[cptbutton]

Something I was told about in college. Back in the 1950s or so they got a serious radioisotope source for research which was stored in its own little out of the way building in an appropriately massive lead container. They stopped using it at some point, but never got around to disposing of it.

In the 1970s sometime someone realized it was still there and blanched at the prospective furor if word got around that it was still there. So there was a careful very quiet effort made to get it shipped off for proper disposal. Which was a bit tricky since you needed a big forklift and truck to move it, and more recent buildings were in the way. They managed it finally, leaving only the story to be shared with physics majors such as myself.

So you could handwave having some such semi-forgotten source at a college or university of the right age.