The submarine is known as Project 20120 in Russian design terminology. She apparently employs the small nuclear reactor -- known to some engineers as a "teakettle" -- to keep a charge on the battery, providing essentially unlimited underwater endurance on relatively quiet electric propulsion. In effect, this is an Air-Indpendent Propulsion (AIP) system.

The "teakettle" concept is not new. The Soviet Navy deployed a Project 651 (NATO Juliett) cruise missile submarine (SSG) in 1986-1991 with a similar diesel-electric/nuclear plant. That craft had a pressurized-water reactor with a single-loop configuration coupled with a turbogenerator. The Soviet report stated that the sea trials "demonstrated the workability of the system, but revealed quite a few deficiencies. Those were later corrected."

However, no follow-on efforts were undertaken at that time. (The Soviets built 16 diesel-electric Juliett SSGs from 1963 to 1968.)

The B-90 was designed by the Rubin design bureau in St. Petersburg. Construction was begun at the Krasnoe Sormovo shipyard in Nizhnii Novgorod (formerly Gor'kiy), and the submarine was then transported through the inland waterways to the Sevmash yard for completion.

There is no available information on the size of the B-90 program. In the past the Soviet Union was an early leader in AIP-type submarines. As early as 1938 the Soviets began development on a "single-drive" submarine that could operate diesel engines while submerged and surfaced. After World War II the Soviets built the Project 617 (Whale), an AIP submarine based on German technology. She was followed by 23 coastal submarines of Project A615 (Quebec), which were torpedo and gun-armed combat craft. Other AIP experiments followed.

Today several navies are operating AIP submarines, with the U.S. Navy having "borrowed" the Swedish AIP submarine Gotland in 2005-2007 to serve as an anti-submarine target for U.S. carrier task forces. The Gotland, according to Swedish officers, could not be located by U.S. naval forces in exercises until the submarine "wanted to be found."

The Soviet B-90 may be a follow-on submarine to the Kilo-class diesel-electric submarines that have been transferred in large numbers to other navies, including China and India. The B-90, especially when operating in coastal or littoral waters, could pose a significant threat to Western maritime interests.