It's an RTG, it uses the heat created by decay of radioactive isotopes to drive a thermocouple. These things have been designed to be incredibly safe, capable of surviving an explosion of their launch vehicle and remaining intact, for example. But even should their protective capsule be breached the radioactive material is in a ceramic (oxidized) form in spheres. If you cracked open an RTG and threw the insides into a very hot fire nothing would happen, you could just come by and collect the intact ceramic spheres after they'd cooled.
So ignoring the safety issues, would there be enough radioactive isotopes on Earth for every house to have it's own RTG? Imagine unlimited energy, and heat your home with the waste heat?
What's wrong with the economies of scale of larger nuclear plants? I'm asking curiously, I don't understand the fascination with home energy sources.
Radioactive decay is a small energy source compared to fission. Look at the binding energies [1]: the energy accessible by fissioning heavy elements is about 200 MeV -- 1 MeV per nucleon. Radioactive decays are two orders of magnitude less, or smaller.
Pu-238 is one of the nicest ones because it is "clean" -- emitting only alpha radiation, which is easy to shield. You can transmute on the order of 1 atom of it per 100 fission events [2]. A fission event is 200 MeV; a Pu-238 decay is 5.6 MeV. So at steady state, you can support less than 1/3,000th as much Pu-238 power as you can fission power. If you have a Pu-238 source, you also have a vastly larger nuclear fission power source, so why not just use that?
More broadly, it's not a likely idea on physics, even if you accept all radioisotopes (difficult-to-shield gamma emitters) and not just "nice" ones. The high-Z radioactive decay chains end quickly at the element lead [3], so only release a fraction of the energy that is accessible by fission. (Look back to the binding energy curve [1]). And many of these decays are unusably, geologically slow. The naturally-occurring radioisotopes have half lives of >10^8 years (otherwise they wouldn't exist anymore -- they were created in ancient history in supernovae). And trying to transmute them doesn't improve much -- i.e. in a fission reactor, the most common [4] high-Z product is Pu-239 (half life >10^4 years), whose next decay is to U-235 (half life >10^8 years). Others are similar. So I don't see any way forwards.
[2] Through the several processes which create Np-237 from uranium -- a relatively rare process [4]. Np-237 can be isolated and transmuted to Pu-238 separately through neutron irradiation
Probably not. The problem with isotopes needed for things like this is that they tend to have short half-lives (short half-lives = high decay rate = lots of heat generated), which means they pretty much don't exist naturally at all. So they will need to be produced artificially as fission fragments or through neutron irradiation of other isotopes. It's easy enough to produce large quantities of "hot" isotopes in a fission reactor which could be used for RTGs (ignoring safety) but the problem there is that if you're operating a nuclear reactor anyway why not use the heat production from the isotopes to produce energy there instead of going the roundabout, and unsafe, route of packaging things up in RTGs?
Well... as much as I like RTGs and even fission reactors there is a little bit of justification there. Of necessity an RTG pretty much needs to contain hazardous radioisotopes. Putting that into the hands of every joe blow is probably not the best idea at present.
Just cribbing from Wiki, the generator contains 11 pounds of Pu-238 and produces ~150W of electricity from ~2000W of heat. It's hard to put an exact price on it, but it's definitely in the $millions per kg.
It's theoretically feasible, but the cost would be, ah, prohibitive.
Plutonium 238 would be your best bet for such devices, if only because it's halflife is 87.7 years. Strontium 90 might also work, with a halflife of 28.8 years.
However plutonium would not be easy to put in every house since you have to make it in nuclear reactors. You get about half a kilowatt per kilogram of it, I don't think production of it could scale that high.
Strontium is naturally occurring and relatively common. Sr 90 is a trace isotope though. Off the top of my head I would suspect you would be facing similar challenges that separation of Uranium isotopes face.
At least three unrelated "nuclear" things are involved:
* Radioisotope power sources (like this, Cassini, New Horizons, etc.)
* Critical fission reactors (popular in the early space race before solar PV, still the best option for high-power deep-space missions like (cancelled) Jupiter Icy Moons Orbiter/Project Prometheus)
* Nuclear pulse propulsion -- setting off nuclear weapons, riding the shockwaves (e.g. Project Orion)
Pulse prolusion is banned, quite pointlessly, under the Partial Test Ban Treaty of 1963. (Note it extends not only to atmospheric explosions, but ones in deep space). The others aren't. Contained, critical nuclear reactions are legal -- NASA chose a nuclear reactor a few years ago for the Jupiter Icy Moons Orbiter, before they cancelled it. And no one cares about radioisotope sources except confused hippies.
Obscure history: a running Soviet nuclear reactor crashed from space into the Canadian wilderness in 1977. In case anyone's curious of the consequences.
You may be thinking of Cassini; it is powered by an RTG and flew back by the Earth for part of its gravity assist, which caused some controversy about if it somehow went off course and re-entered Earth's atmosphere.
