The nuclear reactor aboard France’s fourth Suffren-class nuclear-powered attack submarine has been started for the first time, marking a major milestone in the vessel’s path toward operational service, the French Ministry of Defense said.
On December 12, the ministry confirmed that the submarine De Grasse (S638) successfully achieved divergence, the technical term for the controlled initiation of a nuclear chain reaction in a reactor core. The step clears the way for the submarine to begin its initial sea trials, known as alpha trials, in the coming months.
Inside France’s Suffren-class submarine program
De Grasse is the fourth Barracuda-type attack submarine being built for the French Navy under the Suffren class program, which is designed to replace the aging Rubis-class fleet that entered service in the 1980s.
The nuclear reactor was designed under the authority of the French Atomic Energy Commission, with TechnicAtome overseeing reactor design, construction, commissioning, and long-term maintenance. Naval Group, France’s main naval shipbuilder, was responsible for manufacturing the reactor pressure vessels and integrating the propulsion system into the submarine.
The divergence marks the start of continuous reactor monitoring by specially trained nuclear propulsion officers, which will remain uninterrupted throughout the submarine’s operational life, which is expected to extend into the 2060s.
The submarine is named after François Joseph Paul, Comte de Grasse, a French admiral whose victory over the British Royal Navy at the Battle of the Chesapeake in 1781 was decisive in securing American independence during the Revolutionary War.
France’s defense procurement agency ordered six attack submarines under the Barracuda program. The first three boats, Suffren, Duguay-Trouin, and Tourville, entered service in June 2022, April 2024, and July 2025, respectively. Two additional submarines, Rubis and Casabianca, are currently under construction and are scheduled for delivery by 2030 under France’s 2024-2030 Military Planning Law.
5,100-ton nuclear submarine
Once fully delivered, the six Suffren-class submarines will form the backbone of France’s undersea warfare capability for decades. Navy officials say the class represents a significant leap in performance compared with the Rubis-class, offering greater stealth, endurance, and combat flexibility.
The submarines displace about 4,700 tons on the surface and roughly 5,100 tons submerged. They are 99 meters long, nearly nine meters in diameter, and capable of diving to depths greater than 350 meters. Powered by a pressurized water reactor generating about 150 megawatts, the boats can exceed speeds of 25 knots and remain at sea for up to 70 days, limited primarily by food supplies.
Each submarine carries a crew of 63 sailors and can embark around 15 commandos. Armament includes F21 heavyweight torpedoes, modernized Exocet SM39 anti-ship missiles, naval cruise missiles capable of striking land targets, and naval mines. Future upgrades are expected to include unmanned aerial and underwater vehicles.
French officials say the Suffren class introduces several innovations, including non-penetrating optronic masts that provide high-resolution imagery accessible throughout the combat information center. The submarines are equipped to deploy special forces using a removable dry deck shelter, enabling the launch of swimmer delivery vehicles and combat divers.
This development comes amid increasing attention on undersea warfare and the need for strategic deterrence in contested maritime areas.
