Request for a name to another apocalyptic movie.

Many years ago I saw a French (could only have been Belgian if it was not) movie about the aftermath of a Nuclear war, possibly as long ago as 1980.
I recall only three memories of this piece. The main players in the story hiding from the oncoming terror in a cellar, the blast causes corks from stored wine bottles to pop out (the heat wave).
They have a few bright sparks and one of them uses some camera film left outside during a rainstorm to check for fallout (after developing the film with his photographic solutions).
At the films end some army types (it is unclear as to which nations) arrive in the area and a white horse is seen galloping across a wasteland ( I recall a helicopters downdraft and the noise causing it to panic), why it was there and its significance I dont know.
Enjoy it while it lasts