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K-123 was a Project 705K (NATO reporting name: Alfa) nuclear-powered attack submarine of the Soviet Navy.

Renamed B-123 in 1992, the submarine was the only Project 705 boat to continue serving in the Russian Navy after the dissolution of the Soviet Union. B-123 was decommissioned in 1996 and scrapped in 2006.

Characteristics
The Project 705 "Lira" (Лира, NATO: Alfa) had a double hull made out of titanium alloy and consisted of six compartments, along with an escape chamber for the crew, with a displacement of 2300 t on the surface and 3180 t while underwater. The submarine had a length of 81.4 m, a beam of 9.5 m, and a draft of 7 m. Its test depth was 350 m. The submarine's power source was one 155 MW liquid metal cooled nuclear reactor that used a lead-bismuth alloy as the coolant. Two reactors types were used for the class, with Project 705 boats receiving the OK-550 reactor and those of the 705K variant using the BM-40A reactor. The reactor provided steam for the OK-7 steam turbine that produced 40,000 hp for the one propeller shaft. There were also two auxiliary propellers. This gave the submarine a speed of 14 kn on the surface and 43 kn while submerged. Its armament were six 533 mm torpedo tubes with 12 reloads, which could fire normal torpedoes or RPK-2 Vyuga (NATO: SS-N-15 Starfish) anti-submarine missiles. The crew included 24 officers, 4 warrant officers, and 1 petty officer, for a total of 30.

The Alfa class submarine had many innovative features. To function in its role as a submarine "interceptor" the Alfa class had to be capable of a high speed, and this was achieved by making it relatively small, using titanium for the hull instead of steel, and creating a more dense and efficient nuclear reactor. The submarine also had smaller electrical equipment by increasing electrical power to 400 Hz. The size of the crew was reduced by automating many systems and having everything be operated from the main control room. The crew stayed in one compartment and did not enter the others except when maintenance was needed. The Alfa class also had an escape chamber inside of its sail that would float to the surface after being detached. One of the complications the design had was the need to constantly keep the liquid metal in the reactor heated to at least 123 C-change to prevent it from solidifying and freezing the reactor. At first, when the submarines were docked they were connected to an external source of heat, but eventually it was decided to keep the reactors running at all times.

History
K-123 and the other Project 705K submarines were built at the Zvezdochka shipyard in Severodvinsk. K-123 was laid down on 29 December 1967, and its chosen crew began their training in 1973 at the 270th Training Center of the Soviet Navy. It was launched on 9 April 1976 and entered service on 12 December 1977 under the command of Captain 1st rank A. U. Abbasov. K-123 was initially assigned to the 33rd Submarine Division of the Red Banner Northern Fleet 1st Submarine Flotilla, before joining the newly formed 6th Submarine Division at Zapadnaya Litsa. From April to May 1979 it became the first submarine of the Alfa class to carry out an operation. In September 1979 the submarine reached a depth of 400 meters during a test in the Norwegian Sea, and from January to March 1980, it tracked a foreign submarine. In December of that year the crew of K-123 was rated as the most ready in its division. In April 1981 K-123 participated in the "North-81" military exercise with two other boats of its class (SOVIET SUBMARINE K-373 and SOVIET SUBMARINE K-432).