I believe you are thinking of light helium (helium-3), not heavy hydrogen. Everything I've read about mining on the moon mentions it. Heavy hydrogen, deuteriu

I believe you are thinking of light helium (helium-3), not heavy hydrogen. Everything I've read about mining on the moon mentions it. Heavy hydrogen, deuterium, is (relatively) abundant on earth and can be harvested from seawater. Heavier hydrogen, tritium, is quite rare, on earth its produced by bombarding lithium. It has a half life of 12 years so it wouldn't be accumulate to useful amounts on the moon. It does decay into helium-3 which is stable, that's why it does accumulate. Fusion with helium-3 takes place at a much higher temperature than deuterium / tritium (DT) fusion, that makes the confinement problem more difficult than the much more studied DT pathway to fusion power generation. One advantage is the possibility of direct conversion to electricity. All this is discussed in https://en.wikipedia.org/wiki/Helium-3 https://en.wikipedia.org/wiki/Helium-3