Written by Tim Lash, Focus Fusion Society Contributor.
Often when considering the relative merits of fusion energy reactor designs the focus lands on underlying scientific principles or construction costs. Rightly so since these are the upfront hurdles to reach viability. Less often is the proposed fusion fuel a topic of comparison. A recent article discussing theoretical extra-terrestrial mining operations touched on the availability of Helium-3 on the moon.
Helium-3 is an isotope of helium with one less neutron than the far more abundant helium-4 variant. In fact, there’s only one or two atoms of helium-3 on earth for every million atoms of helium 4. This scarcity makes helium-3 harvested from earth a poor choice for powering a fusion reactor. However, helium-3 has many desirable characteristics as a hypothetical fusion fuel.
Helium-3 provides two common modes of fusion reaction. It can fuse with itself. In that case the products are a helium-4 nucleus, two protons and 12.86 MeV of excess energy. The other potential fusion reaction is to combine helium-3 with deuterium. Deuterium (the isotope of hydrogen with one extra neutron) fuses with helium-3 to form helium-4, one proton and 18.355 MeV of output energy. Note that both of these reactions are aneutronic! The Lawrenceville Plasma Physics (LPP) research has focused on aneutronic fuel as the most sustainable path to fusion energy. The benefits of aneutronic combustion have been well documented by LLP. Aneutronic reactions make helium-3 a particularly clean burning fusion fuel, and an intriguing candidate if it weren’t so rare on earth.
This rarity on earth, coupled with an expectation of abundant helium-3 on the moon, has inspired private ventures to speculate about lunar helium-3 mining. Estimates for the market price of helium-3 run as high as $40,000 per ounce. Dozens of times more expensive than gold. This price for a proposed fusion fuel is remarkable even considering the incredible power that could be derived from a tiny amount of this fuel. It is all the more remarkable considering the alternatives.
The LPP Focus Fusion-1 device intends to use boron-11 and hydrogen as its fuel. Both elements are incredibly abundant on earth, and inexpensive to produce. Better yet, the fusion reaction between boron-11 and hydrogen is aneutronic. Rather than spending untold millions in a race into space to procure hypothetical amounts of helium-3, a monumentally less expensive alternative is at hand here on earth.