I have heard a rumor that nuclear devices are detectable via sattelites and that this detection cannot be prevented.
Russia might have tried bringing nukes into the US by unconvential methods otherwise.
I don't think this is true.
I believe there are now radiation detectors at container ports. There are many millions of containers.
Thanks to NAFTA, a truck can enter the U.S. from Canada or Mexico with reduced or no inspection.
No doubt the scenario is constantly gamed by those tasked with our security.
At least since September 11, 2001, we have a right to expect that it is.