Nuclear Winter is not a guarantee, even in a full nuclear exchange. In fact, I think the whole theory has largely been discredited over the years. The whole idea goes that a full-scale nuclear exchange would create so much smoke and airborne dust to blot out the sun and bring about a catastrophic drop in global temperatures. In truth, many natural disasters throughout the course of earth's history have lofted more smoke and dust into the atmosphere than would occur during a full scale exchange with negligible effects on the weather.
Surviving the initial blast and short term fallout are the keys to survival. Both of those goals also happen to be attainable through a bit of planning and preparation.