Looking for a Certain Movie...

~ They played it at public school back in the late seventies/early eighties.

~ It was probably a short film.

~ It was probably to educate us about nuclear attack, but it could have been about alien abduction (which makes no sense as to why they'd show it to us every year...)

~ It was basically about the anticipation of something. At they end, they open a door and the whole screen flashes to white and it ends.

I think.

Any ideas?