This article is from the Fusion FAQ, by Robert F. Heeter firstname.lastname@example.org with numerous contributions by others.
Some researchers feel the advantages of neutron-free fusion
reactions offset the added difficulties involved in getting
these reactions to occur, and have coined the term
"aneutronic fusion" to describe these reactions.
The best simple answer I've seen so far is this one:
(I've done some proofreading and modified the notation a bit.)
[ Clarifying notes by rfheeter are enclosed in brackets like this.]
>From: email@example.com (John W. Cobb)
>Risto Kaivola <firstname.lastname@example.org> wrote:
[[ Sorry I don't have the date or full reference for this anymore;
this article appeared in sci.physics.fusion a few months ago.]]
>>Basically, what is aneutronic fusion? The term aneutronic
>>confuses me considerably. Could you give me an example of
>>an aneutronic fusion reaction? How could energy be produced
>>using such a reaction? Can there be a fusion reaction in which
>>a neutron is never emitted?
>D + He3 --> He4 + p + 18.1MeV
>(deuteron + helium-3 --> helium-4 + proton + energy)
>p + Li6 --> He4 + He3 + 4.0MeV
>(proton + lithium-6 --> helium-4 + helium-3 + energy)
>D + Li6 --> 2 He4 + 22.4MeV
>(deuteron + lithium-6 --> 2 helium-4's + energy)
>p + B11 --> 3 He4 + 8.7Mev
>(proton + boron-11 --> 3 helium-4's + energy)
>All of these reactions produce no neutrons directly.
[[ Hence "aneutronic." ]]
>There are also other reactions that have multiple branches possible,
>some of which do not produce neutrons and others that do
>(e.g., D + D, p + Li7).
>The question is how do you get a "reactor" going and not get
>any neutrons. There are 2 hurdles here. The first is getting the
>fuel to smack together hard enough and often enough for fusion
>The easiest fusion reaction is D + T --> He4 + n (the D-T fuel
>cycle). A magnetic reactor can initiate fusion in one of these
>things at about a temperature of 10keV.
[1 keV = 1000 eV = 11,000,000 (degrees) kelvin, more or less].
>The other reactions require much higher temperatures (for example
>about 50KeV for the D+He3 reaction). This is a big factor of 5.
>The second hurdle is neutron production via "trash" (secondary)
>reactions. That is, the main reaction may be neutron-free,
>but there will be pollution reactions that may emit neutrons.
[ The products of the main reaction, e.g. He3, can be trapped in
your reactor temporarily, and fuse with other ions in the system
in messy ways. ]
>Even if this is only a few percent, it can lead to big neutron
>emission. For example, the D+He3 reaction will also have some D+D
[ Because in your reactor you will have a lot of Ds and He3s, and
the Ds will collide with each other as well as with the He3s. ]
>At 50Kev temperatures, the reaction
>cross-section for D+D reactions is about 1/2 of the D+He3
>cross-section, so there will be some generation of neutrons from
>the 50% branch reaction of D + D-->He3 + n.
>Also, the other 50% goes to T+p, The triton (T) will then undergo
>a D-T reaction and release another neutron.
[ Because the cross-section for D-T reactions is much higher.]
>If the reactor is optmized (run in a He3 rich mode) the number
>of neutrons can be minimized. The neutron power can be as low
>as about 5% of the total. However, in a 1000 megawatt reactor,
>5% is 50 MW of neutron power. That is [still] a lot of neutron
>irradiation. This lower neutron level helps in designing
>structural elements to withstand neutron bombardment, but it
>still has radiation consequences.
>On the other hand, it is my understanding that the p-B11 reaction
>is completely neutron free, but of course it is much harder