I'm Willing to bet...

...that the original script for this movie did not include the atomic bomb. It was probably scripted to have the nuclear waste cause a nuclear explosion. Once the writers found out that this was impossible, they had to add the bomb to make the disaster work.

There are just too many "plot holes" surrounding the bomb - how did it get into the U. S.; why was it a complete bomb; where was it going? If the bomb was stolen, why were they trying to get rid of it? If it wasn't stolen, there is no way the Russian would have given a complete bomb to anyone. Why was the cargo of nuclear material (Uranium Hexafluoride) never mentioned, even though one of the movie trailers used the term "nuclear waste"?