Japan may develop nuclear-powered submarines following new expert study.

A new government-commissioned expert study has suggested that Japan could develop nuclear-powered submarines as part of its future defense modernization effort.

Japan may explore the development of nuclear-powered submarines after an expert panel formally recommended that the Ministry of Defense research “next-generation propulsion” systems for future Japan Maritime Self-Defense Force vessels. The study, presented on Sept. 19, 2025, marks the first time an official advisory body has implicitly opened the door to nuclear propulsion research, signaling a potential shift in Tokyo’s long-standing self-defense and nuclear policy.
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For now, Japan continues to expand its fleet of conventionally powered submarines, such as the 3,000-ton Taigei-class, which features world-first lithium-ion battery technology allowing longer, quieter submerged operations. (Picture source: Wikimedia/Hunini)

Japan’s long-running debate on nuclear-powered submarines has taken a new step after an expert panel mandated by the government formally recommended that the Japanese Ministry of Defense conduct research and development on “next-generation energy sources,” a term widely understood in Japan’s defense community to include the possibility of small nuclear reactors. The report, submitted on September 19, 2025, by the Advisory Council for the Fundamental Reinforcement of Defense Capability, chaired by Sadayuki Sakakibara, urged the government to pursue propulsion technologies not bound by precedent for the next generation of Japanese submarines. It says these submarines, equipped with vertical launch systems (VLS) for long-range strike missions, should be able to travel farther, stay underwater longer, and operate more quietly, clearly implying propulsion requirements beyond the capacity of conventional diesel-electric or air-independent propulsion systems.

The council recommended that the Defense Ministry carry out detailed technical research, feasibility assessments, and long-term development studies to determine which propulsion system could best meet Japan’s future operational needs, saying that longer underwater endurance is essential for deterrence and persistent monitoring of nearby seas. The experts justified this approach by noting that the maritime environment surrounding Japan is increasingly contested and that the technological base of the JMSDF must adapt to enable persistent operations across the Western Pacific and the East China Sea. Their statement follows the government’s previously announced intention to acquire longer-range missiles capable of striking from sea-based platforms, extending Japan’s deterrence reach beyond its coastline. The document itself represents a formal request for structured research rather than an acquisition decision, but its language marks the first time an official advisory body has implicitly opened the path toward exploring nuclear-powered submarine technology within Japan’s defense planning process.

However, any discussion concerning the military use of nuclear weapons in Japan immediately meets legal and political obstacles. The Atomic Energy Basic Act stipulates that nuclear energy in Japan shall be used solely for peaceful purposes, and the country’s long-held “three non-nuclear principles” rule out producing, possessing, or hosting nuclear weapons. These postwar constraints have defined both policy and public opinion for decades. The last time Tokyo studied nuclear-powered submarines was in 2001, when a classified government study concluded that such a program would face legal difficulties and excessive cost, leading to abandonment of the idea. The government reaffirmed that position in September 2024, when Chief Cabinet Secretary Yoshimasa Hayashi said Japan had “no plans to possess nuclear submarines” and that doing so would be “difficult under current law.” Still, the expert panel’s recent language suggests Tokyo wants to prepare the groundwork for possible long-term changes if circumstances require.

Advocates inside Japan’s naval community argue that nuclear propulsion would solve a growing operational problem: endurance. They note that Japan’s submarines, though among the world’s quietest, must eventually surface or snorkel to recharge batteries, limiting how long they can patrol far from home waters. Former Maritime Self-Defense Force submarine fleet commander Kazuki Yano said nuclear-powered boats offer unmatched underwater mobility, stealth, and range, and that Japan already has the engineering and industrial capacity to build them if a political decision is made. He also disputed the common belief that nuclear submarines are noisier, saying that modern designs can be as quiet as or quieter than conventional ones. Supporters see nuclear propulsion as a way to maintain deterrence as the regional environment surrounding Japan is deteriorating; China continues to expand its naval operations beyond the first island chain, Russia increases its submarine transits in the Sea of Japan and the Pacific, and North Korea accelerates its missile and nuclear programs while exploring sea-based launch capabilities.

On the other hand, opponents of nuclear propulsion stress that the issue is not just technical. They argue that developing nuclear-powered submarines would contradict the spirit of Japan’s constitutional limits on self-defense, raise environmental and safety concerns, and risk diplomatic friction with neighbors. They also point to the cost: Japan’s current five-year defense buildup plan, covering fiscal years 2023–2027, already allocates approximately 43 trillion yen to achieve a defense expenditure equivalent to 2 percent of GDP. Submarine unit costs have increased from about 70 billion yen for earlier hulls to approximately 95 billion yen for the latest ones, even without nuclear propulsion. For comparison, the U.S. Navy’s Columbia-class nuclear ballistic missile submarines cost about $8.8 billion per vessel. Critics warn that a nuclear submarine program would require a domestic nuclear infrastructure, specialized training, and long-term maintenance facilities that would multiply costs. They also question whether such a move would strengthen deterrence or instead provoke regional suspicion, potentially accelerating an undersea arms race.

For now, Japan is focusing on improving its conventional 22-submarine fleet while keeping the nuclear option open for study. The fleet includes the Soryu class and its successor, the Taigei class, both of which employ the latest lithium-ion batteries to significantly increase underwater endurance and reduce noise, eliminating some of the traditional advantages of nuclear propulsion. Tokyo is also developing VLS-equipped variants to accommodate cruise missiles with ranges of approximately 1,500 kilometers, as part of a broader strategy to strengthen deterrence and flexibility. In parallel, cooperation within the AUKUS pact has expanded, but remains limited to Pillar II, which focuses on advanced technologies such as autonomous underwater vehicles, quantum navigation, and artificial intelligence, while Japan remains excluded from Pillar I, which deals with nuclear-powered submarines. This reflects the extreme sensitivity surrounding naval reactor technology, which the United States has shared only once since 1958, with the United Kingdom.

Japan’s renewed interest for nuclear-powered submarines comes as other countries revisit their own nuclear choices. South Korea has witnessed a growing domestic debate about developing nuclear-powered submarines and even reconsidering nuclear weapons, driven by North Korea’s advancing missile capabilities, questions about the long-term credibility of U.S. extended deterrence, and concerns about current restrictions under its 123 Agreement with the United States, which limits the enrichment of uranium for military use. Like Japan, while advocates argue such submarines would enhance survivability and patrol endurance, opponents warn they could violate non-proliferation agreements and destabilize regional diplomacy. In Brazil, the PROSUB program is pushing ahead with the Álvaro Alberto-class submarine, which will use a domestically designed nuclear reactor under IAEA supervision. Both Seoul and Brasília view nuclear weapons as a step toward greater strategic autonomy, showing how interest in such weapons now extends beyond the traditional nuclear powers.

Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.