I have written several papers concerning the possible contribution of heat emitted by radioisotope thermoelectric generators (RTGs) to the anomalous acceleration of the Pioneer spacecraft. Doubtless I’ll write some more.
But those RTGs used for space missions number only a handful, and with the exception of those that fell back to the Earth (and were safely recovered) they are all a safe distance away (a very long way away indeed) from the Earth.
However, RTGs were also used here on the ground. In fact, according to a report I just finished reading, a ridiculously high number of them, some 1500, were deployed by the former Soviet Union to power remote lighthouses, navigation beacons, meteorological stations, and who knows what else. These installations are unguarded, and the RTGs themselves are not tamper-proof. Many have ended up in the hands of scrap metal scavengers (some of whom actually died after receiving a lethal dose of radiation), some sank to the bottom of the sea, some remain exposed to the elements with their radioactive core compromised. Worse yet, unlike their counterparts in the US space program which used plutonium, these RTGs use strontium-90 as their power source; strontium is absorbed by the body more readily than plutonium, so my guess is, exposure to strontium is even more hazardous than exposure to plutonium.
The report is a few years old, so perhaps things improved since a little. Or, perhaps they have gotten worse… who knows how many radioactive power sources have since found their way into unauthorized hands.