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Polonium-210 for thermal control by sdsds on 12 Feb, 2024 21:59
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For background: http://www.spacesafetymagazine.com/aerospace-engineering/nuclear-propulsion/energy-resources-space-missions/Quote
In 1969, the first Lunokhod rovers of USSR were launched from Baikonur with [...] polonium-210,
Setting aside launch mishaps, is polonium-210 still a viable choice for thermal control of deep space missions in general and lunar-night survival in particular? It would seem to scale down pretty well.... -
#1 by Blackstar on 12 Feb, 2024 23:19
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I think it has more drawbacks compared to Pu-238, but I don't know what they are off the top of my head. I remember somebody telling me that one of the benefits of Pu-238 is that it does not dissolve in water. Plus, the US has a lot of experience with Pu-238. That's one of the problems with switching to something else: we know a lot about Pu-238 and all our equipment and processes are geared toward using that.
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#2 by Greg Hullender on 12 Feb, 2024 23:57
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Setting aside launch mishaps, is polonium-210 still a viable choice for thermal control of deep space missions in general and lunar-night survival in particular? It would seem to scale down pretty well....
I think the 138-day half-life rules it out for most purposes. -
#3 by eeergo on 13 Feb, 2024 09:01
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Setting aside launch mishaps, is polonium-210 still a viable choice for thermal control of deep space missions in general and lunar-night survival in particular? It would seem to scale down pretty well....
I think the 138-day half-life rules it out for most purposes.
Unless you load *lead*-210 instead of polonium-210, which (beta) decays with a half-life of 22.2 years to bismuth-210, with a short beta 5-day half-life, leading to 210Po. No idea if that's feasible for a long-duration mission though.
Of course, a 138-day life (or longer, if the system is engineered to operate with under half the heating, which isn't crazy) is not bad for a lunar mission. -
#4 by sdsds on 13 Feb, 2024 16:56
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Setting aside launch mishaps, is polonium-210 still a viable choice for thermal control of deep space missions in general and lunar-night survival in particular? It would seem to scale down pretty well....
I think the 138-day half-life rules it out for most purposes.
Unless you load *lead*-210 instead of polonium-210, which (beta) decays with a half-life of 22.2 years to bismuth-210, with a short beta 5-day half-life, leading to 210Po. No idea if that's feasible for a long-duration mission though.
Of course, a 138-day life (or longer, if the system is engineered to operate with under half the heating, which isn't crazy) is not bad for a lunar mission.
Oh, that's quite clever! But imagine the bright-eyed engineer walking into the program manager's office and saying, "I have a great idea: let's launch lead into space!"
To approximate the mass of lead required to replenish the polonium, is the math as simple as taking the ratio of the half-lives? -
#5 by ccdengr on 13 Feb, 2024 17:10
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I think [Po-210] has more drawbacks compared to Pu-238...
The article goes into some detail about that (the short half-life is Po-210's problem, beta emission is the worst problem of Sr-90), and concludes that Am-241 is the only reasonable alternative, though not that great.
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#6 by eeergo on 13 Feb, 2024 17:38
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Setting aside launch mishaps, is polonium-210 still a viable choice for thermal control of deep space missions in general and lunar-night survival in particular? It would seem to scale down pretty well....
I think the 138-day half-life rules it out for most purposes.
Unless you load *lead*-210 instead of polonium-210, which (beta) decays with a half-life of 22.2 years to bismuth-210, with a short beta 5-day half-life, leading to 210Po. No idea if that's feasible for a long-duration mission though.
Of course, a 138-day life (or longer, if the system is engineered to operate with under half the heating, which isn't crazy) is not bad for a lunar mission.
Oh, that's quite clever! But imagine the bright-eyed engineer walking into the program manager's office and saying, "I have a great idea: let's launch lead into space!"
To approximate the mass of lead required to replenish the polonium, is the math as simple as taking the ratio of the half-lives?
It should, yes, which is also quite probably why this scheme doesn't really appear to be considered: you would need almost 60 times the lead mass than the equivalent polonium mass you strictly need at any particular moment. Still, Lunokhod's "1K" RHUs were about 160 W, which means they contained slightly over a gram of 210Po (in the form of yttrium polonnide, so more actual mass), which would mean completely acceptable amounts of 210Pb - although it would be "interesting" to determine what the equilibrium temperature, stability and heating efficiency of the active lead mass with polonium inclusions would be.
Another even wonkier option would be to embark 222Rn, which is a gas... it would decay alpha to 218Po with a halflife of 4 days, but end up quickly becoming 210Pb in a variety of ways. Spray-on RHU!
(as an aside, I actually did something like this myself! Back in a previous project, we loaded 222Rn aspirated from a radium salt container into a hydrocarbon fluid by dissolving it in the liquid by bubbling, then froze it all with LN2 to avoid having the mix evaporate away, vacuumed the remaining air in the ampoule to avoid 85Kr contamination, and fire-sealed the thing... worked well enough, but we were just interested in the 222Rn decay, not the decay products! I have an 8-year old ampoule in my living room, so I guess I have some nice 210Po RHU in the μW range )
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#7 by deadman1204 on 13 Feb, 2024 18:48
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I wonder if this would be too complicated. You will have significant amounts of different metals with different decay properties, and the amounts are changing.Setting aside launch mishaps, is polonium-210 still a viable choice for thermal control of deep space missions in general and lunar-night survival in particular? It would seem to scale down pretty well....
I think the 138-day half-life rules it out for most purposes.
Unless you load *lead*-210 instead of polonium-210, which (beta) decays with a half-life of 22.2 years to bismuth-210, with a short beta 5-day half-life, leading to 210Po. No idea if that's feasible for a long-duration mission though.
Of course, a 138-day life (or longer, if the system is engineered to operate with under half the heating, which isn't crazy) is not bad for a lunar mission. -
#8 by eeergo on 14 Feb, 2024 07:10
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I wonder if this would be too complicated. You will have significant amounts of different metals with different decay properties, and the amounts are changing.Setting aside launch mishaps, is polonium-210 still a viable choice for thermal control of deep space missions in general and lunar-night survival in particular? It would seem to scale down pretty well....
I think the 138-day half-life rules it out for most purposes.
Unless you load *lead*-210 instead of polonium-210, which (beta) decays with a half-life of 22.2 years to bismuth-210, with a short beta 5-day half-life, leading to 210Po. No idea if that's feasible for a long-duration mission though.
Of course, a 138-day life (or longer, if the system is engineered to operate with under half the heating, which isn't crazy) is not bad for a lunar mission.
It is too complicated in practice, most probably, because nobody has really proposed it, the Soviet probes using 210Po RHUs did not even approach using the scheme (although to be fair they didn't need long heating lifetimes), and I firmly believe there are ample amounts of smarter people than me
However, the reason it is too complicated is not because the amounts are changing, or the metals have different decay properties. That's easily modelled analytically, as you can see in the attached figure. You are even almost ideally close to a secular equilibrium condition in which 210Pb is akin to an inexhaustible source continuously creating 210Po "heating elements" that get exhausted relatively quickly (1/60th as quickly as they are replenished, which is why you'd naively need 60x the amount of lead than the polonium you need to heat up the source).
What is surely more difficult to model is how the heating would be distributed within the lead mass, since 1 g of 210Po will heat to 500ºC at equilibrium due to the alpha particles getting stopped in the material bulk. Will the same be true for the lead bulk, will it actually reach higher temperatures due to the intervening beta decays, will it be less because of there being more material with different stopping powers and the polonium being evenly distributed? What's more: lead has a notoriously low melting point at 327.5ºC (which is why it's used for toy soldiers ), a whole 175ºC lower than the concentrated 210Po equilibrium temperature per gram. This would imply that, unless the last option above is true, the mixture would be in a liquid state. Not an insurmountable problem to have a few tens of grams of molten lead in your capsule, but an additional complication.
Purification of enriched 210Pb is quite possibly also not trivial, given the other long-lived isotopes of the metal around it, which still decay quite fast compared to the 210, but do lead off to other decay chains complicating the non-penetrating "gamma-free" nature of 210Po decay.
However, thanks to both you and sdsds for keeping up the discussion, which prompted me to dig around a bit more and find this paper: https://www.osti.gov/servlets/purl/1524731 whose authors actually modelled and created precisely a 210Po generator from 210Pb for polonium generation away from expensive/slow accelerators. I feel more research into radiological contraptions would easily yield nifty results, but it's just sadly out of fashion given their perceived dangerousness.
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#9 by sdsds on 14 Feb, 2024 18:22
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[...] 1 g of 210Po will heat to 500ºC at equilibrium [...]
Ooh. Does the equilibrium temperature vary at all with the geometry of the sample? For example, would it be lower for a long cylindrical rod than for a sphere?
(I'm hoping there's a 'killer application' for this that would provide economic motivation for developing it. In particular: micro-rovers that use solar-electric power to zip around during lunar day, and then huddle down to keep warm overnight.) -
#10 by deltaV on 16 Feb, 2024 02:00
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[...] 1 g of 210Po will heat to 500ºC at equilibrium [...]
Ooh. Does the equilibrium temperature vary at all with the geometry of the sample? For example, would it be lower for a long cylindrical rod than for a sphere?
(I'm hoping there's a 'killer application' for this that would provide economic motivation for developing it. In particular: micro-rovers that use solar-electric power to zip around during lunar day, and then huddle down to keep warm overnight.)
The equilibrium temperature will be roughly T_0 + R * P where T_0 is the temperature of the environment, R is the thermal resistance between the sample and the environment, and P is the nuclear decay power. (This assumes that a single equilibrium temperature exists for the whole sample, which may not be a good approximation.) Increasing the surface area of the sample would probably reduce the thermal resistance and hence the equilibrium temperature. Increasing insulation would increase the thermal resistance and hence the equilibrium temperature. -
#11 by sdsds on 16 Feb, 2024 02:20
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Thanks! Lead apparently has thermally conductivity ~10x less than copper. and 2x less than iron, so increasing the surface contact area between the remainder of the micro-rover and the Po-210 laced Pb-210 sample might make sense.
Looking at a production process that generates pure Pb-210, the thermal power would seem to change with time something like the attached graph. Does that look at least plausible?
(as an aside, I actually did something like this myself! Back in a previous project, we loaded 222Rn aspirated from a radium salt container into a hydrocarbon fluid by dissolving it in the liquid by bubbling, then froze it all with LN2 to avoid having the mix evaporate away, vacuumed the remaining air in the ampoule to avoid 85Kr contamination, and fire-sealed the thing... worked well enough, but we were just interested in the 222Rn decay, not the decay products! I have an 8-year old ampoule in my living room, so I guess I have some nice 210Po RHU in the μW range 