I really doubt that. I can't imagine that the US and the UK for example, don't have sonar networks and other means to detect subs. It might be nearly impossible

I really doubt that. I can't imagine that the US and the UK for example, don't have sonar networks and other means to detect subs. It might be nearly impossible to track a boomer in open ocean, but they're not designed for littoral operations and sonar networks are much more effective there. You also have to cope with an airforce and patrolling navy. All in all, you're mostly likely to nuke the nearby ocean, rendering it mostly a radiological/environmental weapon. When you consider that you can far more easily launch a combined 100MT in smaller payloads, with a higher resulting degree of destruction than a single 100MT warhead...