The concern is future generations stumbling upon radioactive stuff and spreading it everywhere... months, maybe years down the road, those exposed would get can

The concern is future generations stumbling upon radioactive stuff and spreading it everywhere... months, maybe years down the road, those exposed would get cancers, etc. And, by the time they figure out the cause, the material may have spread too widely. A counter-intuitive solution to this would be to mix extremely radioactive material with the low-grade stuff. Those who stumble upon it will be dead within hours (or at least have immediately obvious symptoms of poisoning.) The ease with which our techno-ignorant descendants will be able to connect death/sickness to the location will ensure they stay away.