France Activates Nuclear Reactor on Barracuda-Class Attack Submarine De Grasse.

France has initiated the first controlled nuclear chain reaction in the propulsion system of De Grasse, its fourth Barracuda-class nuclear-powered attack submarine, according to the Ministry of the Armed Forces. The milestone, known as divergence, moves the submarine into final testing and highlights Europe’s continued investment in undersea deterrence and power projection.

According to the French Ministry of the Armed Forces, on 12 December 2025, France initiated the first controlled nuclear chain reaction in the propulsion plant of De Grasse (S638), the fourth Barracuda-type nuclear-powered attack submarine, a milestone known as “divergence.” The announcement places the boat on a clear track toward initial alpha sea trials, moving the program from industrial integration into the tightly regulated phase where propulsion performance, safety systems, and crew procedures are validated under continuous nuclear watch-keeping.
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Barracuda-class attack submarine Duguay-Trouin during reactor-related industrial work in 2022; the image illustrates a similar nuclear propulsion milestone to the reactor start recently achieved on De Grasse (Picture source: French Ministry of the Armed Forces).

In practice, divergence is far more consequential than a ceremonial start. It is the moment the reactor core becomes critical under strict controls, beginning permanent monitoring by trained nuclear propulsion officers intended to remain uninterrupted through the class’s operational life into the 2060s. De Grasse’s reactor was designed under the responsibility of France’s Atomic Energy Commission, with TechnicAtome supervising reactor design and lifecycle support, while Naval Group manufactures key components such as the pressure vessel and integrates the plant into the submarine at Cherbourg.

The Barracuda plant is a compact pressurized-water reactor, derived from French naval reactor lines used on Triomphant-class SSBNs and the aircraft carrier Charles de Gaulle, paired with a hybrid steam and electric propulsion architecture. Heat from the primary circuit is transferred through the nuclear steam supply system to generate secondary steam that is free of radiation, driving turbines and turbo-generators that feed the boat’s electrical network, including very quiet electric motors optimized for low and tactical speeds. Open sources commonly associate the K15 family with roughly 150 MW thermal output, and the nuclear boiler itself weighs about 400 tons and occupies around a 10-meter section of the hull, with refueling intervals on the order of a decade, a major improvement over early Rubis-class patterns.

The operational advantage is the kind submariners talk about in negatives: fewer moments when you must expose yourself. Nuclear propulsion removes the diesel-electric need to snorkel for battery charging, sharply reducing time spent at periscope depth with masts raised and signatures elevated. It also lets a commander sustain higher underwater transit speeds to reach a patrol box quickly, then shift into slow, ultra-discreet tracking without budgeting every maneuver against battery state. Endurance becomes a matter of food and maintenance planning, not oxygen and energy constraints, enabling longer submerged presence, wider route choice, and greater control over the timing of detection.

De Grasse inherits a broader combat toolkit than the older Rubis boats. Its armament mix includes MdCN naval cruise missiles launched from torpedo tubes for deep land attack while remaining submerged, F21 heavyweight wire-guided torpedoes, modernized Exocet SM39 anti-ship missiles, and mines, with growth room for future unmanned air and underwater vehicles. The class also integrates non-penetrating Safran optronic masts that distribute high-definition imagery across the combat information center, strengthening covert intelligence collection without the vulnerabilities of traditional periscopes.

The Barracuda design was shaped around missions that matter to France’s nuclear deterrent and expeditionary posture: protecting SSBNs, escorting the carrier and amphibious groups, and intelligence gathering. Special operations support represents a defining leap, with automation reducing crew burden and options such as a dry deck shelter to deploy combat swimmers and swimmer delivery vehicles, while published figures indicate a core crew in the low sixties plus embarked commandos. That combination of endurance, sensors, and SOF delivery pushes the class beyond classic sea denial into a flexible undersea node for strike and reconnaissance.

In the wider SSN landscape, France’s approach contrasts with allies and competitors. The U.S. Navy’s Virginia class is significantly larger and built around payload growth, with Block V introducing the Virginia Payload Module, adding four large tubes that can carry dozens of additional Tomahawk cruise missiles while expanding special operations payload options. The Royal Navy’s Astute class emphasizes long-range precision strike and high-end anti-submarine warfare, pairing Tomahawk IV missiles with Spearfish heavyweight torpedoes and one of the most sensitive sonar suites in service. Russia’s Yasen class is configured as a cruise-missile heavy undersea threat, with vertical launch silos and multiple torpedo tubes reflecting a doctrine that prizes high-volume standoff attack. Against that field, Suffren-class boats are tuned for quiet mobility, selective surprise strike, and the protective duties that keep France’s SSBN force credible.

With six Barracuda submarines ordered to replace the Rubis class and deliveries planned by 2030 under France’s current military planning framework, each reactor start is a strategic signal as much as a technical step. De Grasse’s divergence is the moment the program’s promise begins to translate into a warship that can stay down longer, listen farther, strike deeper, and operate with the kind of discretion that only nuclear propulsion and modern quieting can deliver.