The submarine I-52, perhaps? Looks like it was supposed to receive a cargo of Uraniam Dioxide from Germany. But it never made it that far and the use hasn’t been confirmed. So either way that certainly wasn’t sunk with a dirty bomb bound for anywhere in the US.
Don’t misread me, I want your telling of it to be true (it has a really intriguing “what if” factor) but I just can’t find any evidence of it.