Well you could cover the ground around the storage area with highly radioactive material. Anyone who approaches dies. Skeletons tend to be an obvious clue. People tend to avoid areas with lots of skeletons.
Also note that rather than "storing" the nuclear material you could use it as a perpetual power source, driving a heat engine, making the area physically too hot (over 100C) to approach. People tend to avoid volcanos without any signs.
Or, with careful design, it could be stored in a matrix such that moving any member would make it immediately go critical. People tend to avoid things that blow up.
In summary, change from the idea of "store it safely", to "store it danerously". Make it painfully obvious that, like a cliff edge, this is something to avoid.
You don't need s aign if the thing itself is obviously dangerous.
Also note: We may have found a use for a black hole. A "laboratory sized" black hole would be a great place to put stuff. Although I suspect the boundary radiation would be at least as intense as anything thrown into it, radioactive or not.
Also note that rather than "storing" the nuclear material you could use it as a perpetual power source, driving a heat engine, making the area physically too hot (over 100C) to approach. People tend to avoid volcanos without any signs.
Or, with careful design, it could be stored in a matrix such that moving any member would make it immediately go critical. People tend to avoid things that blow up.
In summary, change from the idea of "store it safely", to "store it danerously". Make it painfully obvious that, like a cliff edge, this is something to avoid.
You don't need s aign if the thing itself is obviously dangerous.
Not the most popular solution, I will admit.