The smaller the nuclear weapon, the more difficult it is to keep it working........  Suitcase nukes must be returned to the factory every couple years to have their plutonium refined, as it quickly degrades into uselessness........... nukes the terrorists have stolen years ago are useless to them........it is highly unlikely Iran could build or rebuild them.......
 
Suitcase nukes would blow up several blocks of a city, but that's all......  they are not very powerful....... they were made to blow up bridges, dams, power plants......they were ineffective for anything else.......you can see on video the explosions of nuclear artillery shells from the 1950's........
 
there are nuke sensors all over our highways and cities...... they would not get very far transporting a bomb......
 
Lewis Brackett