B-90 Sarov, also referred to as Sarov class, or Russian designation Project 20120 Sargan, is a Russian special purpose diesel-electric submarine that uses a nuclear reactor as a supplementary power generator. 

That propulsion arrangement is one of the things that makes her so unique and interesting, there isn't even designation for it.

Preorder now and save 50% on Antarctica STORM: https://www.amazon.com.au/gp/product/B0DNZBLM3X

The existence of the submarine was first revealed in 2007, when details about the boat were accidentally published on the Sarov, Nizhny Novgorod region's local government website as part of an account of a meeting with its commander. 

One of the first misinformations on the internet is her size. Because she was based on a KILO Class submarine hull her length is generally quoted as 72m (236ft) long and 9.9m (32ft) wide. In fact she is nearer to 98m (322ft) long. This is immediately apparent from satellite imagery and from photographs of her tied up alongside a KILO Class boat. The extra length fits with the addition of the nuclear reactor section immediately aft of the sail. The sail itself is also much larger than on the KILO and now contains an escape capsule section like on recent Russian nuclear submarines.

It serves in the Russian Navy's Northern Fleet and is being used as a technology demonstrator for testing of upgraded weapons and military equipment or as an intelligence collection boat. 

On 27 November 2016, Sarov was allegedly involved in a testing of the Russia's nuclear-powered, nuclear-armed Poseidon unmanned underwater vehicle, according to the Pentagon officials. 

On 20 February 2019, Russian Defence Ministry released a video showing the Poseidon unmanned underwater vehicle as it is being launched out of a torpedo compartment aboard Sarov.

Otherwise, reports of the Sarov have been scarce and it is questionable whether the Sarov (like the mystical Poseidon torpedo) is still in commission.