Answering this would depend on knowing the following things:

• What would be targeted, and how many targets would be hit, and with what kinds of weapons, and what kinds of targeting choices (surface bursts produce lots of local fallout, airbursts do not)

• The wind patterns of the day of the attack

The first of those is highly contingent on assumptions made about target strategy and capabilities, and both changed of those were very different at different points in the Cold War (and we don't really know the Soviet targeting strategies, so it would be a big assumption there).

The second of those would require knowing the weather of any specific day(s) of the attack and combining those patterns with the fallout data posited in the first one.

So there are a lot of assumptions to be made there and this is not a simple or straightforward question to answer. And not really a historical question.