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?
Yep, it’s called a “radioisotope thermoelectric generator”. Mostly used on satellites.
Related accident: https://en.wikipedia.org/wiki/Lia_radiological_accident
Three men in forest in Georgia during winter found mysterious sources of heat and decided to warm up using them. They turned out to be unlabeled RTG cores. One of the three died as a result of exposure.
The youtube channel @PlainlyDifficult has a whole playlist about “sources in the wild” that also covered this one and many other nuclear accidents and incidents.