Naval Group displays SNLE 3G nuclear submarine to preserve France's second strike capability through 2090.

Naval Group displayed a scale model of France's future L’Invincible-class (SNLE 3G) nuclear-powered ballistic missile submarine at Euronaval 2026 in Paris. The four 150-meter, 15,000-tonne SSBNs will replace the Le Triomphant fleet between 2037 and 2052, integrating a 220 MW TechnicAtome K22 nuclear reactor, X-shaped stern control surfaces, and the Thales ALICIA sonar architecture. Armed with 16 M51 submarine-launched ballistic missiles, the lead submarine L’Invincible will be delivered in late 2037 to sustain France's continuous at-sea strategic deterrence from the Île Longue base.

The new SNLE 3G submarine class will incorporate Thales' ALICIA passive sonar, X-shaped stern control surfaces, improved acoustic isolation, and compatibility with the M51.4 missile to sustain France's sea-based nuclear deterrent when the Le Triomphant-class retires. (Picture source: Army Recognition)

Key Specifications & Capabilities

• Dimensions & Displacement: 147–150 meters in length, displacing over 15,000 tonnes submerged.

• Nuclear Propulsion: Powered by TechnicAtome's 220 MW K22 pressurized water reactor, combining pump-jet propulsion with X-shaped stern control surfaces.

• Sonar & Acoustic Stealth: Integrates Thales ALICIA sonar architecture (including ALRO optical-technology towed array and SYCOBS 3.0 processing using AI for contact tracking) alongside advanced magnetic and acoustic isolation systems.

• Armament: Retains 16 launch tubes for M51 submarine-launched ballistic missiles (transitioning to the M51.4 variant in the mid-2030s), plus four 533 mm tubes for F21 heavyweight torpedoes.

Between November 3 and November 6, 2026, Naval Group displayed a scale model of France's future SNLE 3G nuclear-powered ballistic missile submarine at Euronaval 2026, presenting the design of the four L’Invincible-class submarines scheduled to replace the four Le Triomphant-class boats between 2037 and 2052. President Emmanuel Macron named the first SNLE 3G submarine L’Invincible on March 2, 2026, at the Île Longue submarine base, confirming that the first vessel would sail in 2036, followed by delivery in late 2037. France's next generation of SSBN submarines will measure between 147 and 150 m, displace more than 15,000 tonnes submerged, and retain the 16 M51 ballistic missile tubes.

Major changes include a 220 MW K22 nuclear reactor, X-shaped stern control surfaces, the Thales ALICIA integrated sonar architecture, and improved acoustic and magnetic signatures. The introduction of France's future SSBN submarines will also coincide with the M51.4 missile, expected to enter service in 2035, and France's March 2026 decision to increase its nuclear warhead inventory. The program is intended to preserve France's continuous at-sea nuclear deterrent, maintained since 1972, while replacing submarines that will have accumulated between 27 and 40 years of commissioned service when the first L’Invincible-class enters the fleet. The SNLE 3G submarine program entered full-scale development in February 2021 under a five-year contract covering submarine design, initial pressure-hull components, nuclear reactor equipment, long-lead procurement, and modernization of Naval Group's Cherbourg shipyard.

Related topic: UK deploys three of four Vanguard-class nuclear submarines at sea for first time in more than five years

Early industrial orders included CNIM missile-launch tubes, Aubert & Duval and Framatome nuclear forgings, and Industeel high-strength structural steel. On March 20, 2024, Naval Group cut the first hull steel at Cherbourg, followed by a second contract in November 2025 covering detailed design, complete equipment procurement, and construction of L’Invincible. Design reviews were scheduled during 2026, with an assembly of major hull sections anticipated in 2026-2027. The lead submarine is scheduled for launch in 2036 and delivery in late 2037, leaving a period that will be used for reactor qualification, acoustic measurements, submerged trials, and strategic weapons testing. Subsequent deliveries are planned for 2042, 2047, and 2052, establishing a five-year production interval and requiring overlapping construction, equipment manufacturing, and qualification activities across successive submarines.

The L’Invincible-class will measure 147 to 150 m in length, retain a beam of 12.5 m, and displace more than 15,000 tonnes submerged, compared with 138 m and 14,335 tonnes for the Le Triomphant-class. This represents a length increase of 6.5- 8.7%, while the displacement rises by at least 665 tonnes, or 4.6%, primarily to accommodate larger machinery spaces, acoustic isolation, and reserves for future equipment modifications rather than increasing the number of ballistic missile tubes. Maintaining a beam close to that of Le Triomphant limits the changes required to the existing submarine berthing and maintenance infrastructure at Île Longue. TechnicAtome's K22 pressurized-water reactor, already used on the France Libre aircraft carrier, will produce approximately 220 MW thermal, 46.7% more than the existing K15's 150 MW, although this does not indicate an equivalent increase in propulsion power.

How France's SNLE 3G submarines will improve underwater stealth and detection

Pump-jet propulsion will now be combined with X-shaped stern control surfaces, an arrangement also employed on the Suffren-class attack submarines, to modify underwater maneuvering and stern equipment arrangements. The crew is expected to remain close to 100 to 110 personnel, with mixed-gender accommodation, updated atmospheric regeneration equipment, and redesigned living spaces supporting patrols lasting nine to ten weeks. Underwater surveillance will be centered on Thales' ALICIA architecture, which integrates a new bow sonar, large flank-mounted passive arrays, and the ALRO optical-technology towed sonar with SYCOBS 3.0 acoustic processing. The ALRO deploys between the upper X-shaped stern control surfaces and supports ultra-low-frequency detection of quiet underwater contacts, while flank arrays provide additional acoustic observation angles.

For its part, the SYCOBS 3.0 is designed to process ten times the acoustic data volume of existing French ballistic missile submarines, by using artificial intelligence to assist contact identification, classification, and simultaneous tracking. The Le Triomphant-class currently employs the DMUX 80 bow sonar, DSUV 61B very-low-frequency towed array, and DUUX 5 acoustic ranging system. Additional machinery isolation, acoustic coatings, pump-jet refinements, and magnetic-signature reduction aim to reduce detectability while limiting the SNLE 3G submarine's own interference with passive sensors. Actual detection ranges remain dependent on water temperature, salinity, depth, sound propagation, and background noise, and no quantified reduction in the submarine's acoustic signature has been released to date.

How the M51.4 nuclear missile will equip France's next-generation submarines

Each L’Invincible-class submarine will carry 16 M51 submarine-launched ballistic missiles (SLBMs), maintaining 64 launch positions across the four-submarine fleet, although the French strategic submarine force uses three sets of 16 missiles to arm submarines rotating through the operational cycle, while the fourth submarine undergoes major maintenance. The three-stage, solid-propellant M51 measures approximately 12 m in length and 2.3 m in diameter, weighs 52 to 56 tonnes at launch, and has an estimated maximum range approaching 10,000 km depending on nuclear payload. The current M51.3 entered operational service in 2025, introducing improvements to the third propulsion stage, range, accuracy, and penetration of missile defenses, together with the progressive introduction of the TNO-2 nuclear warhead. The Le Triomphant-class submarine Le Vigilant conducted an unarmed M51.3 launch on October 6, 2026, validating the submarine's ability to employ its strategic missile system after maintenance.

On August 28, 2025, the DGA awarded ArianeGroup the development and production contract for the M51.4, which is expected to enter service in the mid-2030s and equip both existing Le Triomphant-class submarines and the future L’Invincible-class. Safran Electronics & Defense is also developing the CI4 inertial guidance unit for the M51.4, using dual-core hemispherical resonator gyroscope technology to improve navigation accuracy, while an updated submarine navigation system will support submerged positioning without continuous satellite updates. Four 533 mm torpedo tubes will accommodate F21 heavyweight torpedoes for conventional self-defense, with the submarine's principal mission remaining undetected strategic deterrent patrols.

How 500 French companies will build the four L’Invincible-class submarines

Construction involves approximately 500 French companies, 3,000 direct skilled jobs, 1 million components, and an estimated 20 million working hours, with 90% of program added value generated in France. Naval Group manages submarine construction at Cherbourg, TechnicAtome develops nuclear propulsion, Thales supplies sonar equipment, ArianeGroup develops the M51 missiles, and Safran contributes navigation systems, under DGA and CEA oversight. Nearly 100,000 onboard equipment items and hundreds of kilometers of cabling and piping must also be integrated into each submarine's industrial production process. Approximately 50 major nuclear boiler-room forgings were ordered in 2021 for the first two vessels, with Aubert & Duval and Framatome supplying components and Naval Group's Nantes-Indret facility performing propulsion-related machining and integration.

Industeel's six structural connection components required approximately 50,000 working hours, while TechnicAtome plans to begin modular assembly of the first K22 propulsion plant in 2028. The industrial network expanded from more than 400 companies in 2024 to 500 in 2026, but the classified program budget prevents a reliable submarine unit-cost calculation. The operational constraint facing the SNLE 3G submarine program is maintaining France's continuous nuclear deterrence with only four submarines during a replacement period extending to 2052. Similar to the British Dreadnought-class program, this means that one submarine permanently at sea represents at least 25% of the fleet, while the other three must collectively cover the next patrol, the crew training, maintenance periods, and major refits; a prolonged technical failure can therefore reduce the available rotation.

Why replacing France's nuclear submarines by 2052 is essential to continuous deterrence

Of course, the M51's estimated range, which approaches 10,000 km, permits distant targets to be held at risk without requiring patrols close to defended coastlines, while the ALICIA's distributed sensors and reduced machinery noise will improve underwater contact detection without increasing the submarine's acoustic exposure. However, acoustic propagation varies with temperature, salinity, and depth, while opposing passive sonar systems can exploit weak machinery and hydrodynamic signatures over long distances under favorable conditions.

Nevertheless, the principal scheduling risk is, like in the UK and the US, the age of the existing SSBN submarines: Le Triomphant will reach 40 years of service in 2037, while the fourth replacement is not scheduled until 2052. Any delays in reactor qualification, missile integration, acoustic trials, or crew certification could thus extend the service required of older submarines and compress maintenance intervals, even if maintaining M51.4 compatibility across both generations, modernizing selected Le Triomphant systems, and expanding maintenance capacity at Île Longue would help preserve patrol availability during the transition.

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Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, South Korea, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.