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A Solar drive / neutron atmosphere

  1. Mar 18, 2016 #1
    I am wondering, if the sun is a fusion reactor, it should have an "atmosphere" of neutrons ?
    So presumably a craft with suitable fuel eg boron or uranium, in a close pass, would be able to use that to react with the fuel in a suitable drive ? Probably not good for humans - (!) but for an unmanned bulk carrier, it could produce a useful boost maybe... just an idea.
  2. jcsd
  3. Mar 18, 2016 #2


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    Staff: Mentor

    The Sun's primary method of energy generation is Proton-Proton fusion. This reaction chain builds up helium by fusing together a total of 4 protons. During the first reaction in the chain, two protons collide, one is converted into a neutron, and the two fuse together under the strong nuclear force. This creates a deuterium nucleus. Another free proton collides with this deuterium and the two fuse, creating Helium-3. Another three protons combine in the same manner to form another Helium-3 nucleus, and then the two nuclei collide and fuse. Two out of these remaining four protons are ejected during this process, leaving two protons and two neutrons fused together, aka Helium-4. No free neutrons are created during this process.

    The other energy generation process is the CNO cycle. The cycle is more complicated than the P-P chain, but no free neutrons are generated during it either.

    See the following links for more info.

    Long story short, the Sun doesn't have an atmosphere of free neutrons.
  4. Mar 18, 2016 #3
    You're correct, protium fusion does generate some neutrons, especially in side reactions like between deuterium nuclei. However, the sun's fusion reactions take place in its core, which is only 20-25% of its radius. That leaves the rest of the sun's bulk (75-80% of its radius and 65% of its mass) to act as shielding. The sun's self-shielding is so effective that it takes even photon energy about 10,000 to 170,000 years to transfer to the surface, while neutrons are almost immediately used in core nuclear processes. If a free neutron manages to avoid being caught up in nuclear reactions, it has a half-life of 15 minutes before it turns into a proton. (Neutrons are surprisingly unstable outside of nuclei.)

    In short, those core neutrons will never reach the surface.

    It's also pretty easy to stimulate neutron release in a quantity of uranium suited for nuclear propulsion, and the means of doing so (e.g., a critical mass with adjustable control rods or neutron reflectors) give fine control over the nuclear processes and provide a safety mechanism. Depending on an outside, variable, even random neutron flux is tickling the dragon's tail - begging for trouble.

    But it's a good leap of imagination to put those facts together.
  5. Mar 18, 2016 #4

    Ken G

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    Minor nitpick-- the free neutron half-life is 10.2 minutes. The 15 minutes is the mean lifetime, which is a bit longer than the time it takes 1/2 of the neutrons to decay.
  6. Mar 18, 2016 #5
    Heh, I remembered 12.5 minutes but it'd been years since I used the value so I double checked with the source of all teh interwebs knowledge, Wikipedia, and saw 881.5 seconds for, yep, mean lifetime. I had "half-life" on the brain and wrote that instead. Thank you for pointing that out or I'd still be spouting "15 minutes half-life" years from now.
  7. Mar 18, 2016 #6


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    Staff: Mentor

    Those pesky side reactions!! ?:)
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