Sorry, hate to be a wet blanked, but this story does not fly.
Uranium is one thing, and it requires extensive purification to be anywhere near weapons grade. That takes huge plants.
For example, the one Hiroshima bomb consumed all the U235 Oak Ridge produced throughout WWII.
Pu is another matter. Not obtainable.
As far as residual radiation, there would be extensive contamination in the aftermath of any type of fission explosion.
Finally, fission explosions do not have a bright flash. They have a very bright DOUBLE flash unique to atomic explosions.
Meteor blast in the atmosphere also plausible. But, you see, Trump mentioned Russia!
It could have been a clean fusion implosion with slow neutrons.