The most convenient decay source is Plutonium-238 because it does not produce any gammas during the decay which would require lots and lots of shielding to protect people. It has long half life of 87 years so a reduction in power of roughly 1% per year. However, it's weapons material - not okay for wide use. This cannot be overstated. Making nuclear weapons today is very easy when you have the requisite materials (Ted Taylor of Los Alamos used to say it would take 3 guys a few months starting from scratch). With today's off the shelf timing systems, explosives, and manufacturing, it could be even faster.
Next up is Strontium, but that is quite dangerous due to human uptake in the bones. And there's polonium - very poisonous and too short a half life to be used.
I have heard from a friend in the business of a new concept that generates desirable isotopes specifically for decay heat sources in a reactor in an encapsulated form which gets rid of any weapons material or processing of radioactive material.
Next up is Strontium, but that is quite dangerous due to human uptake in the bones. And there's polonium - very poisonous and too short a half life to be used.
I have heard from a friend in the business of a new concept that generates desirable isotopes specifically for decay heat sources in a reactor in an encapsulated form which gets rid of any weapons material or processing of radioactive material.
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...