The shitstorm would be that no country has ever used a nuclear weapon against another country, since Hiroshima and Nagasaki. Every civilized country in the world would turn against them, probably even China. Every nuclear arsenal in the world would go to its most ready condition, and things would get pretty damned tense. I can't even imagine what the repercussion to Russia would amount to, but they'd probably deny that it was a nuclear weapon they used. We do have satellites with sensors that tell us if a nuke goes off anywhere in the world.