With normal torpedoes, the amount of noise (via cavitation, engine noise, etc.) is significant. If you could produce a really quiet weapon, the target wouldn't know it was coming until it was too late. If you can hear the weapon inbound, you can at least make efforts at evasion, and you can probably get off a counterfire to make the other guy's life less pleasant.
With a 200kt nuclear torpedo, all of the standard rules go away. By the time you recognize the noise, it's _way_ too late. There's no time to even make a pretense of evasion. There's not a chance of a counterfire. There isn't even time to send a message saying "We're so f---ed." In the words of a famous coward, "It's game over, man!" Except that you probably wouldn't even have time to say that.
The only realistic hope is poor manufacturing causing failures in the gear. If they use roughly similar quality control on their nuclear weapons as they do on my slide scanner, we have nothing to fear.
