Tokyo’s Ministry of Defense received a report by the Advisory Council for the Fundamental Reinforcement of Defense Capability, chaired by Sadayuki Sakakibara, urging the Government to pursue propulsion technologies, including nuclear, for the next generation of Japanese submarines.

The study, presented on September 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.

In detail, the report formally recommends that the Japanese Ministry of Defense conduct research and development on “next-generation energy sources,” a term widely understood within Japan’s defense community to include the possibility of small nuclear reactors. The report states that 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 (AIP) systems.

The study further recommends conducting detailed technical research, feasibility assessments, and long-term development studies to determine which propulsion system could best meet Japan’s future operational needs, noting that longer underwater endurance is essential for deterrence and persistent monitoring of nearby seas. The experts, who produced the report, justified this approach by observing that the maritime environment surrounding Japan is increasingly contested and that the technological base of the Japan Maritime Self-Defense Force (JMSDF) must adapt to enable persistent operations across the Western Pacific and the East China Sea. Their statement aligns with 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.

However, any discussion concerning the military use of nuclear weapons in Japan immediately encounters 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. However, while nuclear propulsion would not, in legal terms, violate Japan’s Three Non-Nuclear Principles - since it does not involve the possession or deployment of nuclear weapons - it would nonetheless represent a profound political and symbolic departure from Japan’s traditional non-nuclear stance.

The last time Tokyo examined the nuclear-propulsion option 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.”

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