Japan is edging closer to a once-unthinkable choice: whether nuclear propulsion should underpin its next generation of undersea deterrence.
This month, the Japan Times reported that Japanese Defense Minister Shinjiro Koizumi confirmed that Japan is keeping the option of acquiring nuclear-powered submarines on the table, following encouraging signals from a US Navy official.
Speaking at a regular press conference, Koizumi stated that Japan will not rule out the capability as it prepares revisions to three key national security documents by year-end.
The remarks came after an unnamed US Navy official, later corroborated by the US Pacific Fleet, suggested discussions should proceed given Japan’s advanced nuclear expertise, noting that asset procurement must align with national operational needs.
The submarine debate also follows Koizumi’s July 2026 call for Japan to discuss nuclear deterrence without taboos, saying the country could no longer avoid confronting nuclear weapons policy amid its worsening security environment.
While Japan currently relies on an all-diesel fleet of 23 conventional attack submarines (SSK), security hawks and coalition leaders from the Liberal Democratic Party and Japan Innovation Party have pushed to modernize naval capabilities amid China’s rapid naval expansion in the Pacific.
Koizumi acknowledged the trade-off: nuclear submarines offer far greater speed and endurance but at higher financial, acoustic and technical cost. His remarks sharpen a broader question: whether nuclear-powered submarines would strengthen Japan’s undersea deterrent enough to justify their costs and operational burdens.
His statement follows Japan’s announcement that it plans to include submarine-launched hypersonic weapons in its fiscal year 2027 budget, aiming to strengthen its long-range counterstrike capabilities against enemy bases.
That missile program raises the stakes of the propulsion debate, because longer-range weapons become more strategically useful when their launch platforms can patrol farther, faster and for longer periods.
Amid mounting regional threats from China across the First Island Chain, Japan plans to field vertical launch systems capable of firing solid-fuel Hyper Velocity Gliding Projectiles (HVGP) and scramjet-powered cruise missiles from submarines and unmanned underwater vehicles.
While intended to provide a survivable conventional deterrent that bypasses vulnerable air bases and surface ships, the same infrastructure would narrow the technical gap between conventional undersea strike and a future sea-based nuclear deterrent.
In a November 2025 report for the S. Rajaratnam School of International Studies (RSIS), John Bradford and other writers note that advocates argue nuclear propulsion provides unmatched underwater endurance, higher speeds, and the power to defend vital Indo-Pacific sea lanes independently against expanding Chinese and North Korean threats.
According to US Office of Naval Intelligence Director Rear Admiral Mike Brookes’ March 2026 testimony before the US Congress, China’s People’s Liberation Army Navy (PLAN) is making a major strategic shift from diesel-electric to a fully nuclear submarine fleet.
Brookes points out that this push is driven by shipyard expansions that more than doubled production capacity, with China now fielding advanced platforms like the Type 093B nuclear cruise missile submarine (SSGN) and the compact Type 41 nuclear attack submarine (SSN) for regional patrols.
Looking ahead, Brookes projects that China’s fleet will reach roughly 80 vessels by 2035—about half nuclear-powered—culminating in next-generation Type 095 SSNs and Type 096 ballistic missile submarines (SSBNs) designed to contest blue-water approaches beyond the First Island Chain.
For Japan, that trajectory strengthens the case for submarines able to shadow Chinese forces across much greater distances than its current coastal defense doctrine demands.
North Korea is pursuing its own sea-based second-strike capability, combining modified conventional submarines with ambitions for true SSBNs. Suspected Russian technical assistance could accelerate that effort, although reactor, acoustic and anti-submarine warfare constraints are likely to keep North Korea’s fleet largely confined to coastal bastions for years.
Even then, a deeper question remains about strategic intent. Nuclear-powered submarines derive much of their strategic value from remaining submerged and difficult to locate for extended periods far from home waters—advantages most consequential when paired with a survivable nuclear deterrent.
If Japan remains committed to a purely conventional arsenal, absorbing the extraordinary cost and complexity of nuclear propulsion becomes harder to justify.
Still, Japan has the scientific and industrial base to shorten the path to a nuclear deterrent if policy shifts. Its nuclear fuel-cycle expertise, combined with advanced high-energy-density and laser-fusion research at Osaka University’s Institute of Laser Engineering, underscores its significant latent technical capacity.
Technical capacity, however, does not settle whether nuclear propulsion is the best fit for Japan’s likely operating environment. Bradford and his co-authors note that Japan’s lithium-ion SSKs are quieter and better suited to shallow coastal waters.
Powered by diesel-electric machinery and lithium-ion batteries, Japan’s Taigei-class SSKs can operate extremely quietly and are optimized for prolonged submerged operations.
Conversely, nuclear submarines are constrained in confined coastal environments: their massive hulls increase sonar detection risks, coolant intakes risk fouling near the seabed, and active reactor machinery generates persistent acoustic noise, making smaller conventional submarines particularly well suited to coastal defense, reconnaissance and chokepoint interdiction.
Cost also differs when comparing nuclear to conventional submarines. At roughly US$700 million per boat versus an estimated US$3–4 billion for an SSN, the Taigei class also lets Japan preserve submarine numbers at a much lower cost.
Despite the debate over nuclear submarines, Japan faces significant challenges if it considers acquiring such assets. Personnel shortages are a particularly pressing issue.
The Asahi Shimbun reported in April 2026 that Japan Self-Defense Force (JSDF) personnel dropped to roughly 220,000 members in fiscal 2024—hitting 89.1% of capacity and falling below 90% for the first time since 1999—with internal ministry projections warning active ranks could plunge to 130,000 by 2045.
That personnel shortage could affect Japan’s plans to operate nuclear submarines, as a Virginia-class SSN requires a crew of 132 specially trained personnel, compared with the Taigei-class SSK, which needs only 70.
Automation could trim onboard crews, but nuclear propulsion would still create new demands for reactor specialists, maintenance personnel and shore-side support—precisely the personnel categories Japan may struggle to expand.
Japan would also face a legal and political hurdle. Japan’s Atomic Energy Basic Act limits the use of nuclear energy to peaceful purposes, meaning any move toward naval nuclear propulsion would require Japan to clarify or potentially revise how that principle applies to military propulsion.
Those constraints are unlikely to end the debate, however, because Japan’s deteriorating strategic environment is pushing policymakers toward options once considered politically untouchable. Richard Weitz describes that environment in a Hudson Institute article this month as a “three-front dilemma.”
Weitz frames that dilemma around simultaneous pressure from China, North Korea and Russia, compounded by doubts over the long-term reliability of US defense commitments.
He says that those pressures are compelling Japanese policymakers to weigh unprecedented strategic options, including autonomous counterstrike capabilities, targeted diplomatic accommodation and reconsideration of longstanding taboos surrounding nuclear deterrence.
Thus, the real test will be whether Japan can turn its expanding undersea ambitions into a sustainable force before regional threats outpace the institutions, personnel capacity and political consensus needed to sustain it.
