Japan’s ruling coalition has opened discussions on introducing nuclear-powered attack submarines, placing a potential US$25 billion naval procurement programme at the centre of a major security policy review scheduled for December 2026. The debate reflects growing concern in Tokyo over China’s expanding naval operations across the Western Pacific and the limitations of Japan’s conventional submarine fleet in maintaining prolonged underwater surveillance far from its home waters.
The discussions, which began in November, could lead to naval nuclear propulsion being explicitly included in Japan’s revised National Security Strategy, National Defense Strategy and Defense Buildup Program. However, placing nuclear-powered submarines on the government’s research agenda would not automatically mean that Japan has decided to acquire them. A formal procurement decision would require further assessments of operational requirements, financial commitments, legal restrictions, technological capabilities and cooperation with the United States.
The potential programme comes as China continues expanding its aircraft carrier operations, nuclear-powered submarine capabilities and long-range naval deployments beyond the First Island Chain. Japanese defence planners increasingly face the question of whether the Maritime Self-Defense Force (MSDF) can maintain sufficient underwater surveillance across the Western Pacific as Chinese naval forces operate farther from their immediate maritime approaches.
Prime Minister Sanae Takaichi signalled a shift in the government’s approach on October 14, telling parliament that her administration would “seriously consider” nuclear-powered submarines “in a concrete and realistic manner”.
Her statement placed particular emphasis on Japan’s intelligence-gathering requirements in the Pacific, where increasing military activity by neighbouring countries is creating new challenges for maritime surveillance.
Defence Minister Shinjiro Koizumi has also identified submerged endurance, operational mobility and crew safety as important considerations in the debate. However, he has acknowledged that nuclear-powered submarines would require larger hulls, specialised personnel and substantially higher operating costs, raising questions about whether they would be appropriate for all of Japan’s existing submarine missions.
The November coalition negotiations are therefore testing how far the operational arguments can translate into an actual government commitment. The Japan Innovation Party (JIP) is advocating the swift introduction of nuclear-powered submarines, while the Liberal Democratic Party (LDP) is approaching the proposal more cautiously, considering its financial, political and legal implications.
Japan currently operates a sophisticated conventional submarine fleet, including the Taigei-class boats equipped with lithium-ion batteries. These submarines offer improved submerged endurance and extremely quiet operating characteristics, making them particularly suitable for surveillance, intelligence gathering and sea-denial missions around Japan’s maritime approaches.
However, conventional diesel-electric submarines face operational limitations when deployed over long distances.
Although lithium-ion batteries allow submarines to remain submerged longer than earlier conventional designs, they must periodically snorkel to recharge their batteries. Snorkelling can expose a submarine to detection, particularly during extended surveillance missions in waters where adversary aircraft, surface vessels and underwater sensors are operating.
Nuclear propulsion would fundamentally change that operational equation.
A nuclear-powered submarine can remain submerged for months, with food supplies, crew endurance and maintenance requirements generally becoming more important constraints than propulsion energy. It can also sustain higher underwater speeds over extended distances, allowing it to reach distant operational areas more quickly and spend a greater proportion of its deployment conducting surveillance.
For Japan, this capability could become increasingly valuable as Chinese naval operations extend across the Western Pacific.
The difference is particularly significant when comparing missions around the Ryukyu Islands and the East China Sea with operations farther into the Pacific. In confined waters close to Japan, conventional submarines can exploit their quiet battery-powered operation and local geographic advantages. For distant patrols, however, transit time and endurance become more important.
Nuclear propulsion could therefore allow Japan to establish a more persistent underwater presence in areas where Chinese naval deployments are becoming increasingly frequent.
Nevertheless, nuclear propulsion does not automatically guarantee superior acoustic performance. A conventional submarine operating on batteries can be exceptionally quiet, while the acoustic signature of a nuclear-powered submarine depends on reactor machinery, propulsion systems, engineering design and operating conditions.
Larger nuclear-powered submarines could also face disadvantages in confined waters, where manoeuvrability and the ability to operate close to coastlines and maritime chokepoints are important.
These differences have strengthened arguments for considering a mixed fleet rather than replacing Japan’s existing conventional submarines.
Former defence minister Gen Nakatani has argued that Japan will eventually need nuclear propulsion because conventional submarines face fundamental disadvantages in speed and endurance when operating against nuclear-powered fleets.
The operational question, however, is whether Japan needs a separate submarine class specifically designed for extended Western Pacific deployments.
Takaichi’s government is reviewing three central security documents adopted in December 2022 under former prime minister Fumio Kishida: the National Security Strategy, National Defense Strategy and Defense Buildup Program.
The review reflects changing regional and international security conditions, including China’s military expansion, Russia’s war in Ukraine, North Korea’s weapons programmes and uncertainty over the future scale of American military engagement in the Indo-Pacific.
An expert panel began examining the country’s defence requirements in spring 2026, with its report expected in early December. Government and ruling-party discussions are expected to follow.
The September 2025 Defence Ministry advisory panel had already recommended additional research into submarines equipped with vertical launch systems and next-generation propulsion technologies.
The October 2025 coalition agreement between the LDP and JIP also established a political basis for further submarine research, although the two parties have different levels of urgency regarding nuclear propulsion.
In June 2026, the JIP proposed a nuclear-powered submarine equipped with a vertical launch system, arguing that nuclear propulsion would be essential for extended operations across the Pacific.
The two technologies, however, address different operational requirements.
A vertical launch system increases the submarine’s ability to carry and launch missiles, potentially supporting Japan’s expanding counterstrike capabilities. Nuclear propulsion, meanwhile, primarily addresses underwater mobility and endurance.
The government must therefore determine whether Japan requires a submarine combining long-range missile capabilities with extended underwater surveillance or whether these missions can be performed more effectively through separate platforms and existing defence networks.
The wording adopted in December will be particularly important. An explicit mandate to study naval nuclear propulsion would allow defence planners to examine potential missions, infrastructure requirements, technological partnerships and long-term procurement costs.
The financial implications represent one of the biggest obstacles to introducing nuclear-powered submarines.
An illustrative estimate of US$8.4 billion per submarine would put a three-boat fleet at approximately US$25.2 billion. Other estimates place the cost of an individual nuclear-powered submarine between US$6 billion and US$6.5 billion.
By comparison, a conventional Taigei-class submarine is estimated to cost approximately US$450 million to US$800 million, depending on the accounting basis and associated equipment.
The comparison demonstrates the substantial financial difference between the two propulsion systems.
Moreover, the construction price would represent only part of the overall programme. Japan would also need to develop specialised shipyard facilities, nuclear-certified maintenance infrastructure, reactor-training centres, fuel-management arrangements and a dedicated regulatory system.
A three-submarine fleet would also require sufficient vessels to maintain operational availability while other boats undergo maintenance, crew training and periodic overhauls.
A submarine undergoing extensive maintenance cannot contribute to operational surveillance. Consequently, the actual number of boats required would depend on deployment schedules, maintenance cycles and the level of continuous presence Japan expects to maintain in the Pacific.
Personnel requirements would add another complication.
Illustrative estimates suggest that a nuclear-powered submarine could require approximately 100 crew members, compared with around 70 aboard a Taigei-class submarine. The additional personnel would include specialists responsible for reactor operation, engineering and nuclear safety.
The burden would extend beyond the vessels themselves. Japan would need reactor-trained technicians, maintenance specialists, regulatory personnel and shore-based operational support teams.
The Self-Defense Forces already face recruitment and retention challenges, making the expansion of a highly specialised submarine workforce particularly difficult.
Japan’s legal framework presents another significant challenge.
The Atomic Energy Basic Law restricts the use of nuclear energy to peaceful purposes. Previous government interpretations have created obstacles to the introduction of naval nuclear reactors by the Self-Defense Forces.
A government decision to pursue nuclear-powered submarines would therefore require a clear legal interpretation establishing whether naval propulsion could be accommodated under the existing framework or whether new legislation would be necessary.
A government position dating back to 1965 linked permissible nuclear propulsion to established civilian nuclear applications. Tokyo would need to determine whether that interpretation could provide a sufficient legal foundation for a military submarine programme.
International nuclear safeguards would also require careful consideration.
The International Atomic Energy Agency’s safeguards framework raises questions about accounting for nuclear material used in naval propulsion while protecting sensitive information about military reactor technology.
Japan’s nuclear cooperation agreement with the United States would also become relevant if Tokyo sought American reactor technology, nuclear material or technical assistance.
The government would need to establish an appropriate system for nuclear fuel management, operational safety, maintenance and regulatory supervision.
Domestic political acceptance could be equally difficult.
Japan’s experience as the only country to have suffered atomic bomb attacks during wartime continues to shape public attitudes towards nuclear technology. The distinction between nuclear propulsion and nuclear weapons would therefore be central to any political debate.
The 1974 incident involving Japan’s nuclear-powered ship Mutsu, which experienced a radiation leak during testing, also remains relevant to public concerns about nuclear safety.
Any submarine programme would require a suitable home port with specialised maintenance facilities and emergency-response arrangements. Securing local government and community acceptance could become a major obstacle, particularly if residents raise concerns about reactor safety and nuclear waste management.
Japan’s debate is taking place as other US allies examine nuclear-powered submarine capabilities.
Australia’s AUKUS programme provides the most prominent comparison.
Under the agreement with the United States and Britain, Australia plans to host rotational deployments of American nuclear-powered submarines from 2027, acquire Virginia-class submarines during the 2030s and eventually operate jointly developed SSN-AUKUS submarines during the 2040s.
The programme demonstrates the extensive preparation required before a country can independently operate nuclear-powered submarines.
Training, industrial development, infrastructure construction and technology-sharing arrangements must begin years before the first domestically operated submarine enters service.
South Korea is pursuing a different approach.
Its reported Jangbogo-N programme envisages domestically constructed nuclear-powered submarines using low-enriched uranium, with an initial launch targeted for the mid-2030s and operational service expected later in the decade.
However, Seoul’s primary strategic concern is North Korea’s growing missile and submarine capabilities, whereas Japan’s debate is increasingly focused on long-distance surveillance and China’s expanding naval operations.
Both programmes illustrate how nuclear propulsion can serve different strategic requirements depending on geography, operational priorities and available industrial capacity.
For Japan, cooperation with Washington would be particularly important. However, American shipbuilding capacity is already under pressure from domestic submarine requirements and existing AUKUS commitments.
Tokyo cannot assume that the United States would automatically provide the technology, industrial assistance or training necessary for a Japanese programme.
A predominantly domestic development programme would offer greater technological independence but could require substantially more time and investment.
Some assessments suggest that developing and integrating a naval reactor could take 15–20 years, even with foreign assistance. This would place the possible introduction of Japan’s first nuclear-powered submarine well beyond the immediate security review.
China has criticised Japan’s discussion of nuclear-powered submarines, warning that such a move could undermine regional stability and contribute to military tensions in East Asia.
Beijing has portrayed Tokyo’s defence expansion as part of a broader shift towards remilitarisation, particularly amid continuing disagreements over Taiwan and Japan’s growing security cooperation with the United States.
Japan, meanwhile, views China’s expanding naval operations as a growing intelligence and surveillance challenge.
The disagreement reflects competing interpretations of military modernisation in the Indo-Pacific. Beijing describes its naval expansion as defensive, while Tokyo increasingly considers the geographical reach and persistence of Chinese naval forces important factors in its own defence planning.
Russia’s existing nuclear-powered submarine fleet and North Korea’s announced ambitions to develop nuclear-powered submarines add further complexity to Japan’s assessment of the regional underwater balance.
However, the maturity and operational capabilities of these programmes differ considerably, and their strategic implications should not be treated as equivalent.
Ultimately, Japan’s December 2026 security review will determine whether nuclear-powered submarines become a formal subject of long-term defence planning or remain an option for further technological research.
The central issue is not simply whether Japan can build a nuclear-powered submarine. It is whether the operational advantages of sustained underwater surveillance across the Western Pacific justify the enormous financial, technological, legal and personnel commitments required to develop and maintain an entirely new submarine fleet.
For Tokyo, the decision could shape the future direction of its maritime defence strategy for decades, particularly as China’s expanding naval presence continues to challenge Japan’s ability to monitor the increasingly contested waters of the Indo-Pacific.
