Up on the roof.
"Let no one on the roof of his house go down to take anything out of the house. Let no one in the field go back to get his cloak. How dreadful it will be in those days for pregnant women and nursing mothers!"
I wonder if this roof is what Matthew was talking about.. From here, the shining cloud above the reactor must have been a staggering sight.
Standing on the roof of the highest building in this empty town brings a feeling of being completely alone in the world - like this whole town is.