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Atomic batteries powered by plutonium decay have been running pacemakers and space probes for over sixty years. But the raw material is scarce: worldwide, only a few kilograms of plutonium-238 are produced each year, barely enough for the most critical missions. The authors of the new study propose an unexpected source: a fusion reactor that hasn't been built yet but is already designed down to the last detail.

Inside such a reactor, the fusion of deuterium and tritium produces neutrons with an energy of 14 million electronvolts. When these particles hit specially selected materials (thorium, neptunium, protactinium) they cause their nuclei to capture neutrons and turn into isotopes that barely exist in nature. Calculations with the OpenMC computer model show that one gigawatt-year of reactor operation yields between 11 and 57 kilograms of plutonium-236. A gram of this substance releases 18 gigajoules of energy over a century, like a tiny speck keeping a kettle warm for decades.

Besides plutonium, the reactor could produce uranium-232, thorium-228, actinium-227, and lead-210—a whole set of fuels that are ten times more energy-dense than the familiar plutonium-238. For a spacecraft, this means a light and powerful source of electricity: actinium-227 and lead-210 require less radiation shielding than current materials when power exceeds a few hundred watts. Thus, a fusion plant designed to generate electricity becomes, in passing, a factory for battery fuel that will power watches, sensors, and Mars rovers.

Fusion reactors haven't yet left the lab, but the calculation shows that a single plant could replace an entire isotope production facility. Then atomic batteries would stop being a niche product and become an ordinary energy source — quiet, long-lasting, and almost eternal.