[1] https://en.wikipedia.org/wiki/Stirling_radioisotope_generato...
“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.
http://www.planetary.org/multimedia/planetary-radio/show/201...
In light of the current geopolitical climate, it's probably better that NASA's (et al) plutonium should come from Canada vs. Russia.
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."
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-4229-9f9...
[1] 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
Downvote me if you like but our fuel mix in this country is still ~40% combustion and the calculations on wind power say that at current power requirements civilization will harvest enough energy from the air currents to change the climate again. Maybe it stops hurricanes, I have no idea.
Also, steel, like atomic fuel, is toxic to produce and reprocess!
I know it is much-despised by but I am curious as to the lifetime energy footprint of a fission reactor facility with onsite fuel-reprocessing and waste transmutation. And then? Tokamak. Won't even need a turbine!!
Parts of that system don't even exist yet but me I believe one day there will be clean atoms, fission or fusion, and they will require even less energy to produce and maintain than the wind and solar grid.
But then, I have lived among the fission reactors my entire life. Perhaps I am "mad from the rads" ;)
?
Wind doesn't circulate through the atmosphere forever with perfect efficiency, it eventually dumps its energy into the ground via friction.
There's no difference between slowing wind down with a wind turbine and slowing it down with a tree, they both end up as heat eventually.
However, containment vessels like that are usually for refined ores, which end up in reactors, smoke detectors, medical devices, radiotherapy machines, you name it. Too expensive and possibly hazardous to airlift.
As far as I know, production of atomic weapons has stopped in this country and we're not even sure if they still work (half-lives--they rot). Could've been materiel for a stewardship program at the Hanford Site, or waste being evacuated from the Hanford Site.
Generally speaking, the remaining atoms transported and transmuted are for peace. That's why Russia brokers Uranium to the States, and why people in Kazakhstan and elsewhere continue to mine fissile ore.
If you were to shut down the remaining fission plants of Planet Earth, civilization as we know it, would end almost immediately, such is our need for these fuels. It would not be pretty--consider the amount of electric heat, the number of electric stoves with 50+ year duty lifetimes. That's why Japan recycles spent fuel at the French reactor.
[1] http://www.wipp.energy.gov/WIPPCommunityRelations/images/pho...
Still hard to imagine
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.
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.
Note that while implementations usually show a crank and rod with a flywheel, it could just as easily use a magnet and coil to generate electricity.
That get's you down to a single part.
Then you have this:
https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine
...that gets you down to something that can generate sound from a heat differential, and you could couple that sound to some kind of transducer to generate electricity. Still a moving part, though.
You probably can't get zero moving parts and yet have it do useful work, but you can get really close I think.
The RTGs on both probes have decayed A fair amount at this point and are producing a lot less power.
10 years may seem short, but combined with an electrically powered thruster there is potential for doing types of missions we have not really been able to do before. That 10 years could be spent doing propulsion.
If you want to use it more in the science phase, use chemical rockets to get up to speed and then boot up the reactor in time to say, decelerate into orbit and you are looking at having the majority of that 10 years used at the destination.
My main concern isn't really the duration, but the reliability of the moving parts. But without plutonium, there are not a whole lot of other options for powering missions to Uranus and Neptune.
There must be some way to directly turn radioactivity (or heat) into thrust with sufficiently high exhaust velocity!?
You get around 15 years of useful work and 15% efficiency with current day technology.
It has no moving parts as RTG, has better power to weight than RTG, and has efficiency comparable to simple Stirling
https://en.wikipedia.org/wiki/Optoelectric_nuclear_battery
Its major, still undressed downsides are the requirement of expensive beta emitters, and synthetic diamond PV cells (everything else will die to beta particles)
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: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: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.
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.