Yeah, a nuclear blast is gonna be totally deadly within a particular radius, no matter what you do. And then at some larger radius, everything outside that radius will be safe, regardless of what you do. So the area in between is going to be the area where the response can make a difference.

And as you mention, the area of the "can actually make a difference" zone is much larger than the "dead-no-matter-what" zone, because it scales by the square of the distance. So if the outer safe radius is twice the inner death radius, the area of the in between zone is gonna be about 3 times the size of the death zone (π(2r)^2 - πr^2 = 3πr). If it's 3 times the radius, it'll be 8 times the area.