TVTropes has a pretty good article about the detonation of a nuclear bomb (1 megaton) at Walt Disney Headquarters (for no reason in particular... probably just to be funny) in Los Angeles and it's affect in the LA area. It's a good read if you can find it.
Thermo-nuclear is a H-Bomb and basically uses a fission explosion to generate heat necessary to create a fusion explosion... which is far more destructive. A lot of this will be based on how close to the ground the bomb is. Most of the radioactive fallout of a nuclear blast is based on how much of the fireball intersects with the ground. The more intersection, the more radioactive material will be vaporized and then fall out. This isn't to say that it is all that will be destructive, as the overpressure or blast, and the radiant heat (the light) will still kill for some distance outside of ground zero. The greater the height, the more threat it makes to more stuff.
The radiant heat is pretty destructive as it will start a "Simba's Prideland" of fires... everything the light touches will burn. Famed negative shadows of Hiroshima are people shaped light patches on walls that were saved from charring because there was a person casting a shadow at the time of the radiant heat. In Nagasaki, despite the more powerful of the two bombs dropping, it was less destructive to the overall city because it detonated in the valley of two mountains... essentially blocking much of the radiant heat and over-pressure from destroying the rest of the city.
It's important to remember this is "Simba's Pridelands" so you can survive pretty close to the epicenter in the right conditions, Akiko Takakura entered the history books as being the closest survivor to ground zero in Hiroshima, at a distance of about 300 meters. She survived because at the time of the blast, she was in the vault of her bank and the over-pressure was weak enough not to blow apart the vault structure and the light had already burnt the building's exterior.
It's also important to note that it will be light first than over-pressure... for the same reason it's always lightning and thunder.
So with that out of the way, FEMA answers this with the 7-10 rule. For every measure of 7 hours of time, radiation levels decrease by 10%. If you know your initial rads per hour, this can be mathed out so that 7*.1X = y. This is also rendered as the 7 X 7 X 7 rule. After 7 hours, the initial rads have decreased by 90%. After 49 hours, the initial rads decrease by 99%. After 343 hours (a little over two full weeks) the rads decrease by 99.9%. Ground Zero of Hiroshima today is so safe, it's a tourist attraction.