reactors running almost nonstop. The rest comes from improving the plasma and making the machines themselves cheaper.
This creates a “neutron ladder”: first, a reactor pays for itself by producing expensive isotopes, and once the technology becomes widespread, it shifts to cheap electricity. That’s a stepwise path from today’s experimental machines to gigawatt power plants.
Actinium-225 is one of the most expensive materials on Earth: its price can reach billions of dollars per gram, because it helps treat certain types of cancer.
In Larry Niven’s novel Ringworld, fusion engines are used to move entire planets — neutrons there are working at full capacity.