Paul F. Dietz [[EMAIL PROTECTED]] wrote: > Bob, > > You are missing the point I was making in this > thread. The argument went something like this: > > P1: Look at this link for an antimatter rocket to Mars! > P2: WTF?! That quantity of antiprotons only gives you > 25 megajoules of energy. Must be a small spacecraft! > P3: But it uses fission too! Maybe that's where the extra > energy is coming from. > Me: Adding fission increases the total energy output > only marginally, if at all.
I think I missed the original article.
> Now, to address some of your points...
>
> > Injecting a small amount of anti-protons can greatly increase the
> > fission rate. For comparison, imagine adding a very short half-life
> > substance (like Americium) to a longer-half-life substance (like
> > plutonium). The aggregate would have a reduced half-life with only a
> > tiny amount of Americium.
>
> What injecting the antimatter *CANNOT* do is change the
> criticality of a mass of fissionable material. If the assembly
> is very subcritical -- as it would have to be in this case --
If it is very subcritical - true. Maybe I missed this from the article.
I assumed they were increasing the reaction rate of an already critical
core.
> then any neutrons (or other energetic particles) emitted by
> the antimatter-fissioned nuclei will not have much additional
> effect; most will just escape. In this situation, you're going
> to get roughly 1 fission per injected antiproton, so the extra
> energy yield is small (10% at most, as I argued before; perhaps
> less if energy is soaked up in nuclear spallation instead of
> fission).
Yes, you have to be near critical for that 10% to matter.
> If the mass *is* critical (or supercritical) then injecting
> antiprotons will also have no substantial effect; you could
> start the chain reaction just as easily by adding neutrons.
> It's also far too big for this scheme.
Neutrons are hard to come by. Anti-protons being charged can be stored
in a magnetic bottle. You could carry around a small neutron source and
bring it close to the core to initiate fission...
> If the mass is just subcritical (so that substantial multiplication
> occurs without a self-sustaining chain reaction) then it is still
> too big, and you might as well make it slightly larger so that it
> is truly critical.
>
> > As you said, they create atomic fragments/isotopes that slam into other
> > fissile material, thus reducing the aggregate's half-life, and
> > increasing heat output.
>
> No. Fission fragments, btw, do diddlysquat in inducing fission
> in other nuclei (the Coulomb barrier is just too high); it's neutrons
> that propagate a chain reaction.
Yes, I was oversimplifying. A neutron is a fission fragment too. ;)
Many of those fragments also have very short half-lives and release
neutrons...
> > You shoot the anti-protons in while the engine is running to increase
> > the heat output. Think it as the opposite of the carbon control rods
> > used in reactors.
>
> Control rods change the criticality of a reactor; injecting antimatter
> does not. So this analogy is completely confused.
It should change the criticality, even if just a little bit (as you
argue above). If you're not changing the criticality, then I fail to
see how antimatter helps at all. In other words, I think we agree. ;)
> Control rods are also typically not made of carbon, which doesn't
> absorb neutrons very well.
Err...Boron Carbide. Boron has a high cross section to absorb neutrons.
Cheers,
-- Bob
Bob McElrath ([EMAIL PROTECTED])
Univ. of Wisconsin at Madison, Department of Physics
"The purpose of separation of church and state is to keep forever from
these shores the ceaseless strife that has soaked the soil of Europe in
blood for centuries." -- James Madison
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