Aneutronic Fusion - Candidate Aneutronic Reactions

Candidate Aneutronic Reactions

There are a few fusion reactions that have no neutrons as products on any of their branches. Those with the largest cross sections are these:

The two of these which use deuterium as a fuel produce some neutrons with D–D side reactions. Although these can be minimized by running hot and deuterium-lean, the fraction of energy released as neutrons will probably be several percent, so that these fuel cycles, although neutron-poor, do not qualify as aneutronic according to the 1% threshold.

The next two reactions' rates (involving p, 3He, and 6Li) are not particularly high in a thermal plasma. When treated as a chain, however, they offer the possibility of enhanced reactivity due to a non-thermal distribution. The product 3He from the first reaction could participate in the second reaction before thermalizing, and the product p from the second reaction could participate in the first reaction before thermalizing. Unfortunately, detailed analyses do not show sufficient reactivity enhancement to overcome the inherently low cross section.

The pure 3He reaction suffers from a fuel-availability problem. 3He occurs in only minuscule amounts naturally on Earth, so it would either have to be bred from neutron reactions (counteracting the potential advantage of aneutronic fusion), or mined from extraterrestrial sources. The top several meters of the surface of the Moon is relatively rich in 3He, on the order of 0.01 parts per million by weight, but mining this resource and returning it to Earth would be relatively difficult and expensive. 3He could in principle be recovered from the atmospheres of the gas giant planets, Jupiter, Saturn, Neptune and Uranus, but this would be even more challenging.

The p –7Li reaction has no advantage over p –11B, given its somewhat lower cross section.

For the above reasons, most studies of aneutronic fusion concentrate on the reaction, p –11B.