A rare, non-radioactive helium isotope that enables aneutronic fusion reactions, helium-3 is scarce on Earth but potentially abundant on the Moon.
Helium-3 (3He) is a stable, non-radioactive isotope of helium with two protons and one neutron. In fusion energy, it is prized as a fuel for the D–3He reaction, which produces a proton and an alpha particle totalling 18.3 MeV — with no neutron as a primary product.1
D + 3He → 4He (3.6 MeV) + p (14.7 MeV). Because both products are charged particles, their energy can in principle be captured electromagnetically or deposited directly in the plasma. The absence of a primary 14.1 MeV neutron dramatically reduces structural activation and the need for massive shielding, making D–3He attractive for compact, low-activation reactors.2
Helium-3 is extremely rare on Earth. The atmospheric concentration is about 7 parts per trillion. The U.S. stockpile, largely a byproduct of tritium decay in nuclear weapons programs, has been depleted by demand from neutron detectors for homeland security. Current prices exceed $2,000 per litre (STP), and global annual production is measured in tens of thousands of litres — far below what a power plant fleet would require.3
The solar wind implants 3He into the regolith of airless bodies. Lunar soil samples returned by Apollo missions contain 3He at concentrations of 4–20 parts per billion by mass. Estimates suggest the Moon's regolith holds on the order of 106 tonnes of 3He, enough to power human civilisation for centuries — though extraction would require processing vast quantities of lunar soil.4
Helium-3 is also a product of tritium beta decay and of the D–D reaction. In tritium-breeding fusion blankets, 3He can accumulate and must be managed. Additionally, 3He–3He reactions (producing 4He + 2p at 12.9 MeV) are fully aneutronic but require still higher temperatures and have very low cross-sections, placing them further in the future.