Using Stirling Americium RTG for Space Station electricity

[fredinno]

How worth it would it be to use a Americium RTG for a Space Station's power? By the looks of it, it could be a lot lighter, and also possible, due to being way more common than Plutonium 238.

[Shpaget]

Stirling engines are significantly more efficient at converting heat into mechanical energy and then electricity, than thermocouples, so I'm all for it.

There are some issues with stirlings, though. It being a mechanical machine it requires more maintenance than a thermocouple which has no moving parts.

[RainDreamer]

Well, if it is used for the space station, we do have astronauts up there to do regular maintenance. Though is it worth it? Anyone know how much energy the ISS uses and where it gets its energy form? Can its massive solar panels supply all of that already?

[fredinno]

My reason was that RTGs would be less massive, requiring fewer launches.

I also have another question- if RTGs are used for such a mission, would they require more radiators? How far should they be placed away from the station?

[Shpaget]

Solar panels can provide enough power to the ISS, but they are a major source of atmospheric drag and orbital decay. Getting rid of them, or reducing their size would significantly reduce the amount of propellant needed for boosting.

For example, just the fact that they orient the panels edge on to the direction of travel when in the Earth shade, reduces the propellant usage by about 1000 kg per year.

[RainDreamer]

I think it would need its own radiator and placed on a boom far away from any human habitable area. Though it would need robot arms or astronaut EVA to do maintenance on it though... Can our current space suit block the amount of radiation near a broken RTG? I would assume it can?

[cryogen]

How worth it would it be to use a Americium RTG for a Space Station's power? By the looks of it, it could be a lot lighter, and also possible, due to being way more common than Plutonium 238.

Radioisotopes are much too rare, expensive, and heavy for anything except tiny probes. There's no reason not to use solar power if it's available.

It's not lighter either. Space-grade PV panels can get specific power as high as 200 W/kg (in earth orbit). Pu-238 generators at best only get 5 W/kg. Am-241 would be worse, since it has a slower decay rate, hence lower power density.

http://esto.nasa.gov/conferences/nstc2007/papers/Banicevich_D1P3_NSTC-07-0048.pdf

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Space

http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_087118.pdf

Am-241 isn't "way more common" than Pu-238 anyway; they're both heinously expensive to make.

Edited September 18, 2015 by cryogen

[fredinno]

Radioisotopes are much too rare, expensive, and heavy for anything except tiny probes.

There's no reason not to use solar power, if it's available. Space-grade PV panels can get specific power as high as 200 W/kg (in earth orbit). Pu-238 generators at best only get 5 W/kg. Am-241 would be worse, since it has a slower decay rate, hence lower power density.

http://esto.nasa.gov/conferences/nstc2007/papers/Banicevich_D1P3_NSTC-07-0048.pdf

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Space

http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_087118.pdf

Am-241 isn't "way more common" than Pu-238 anyway; they're both heinously expensive to make.

That's why I was proposing Stirling generators. Yes, Amercium 241 IS way more common, at least compared to Pu-238.

[Kryten]

The decay heat of Americium-241 is 114w/kg. Therefore, it loses to modern solar arrays even if the generator attached to it is 100% efficient and of negligible mass.

[sgt_flyer]

Beware though only taking into account the electric power generated by thermocouples for calculating the power density is not accurate, because the spacecrafts who have it can also use the thermal power for temperature control (notably for probes sent towards jupiter and beyond, were solar panels power density becomes really bad) - russian moon rovers also used heat decay to keep the rovers warm during the moon night)

Solar panels based vessels need additional electric heaters for temperature control of their components and RCS systems

[passinglurker]

No one's mentioned that americium emits gamma radiation huh? Nor the public's reaction to all things nuclear?

[AngelLestat]

From what I know, at earth-sun distance, any PV solution will provide more energy density than RTG, and is cheaper.

At that height you dont even need lot of batteries, because your day is 90min. So you just use the amount of batteries that almost any system will have in case of problems.

[Shpaget]

Using nuclear+stirling (not RTG) you produce power even in shade, so PV advantage diminishes.

Nuclear doesn't need as much batteries. PVs get another hit.

As I mentined earlier PVs are a major source of drag and increase propelant usage for station keeping.

Speaking of radiators, there is very little difference. Power generated by PVs eventually turns to heat which also needs to be dealt with. The difference is only the efficiency of sterling.

Public opinion on nuclear? You do realize how many nuclear power sources we have already flown?

Americium or something else... Unless we see a dramatic increase in PV efficiency, nuclear is the future.

[fredinno]

Using nuclear+stirling (not RTG) you produce power even in shade, so PV advantage diminishes.

Nuclear doesn't need as much batteries. PVs get another hit.

As I mentined earlier PVs are a major source of drag and increase propelant usage for station keeping.

Speaking of radiators, there is very little difference. Power generated by PVs eventually turns to heat which also needs to be dealt with. The difference is only the efficiency of sterling.

Public opinion on nuclear? You do realize how many nuclear power sources we have already flown?

Americium or something else... Unless we see a dramatic increase in PV efficiency, nuclear is the future.

But is Amercium too expensive to make to make it economical?

[AngelLestat]

Using nuclear+stirling (not RTG) you produce power even in shade, so PV advantage diminishes.

Nuclear doesn't need as much batteries. PVs get another hit.

As I mentined earlier PVs are a major source of drag and increase propelant usage for station keeping.

Speaking of radiators, there is very little difference. Power generated by PVs eventually turns to heat which also needs to be dealt with. The difference is only the efficiency of sterling.

Public opinion on nuclear? You do realize how many nuclear power sources we have already flown?

Americium or something else... Unless we see a dramatic increase in PV efficiency, nuclear is the future.

You need radiators.. the thermal efficiency will be low, you can not work at such big temperatures with so many ISS components around, if is not very well designed you will radiate that heat and it will hit the other PV, which you will decrease the efficience of those PV.

PV are cheap and they dont need maintenance.

[cryogen]

But is Amercium too expensive to make to make it economical?

Much too expensive. Here's some figures:

http://www.world-nuclear-news.org/F-can-americium-replace-plutonium-in-space-missions28071401.html

It costs about $1.5 million per kg (this at a world production of only several kg/year). Its decay heat is 110 W/kg, so a 30%-efficient Stirling generator would get you about 30 W/(kg Am-241) of electricity. Duplicating the ISS' 100,000 watt solar array would take 3 tons of Am-241, at an extrapolated cost of $4.5 billion. (Extrapolated, because this is far more Americium then anyone's ever made before).

3 tons roughly as much Americium as exists in Sellafield's entire plutonium stockpile. This isn't sustainable.

It's perfectly reasonable to use Americium RTG's as a power source -- for small robotic probes, ones that use a few watts of electricity. The ESA is planning to build a few (thermoelectric -- not Stirling); their target size is 5 - 50 watts.

http://www.lpi.usra.edu/meetings/nets2012/pdf/3043.pdf

Their planned rate of production is 10-17 kg/year, enough to support 20-40 watts of new RTG's, per year.

http://www.lpi.usra.edu/meetings/nets2012/pdf/3029.pdf

Edited September 18, 2015 by cryogen

[SciMan]

Am241 at any price and/or inefficiency STILL beats the 0kg/year current and proposed future production of Pu-238 based RTGs.

Stop comparing Am241 RTG/RHU of any type to Pu238 based RTG/RHU.

One of them we can get. The other one we can't.

Also, any higher-efficiency power conversion mechanisim that can be used by an Am241 RTG can be used by a Pu238 RTG.

Russia might have some Pu238 left, not sure. Either way, once that's gone, that's the end of it.

Wonder if Tritium based Betavoltaics could be used instead?

Tritium's half life is 12.32 years, and decays via Beta emission (spits out a high speed electron).

In betavoltaics, this electron is captured and used to power something. The power produced in current betavoltaics is tiny but that's because the devices themselves are tiny.

Early pacemakers used Promethium based Betavoltaics, but modern semiconductors can use the lower-energy beta particles from Tritium.

Tritium also doesn't have any of those other pesky decay modes that emit Alpha particles or Gamma rays, so servicing of a "tritium battery" would be no more dangerous than many other EVA maintenance tasks, and even less dangerous than some (eg. servicing an Ammonia coolant loop on the ISS).

And if containment of the Tritium is lost for whatever reason, there's a hard vacuum separating the tritium battery from the crew compartment, so it dissipates quickly. Likely too quickly to pose a serious threat to an astronaut in an EVA suit, which already provides at least some protection from Beta radiation. Certainly not hazardous to crew in the station itself.

There's even a side benefit: The beta decay of Tritium leaves behind Helium-3.

There's probably a scientific use for that while on a space station. Even if there's not, with the SpaceX Dragon able to carry downmass cargo, we could bring the He3 back for use on Earth.

Tritium isn't even all that rare, Heavy-water fission reactors produce Tritium as a byproduct of operation.

Canada uses a lot of CANDU reactors, which are all heavy water moderated uranium reactors.

One facility in Ontario that sorts the Tritium out of heavy water that's been used in a reactor produces 2.5kg of Tritium a year.

That's quite a large amount of Tritium, remember Hydrogen's low density? (then triple it because 2 more neutrons, but still...).

And that's just one facility in Canada.

[Kryten]

Am241 at any price and/or inefficiency STILL beats the 0kg/year current and proposed future production of Pu-238 based RTGs.

Stop comparing Am241 RTG/RHU of any type to Pu238 based RTG/RHU.

One of them we can get. The other one we can't.

Pu-238 production restart was ordered years ago, DoE are producing test batches right now. Full production should be up in 2021.

[cryogen]

Am241 at any price and/or inefficiency STILL beats the 0kg/year current and proposed future production of Pu-238 based RTGs.

The Pu-238 restart is already funded. It's a small amount, but it's still more than anyone is doing.

The new contract with the DOE will for the first time provide NASA with a steady supply of the isotope. The goal is for the DOE to produce 1.5 kilograms of plutonium dioxide a year by 2021, which translates to about 1.1 kilograms a year of 238Pu. With that small influx, NASA should have enough to fuel about two missions a decade, says David Schurr, deputy director of NASA’s planetary-sciences division in Washington DC. “We’re probably good for the next 20 years for foreseeable missions,†he says.

http://www.nature.com/news/nuclear-power-desperately-seeking-plutonium-1.16411

- - - Updated - - -

One facility in Ontario that sorts the Tritium out of heavy water that's been used in a reactor produces 2.5kg of Tritium a year.

That's quite a large amount of Tritium, remember Hydrogen's low density? (then triple it because 2 more neutrons, but still...).

And that's just one facility in Canada.

Isn't that the entire world's supply of tritium? That's the output all of Canada's heavy water reactors combined, and no one else makes it AFAIK.

http://fire.pppl.gov/fesac_dp_ts_willms.pdf

Edited September 19, 2015 by cryogen

[fredinno]

The Pu-238 restart is already funded. It's a small amount, but it's still more than anyone is doing.

http://www.nature.com/news/nuclear-power-desperately-seeking-plutonium-1.16411

- - - Updated - - -

Isn't that the entire world's supply of tritium? That's the output all of Canada's heavy water reactors combined, and no one else makes it AFAIK.

http://fire.pppl.gov/fesac_dp_ts_willms.pdf

Either way, Amercium RTGs need to be developed and used. 2 missions per decade is pretty small.

[SciMan]

I forgot about the Pu238 restart thing. *DUH*

In that case, Am241 is still perhaps less expensive due to the lack of supply interruptions? I don't know.

Pu238 is obviously the preferred thing due to the superior power per mass.

Agreed that it's not something that should make missions wait, but perhaps there's another isotope out there that's better suited than Am241. Something with half-life in the 10s of years, and doesn't emit gamma or neutrons would be nice. Not sure if one of those exists.

Isn't that the entire world's supply of tritium? That's the output all of Canada's heavy water reactors combined, and no one else makes it AFAIK.

http://fire.pppl.gov/fesac_dp_ts_willms.pdf

That just means that Canada's the only one making it RIGHT NOW.

Heavy water reactors aren't exclusive to Canada.

Fact is, water with one or two tritium atoms is more dense than water with two deuterium atoms.

So a centrifuge can separate them. Might be a moderately special kind of centrifuge, but it's not so special that it could be used for uranium enrichment.

Any country with a heavy water reactor that is unwilling to pay Canada's prices, can set up their own plant. It's just a glorified centrifugal water filter.

Edited September 19, 2015 by SciMan

[passinglurker]

Either way, Amercium RTGs need to be developed and used. 2 missions per decade is pretty small.

That's not counting missions that can be done with solar panels mind you

I forgot about the Pu238 restart thing. *DUH*

In that case, Am241 is still perhaps less expensive due to the lack of supply interruptions? I don't know.

Pu238 is obviously the preferred thing due to the superior power per mass.

Am241 makes sense for ESA, JAXA, or China who don't have domestic Pu238 production (as far as I know of) and don't have even the slightest chance of buying some from NASA or Russia. Am241 on the other hand is more readily available because they already use it for smoke detectors (no you can't crack enough of these open to make your own RTG the quantities you'd need would get you on every watch list ever)

Agreed that it's not something that should make missions wait, but perhaps there's another isotope out there that's better suited than Am241. Something with half-life in the 10s of years, and doesn't emit gamma or neutrons would be nice. Not sure if one of those exists.

Not sure about how radioactive it is but russia has made Strontium-90 RTG's for lighthouses those have a half life of ~28years

[SciMan]

Odd, I could have sworn I'm the author of that last quote box.

Forum oddities aside, Sr90 seems like a nice option for things landed on the Moon (remember, 14 days in the shade).

It's close enough that replacement could be done (either send a new generator to a base, or send a whole new unmanned thing).

Also, the short half life means that the decay energy is released faster, which means you can use less of it for the same power output compared to an isotope with similar total decay energy but a longer half-life.

[fredinno]

Odd, I could have sworn I'm the author of that last quote box.

Forum oddities aside, Sr90 seems like a nice option for things landed on the Moon (remember, 14 days in the shade).

It's close enough that replacement could be done (either send a new generator to a base, or send a whole new unmanned thing).

Also, the short half life means that the decay energy is released faster, which means you can use less of it for the same power output compared to an isotope with similar total decay energy but a longer half-life.

Is Sr 90 common enough for lunar and station operations?

[Shpaget]

When speaking of sterlings and PVs, let's not forget concentrated solar thermal power. Kind of compromise.