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.
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.
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.
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."
"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]
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" ;)
> 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
?
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.
When I lived in Seattle I was sitting at a cafe one day and a government motorcade came down the street with an 18 wheel truck carrying a dumbell-shaped canister with a giant yellow nuclear symbol on the side. I've never been able to figure out what they were transporting. Does anybody here know what it might have been? It was post 9/11 so I was surprised they were still moving radioactive material on the surface streets of a major city.
To some extent we are radioactive material on the surface streets of the major cities, because we have consumed soil!! Even our steel tools can be somewhat radioactive, refinement concentrates point sources.
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.
Waste from the Hanford Site? Why would it be necessary to transport it through the most populous area of WA state? (I'm asking as somebody who has lived near Hanford as well as in the Seattle metro area)
Eh, it also doesn’t make sense since Hanford is a waste storage site...where else could the waste be going, and even if it’s high grade, why does that involve the Pacific Ocean rather than a trip to Utah or neveda in the other direction? I’ll assume that whatever it was, it was unrelated to Hanford, more likely something to do with nuclear submarines or the training reactor they had at UW.
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.
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.
The 40+ years the Voyager probes have stuck around for is impressive... but the actual primary mission duration was about 12 years (for Voyager 2), and that was with two (planned but not guaranteed) extensions for Uranus and Neptune.
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.
If you want to use the RTG to power thrusters, then I'd think that doing the "detour" via a stirling engine to create electrical current to turn into thrust might be unnecessary.
There must be some way to directly turn radioactivity (or heat) into thrust with sufficiently high exhaust velocity!?
Let me introduce you to the NERVA rocket. Nuclear thermal propulsion exists, but it's a real pain to make work and they had to search for coatings that could stand the heat and fuel.
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?
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.
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.
...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.
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.
Its major, still undressed downsides are the requirement of expensive beta emitters, and synthetic diamond PV cells (everything else will die to beta particles)
DTR operations were terminated on Voyager 2 in 2007, and Voyager 1 in 2015, but I don't know if this was related to mechanical problems with the tape recorders or just due to the lack of need given the data collected by the remaining instruments.
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.
> 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]):
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.
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.
“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.