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Fortunately, the lousy persian golf oil tanker smugglers don't come to our shores. The big tankers are run by the oil companies and they have sufficient security to prevent someone from sequesting a nuke. Not easy at all to do that. Would require quite a bit to get one on board undetected, I think; and require replacement of the crew with suicidal fanatics who would have to successfully sail the ship for several days to get it to our shores. Pretty risky for them.

Any such nuke would not come here on an oil tanker. It would come here in a sealed container on a container ship along with cheap Asian-manufactured junk on it's way to your local WalMart.

Any one of these containers (maybe that green one, far aft, starboard side on the Mexican Line ship) could have gone from Iraq to Saudi Arabia to Indonesia to Mexico to San Francisco without ever once being opened and inspected.

Yes - that is a bigger problem. It is a far more likely scenario than trying to get a nuke on an oil tanker.
And I don't think it would be easy to stop if the cargo container was down in the hold, below the water line, with shielding containers stacked around it.
Might be possible.