Fears of terrorists gaining access to a nuclear weapon have been promoted by some of our most respected thinkers. The complexity involved in such an action dwarfs the previous terrorism actions and limits its likelihood to pure speculation and more likely the promotion of fear. To achieve a successful nuclear event would first require the technical capacity to undertake the action and reach its maximum conclusion. This must be followed by acquisition of the materials and machinery required for the explosion. These elements must be delivered to an arena deemed effective for a demonstration of the perpetrator's power. The device then must be successfully detonated at a time that produces the greatest casualties and destruction to make their point. Each of these stages are more complicated and expensive that any successful action to date. 

Repercussions

If we assume such a detonation were to be successful in a highly populated area the results would be significant destruction and immediate casualties. Fear and radiation would expand the affected region greatly, but the overall number of dead including long-term would be limited due to the overwhelming complexity of igniting a large nuclear device. The response from the world would be swift and violent. Borders will be affected and we would see regional and political changes not seen since the 1940s.