Thousands on a container ship. Put it on the bottom and shield it with more containers full of lead or another suitable shielding material and just be patient. Track the ship over the internet and call the special cell phone number when it ties up to be unloaded.
https://www.marinetraffic.com/en/ais/home/centerx:-74.140/centery:40.671/zoom:14
Even not very powerful it would vaporize the ship and probly the terminal and make a big radioactive hole in New Jersey (where it belongs) and cause mass hysteria. Maybe a little more $$ than 9-11 but they have just about everything they need to pull it off already in place.
waiting for carpet bombing of Iran and a naval blockade.
I just looked at the specs of the tsar bomba. That thing would fit inside a 40 foot shipping container easily, and would ruin the east coast...