What would happen should a terrorist group manage to get their hands on the nukes? someone actually use them?
Many deaths. All will be rebuilt. Terrorists wouldn't even be able to operate most nukes, since the only people who have the capability and who have built suitcase nukes are Israel, America and Russia. And they sorta guard their nukes really well. In fact, a situation where their nukes are jeopardized is probably the moment they're about to be launched.
All this craze about 'terrorists' really needs to die, fast. It's always been the countries in charge who have held this awesome power.
People really overplay the threat of nukes sometimes. 10,000 nukes isn't enough to wipe out humanity. Cripple civilization? Yes. But it's conventional weapons that still hold the ability to end life on Earth.
*EDIT
Just realized I was downplaying the threat of the cessation of civilization. The human mind is a strange thing when it encounters powers beyond its comprehension.
On this: and interesting fact is at the end of the First world war, the Allies had enough in Small Arms Ammunition to kill every single person alive on the planet then, or could at least purchase the needed amount without trouble. The difficulty is speed.
Now, we have an unknown number of nukes on the planet, ranging from 10,000 to 30,000. But it is safe enough to say that a instance where 10,000 nukes are used would be a complete nuclear war between all factions that are nuclear armed. In terms of the explosions, no. We do not have nearly the yield to wipe out human life, even given most live in cities. However, given that in poorer countries and rural places, there are no Bunkers, a careful allowance of More high-yield weapons to cities and Nuclear weapons with a higher radiation to high-alitude detonations, we might be able to do it.