Yep, thats the challenge. The only ship ever sunk by a Nuclear Submarine was the General Belgrano. But lets change that, get some other powers to be involved in Wars where Nuclear Submarines are used.
Only 2 Limitations
1-This all has to occur with a POD no earlier then 1945
2-It cannot involve a Soviet-NATO War. Chinese America or Chinese Soviet Wars are fine, though frowned upon.