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