You do not need neutrino detector.
There is no need for exotic tech. You just need to pay attention to the "enemy" presentations in their native language and filter out the rubbish interpretation the press is feeding you.
https://www.defensenews.com/space/2018/01/12/russias-nuclear-underwater-drone-is-real-and-in-the-nuclear-posture-review/ - a lot of this is from pictures of a preso taken when one of the (i do not envy the poor thing) assistants of the Russian general staff forgot to cover it.
The press (being the press) on both sides of the fence presented us this as some scary doomsday cobalt supernuke. One off super duper weapon. Doomsday device.
It ain't. Read carefully. Slide on the right top (translating for those not proficient in великий и могучий):
Left: Primary applications of this system are: engagement with torpedoes of capital ships, mixed rocket-torpedo load missions, setting of minefields (including clandestine), hunting of opponent atomic submarines.
Right: Out of the multitude of possible applications the publicly disclosed one is carrying a warhead of up to 100Mt with a possible "cobalt bomb" (emphasis on multitude - mine).
And suddenly the whole idiotic superweapon malarkey makes sense.
It is not. It is a cut down, cheap, semi-automated and semi-autonomous (possibly going autonomous in the future) unmanned hunter-killer submarine. No frigging 100Mt cobalt bombs - basic torpedoes and missiles. That also explains the gigantic sonar array in the bow. All tech is available - this is just the old Alpha/Lira class (the one which necessitated the development of the Spearfish torpedo) with the meatware removed and a further improved reactor. It is cheap too. Way cheaper than a manned hunter killer. It also fits the way they are building stuff - as a design philosophy it is the navy equivalent of a Uran-9. Cheap, unmanned, designed for manufacture in quantity.
So what can a submarine coming out of port do against an opponent which is doggedly following it constantly shining on it with active sonar? What can it do if it is doing it from deeper than its maximum torpedo engagement depth (up to 500m)? What can it do if the "target" is 40% faster than its fastest torpedo ~ up to 100 knots (not difficult if you do not need to carry meatsacks)? How do you think it will perform if the crew knows that they have a gun pointed at their temple 24x7x365?
If (there is an if, but an a big one here) they start building these, the Agincourt in 5-10 years or so it will be near-obsolete. Still better than the Dreadnought class which may end up being obsolete before even leaving for sea trials.