Pretty good overview.
Regarding shooting it into the sun, there are a couple of cheaper ways to do it:
1: It takes less delta-v to reach solar escape velocity than to launch directly into the sun, but you could send a rocket to the outskirts of the solar system for slightly less cost, at the apoapsis the velocity would be incredibly low, at which point you can spend a tiny amount of delta-v to lower your periapsis beneath the surface of the sun.
2: Alternatively, a solar sail, counter-intuitively, can lower your apoapsis more efficiently than it can raise it, and would only need solar power to get you into an orbit lower than Mercury's. It might not get you all the way to the sun before it fails, though.
Regarding burying it in subduction zones or geologically inactive areas, well that's fine, but it seems to me as if we are squandering a free energy source by doing that. If you have a lump of matter that's going to be hot for thousands of years, it seems to me like we could use that. If not on Earth, then definitely in space. It should be a good energy source for interstellar probes, for example.
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brb Monkhouse Stampede, Tue 8 Nov 2022, 21:04,
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The trouble with RTGs in space is that they need huge radiators. Much less so where there’s an atmosphere.
Throwing nuclear “waste” away is indeed silly though. Stupid non-proliferation treaties.
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jonbob loves you, Tue 8 Nov 2022, 21:19,
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The temperature differential is what makes the thermocouples generate electricity...
Look at the Voyagers, they have no large radiators, just the standard RTG fins. The RTG is an internal part of the Cassini probe, which has no large radiator (unless the rear side of the solar panel counts..?). Are you thinking of fission reactors?
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brb Monkhouse Stampede, Tue 8 Nov 2022, 22:54,
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There’s no temperature difference unless you can keep one side cooler than the other
You need a radiator for that in space. If you don’t remove heat from the cold side one way or another you eventually lose the temperature differential by simple conduction. Beta decay batteries don’t work like that but I don’t think they were used at that point.
If you look at the Cassini example you’ll see the fins going around the outside. That’s the radiator.
Ed: I should have been clearer: you need huge radiators if you want to generate more than a hundred watts or so. RTGs are great for longevity away from the sun but they’re not a practical source of large amounts of power.
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jonbob loves you, Wed 9 Nov 2022, 6:19,
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Could you point to the radiator fins on Cassini?
www.rocketstem.org/wp-content/uploads/2014/05/97pc1110.jpg
The only ones I see are the standard RTG fins in cutaway diagrams. Cassini also has a number of other isotopic heaters to keep the electrics warm, rather than worrying about overheating or not being able to shed enough heat with radiators.
"The spacecraft also carried 82 strategically placed radioisotope heater units (RHUs), which provided focused warmth in the form of one watt of thermal power each using a pencil eraser-sized pellet of plutonium dioxide. The Huygens probe used 35 similar RHUs to keep it warm on its descent to the frigid surface of the frigid Titan."
solarsystem.nasa.gov/missions/cassini/radioisotope-thermoelectric-generator/
My searches for "large radioisotope thermal generator radiators" is returning nothing relevant to your assertion.
I'm not calling you out, just confused.
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brb Monkhouse Stampede, Wed 9 Nov 2022, 14:47,
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The black bits that you already know about. I wasn’t referring to the probe as a whole.
commons.m.wikimedia.org/wiki/File:Cassini%27s_RTG.jpg
If you wanted to get more than a hundred watts out of it (which you would definitely need for anything interstellar) you’d need much bigger fins to make the temperature different larger; Seebeck generators are laughably inefficient to start with, which is probably why NASA started to look at Sterling engines instead.
Probably just a case of crossed wires here.
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jonbob loves you, Wed 9 Nov 2022, 15:31,
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Ah okay.
I'm not sure you'd need that much more wattage for interstellar probes... I'm assuming either chucked potatoes or laser sails, with no exotic propulsion systems or even ion drives, just some mostly passive sensors and a laser or radio transmitter to send data back home (or to the nearest network node). That would just need enough battery storage to use sparingly, punctuating long recharge times.
The waste will be hot for thousands of years, so there's no need to rush. We could make shitloads of such probes.
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brb Monkhouse Stampede, Wed 9 Nov 2022, 17:55,
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Depends what you mean by interstellar.
Lasers as you will know are neither perfectly collimated nor immune to being scattered by space dust. The last time I read about the subject they were talking about needing megawatts.
It’s all (literally) academic really.
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jonbob loves you, Wed 9 Nov 2022, 18:19,
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A proposed comms laser for Breakthrough Starshot is a mere 100W:
phys.org/news/2020-05-interstellar-probes-starshot.html
It would need at least one 30m receiver at this end, or possibly smaller assuming a relay network of probes. The denser the relay network, the less powerful and accurate each laser needs to be.
There's talk of drawing power from the interstellar medium, which sounds far fetched compared to nuclear waste. ;)
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brb Monkhouse Stampede, Thu 10 Nov 2022, 20:35,
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Pointing at Earth will be tricky to say the least!
I notice it also assumes improvements in detectors, which reminds me of something Carl Sagan once said about radio telescopes. Apparently if you summed up all the energy picked up by all the world’s radio telescopes it would amount to about the same as a single falling snowflake.
Let’s just build a few self-sustaining habitats, strap a few fusion rockets to them and fling 'em into the void eh?
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jonbob loves you, Thu 10 Nov 2022, 21:12,
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