Nuclear materials already move intercontinentally on commercial ships. Somalian pirates can already pull an Exxon Valdez using commercial oil tankers.

Also note that the potential for catastrophic meltdown / contamination is proportional to how much fuel you have on board. If you only have ~3 months fuel for a 20MW reactor, then it's not like a 3-mile-island or Chernobyl situation. There are plenty of modern reactor designs that don't turn into a giant cloud of radioactive smoke under duress, but shut down relatively calmly and safely.