In the glowing sea there are buildings, also the crater in Cambridge have buildings all around it (probably only a tactical sortie was launched at that place, which doesn't make sense, but meh)
As for your previous question, I looked up on google, and the largest bomb Russia could drop, 100 megaton, would have a fireball radius of 1,88 miles, radiation radius would be 4,65 miles, altho, the hot air blast would kill everyone in a radius of 20,51 miles, and the thermal radiation would cover 47,88 miles. So Nuclear shadows, I would assume, would occur after the lethal radius of the air blast, and within the thermal radiation range, altho not at max or near max range (I will assume)
Now, if we assume a 20 megaton bomb fell in the glowing sea (not unlikely size imo) it would be rougly a fifth of the numbers above, altho I have no clue if it scales so perfectly (probably not) But an estimated guess would then be beyond 4 miles, nuclear shadows could occur, with diminishing probability, and probably not beyond ca. 6-7 miles.
I am no mathematician, so people, feel free to correct me!