Signs of the times

I rode past the Atomic Weapons Establishment at Aldermaston today and this collection of signs outside one of the gates intrigued me.

Why do all these people have to wear cycle helmets? Are there things falling out of the sky that the rest of us don't know about? Or do cycle helments provide some kind of protection from mysterious rays?

And why are their helmets so big? They must need very clever staff, so perhaps they only recruit people with enormous heads. Or do their heads expand as a result of working on nuclear warheads?

Or maybe I've got this bit wrong, and they make them ride tiny little bicycles for some reason.

I can understand why a brand new pass would be viewed a bit suspiciously, but why aren't new cycle helmets any good. How worn do they have to be? My own is a couple of years old now, and it's a bit battered, but I wouldn't describe it as particularly worn. Does that mean it won't work? Where do they get their worn helmets from?

There is clearly something odd going on, and it seems to me that (despite the official secrets act) the great British public should be told what it is.