Are you sure p-B11 fusion isn’t fission?


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Posted by Admin on May 14, 2006 at 05:20 PM
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Q:

Isn't the p-B11 reaction, strictly speaking, nuclear fission, as the reaction product immediately splits into three He4 atoms? Is it called fusion because "fission" is a dirty word in the nuclear community? [Question sent by R. Curnow]

A:

Yes, technically, the p-B11 reaction is a fusion-fission reaction.  The proton and boron-11 fuse to temporarily create an unstable isomer of carbon-12, which then fissions to create three helium-4s.  However, the same can be said about the deuterium-tritium reaction, which fuses to produce an unstable helium-5 and then fissions into a helium-4 and a neutron.  Like wise with deuterium-helium-3, or practically any fusion reaction.  There are a few pure fusion reactions.  For example, when two deuteriums fuse they will usually either produce a Tritium and a proton, or a helium-3 and a neutron, but there is a very small chance that they will stick together to form a helium-4 with the excess energy carried off by a gamma ray.  This would be a fusion reaction with no subsequent fission.  But this reaction happens purely by chance.  There is no way to make this particular outcome happen more often.  It’s just part of the random nature of quantum mechanics that decides which outcome occurs.  There is no fuel combination that produces only these kind of pure fusion reactions.  And even if there were this reaction really isn’t any safer than the p-B11 fusion-fission reaction which already produces no neutrons.

And yes, you are right that these reactions are called fusion and not fusion-fission to distinguish them from conventional heavy element fission.  There are important differences between heavy element fission and light element fusion-fission.  In particular, Light element fusion-fission cannot produce a self sustaining chain reaction.  The plasma environment in which the reactions occur must be created by some external process.  So there can be no runaway reactor meltdown.  Also, the heavy element fission products are radioactive while the light element fusion-fission products are not (except for neutrons, which either decay into a proton and electron or are captured by an atom turning it into a different isotope which might be radioactive)  This is why we make sure to call p-B11 aneutronic fusion, which is basically the safest nuclear reaction that exists.  The difference between fission and fusion-fission is an important distinction to make.  It’s a lot easier to make that distinction to the public if their names are a little less similar.

Put another way:  Yes, you could think of it like a fusion of B11 + H to make an atom of C12 which then fissions into three He4.  But it doesn’t happen in two separate steps.  It’s like doing a “break” in pool, but with sticky pool balls that fly off stuck together in certain combinations.  I suppose calling it fusion is mostly convention, but it shares much more in common with other types of fusion than with Uranium or Plutonium fission.  It starts with light elements that cannot undergo fission by themselves until the fusion reaction occurs.


 
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Brian H's avatar

Hm, I can certainly see the other point of view, though; it’s like the B11 “captures” a proton and then fissions from the C12 state, just as heavy radioactives “capture” slow neutrons and then fission. So the distinction could be thought of purely in terms of neutron capture vs proton capture.  The thing about proton capture is that it doesn’t produce just another isotope, but another element—sort of “bump” transmutation.  Neutron capture simply makes the nucleus (more) unstable (in fission reactions).


Ok.  But, what does “isomer of Carbon 12” mean?  I thought Carbon 12 was the regular, stable form of Carbon.

Is this because the structure of the Boron 11 nucleus is sufficiently different to that of the stable Carbon 12 that the chances of stable Carbon 12 forming are minimal?  Or is it just the excess energy from the fusion causes the thing to pop apart again?


maihem's avatar

@samv: The former.

Each member of a nucleon species can be in one of two states (ie, each neutron can be up or down and each proton can be up or down). Some ratio’s of the number of up/down neutrons and up/down protons are stable (low energy/mass) and some are not (high energy/mass), some unstable ratios are likely to decay into another configuration and some are likely to split apart instead.

The pB11 reaction produces a very large number of unstable ratios that tend to split apart (practically all of them are this type).


Brian H's avatar

@maihem

Elsewhere, Eric says it’s the latter.  But the former is far more interesting!


maihem's avatar

@Brian H

I think Eric was simplifying it for the lay audience. In fact, the former is one of the many ways in which the latter can be true… from my comment:

“Some ratio


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