[AR] Re: Mars Rover Power source
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 10 Mar 2021 18:57:14 -0500 (EST)
On Wed, 10 Mar 2021, John Stoffel wrote:
Henry> (...anything to do with RTGs is expensive, and
Henry> their Pu-238 fuel is in short supply, so doing a flight-demo
Henry> mission for Stirling RTGs would face unusual obstacles.)
I wonder if the switch to americium or some other radioisotope in
larger supply, even with less heat per/cm3 would make sense?
Potentially, yes, as a way of flight-testing the Stirling technology...
but then you've got to pioneer the alternate isotope, and accept its
issues. And alas, Pu-238 may be in short supply but in many ways it's the
clear winner.
I found an ESA presentation from some years ago that talked a bit about
how they decided to focus on Am-241. They put out two parallel research
contracts to identify and assess usable isotopes, and the two contractors
came up with quite similar shortlists. The presentation doesn't discuss
the criteria used for selection, but presumably they were looking for
something with a half-life in the right range, and not too many energetic
gamma rays emitted as part of alpha or beta decay (either by the chosen
isotope itself or by its near-term decay products -- e.g., U-232 looks
interesting until you check on the decay products, at which point you
quickly cross it off the list). Almost all radioisotopes decay to an
"excited" state of the decay product, with extra energy, and it then
unloads that excess as gamma rays. But the numbers vary greatly, and
you'd prefer few and low-energy gamma rays for the sake of both prelaunch
handling and inflight radiation damage to electronics.
The combined shortlist was Cm-244, Sr-90, Gd-148, Cd-113m, H-3 (tritium),
Pu-238, and Am-241.
Curium 244 produces too many neutrons, presumably from spontaneous
fission.
Strontium 90 was actually used in a few early RTGs, but it emits energetic
beta particles, and those have an extra problem: bremsstrahlung X-rays
emitted as they decelerate, far too many of them.
Gadolinium 148 and cadmium 113m can't be made with neutron irradiation,
i.e. in a reactor, and have to be made in a particle accelerator, which is
impossibly slow and expensive.
Tritium is easily made -- in fact it's a waste product from reactors --
but it has no compounds that are stable solids at high temperatures, and
storing it as a gas means very low power density, making it hard to reach
the high temperatures needed for efficient energy conversion.
Plutonium 238 is odd in that it *does* almost always decay to the "ground"
state (of U-234, which has a very long halflife and so its radiation is
negligible), with no excess energy. And when it doesn't, almost always
the gamma rays have very low energy, i.e. are easy to stop. There are
still a few heftier gamma rays, and occasional neutrons, but not many.
The main snag ESA saw is production requiring multiple reactor irradiation
steps and multiple chemical separations of the radioactive results, so
it's expensive to make, and expensive to get set up to make.
That leaves americium 241, with 1/4 the power density of Pu-238 and more
gamma emission, but both were considered manageable problems, and it's
less complicated to make, with some existing production already.
I wonder if some sort of stirling cycle for a lunar rover in the polar
regions would be a good first step.
Only if you can fly it as an otherwise low-cost mission, so people will
feel comfortable taking a chance on it. But the first deep-polar-crater
rover is all too likely to turn into a flagship mission that won't be
allowed to take unnecessary risks.
Henry
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