On the Beach is a great novel (the best bit is the motor racing), but actually implausable. Neville Shute was a great novelist, but his knowledge of nuclear physics and the persistence of radiation was rudimentary (as was that of even those involved in developing the atom bomb at the time). The idea that massive contamination would slowly (over more than a year, if my memory serves) spread from the nornthern hemisphere and overwhelm the south was popular in the 50s but doesn't hold water. It was replaced by the more plausible nuclear winter theory that would arise from a massive nuclear exchange. This is not meant to be encouraging ...