The tiles had enough natural radiation to "trip" the warning on a scanning, broad-spectrum detector.

The tiles did not have the type radiation needed to "trip" the warnings on the more sophisticated detectors the SEALS would use.

Don't think they could have been stopped from dumping whatever they wanted. I'm sure the ship would have been modified to make that possible, and the material could have been marked for later pickup.

OR - The Feds wanted to search that ship, and used the radiation the way they did the TNT on the guys in Florida. "Oops, sorry. Our mistake. Nice ship you got here, lotsa interesting stuff. Gotta go. Adios, or whatever they say in Liberia. I'll just be shoving off here."

But they could have done that in port. May have wanted a SEAL exercise.

Arf