The United States and Canada have some pretty fucking insanely good radioactive material detection grid. It can find people who have had radioactive cancer treatment 8~10 months after the fact. And just to be clear here its cancer treatments from nations who don't share medical data with the U.S. so there end up being false positive nuclear material warnings, the people who have had legit cancer treatments and set off the nuclear warnings and the U.S. can confirm it was genuine treatment never see the interviewer and are never asked about it because its meant to be a largely invisible process.

Part of the reason the sale of a port was considered "viable" was the fact security has moved away from direct inspections, into really elaborate surveillance and detection grids. A lot of major airports also have the same kind of hidden border controls. Thats part of the reason a smart border wall between Mexico is 110% viable and necessary, parts of it already exist in certain remote hotspots for border crossings there just isn't the political will to see a real physical barrier going up.

Am I saying China wouldn't try it? oh they probably already have and theres probably worse things moving around the west than anything the Chinese have successfully managed to get into the west. There was some pretty alarming cold war reports of Soviet agents traveling around Europe and North America carrying dirty bombs, not nuclear bombs of course but something that would irradiate sections of a city or lets say air/navy base should the cold war turn hot. From what I understand their instructions were to keep it moving and pass it from agent to agent in a city then if nothing happened during their mission dispose of it in a way that another agent could recover it. Fuck knows if its real but China would be the type of just buy the information from Russia.