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Nuclear reactor in zero gravity

  1. Nov 16, 2009 #1
    How do nuclear reactors used to power satellites and such operate in zero gravity?
  2. jcsd
  3. Nov 16, 2009 #2
    Hi there,

    I don't think I fully understand your question. There is absolutely no reason for heavy nuclei not to fission in zero gravity. The fission of a nucleus is a stochastic event, that depends on the potential energy of the nucleons and the nuclear strong interaction, ok mixed with a bit of quantum physics.

    For powering sattelites, since they are in almost without gravity, you need very little force to correct their trajectory. Therefore, with just a little bit of power, you can deviate a satellite, and put it back into a correct trajectory.

  4. Nov 16, 2009 #3
    google "nuclear powered satellites"
    here's the first hit:

    Think of a satellite orbiting at a altitude of, say, 200 miles. The gravitational force on the satellite is (4000 / 4200)^2 or 91 percent of the force of gravity it feels at sea level. Freefall isn't the same as "no gravity."
  5. Nov 16, 2009 #4


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    Uh ! That's not true ! Locally, of course, everything in the satellite will behave as in gravity-free outer space.

    Free fall IS the same as no gravity (apart from tiny tidal effects). That's what the equivalence principle is about.

    I guess the OP wants to know how the mechanics of a reactor operates in "zero gravity" or "free fall". Of course it cannot be a BWR, and even a PWR would be hard to do, because you need gravity in those systems to separate vapor from liquid water (for a BWR, in the reactor vessel, and for a PWR, in the steam generators).

    Also the workings of a cooling tower are probably jeopardized in space :wink:
  6. Nov 16, 2009 #5
    What I was trying to say is, moving to a higher orbit requires nearly as much work (91% in the case of an orbit initially at 200 miles above the surface) as it does at sea level. Contrary to the previous post which says that in the absence of gravity such maneuvers require little power. Am I wrong about that?
  7. Nov 16, 2009 #6


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    Yes - maneuvers to put a satelite in a higher orbit require energy. So to move a sateleite from a 200km orbit to 400km requires almost as much energy as to move it from the ground to 200km - except for all the losses getting through the atmosphere.

    But most satelite orbital maneuvers don't change the energy very much - they are to slightly shift the inclination of the orbit to put it over a new target or to avoid another object.

    The bigger problem using reactors for thrust is that you still need a propellant which is used up - if you are going to use a reactor to heat an inert propellant and throw it out of the back to create thrust - you might as well just carry a chemical fuel. The exception to this is ion drive engines that expel charged ions at very high speed and so use very little propellant mass - but the thrust of these engines is to low for them to be practical for maneuvering satellites.

    The main use of reactors in satelites is for low orbits where large solar panels would cause too much drag and you need a lot of power.
  8. Nov 16, 2009 #7


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  9. Nov 16, 2009 #8


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    Correct. Nuclear power systems in space are typically RTGs, which have traditionally used isotopic decay, e.g. alpha decay of Pu-238 as a heat source.

    The thermal to electrical conversion is accomplished by thermoelectric systems which are solid state, so lack of gravity is not an issue. One could use single phase cycles like a Brayton cycle, and likewise, lack of gravity is not an issue.

    If one wishes to use a two-phase cycle like the Rankine cycle, it's possible, but requires a clever design to deal with the lack of bouyancy in the absence of gravity. One could spin the part of the vehicle with the power plant in order to induce 'artificial' gravity, but then one must consider the v2/r relationship.

    Out in space, radiation or radiative heat transfer is the process by which heat is rejected from the system to the environment.
  10. Nov 16, 2009 #9


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    Ah, sorry, I thought you made the silly mistake of Jules Verne :smile:
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