Fusion of Neutrons and Hydrogen Nucleus: Mechanism and Possibility Explained

[Denton]

It never occurred to me until now why instead of using two charged protons to fuse together, rather fuse neutrons to a hydrogen nucleus.

Yes, it seems like cheating the system and therefore that's why I've not heard about this before, but what is the exact mechanism that prevents this from occurring? Neutrons share the strong nuclear force just as much as the protons, and considering there's at least one proton, shouldn't it be possible to fuse 1-2 neutrons to it?

 

[Astronuc]

On the other hand, while we observe a deuteron (pn), we do not observe a (pp) particle, which we attribute to the very strong Coulomb repulsive force, but we also do not observe (nn), which would not be subject to the Coulomb force. Nature does not seem to favor n-fusion - at least not in our part of the universe.

 

[vanesch]

It never occurred to me until now why instead of using two charged protons to fuse together, rather fuse neutrons to a hydrogen nucleus.

This is perfectly possible, and is actually a pain in most applications. Only, one doesn't call it "neutron-hydrogen fusion" but rather: neutron capture by hydrogen. It does liberate some energy (in the form of gamma radiation), of the order of 2.2 MeV if I remember well. The question is: where do you get the free neutrons from ?

The H + n -> D reaction has a non-neglegible probability, and is actually the reason why one cannot have nuclear power reactors with light water and natural uranium.
The reaction D + n -> T is much less probable (but does happen!) and is the reason why heavy water reactors CAN(DU) work with natural uranium.

This makes me think that nobody has got it ever in his (sick) mind to try to make a tritium-water reactor...

 

[Denton]

I don't understand why you wouldn't want the H + n -> D reaction in nuclear reactors.. more energy is not better?

 

[Astronuc]

In a fission reactor (LWR), the H + n -> D reaction does occur, and so does the D + n -> T reaction, but at a low rate. The problem with tritium is the radioactivity which one would like to minimize.

The H + n reaction is not a primary reaction in an LWR. It only produces ~2 MeV as opposed to ~200 MeV per fission of U-235 or Pu-239 which are desirable reactions in an LWR.

 

[vanesch]

I don't understand why you wouldn't want the H + n -> D reaction in nuclear reactors.. more energy is not better?

It happens of course, a little bit. The reason I stated that was that for natural uranium, the neutron balance is extremely tight. You only have about exactly the right number of neutrons to keep the chain reaction going (meaning, that 1 neutron produced in a fission will on average give rise to 1 other fission), and to achieve that, you have to slow down the neutrons which are produced in fission at high energy to low thermal energies WITHOUT LOOSING THEM. Water can slow them down, but the reaction H + n -> D makes that one also looses some, and just too many are lost to be able to sustain a chain reaction.