The nuclear reactor of the fourth Suffren-class nuclear-powered attack submarine (SSN) was fired up for the first time, according to a French Ministry of Defense release issued 14 December, 2025. With this new milestone, known as “divergence”, the De Grasse (S638) is on track to start alpha sea trials.
De Grasse is the fourth Barracuda type SSN for the French Navy
Designed under the responsibility of the French Atomic Energy Commission (CEA), the nuclear reactor was produced under the supervision of TechnicAtome (who specializes in the design, construction, commissioning and operational maintenance of compact nuclear reactors) and Naval Group (being responsible for the production of the pressure vessels and for the integration of the nuclear reactors on board all Barracuda type SSNs).
The divergence of a nuclear reactor, particularly on a Ship Submersible Nuclear (SSN), is the term used to describe the initial startup of the controlled nuclear chain reaction within the reactor core. Divergence marks the beginning of permanent monitoring of the nuclear reactor by nuclear propulsion officers. This monitoring will be uninterrupted until the final shutdown of the submarines, scheduled for the 2060s.
The fourth Suffren-class submarine is named after Comte de Grasse (full name: François Joseph Paul, Comte de Grasse, Marquis of Grasse-Tilly). He was a French naval officer who became one of the most important figures in the American Revolutionary War. His most significant contribution was his crucial victory over the British Royal Navy at the Battle of the Chesapeake (also known as the Battle of the Virginia Capes) in September 1781.
The six submarines ordered by the DGA (French Defence Procurement Agency) under the Barracuda program are replacing the Rubis-class SSN fleet, which entered service in the 1980s. The first three Barracuda SSNs, Suffren, Duguay-Trouin, and Tourville, were commissioned in June 2022, April 2024, and July 2025, respectively. The last two submarines in the series, the Rubis and the Casabianca, are under construction at various stages of completion, with deliveries scheduled by 2030, in accordance with the 2024-2030 Military Planning Law.
About Suffren-class SSN
Six new attack submarines will form the vanguard of the French Navy (Marine Nationale) for the coming decades. Developed as part of the Barracuda program, the new submarines will offer a massive capability leap over the current Rubis-class boats.
Main specifications
- Surface displacement: 4,700 tons
- Diving displacement: 5,100 tons
- Length: 99 meters
- Diameter: 8.8 meters
- Maximum depth: > 350 meters
- Speed: > 25 knots
- Armament: naval cruise missiles, F21 heavy-weight wire-guided torpedoes, modernized Exocet SM39 anti-ship missiles, FG-29 mines, UAV and UUV (future development)
- Hybrid propulsion: pressurized water reactor (150MW) derived from the reactors on board the Triomphant-class SSBN and Charles-de-Gaulle aircraft carrier, two propulsion turbines, two turbo generators and two electric motors
- Crew: 63 crew members + approx. 15 commandos
- Endurance: 70 days at sea (or until food supplies run out)
Innovation for naval combat
Suffren-class SSNs are equipped with several innovations that allow them to demonstrate differentiating capabilities in many areas. The French Navy’s new class of submarines is able to strike deep behind enemy line all while remaining stealthy thanks to the torpedo tube-launched naval cruise missile (MdCN). The integration of state-of-the-art sensors also gives it superior anti-submarine warfare and intelligence, surveillance, reconnaissance (ISR) capabilities. The non-penetrating optronic masts (by Safran Electronic and Defense) are a disruptive technology. They bring unmatched high quality (4K) imagery and every sailor in the CIC can access them. Finally, these submarines comes with systems that facilitate the deployment of naval special forces. In particular, the Dry Deck Shelter, a removable deck hangar, allows the deployment of the new PSM3G swimmer delivery vehicle (SDV) and a dozen combat swimmers.
