I read the graphic novel version of "When the Wind Blows" as a child about 40 years ago.
I don't remember being scared but to this day I still think about how I would improvise a fall-out proof shelter in my own house.
It becomes exponentially less radioactive due to decay. The long-lived isotopes that persist might eventually give you cancer or cataracts, but the short-lived isotopes present in the immediate aftermath of a blast will kill you. The dose makes the poison and fallout shelters vastly reduce the absorbed dose of radioactivity.