Will Japan Equip Its Future Submarines with Nuclear Propulsion Like Its Regional Neighbors?.

Japan’s Defense Minister Shinjiro Koizumi has called for an open national debate on whether to adopt nuclear propulsion for the next generation of submarines, reflecting a changing security environment. His comments highlight growing pressure from regional developments, including new U.S.-backed and Chinese nuclear-powered submarine programs.

According to Japan’s Ministry of Defense, on November 7, 2025, Defense Minister Shinjiro Koizumi confirmed that no decision has been taken on submarine propulsion for the next generation of boats, yet he urged an open debate as the regional balance tilts toward nuclear-powered fleets. His remarks followed a November 6 appearance on national television where he said, “There are new developments, and all the surrounding countries are set to possess nuclear submarines,” adding that nuclear submarines are “nothing particularly unusual.” He framed the choice starkly, “The environment surrounding Japan has become so severe that we need to discuss whether to continue using diesel power for submarines as before, or to switch to nuclear power.”
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Japan's Defense Minister Shinjiro Koizumi has opened debate on nuclear propulsion for future submarines as Tokyo faces a rapidly shifting regional balance, with China, North Korea, and now South Korea all moving toward nuclear-powered fleets (Picture source: Japanese MoD).

Koizumi’s comments came as he referenced reports from the October 29 leaders’ discussions at APEC in Gyeongju, where he noted U.S. approval for South Korea to build its own SSN. In East Asia, China is steadily adding attack boats and expanding SSBN operations, while North Korea has declared its intent to field nuclear-powered submarines under its 2021 five-year defense plan. The regional trend line is clear, and Tokyo’s calculus is changing accordingly. At his November 7 press conference, Koizumi reiterated the formal position, “At this point, nothing has been decided regarding the next generation of propulsion systems for submarines.”

The political context inside Japan is moving in tandem with the security picture. A blue ribbon panel convened by the Ministry of Defense in September recommended exploring “next generation propulsion systems” for VLS-equipped submarines able to fire long-range missiles and remain submerged for extended periods. At the press briefing that followed, officials cited solid-state batteries and fuel cells as primary candidates, while refusing to rule out nuclear propulsion. A coalition agreement on October 20 between the Liberal Democratic Party and Nippon Ishin committed the government to pursue VLS boats with next-generation power, a formulation that keeps nuclear propulsion in scope without declaring it the default.

Three credible nuclear pathways exist if Japan moves from study to requirement. A UK-designed SSN AUKUS solution would plug Japan into a maturing allied enterprise centered on a compact hull with the latest British reactor technology and U.S. combat systems, delivering common training, stewardship, and safety templates. It would maximize allied integration for patrols across the Philippine Sea and along the first island chain, although access, timing, and technology transfer constraints would be heavy. A French route built around a Barracuda derivative offers a quiet pump jet platform of roughly 5,300 tons submerged with K15 reactor lineage, high sprint and long on-station endurance, and a loadout of F21 torpedoes and tube-launched cruise missiles, with the added attraction of deep industrial workshare in Japanese yards. Paris has never exported naval nuclear propulsion, so the safeguards regime would be complex. A U.S. option centered on the Virginia class, especially Block V with the Virginia Payload Module, brings theater-wide strike, ISR, and close interoperability with U.S. networks, but would face the hardest export control and shipyard capacity barriers.

The gain for the Maritime Self-Defense Force is straightforward. Today’s Taigei class lithium-ion SSKs are among the quietest conventional boats afloat, with impressive underwater endurance for days at a time, rapid recharge, and excellent coastal denial performance. They still must snorkel to generate power and cannot sustain high submerged speeds for long transits or multi-axis tasking. A nuclear boat remains submerged for months, holds 25 knots plus for as long as the mission demands, and can shadow adversary surface groups, hunt submarines, and posture for land attack in a single patrol cycle. In practical terms, nuclear propulsion turns episodic presence into continuous pressure.

If nuclear is a twentieth-century technology, why is it being discussed under the banner of next-generation power? The frontier has shifted to microreactors and small modular reactors that promise intrinsic safety, flexible basing, and simplified logistics. Japanese industry is active in this space, including microreactor concepts under development that could underpin a domestic nuclear stewardship pipeline. On the British side, the Rolls-Royce SMR ecosystem is advancing, and the technologies it matures would overlap with any allied naval program. The direction of travel is toward safer, smaller, more efficient reactors, which reframes nuclear propulsion as a forward-leaning option rather than a legacy one.

The obstacles remain significant. The Atomic Energy Basic Act and the legacy of the Three Non-Nuclear Principles shape public attitudes that were further hardened by Fukushima, and any decision touching nuclear technology faces rigorous scrutiny by regulators and local communities hosting naval bases. Industrially, Mitsubishi Heavy Industries and Kawasaki Heavy Industries would require new nuclear infrastructure, a national training pipeline for nuclear-qualified officers and engineers, and a cradle-to-grave safety regime. Koizumi’s message, however, is that Japan must weigh these costs against a strategic environment that is moving fast. The debate is not whether nuclear propulsion is unusual; it is whether Japan can afford to remain the only major maritime power in its neighborhood without it.

SSN AUKUS offers the clearest allied integration, Barracuda the most compact and potentially sovereign industrial path, and Virginia the highest immediate interoperability with U.S. strike and ISR architecture. Any of the three would shift Japan from a coastal denial specialist to an undersea force with continuous theater-wide presence. That is the real change a nuclear submarine brings, not a single performance metric but a different way of fighting and deterring at sea.

Written by Evan Lerouvillois, Defense Analyst, Army Recognition Group.

Evan studied International Relations, and quickly specialized in defense and security. He is particularly interested in the influence of the defense sector on global geopolitics, and analyzes how technological innovations in defense, arms export contracts, and military strategies influence the international geopolitical scene.