I had an internship at Rutherford-Appleton Laboratory almost two years ago; we got similar directions related to alarms, klaxons, etc. They had two kind of alarms. A ringing bell meant basically "there's a fire, leave the building". The other one was a high-pitched alarm that meant "something bad (likely nuclear) is going on, find a nearest building (or stay in it if you're already there), close all windows, shut down air conditioning and wait".

Another funny anecdote from that place: one day we got an e-mail related to some power problems we experienced earlier. The message suggested to avoid using some part of the parking lot near the transformers, as there was a slight chance that an explosion could eject the doors of the transformer station.