No they are used as ammo for the Fat Man/Experimental MIRV nothing else Play me Again says:
Actually there is a place in the DC area where a person is holding an alley hostage, what happens is, if he sees you the whole alley goes boom, if you manage to kill him and explore the alley, you'll notice the place is filled with land mines, mini nukes and grenades laying on the floor and if you accidentally set of the mines of, it creates a chain reaction and the whole alley blows.
So technically, you can set the mini nuke off, but only with another explosion, same goes with grenades, like example if you want to set a trap for a patrolling enemy, but only have 1 land mine, but a lot of grenades (and/or mini nukes) you can set the land mine, and drop you grenades and/or mini nukes on the floor, so that when the land mine is activated the explosion will cause the other explosives to go off as well, i've tried a few times with grenades, but never dared with the nuke.
^^^^^^^^^^^^^^ You cannot set off a mini nuke with other explosives. The man in the alley is scripted to make them blow up if you get to close. That is the only time in the game that mini nukes will explode outside of the fat man/ MIRV.