Before the Chinese A-Sat weapon tests, the #1 source of space junk was radioactive coolant leaking from old Soviet nuclear reactors in space (not RTGs, literally nuclear reactors)
"The Soviet Union has been flying nuclear power sources in space since about 1965. For the most part these nuclear power sources have been low-power nuclear reactors using a thermoelectric conversion principle."
https://ntrs.nasa.gov/api/citations/20190033494/downloads/20... (pdf) ("The NaK Population: a 2019 Status")