We've had a smoke detector fail. I'm sure it's not 7500 years since it was installed; that being the half-life of the radioactive material they use to make them.
So, what do you do with a busted smoke alarm ? Apparently the best thing to do is throw it in the rubbish bin - that seems to be the recommended practice. The radioactive material costs about £1000 per gram, so you can imagine how little of it there is in the smoke alarm (clue: about 0.0002g). Allegedly it's even safe to swallow it - it doesn't get absorbed by the gut as it's not soluble in water or fat, so apart from a few stray gamma rays passing straight through you, there should be no harm done.
Still, I'm not entirely comfortable with chucking it into landfill - or even incineration.