If I had seen this invasión telegraphed since 2014 and I was a military with access to multiple nuclear reactors, possibly the odd plutonium core (100 year best before) misplaced and Chernobyl, many sympathisers who hate Putin in Russia...
I'd definately have done this, MAD works as well on the understanding that neither side, especially the bigger bully, want to take even a small hit and there could be many. It's a possibility at least. The movies portray helicopters detecting radiation etc. but that's BS, simple heavy shielding will completely conceal a radioactive source.
http://content.time.com/time/magazine/article/0,9171,1001206,00.html
Were the Russians Hiding a Nuke in D.C.?
By Hugh Sidey Monday, Nov. 12, 2001
No, Kennedy continued, it was his understanding that the Soviets had brought the components of an atomic device into the building in inspection-free diplomatic pouches and assembled it in the upstairs attic. "If things get too bad and war is inevitable," he said, "they will set it off and that's the end of the White House and the rest of the city." I laughed. Still suspending his bite of fish, Kennedy said, "That's what I'm told. Do you know something that I don't?" No sign of mirth. The conversation moved on.
Five years ago I was lecturing in Staunton, Va., and retold the story. In the question session, a man in the audience rose and said, "You may not believe that story about the bomb in the attic, but I do. I worked for 25 years at the Defense Intelligence Agency, and that was our understanding." And now I can hear Kennedy asking again, "Do you know something I don't?"