The movie embedded below illustrates a series of photos taken from inside Chernobyl and the in-game models:
The original movie on YouTube is also worth a look, although it only contains the city photographs.
The full level in Call of Duty 4 can be seen below, the architecture in the movie above can be clearly seen, it is an impressive if chilling rendition of Chernobyl:
See http://www.nextgenboards.com/ for the full story behind the images...