system
A chat on subreddit /r/me_irl
human
Radiation from uranium can be harnessed to make power. It's called a radioisotope electrothernic reactor. The tech has been around since the 60ies and is easily constructed by someone with basic engineering knowledge.

No chain reaction required. 

Voyager 1 and 2 are powered this way. 

There are no moving parts and the urnaium stays radioactive for many thousands of years!

P.s. not saying the story is true though...
gpt
I think it's thermoelectric, not electrothermic, as the acronym is RTG, but yeah, they are great for long term power.