Helium-3; Svaret på miljövänlig energi?

Data collected from the Apollo Moon landings have indicated that large deposits of an extremely rare gas called helium 3 are trapped in the lunar soil.

Scientists believe that this helium 3 could be used to create a new source of almost inexhaustible, clean, pollution-free energy on Earth.

One of them is Dr Harrison Schmitt, a member of the 1972 Apollo 17 mission and the only trained geologist ever to walk on the Moon.

"A metric ton of helium 3 would supply about one-sixth of the energy needs today of the British Isles," he claims.

Helium-3 undergoes the following aneutronic fusion reaction, among others, although this is the one most promising for power generation:

2H + 3He → 4He (3.7 MeV) + p (14.7 MeV)

The appeal of helium-3 fusion stems from the nature of its reaction products. Most proposed fusion processes for power generation produce energetic neutrons which render reactor components radioactive with their bombardment, and power generation must occur through thermal means. In contrast, helium-3 itself is non-radioactive. The lone high-energy proton produced can be contained using electric and magnetic fields, which results in direct electricity generation.

However, since both reactants need to be mixed together to fuse, side reactions (D + D and 3He + 3He) will occur, the first of which is not aneutronic. Therefore in practice this reaction is unlikely to ever be completely 'clean'. Also, the temperatures required for D + 3He fusion are much higher than those of conventional D + T fusion, so it is unlikely that this type of fusion will be achieved before the problems with conventional fusion are worked out. Finally as mentioned in the manufacturing section below helium-3 mining is impractical on earth and manufacture requires the high-energy neutrons that proponents of this fusion reaction seek to avoid.

The amounts of Helium-3 needed as a replacement for conventional fuels should not be underestimated. The total amount of energy produced in the 3He + D reaction is 18.4 MeV, which corresponds to some 493 megawatt-hours (4.93e8 Wh) per three grams (one mole) of 3He. Even if that total amount of energy could be converted to electrical power with 100% efficiency, it would correspond to about 30 minutes of output of a thousand-megawatt electrical plant; a year's production by the same plant would require some 17.5 kilograms of Helium-3. Currently, the practical conversion efficiency between the fuel source and the customer's wall-socket is about 30%, so the actual amounts of Helium-3 would be two or three times higher than the above estimates.

The amount of fuel needed for large-scale applications can also be put in terms of total consumption: According to the US Energy Information Administration, "Electricity consumption by 107 million U.S. households in 2001 totaled 1,140 billion kWh" (1.114e15 Wh). Again assuming 100% conversion efficiency, 6.7 tonnes of Helium-3 would be required just for that segment of one country's energy demand, 15 to 20 tonnes given a more realistic end-to-end conversion efficiency.