Japan Is Considering Nuclear Submarines in a Major Shift in Defense Policy

Japan is considering studying the acquisition of nuclear-powered submarines as it prepares to revise its national security strategy, a move that could mark a significant shift in its maritime defense posture as Chinese military activity expands and regional powers strengthen their undersea capabilities.

The government is weighing whether to include language committing it to “investigating” and “researching” the feasibility of nuclear-powered submarines in three security documents due to be revised by the end of the year, Nikkei reported, citing sources familiar with the discussions.

The proposal falls short of a decision to acquire the vessels, but would keep the option open as Tokyo assesses the operational advantages, financial burden and legal implications of introducing nuclear propulsion into its submarine fleet.

“If we didn’t write anything [about nuclear submarines], it would mean we won’t go forward with it, but if we write about investigation or research, we can take more time to reach a conclusion while determining the feasibility,” a source close to the government said.

Prime Minister Sanae Takaichi told parliament on Wednesday that she would “deepen concrete and realistic considerations” regarding the defence of sea lanes, including the introduction of nuclear submarines.

Defence Minister Shinjiro Koizumi told reporters that he believes “the public understands the need for a discussion” on nuclear submarines.

The vessels “will enable us to engage in necessary activities while ensuring the safety of the crew at a higher level,” Koizumi said.

The debate comes as Japan seeks to strengthen its ability to monitor and deter military activity across the western Pacific, including around Taiwan and the East China Sea, while maintaining close defence cooperation with the United States.

A shift in Japan’s postwar defence posture

Japan’s consideration of nuclear-powered submarines would represent another step in its gradual expansion of defence capabilities since the end of World War Two, when its postwar constitutional framework and political commitments placed strong emphasis on pacifism.

The country has long relied on conventionally powered submarines operated by its Maritime Self-Defence Force. These vessels are designed for surveillance, intelligence gathering and maritime denial, but diesel-electric submarines generally face greater endurance constraints than nuclear-powered vessels because they must periodically surface or use snorkels to replenish batteries.

Nuclear propulsion allows submarines to remain submerged for much longer periods and sustain higher speeds over extended distances. Their endurance is constrained by provisions, crew requirements and maintenance rather than the need to replenish conventional propulsion fuel during a patrol.

Japan’s three core security documents — the National Security Strategy, National Defense Strategy and Defense Buildup Program — were last revised in 2022. The documents set out a major expansion of defence capabilities, including a five-year spending programme worth about 43 trillion yen.

The government said at the time that it intended to bring defence spending and related security expenditure to around 2% of gross domestic product by fiscal 2027. The programme also included plans to strengthen long-range strike capabilities, missile defences and the country’s posture in the southwest islands. 

The current Defense Buildup Program calls for the development and procurement of vertical launch systems that can be installed on submarines. Proponents of nuclear propulsion argue that the vessels’ speed and endurance would make them particularly suitable for operating such systems across distant maritime areas.

The government’s expert panel tasked with reviewing the three security documents has held four meetings so far. Some members support examining nuclear-powered submarines, while others remain cautious. Its recommendations are not expected to prescribe a definitive policy on acquiring the vessels.

“I believe the wording will be such that it won’t restrict the government’s decision,” an expert said.

China, Russia and the race beneath the Pacific

The proposal reflects growing concern in Tokyo about the regional military balance, particularly China’s expanding naval capabilities and Russia’s continued military presence in the northern Pacific.

Japan’s geography makes the security of surrounding seas central to its defence planning. Its trade and energy supplies depend heavily on maritime routes, while its southwestern islands lie close to Taiwan and key passages linking the East China Sea to the wider Pacific.

A nuclear-powered submarine fleet could give Japan greater flexibility to operate farther from home waters, maintain a sustained presence in contested maritime areas and work alongside US submarines during extended missions.

A December 2025 commentary by Brent Sadler, a senior research fellow at the Washington-based Heritage Foundation, estimated that a nuclear-powered submarine operating from Yokosuka could spend 79 days in the South China Sea. That would be 4.6 times as long as a diesel-powered conventional submarine, according to the analysis.

The commentary estimated that the endurance advantage would be 2.4 times in the Philippine Sea and 2.5 times in the Taiwan Strait.

Those figures are estimates based on the commentary’s assumptions, not guarantees of operational performance. Actual patrol duration depends on mission requirements, logistics, maintenance and the characteristics of individual submarines.

Japan’s debate is also unfolding alongside a wider regional shift towards nuclear propulsion.

Australia is pursuing nuclear-powered, conventionally armed submarines through the AUKUS partnership with the United States and Britain. Under the announced pathway, Australia plans to acquire US Virginia-class submarines from the early 2030s before moving towards the development and construction of SSN-AUKUS vessels. 

South Korea has also sought greater submarine capabilities amid concerns about North Korea’s nuclear and missile programmes. The Japanese report said Seoul had reached an agreement with the Trump administration concerning the acquisition of nuclear-powered submarines.

The expansion of these programmes could strengthen cooperation among US allies, but it also raises questions about the cost of maintaining advanced fleets, the availability of specialist personnel and the management of nuclear materials.

Nuclear-powered submarines are not necessarily nuclear-armed. Their propulsion systems use nuclear energy to generate power, while their weapons depend on the vessel’s configuration and the decisions of the government operating it.

The price of nuclear propulsion

Cost is likely to be one of the biggest obstacles facing Japan.

Building a nuclear-powered submarine in Japan is estimated to cost more than 1 trillion yen ($6.29 billion), according to the Nikkei report. That compares with approximately 70 billion yen for the Maritime Self-Defence Force’s most advanced conventional submarines.

In the United States, the construction cost of a nuclear-powered submarine is estimated at more than $4 billion per vessel, the report said.

The purchase price is only part of the financial commitment. Nuclear-powered fleets require specialised shipyards, nuclear engineering expertise, secure fuel-handling arrangements, dedicated maintenance facilities and extensive training for crews and shore-based personnel.

Japan would also need to establish the regulatory and institutional framework necessary to operate naval nuclear propulsion safely over decades.

The government faces competing demands on public finances as it seeks to expand military capabilities while funding other national priorities. The existing five-year defence programme already represents a substantial commitment, and a nuclear-submarine programme would require long-term funding beyond the initial construction phase.

The workforce challenge is equally significant. The Maritime Self-Defence Force faces personnel shortages, while nuclear-powered submarines require highly trained specialists capable of operating and maintaining complex propulsion systems.

Even if Tokyo approves a feasibility study, developing the industrial base, recruiting and training personnel and establishing the necessary regulatory oversight could take years.

Legal and political obstacles

Japan’s postwar nuclear policy presents another challenge.

The country’s longstanding Three Non-Nuclear Principles state that Japan will not possess, produce or permit the introduction of nuclear weapons. Nuclear-powered submarines are not inherently prohibited by those principles because nuclear propulsion does not itself constitute possession of nuclear weapons.

However, the government would need to address how naval nuclear propulsion fits within Japan’s domestic legal framework, which has historically limited nuclear energy to peaceful purposes, as well as its international non-proliferation obligations.

The distinction between nuclear propulsion and nuclear armament would be central to any political debate. The use of nuclear material in naval reactors would also raise questions about oversight, safety, fuel management and transparency.

A senior official in the ruling Liberal Democratic Party’s Research Commission on Security expressed caution about acquiring nuclear submarines, citing the expense and the difficulty of gaining international acceptance.

“There is little inclination for the research commission to accept the argument for the acquisition” of nuclear submarines, the official said.

The Japan Innovation Party, the LDP’s coalition partner, proposed in June that the country should acquire nuclear submarines “promptly.” A separate proposal issued around the same time by the LDP did not mention the vessels, underscoring the lack of consensus within the governing camp.

The disagreement leaves the government with an intermediate option: authorise further research without committing to procurement.

That approach would allow officials to examine technical feasibility, costs, legal requirements and potential cooperation with the United States before deciding whether to proceed.

A decision with regional consequences

For Washington, Japanese nuclear-powered submarines could provide additional undersea surveillance and operational capacity in the Indo-Pacific, potentially strengthening coordination among allies facing pressure from China.

For Beijing and Moscow, the prospect of another nuclear-powered submarine operator in the region could be viewed as part of a broader expansion of allied military capabilities.

The consequences would depend on the final design, weapons configuration, deployment patterns and the extent of operational integration with US forces. Nuclear propulsion alone does not determine whether a submarine is offensive or defensive, but it can expand the distance, speed and duration over which it operates.

Tokyo must also weigh the diplomatic costs of a programme that could be interpreted by neighbours through the lens of Japan’s wartime Hiroshima and Nagasaki history and its postwar commitment to restraint.

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