An accessible guide to the fuel supply for fusion energy — deuterium from seawater is essentially unlimited, but tritium must be manufactured, and helium-3 is extraordinarily rare.
Deuterium (2H or D), the heavy isotope of hydrogen, makes up about 0.015% of all hydrogen on Earth. It is extracted from ordinary water through electrolysis or chemical exchange processes (the Girdler sulfide process). A single gallon of seawater contains enough deuterium to produce energy equivalent to 300 gallons of gasoline through fusion.[1]
Tritium (3H or T) is radioactive (half-life 12.3 years) and does not exist in useful quantities in nature. The world’s supply (~25 kg, mostly from Canadian CANDU fission reactors as a byproduct) is shrinking through decay. A fusion power plant burning D–T fuel would consume 100–200 kg of tritium per year and must breed its own supply from lithium in the reactor blanket.[2]
Helium-3 (3He) is proposed for advanced D–3He fusion, which produces charged particles instead of neutrons. However, 3He is exceedingly rare on Earth (~15,000 litres/year from tritium decay in nuclear weapons stockpiles). The Moon’s regolith contains ~1 million tonnes of 3He implanted by the solar wind, but extraction would require mining billions of tonnes of lunar soil.[3]
Since tritium is bred from lithium, the practical fuel for D–T fusion is deuterium and lithium. Global lithium reserves exceed 22 million tonnes — enough for thousands of years of fusion power, even accounting for growing battery demand.