Underwater long-range detection is (mostly) done by sound -- that entails: Reducing as much as possible the emission of noises (specially regular, periodical noises) to avoid detection, and; Being able to listen better -- and farther away -- than one's adversary. Nuclear Reactors for submarines are a breed apart. read more
Detecting and track nuclear subs at sea by sonar has been possible since the first nuclear submarine. It certainly isn't easy but it is possible. There must be more to this briefing you reference since submarine detection was pioneered by the U.K. in WWI. read more