France has reached a major milestone in its naval modernization program with the first start-up of the nuclear reactor aboard De Grasse (S638), the fourth Suffren-class nuclear-powered attack submarine (SSN). In a statement released on 14 December 2025, the French Ministry of the Armed Forces confirmed that the reactor has achieved “divergence,” clearing the way for the submarine to begin preparations for its initial alpha sea trials.
The achievement places De Grasse firmly on track to join the French Navy’s Barracuda-class fleet, which is progressively replacing the aging Rubis-class submarines that have been in service since the 1980s. Once operational, De Grasse will further strengthen France’s undersea warfare capabilities at a time when naval competition and submarine operations are gaining renewed strategic importance.
Reactor Divergence: A Critical Step in Nuclear Propulsion
Divergence marks the moment when a controlled nuclear chain reaction is initiated inside the reactor core for the first time. For a Ship Submersible Nuclear (SSN), this is one of the most critical phases in construction and commissioning. From this point onward, the nuclear reactor enters permanent monitoring by qualified nuclear propulsion officers, a process that will continue without interruption until the submarine’s final shutdown, currently planned for the 2060s.
The nuclear reactor aboard De Grasse was designed under the authority of the French Atomic Energy Commission (CEA). Its production and commissioning were overseen by TechnicAtome, the French specialist in compact naval nuclear reactors, while Naval Group was responsible for manufacturing the reactor pressure vessels and integrating the propulsion plant into the submarine. This division of responsibility reflects France’s long-standing, sovereign approach to naval nuclear propulsion, combining civilian nuclear expertise with military shipbuilding know-how.
The Fourth Boat of the Suffren Class
De Grasse is the fourth submarine of the Suffren class, also known as the Barracuda-type SSN. The class represents a generational leap for the Marine Nationale, offering improved stealth, greater endurance, enhanced automation, and a broader mission profile compared with the Rubis class.
The submarine is named after François Joseph Paul, Comte de Grasse, Marquis de Grasse-Tilly, one of France’s most celebrated naval commanders. His decisive victory over the British Royal Navy at the Battle of the Chesapeake in September 1781 played a crucial role in securing American independence. By carrying his name, De Grasse continues the French Navy’s tradition of honoring historic maritime leaders through its capital warships.
Progress of the Barracuda Program
Under the Barracuda program, six Suffren-class SSNs were ordered by the Direction Générale de l’Armement (DGA), France’s defense procurement agency. The first three boats—Suffren, Duguay-Trouin, and Tourville—were commissioned in June 2022, April 2024, and July 2025, respectively, and are already integrating into operational service.
The final two submarines of the series, Rubis and Casabianca, are currently under construction at different stages at Naval Group’s Cherbourg shipyard. According to current planning under the 2024–2030 Military Planning Law, all six submarines are scheduled for delivery by 2030, ensuring continuity in France’s attack submarine force as the Rubis-class boats are retired.
A New Standard for French Attack Submarines
With a surface displacement of around 4,700 tons and more than 5,100 tons submerged, the Suffren class is significantly larger than its predecessors. Measuring approximately 99 meters in length with a hull diameter of 8.8 meters, the submarines are designed for deep-water operations beyond 350 meters and sustained high-speed transit in excess of 25 knots.
Propulsion is provided by a pressurized water reactor generating roughly 150 megawatts of thermal power, derived from proven reactor designs used on the Triomphant-class SSBNs and the aircraft carrier Charles de Gaulle. The reactor drives two propulsion turbines, supported by turbo-generators and electric motors, forming a hybrid propulsion architecture optimized for both performance and acoustic discretion. Endurance is estimated at up to 70 days at sea, limited primarily by food supplies rather than fuel.
Each submarine is operated by a core crew of around 63 sailors, with additional capacity for approximately 15 naval commandos, reflecting the class’s expanded role in special operations.
Enhanced Strike, Stealth, and Special Operations Capabilities
The Suffren class introduces a wide range of technological innovations intended to give the French Navy a decisive edge in undersea combat. One of the most significant is the integration of the MdCN naval cruise missile, launched from standard torpedo tubes. This capability allows the submarine to strike high-value targets deep inland while remaining covertly submerged.
Advanced sonar systems and sensor suites provide superior anti-submarine warfare (ASW) and intelligence, surveillance, and reconnaissance (ISR) capabilities. The submarines are also equipped with non-penetrating optronic masts developed by Safran Electronics & Defense, delivering high-resolution 4K imagery directly to the combat information center (CIC). Unlike traditional periscopes, these masts enhance stealth and allow multiple operators to access visual data simultaneously.
Special forces operations are another core mission. The Suffren class can be fitted with a removable Dry Deck Shelter, enabling the deployment of swimmer delivery vehicles (SDVs) such as the new PSM3G and teams of combat divers. This flexibility supports covert insertion, reconnaissance, and maritime counter-terrorism missions far from home waters.
With reactor divergence now complete, De Grasse will move into the next phase of testing, culminating in alpha sea trials to validate propulsion, safety, and core systems at sea. Full operational commissioning will follow after an extensive period of trials and crew training.
As France continues to roll out the Barracuda program, the firing up of De Grasse’s nuclear reactor underscores the steady progress of one of Europe’s most ambitious naval projects. Once fully operational, the Suffren-class submarines are expected to form the backbone of French undersea power well into the second half of the 21st century, reinforcing the country’s strategic autonomy and global maritime presence.
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