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Couldn't we have a lead box lined with these radiation to electricity converters with a small amount of radioactive material in the center, and have an energy generating device that would last for thousands or even millions of years? Imagine putting the sun in a box lined with solar cells, but on a much smaller scale.
Is there a reason this wouldn't work?
Since they also rely on the radioactive decay, the power output isn’t constant. It will also decay as time goes on. Sure, it will stay hot (not to mention completely lethal) for a very long time, but that might not be enough for all applications.
Maybe it was enough to keep a lighthouse operational for decades, but eventually it won’t be enough for that. What do you do with an RTG like that? Instead of powering a large light with that, you could probably power a smaller light or a small water pump. After a few more decades, that small pump is once again too powerful for your legacy RTG, so you’ll have to settle for running a smaller pump or a street light with it. It’s going to take a very long time until you get to that point, so it’s highly likely that the RTG will be forgotten, abandoned or stolen by then.