If they are going to do this I don't think they are going to worry too much about Radioactive Half-life, remember they aren't planning on many of us surviving (500 Mill) and they can choose exactly where they want to live. London, NY etc are tiny, so I doubt they will worry too much if they cant go there again.
Plus if they do hit the major cities imagine the fear they can use against those still asleep.
Also if they hit say Washington they will get all the backing they need to start a war against who ever they chose to be the enemy.
Oh well theres not much any of us can do about it
