If you want, I can provide you with all of Fallout: New Vegas Weapons sounds directly ripped from game files.
I pretty much have the Geck properly set up and all the nv core files archived, so I can access the sound files at any moment. Otherwise, the fallout wiki usually has the sounds available
Maybe you can work with this for the fat man?
For now, i'm okay. I'll either make my own nuke explosion, or use the mininuke explosion by bushido and modify it a bit.