Almaz / X-Ray

Almaz / X-Ray is the Soviet/Russian NATO-reporting-name family for the Project 1851 Nelma deep-diving special mission submarines. Open sources describe the 40-meter, titanium-hulled craft as carrier-borne vehicles with a diver lock-out, used for reconnaissance, cable tapping, rescue, and seabed recovery at about 1,000 meters.

Various Conflicts🏳️ Unspecified

Almaz / X-Ray, Deep-diving nuclear special mission submarine, Naval Systems

Image source: Russian X-RAY Class Special Mission submarine

Profile

Origin: Soviet Union
Built by: Admiralty Shipyards
Type: Deep-diving nuclear special mission submarine
Service note: 1986-present
Designer: SPMBM Malakhit
Designed: 1970s
Produced: 1981-1995
Number built: 3 hulls
Variants: AS-23, AS-21, AS-35
Developed into: Project 18511 Halibut / Paltus

Also Known As

Project 1851
Project 1851 Almaz
Project 1851 Nelma
Nelma class
X-Ray class
AS-23

naval submarine nuclear-powered special operations surveillanceundersea warfarerescue

Specifications

Length: 40 m
Beam: 5.3 m
Draft: 5.0 m
Displacement: 550 tons surfaced; 1,000 tons submerged
Speed: 20 knots submerged
Operating depth: 1,000 m
Crew: 6 officers
Propulsion: 10 MW reactor, electric motor, stern propeller, and bow thruster
Armament: None

Service And Conflict Use

Service History

In service: AS-23 entered service on December 30, 1986; the follow-on AS-21 and AS-35 boats joined the family in the 1990s, and open sources still place the class with the Northern Fleet.
Used by: Soviet Navy, Russian Navy

Conflict Usage

Side: 🏳️Unspecified
Role: Deep-sea special operations and reconnaissance

RUSI places X-Ray among Russia's specialised GUGI submarines used for seabed warfare and critical undersea infrastructure threats, but the open-source record does not tie the class to a single named post-2015 conflict.

Timeline

Almaz / X-Ray Key Events

Sep 25, 1981
AS-23 laid down
GlobalSecurity says the first Project 1851 submarine, AS-23, was laid down at Sudomech in Leningrad in September 1981.
Sources: Project 678 / Project 1851 - Nelma class / X-Ray class
Dec 30, 1986
First boat enters service
GlobalSecurity and Modellmarine both place AS-23 in service in 1986, making it the first operational X-Ray-family deep-water station.
Sources: Project 678 / Project 1851 - Nelma class / X-Ray class, Russisches Spezial-U-Boot des Typs Almaz/Projekts 1851 (1/700, L'Arsenal) von Eberhard Sinnwell
Dec 28, 1991
Follow-on Project 18511 boat commissions begin
GlobalSecurity records AS-21 entering service in December 1991 and AS-35 joining in October 1995 as the improved Project 18511 follow-ons.
Sources: Project 678 / Project 1851 - Nelma class / X-Ray class
Jun 1, 2023
Seabed-warfare relevance remains current
RUSI's analysis of Russian activity against undersea infrastructure still treats X-Ray as part of the country's specialised GUGI capability set.
Sources: Stalking the Seabed: How Russia Targets Critical Undersea Infrastructure

Almaz / X-Ray Images

Almaz / X-Ray, Deep-diving nuclear special mission submarine, Naval Systems - view 1 — Source: Russian X-RAY Class Special Mission submarine

Almaz / X-Ray, Deep-diving nuclear special mission submarine, Naval Systems - view 2 — Source: Russian X-RAY Class Special Mission submarine

Almaz / X-Ray, Deep-diving nuclear special mission submarine, Naval Systems - view 3 — Source: Russian X-RAY Class Special Mission submarine

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Sources

Russian X-RAY Class Special Mission submarinePublisher: Covert Shores | Note: Supports the X-Ray name, Malachite design context, special-mission role, operating depth, and the gallery images hosted on the article page. | Accessed: 2026-06-25 Project 678 / Project 1851 - Nelma class / X-Ray classPublisher: GlobalSecurity.org | Note: Supports the Project 1851 / X-Ray naming, the carrier-submarine program history, AS-23 launch and service dates, the 40 m / 1,000 t class dimensions, and Admiralty Yard builder context. | Accessed: 2026-06-25 Russisches Spezial-U-Boot des Typs Almaz/Projekts 1851 (1/700, L'Arsenal) von Eberhard SinnwellPublisher: Modellmarine | Note: Supports the Almaz / Project 1851 identification, the three-boat family, AS-23 service date, the Project 18511 follow-ons, and the compact technical summary used on the page. | Accessed: 2026-06-25 Stalking the Seabed: How Russia Targets Critical Undersea InfrastructurePublisher: Royal United Services Institute | Note: Supports Various Conflicts direct fielding/operation context: RUSI places X-Ray among Russia's specialised GUGI submarines used for seabed warfare and notes the diver lock-out, but it does not tie the class to a single named post-2015 conflict. | Accessed: 2026-06-25