8 ms·

This is similar to the "Stirling radioisotope generator" [1] but it is an actual nuclear reactor (albeit a small one). Both systems are a response to the same

by curtis 9y ago

This is similar to the "Stirling radioisotope generator" [1] but it is an actual nuclear reactor (albeit a small one). Both systems are a response to the same fundamental problem, the dwindling supply of Plutonium-238 [2]. The Stirling radioisotope generator uses Pu-238 more efficiently than thermoelectric RTGs, and the nuclear reactor from the article dispenses with Pu-238 altogether. [1] https://en.wikipedia.org/wiki/Stirling_radioisotope_generator https://en.wikipedia.org/wiki/Stirling_radioisotope_generato... [2] https://en.wikipedia.org/wiki/Plutonium-238 https://en.wikipedia.org/wiki/Plutonium-238

gozur88 9y ago
The other problem with RTGs is they scale very badly.
- Faaak 9y ago
  how ?
  - andygates 9y ago
    They're very low power, make a lot of waste low-grade heat, and use an inefficient electricity generation approach.
    - masklinn 9y ago
      > make a lot of waste low-grade heat That's probably the biggest issue for deep space it's extremely difficult to shed heat in hard vacuum (without ejecting mass and its heat with it) as you can only radiate it away.
      - acidburnNSA 9y ago
        And radiative heat transfer scaled with temperature to the fourth power so it would be easy to do if you had materials that could handle really high temperatures for long times.
        wbl 9y ago
        But then your Carnot efficiency goes way down.
wolfram74 9y ago
Also it is my understanding that /because/ it dispenses with the plutonium out right it's more benign until you turn it on. Which you would presumably do after the potential for RUD in atmosphere has diminished considerably.
- curtis 9y ago
  > Also it is my understanding that /because/ it dispenses with the plutonium out right it's more benign until you turn it on. Which you would presumably do after the potential for RUD in atmosphere has diminished considerably. This is a good point. Here's a picture of a Plutonium-238 oxide pellet (referenced from [1]): https://en.wikipedia.org/wiki/Plutonium-238#/media/File:Plutonium_pellet.jpg https://en.wikipedia.org/wiki/Plutonium-238#/media/File:Plut... It will always look like that if you have no way to dump the heat -- basically it's a heat source which is always on. On the other hand a nuclear reactor that's never been activated will have fuel that looks like this (referenced from [2]): https://en.wikipedia.org/wiki/Uranium-235#/media/File:HEUraniumC.jpg https://en.wikipedia.org/wiki/Uranium-235#/media/File:HEUran... This is no doubt oversimplifying things, since Pu-238 is a pure alpha emitter, whereas once it's been activated, the fuel in a nuclear reactor will generate all sorts of nasty radioactive isotopes which in turn release alpha radiation, beta radiation, neutrons, and gamma rays. It will be pretty safe while it's still cold though -- in fact if it ends up in the ocean it's really unlikely to hurt anything, since there is already uranium dissolved in sea water. [1] https://en.wikipedia.org/wiki/Plutonium_238 https://en.wikipedia.org/wiki/Plutonium_238 [2] https://en.wikipedia.org/wiki/Uranium-235 https://en.wikipedia.org/wiki/Uranium-235
yborg 9y ago
The problem with Stirling engines is they have moving parts. It's highly unlikely a Stirling cycle generator on the Voyagers would have operated for 40+ years as their RTGs have.
- raverbashing 9y ago
  Yup, I wonder how they can guarantee the engine will keep turning for 10 years even. Maybe if everything is sealed in a (very long) lived lubricant that stands the harshness of space? Still hard to imagine
  - rm445 9y ago
    That's the usual way things are done. If you buy for instance a simple 5:1 gearbox for industrial use, it will quite likely be a sealed casing 'greased for life'. Not that I'm disagreeing with you really - mechanical devices have high failure rates. 10 years maintenance-free is a bit of a dream. As a mechanical engineer I'm interested to see whether the Stirling engines involved have radically different scaling to optimise for reliability, or NASA just plan to do the engineering really well. Of course manned maintenance may be possible - they are touting this technology for Mars bases. Plus robotic maintenance is going to become more of a thing over time.
  - jacquesm 9y ago
    Stirling engines have a pair of seals that are extremely hard to make durable, and to my knowledge only Whisper Systems has cracked this problem to the point where you won't lose working gas (or seals) in a timespan much shorter than those 10 years. It's a stupidly hard engineering issue, without it there likely would have been far more adoption of the Stirling cycle for production machinery.
  - Manglano 9y ago
    There's some work being done in superfluid discovery which would create the sort of conditions you're looking for but physics discoveries move incredibly slowly and some of the materials are rare or difficult to create. However, that sort of "lubricate, seal, and forget" kinetic system is essential for a lot of surface work in space. Increasingly important on Earth as well. As a heuristic, the less mechanical parts in any system, the more efficient it is over time. Ideally, everything that's built is engineered like a spacecraft, because to some extent it is, and is onboard one.
tpurves 9y ago
For anyone else wondering why Pu-238 is dwindling, from the linked wikipedia: “The United States stopped producing bulk Pu-238 with the closure of the Savannah River Site reactors in 1988.[12][13][14] Since 1993, all of the Pu-238 used in American spacecraft has been purchased from Russia. In total, 16.5 kilograms (36 lb) has been purchased but Russia is no longer producing Pu-238 and their own supply is reportedly running low.[15][16]” Interestinly, it looks like Canada (Ontario, Darlington) are starting set up operations to make some in the near future.
- cptaj 9y ago
  Here's an episode of Space Policy from Planetary Radio. They go very in depth on the issue of pu-238, the efforts to restart production and why it is so complicated. http://www.planetary.org/multimedia/planetary-radio/show/2016/august-2016-space-policy-edition-sls-europa-cost-of-next-mars-rover-return-of-plutonium-238.html http://www.planetary.org/multimedia/planetary-radio/show/201...
- 52-6F-62 9y ago
  This was my very first question. Thanks for pulling the basics out. I've been loosely following the proliferation of nuclear power in Ontario for a little while now, it is interesting to learn they'll be starting production nearby. In light of the current geopolitical climate, it's probably better that NASA's (et al) plutonium should come from Canada vs. Russia.
- Manglano 9y ago
  US production has resumed, according to Wikipedia, as Cobalt 60 is coproduced and useful for medical sterilization. As an aside: There are some people who are very skeptical of both atoms and space exploration. I'm not sure what their motives are but given the amount of steel necessary to produce wind turbines, I'm unconvinced that "renewable" is actually "greener."
  - lloyd-christmas 9y ago
    "over 20 years, a three-megawatt wind turbine can deliver 80 times more energy than is used in its production and maintenance." [0] Beyond that, Steel is 100% recyclable^, and that accounts for over 65% of US steel production. Recycled steel can be done in arc furnaces, requiring no coal coke. [1] Info like this is at the tip of your fingers. [0] https://www.worldsteel.org/en/dam/jcr:f07b864c-908e-4229-9f92-669f1c3abf4c/fact_energy_2016.pdf https://www.worldsteel.org/en/dam/jcr:f07b864c-908e-4229-9f9... [1] https://seekingalpha.com/article/3785906-metallurgical-coal-future https://seekingalpha.com/article/3785906-metallurgical-coal-... ^ This rate is never achieved. Global average is ~90%, as some countries aren't efficient: http://www.steel.org/sustainability/steel-recycling.aspx http://www.steel.org/sustainability/steel-recycling.aspx

by curtis 9y ago

gozur88 9y ago

The other problem with RTGs is they scale very badly.

Faaak 9y ago
how ?
- andygates 9y ago
  They're very low power, make a lot of waste low-grade heat, and use an inefficient electricity generation approach.
  - masklinn 9y ago
    > make a lot of waste low-grade heat That's probably the biggest issue for deep space it's extremely difficult to shed heat in hard vacuum (without ejecting mass and its heat with it) as you can only radiate it away.
    - acidburnNSA 9y ago
      And radiative heat transfer scaled with temperature to the fourth power so it would be easy to do if you had materials that could handle really high temperatures for long times.
      - wbl 9y ago
        But then your Carnot efficiency goes way down.

wolfram74 9y ago

Also it is my understanding that /because/ it dispenses with the plutonium out right it's more benign until you turn it on. Which you would presumably do after the potential for RUD in atmosphere has diminished considerably.

curtis 9y ago
> Also it is my understanding that /because/ it dispenses with the plutonium out right it's more benign until you turn it on. Which you would presumably do after the potential for RUD in atmosphere has diminished considerably. This is a good point. Here's a picture of a Plutonium-238 oxide pellet (referenced from [1]): https://en.wikipedia.org/wiki/Plutonium-238#/media/File:Plutonium_pellet.jpg https://en.wikipedia.org/wiki/Plutonium-238#/media/File:Plut... It will always look like that if you have no way to dump the heat -- basically it's a heat source which is always on. On the other hand a nuclear reactor that's never been activated will have fuel that looks like this (referenced from [2]): https://en.wikipedia.org/wiki/Uranium-235#/media/File:HEUraniumC.jpg https://en.wikipedia.org/wiki/Uranium-235#/media/File:HEUran... This is no doubt oversimplifying things, since Pu-238 is a pure alpha emitter, whereas once it's been activated, the fuel in a nuclear reactor will generate all sorts of nasty radioactive isotopes which in turn release alpha radiation, beta radiation, neutrons, and gamma rays. It will be pretty safe while it's still cold though -- in fact if it ends up in the ocean it's really unlikely to hurt anything, since there is already uranium dissolved in sea water. [1] https://en.wikipedia.org/wiki/Plutonium_238 https://en.wikipedia.org/wiki/Plutonium_238 [2] https://en.wikipedia.org/wiki/Uranium-235 https://en.wikipedia.org/wiki/Uranium-235

yborg 9y ago

The problem with Stirling engines is they have moving parts. It's highly unlikely a Stirling cycle generator on the Voyagers would have operated for 40+ years as their RTGs have.

raverbashing 9y ago
Yup, I wonder how they can guarantee the engine will keep turning for 10 years even. Maybe if everything is sealed in a (very long) lived lubricant that stands the harshness of space? Still hard to imagine
- rm445 9y ago
  That's the usual way things are done. If you buy for instance a simple 5:1 gearbox for industrial use, it will quite likely be a sealed casing 'greased for life'. Not that I'm disagreeing with you really - mechanical devices have high failure rates. 10 years maintenance-free is a bit of a dream. As a mechanical engineer I'm interested to see whether the Stirling engines involved have radically different scaling to optimise for reliability, or NASA just plan to do the engineering really well. Of course manned maintenance may be possible - they are touting this technology for Mars bases. Plus robotic maintenance is going to become more of a thing over time.
- jacquesm 9y ago
  Stirling engines have a pair of seals that are extremely hard to make durable, and to my knowledge only Whisper Systems has cracked this problem to the point where you won't lose working gas (or seals) in a timespan much shorter than those 10 years. It's a stupidly hard engineering issue, without it there likely would have been far more adoption of the Stirling cycle for production machinery.
- Manglano 9y ago
  There's some work being done in superfluid discovery which would create the sort of conditions you're looking for but physics discoveries move incredibly slowly and some of the materials are rare or difficult to create. However, that sort of "lubricate, seal, and forget" kinetic system is essential for a lot of surface work in space. Increasingly important on Earth as well. As a heuristic, the less mechanical parts in any system, the more efficient it is over time. Ideally, everything that's built is engineered like a spacecraft, because to some extent it is, and is onboard one.

tpurves 9y ago

For anyone else wondering why Pu-238 is dwindling, from the linked wikipedia: “The United States stopped producing bulk Pu-238 with the closure of the Savannah River Site reactors in 1988.[12][13][14] Since 1993, all of the Pu-238 used in American spacecraft has been purchased from Russia. In total, 16.5 kilograms (36 lb) has been purchased but Russia is no longer producing Pu-238 and their own supply is reportedly running low.[15][16]” Interestinly, it looks like Canada (Ontario, Darlington) are starting set up operations to make some in the near future.

cptaj 9y ago
Here's an episode of Space Policy from Planetary Radio. They go very in depth on the issue of pu-238, the efforts to restart production and why it is so complicated. http://www.planetary.org/multimedia/planetary-radio/show/2016/august-2016-space-policy-edition-sls-europa-cost-of-next-mars-rover-return-of-plutonium-238.html http://www.planetary.org/multimedia/planetary-radio/show/201...
52-6F-62 9y ago
This was my very first question. Thanks for pulling the basics out. I've been loosely following the proliferation of nuclear power in Ontario for a little while now, it is interesting to learn they'll be starting production nearby. In light of the current geopolitical climate, it's probably better that NASA's (et al) plutonium should come from Canada vs. Russia.
Manglano 9y ago
US production has resumed, according to Wikipedia, as Cobalt 60 is coproduced and useful for medical sterilization. As an aside: There are some people who are very skeptical of both atoms and space exploration. I'm not sure what their motives are but given the amount of steel necessary to produce wind turbines, I'm unconvinced that "renewable" is actually "greener."
- lloyd-christmas 9y ago
  "over 20 years, a three-megawatt wind turbine can deliver 80 times more energy than is used in its production and maintenance." [0] Beyond that, Steel is 100% recyclable^, and that accounts for over 65% of US steel production. Recycled steel can be done in arc furnaces, requiring no coal coke. [1] Info like this is at the tip of your fingers. [0] https://www.worldsteel.org/en/dam/jcr:f07b864c-908e-4229-9f92-669f1c3abf4c/fact_energy_2016.pdf https://www.worldsteel.org/en/dam/jcr:f07b864c-908e-4229-9f9... [1] https://seekingalpha.com/article/3785906-metallurgical-coal-future https://seekingalpha.com/article/3785906-metallurgical-coal-... ^ This rate is never achieved. Global average is ~90%, as some countries aren't efficient: http://www.steel.org/sustainability/steel-recycling.aspx http://www.steel.org/sustainability/steel-recycling.aspx