Short summary: Soviet engineers installed RTG powered radio relays to support the construction of a damn in Georgia. Political instability lead to the abandonment of the RTGs. Someone scavenged the generators and removed the radioactive cores from them.
Two of the radioactive sources were discovered by men gathering firewood in the forest. They decided to bring them to their camp(!) and cozy up to them to keep warm during the night(!!). Despite showing symptoms of radiation poisoning they kept the cores on their person while loading their truck(!!!). They all suffered terrible radiation injuries.
There are more sources "lost" from the same batch which remains unaccounted for to this day.
https://m.imdb.com/title/tt1588875/
Only the Soviets were daft enough to build RTGs using Strontium 90.
The United States did too.
Oak Ridge National Laboratory technical report "Strontium-90 Heat Sources"
https://technicalreports.ornl.gov/1971/3445605716035.pdf
Introduction
Compact electrical generators powered by heat from radioisotopes have been under development in the United States since the early 1950s for space, marine, and terrestrial uses. Essentially all the generators developed for marine and terrestrial uses have been powered by 90 Sr. This report summarizes the development work done by Oak Ridge National Laboratory (ORNL), Hanford Atomic Products Operation, and Martin Company, Nuclear Division, which led to the production of 90 Sr heat sources for use in the generators.
It was a natural choice since strontium 90 is an inevitable byproduct of operating any fission reactor, and was readily available as a coproduct from weapons plutonium production reactors. Making better RTG isotopes like plutonium 238 required additional infrastructure.
I mean, Sr90 is super cheap, and as long as it stays inside the RTG you're fine. The AEC actually tried Polonium RTGs in the late 50s.
The shorter half-life of Sr compared to Pu also means it's a bit less of an issue when you lose the source.
https://www.emergencylights.net/collections/self-luminous?gc...
They aren’t generating electricity though.