Japan has said it is keeping the door open to acquiring nuclear-powered submarines, a potentially major shift for a country whose submarine force has historically relied exclusively on conventional propulsion. Defense Minister Shinjiro Koizumi confirmed recently that Tokyo is not ruling out nuclear-powered submarines as it considers the future requirements of the Japan Maritime Self-Defense Force (JMSDF).

Should Japan ultimately pursue the capability, it would join an exclusive group of nations operating nuclear-powered submarines. China, France, India, Russia, the United Kingdom, and the United States are currently the only countries with nuclear-powered submarines in active service. Brazil is already working to join the club, while Australian and South Korean nuclear-powered submarine plans are somewhat longer-term.

Japan’s submarine fleet is already regarded as among the most capable conventionally powered forces, and is very much at the forefront of advanced naval propulsion technologies. Even with that considerable expertise, however, developing and sustaining a nuclear propulsion capability would represent a huge challenge, requiring expertise and infrastructure far beyond those needed to build Japan’s existing conventionally powered submarines.

Koizumi’s comments do not represent a decision to acquire nuclear-powered submarines, nor do they indicate that Japan has begun a procurement program. They do, however, mark a significant evolution in the country’s public debate over submarine propulsion and point to a growing willingness within the Japanese defense establishment to consider capabilities that were previously largely outside the mainstream of defense planning.

Currently, Japan operates one of the world’s most capable conventional submarine fleets, centered on increasingly sophisticated diesel-electric boats equipped with air-independent propulsion and advanced lithium-ion batteries. These submarines are well suited to operations around Japan and the country’s surrounding waters.

The JMSDF currently operates 22 conventionally powered submarines, maintaining the force structure established in the 2010s while progressively replacing older boats with the more capable Taigei class. Five Taigei class submarines are now in JMSDF service, with additional boats in the pipeline.

Nuclear propulsion, however, would offer substantially greater endurance and sustained underwater speed, allowing Japanese submarines to conduct longer-range operations across the broader Western Pacific with minimal support. This is a requirement that is becoming increasingly relevant as Tokyo’s strategic focus expands beyond its immediate maritime approaches.

China’s rapidly expanding naval force, including its growing submarine fleet and increasingly distant maritime operations, is forcing Tokyo to consider how it would monitor and potentially counter Chinese forces farther from the Japanese archipelago. A nuclear-powered attack submarine could remain submerged for extended periods without the endurance limitations imposed by conventional propulsion, making it particularly useful for long-range surveillance, tracking, and sea-denial missions.

Japan’s debate is also unfolding alongside a significant development in neighboring South Korea. Seoul has confirmed plans to develop a new class of nuclear-powered submarines, a move that could add further momentum to an emerging undersea competition in Northeast Asia. South Korea’s decision is particularly relevant to Tokyo because it demonstrates that nuclear propulsion is no longer merely a theoretical capability for U.S. allies in the region. While the missions and strategic priorities of the Japanese and South Korean navies are different, both countries face the challenge of operating in waters increasingly contested by China and North Korea.

The development could also create new opportunities for Japan to work more closely with Australia and the other AUKUS partners. Australia is in the process of acquiring its own conventionally armed, nuclear-powered submarine capability under AUKUS Pillar I, with U.S. and U.K. nuclear-powered submarines also scheduled to operate from Australia. Japan has separately been identified as a potential partner for AUKUS Pillar II, which covers advanced capabilities and technology cooperation.

A Japanese decision to pursue SSNs could eventually create a much deeper basis for cooperation with Australia, including submarine interoperability, undersea warfare, workforce and technical expertise, infrastructure, sustainment, and potentially aspects of the industrial base. Analysts have already examined how Japan could fit into the wider AUKUS submarine ecosystem as Australia’s nuclear-submarine program develops.

A Japanese nuclear-powered submarine with a robust capacity to launch land attack cruise missiles would also be a powerful component in the country’s emerging “counterstrike” posture, which is increasingly centered on acquiring the ability to strike targets at longer ranges.

Japan has already begun deploying domestically developed long-range missile systems, including the Upgraded Type 12 surface-to-ship missile, while also acquiring the U.S.-made Tomahawk cruise missile.

An SSN would potentially add a highly survivable, mobile platform to this emerging strike architecture, allowing Japan to position long-range weapons farther from its home territory and complicating an adversary’s efforts to locate and neutralize them. That could be particularly valuable in a conflict in which Japanese air bases, missile batteries, and other fixed infrastructure were under attack.

The potential SSN requirement also fits with Japan’s expanding expeditionary ambitions. As the JMSDF converts its Izumo class helicopter destroyers to operate F-35B fighters, giving Japan a more credible carrier aviation capability, nuclear-powered attack submarines could provide a valuable escort for these increasingly important surface groups. Their speed, endurance, and ability to remain submerged for extended periods makes SSNs particularly well suited to accompanying carrier forces on longer-range deployments and supporting contingencies farther from Japanese waters.

Looking at the bigger picture, it is highly significant that nuclear propulsion was effectively excluded from Japan’s conventional defense debate for decades. More recently, however, Japanese defense officials and policymakers have begun examining whether future submarines will require a different propulsion system. The issue is now one that can be discussed openly at the ministerial level.

Nevertheless, pursuing a nuclear-powered submarine program would come with huge challenges.

Building and operating nuclear-powered submarines would require Japan to establish an entirely different industrial, regulatory, and training infrastructure. The vessels would also be considerably more expensive and complex to build and operate than Japan’s existing conventionally powered submarines. Tokyo would have to resolve questions involving nuclear regulation, reactor technology, maintenance facilities, crew training, and the handling of nuclear material.

There would also be a significant political dimension. Although naval nuclear propulsion is fundamentally different from possessing nuclear weapons, any move in that direction would inevitably receive scrutiny in a country whose postwar security policy has been shaped by its experience with nuclear weapons and its longstanding non-nuclear principles.

For those reasons, Koizumi’s statement should be viewed less as an announcement of a Japanese nuclear-submarine program than as evidence that the political barrier to discussing such a program is falling.

The next question is whether the Japanese government will move from keeping the option open to actually studying a nuclear-powered submarine design, propulsion technology, and acquisition pathway in detail.

If it does, the consequences could extend well beyond Japan’s submarine fleet. A Japanese nuclear-powered submarine capability would represent a major enhancement of Tokyo’s ability to operate across the Western Pacific and could alter the undersea balance between Japan, China, and the United States’ other regional allies. At the very least, the idea is one that is now part of wider discussions at the highest levels.

Contact the author: thomas@thewarzone.com

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The first of Japan’s two new cruiser-sized Aegis System Equipped Vessels (ASEV) is continuing to take shape, recently receiving its huge main integrated mast, giving the vessel a new and remarkably imposing silhouette. The four very large fixed arrays for the ship’s powerful AN/SPY-7(V)1 radar will eventually be integrated onto this structure. The ASEV’s stated primary mission is missile defense, but Japanese authorities plan to give the vessels additional counter-drone, anti-ship, and land attack capabilities in the future. Tomahawk cruise missiles are notably set to be added to the ship’s arsenal down the line.

The ASEVs are large surface combatants, set to be some 620 feet long, have a beam (the widest point of the hull) of some 82 feet, and a standard displacement of 12,000 tons. For comparison, Japan’s largest surface combatants today, its Maya class destroyers, are just over 557 feet long, have a beam of some 73 feet, and a standard displacement of around 8,200 tons. It is worth noting here that the size of new and advanced destroyers, often optimized for anti-air warfare missions, has been trending upward globally in recent years. This, in turn, has caused the line between destroyers and what have more historically been categorized as cruisers in certain cases.

The first ASEV was laid down at Mitsubishi Heavy Industries’ (MHI) shipyard in Nagasaki in July 2025. The second was laid down in February of this year at Japan Marine United Isogo shipyard, which is situated in Yokohama City near Tokyo, according to a past report from Naval News. The Japanese Ministry of Defense is currently aiming to have the first of these vessels in service before the end of the country’s 2027 Fiscal Year, which runs from April 2027 to March 2028. The second is expected to follow in the 2028 fiscal cycle, which extends into 2029.

Significant progress in the overall construction of the first ship has been made just this year. The addition of the main mast structure, installed on top of the main superstructure, as seen in pictures at the top of this story and below, has drawn particular attention in recent weeks.

Japanese are very fast with ship building.

March 26 and today. ASEV-class destroyer.

190 m hull, SPY-7 Aegis, 128cell Mk 41 VLS. SM-3 for ballistic missiles, SM-6, plus Japan’s long range Type 12 SSM. pic.twitter.com/RQZixtqWwe

— 信濃 Æýà (@Shinano_AEya) August 30, 2026

The especially large size of the ASEV’s integrated mast is driven by that of the AN/SPY-7(V)1 active electronically-scanned array (AESA) radar from Lockheed Martin. Each one of the radar’s four rectangular arrays is reportedly around 14 feet (nearly 4.3 meters) tall. This would put them in the same general size category as the arrays for Raytheon’s AN/SPY-6(V)1 Air and Missile Defense Radar (AMDR) found on U.S. Navy Flight III Arleigh Burke class destroyers. That being said, the pictures we have now raise the question of whether the AN/SPY-7(V)1 arrays are bigger than previously assessed. The AMDR is another notably large design heavily optimized for missile defense.

The size and location of the mast provide a centralized, elevated position for the AN/SPY-7(V)1’s large arrays, which will also require significant power generation capacity and thermal management to keep everything cool. This is a notable departure from the placement of the arrays on the main superstructure on existing Japanese destroyers, as well as the U.S. Arleigh Burke class, and offers an unobstructed, elevated line-of-sight to the horizon. Beyond the ballistic missile defense mission, this view farther out over the horizon would be important for tracking and engaging low-flying drones and cruise missiles, which might be targeting the ship itself. This is something we will come back to later on.

“With five times the tracking capability of the SPY-1, it [the AN/SPY-7(V)1] counters lofted trajectories and simultaneous multiple ballistic missile threats,” according to a machine translation of a section of the Japanese Ministry of Defense’s 2026 defense policy white paper, which was released earlier this month.

The passive electronically-scanned array (PESA) AN/SPY-1 was the first radar to be paired with the Aegis Combat System, and both were first introduced in the 1970s. Variants of the SPY-1 are still found on a variety of U.S. and foreign Aegis-equipped warships, including Japan’s Kongo, Atago, and Maya class destroyers.

The AN/SPY-7(V)1 is also a derivative of the U.S. military’s static, land-based Long Range Discrimination Radar (LRDR) in Alaska. Japan also originally planned to employ the AN/SPY-7(V)1 on land at a pair of Aegis Ashore missile defense sites. However, that effort was subsequently cancelled, with the ASEV emerging to fill the resulting gap.

Additional versions of the AN/SPY-7 with much smaller arrays are also being integrated onto new F110 Bonifaz class frigates for the Spanish Navy and the Royal Canadian Navy’s forthcoming River class destroyers (also known as the Canadian Surface Combatant). The use of the AN/SPY-7(V)1 on the ASEV could impact decisions regarding new radars for other future and existing Japanese warships. Raytheon is also pitching versions of its increasingly popular AN/SPY-6 family for use on a successor to the Kongo class and for refit on the Maya class, according to a recent report from Naval News.

When it comes to the ASEV, as already noted, the AN/SPY-7(V)1 is central to the ship’s main stated missile defense mission. As the name makes clear, the ship will also feature the Aegis Combat System. The vessel will be capable of “remote engagement and guidance against anti-air targets tracked by other vessels” via a “Cooperative Engagement Capability (CEC),” per the Japanese Ministry of Defense’s most recent annual defense white paper.

The video below provides details about the U.S. Navy’s Cooperative Engagement Capability (CEC) developments, which are likely very similar to what Japan has planned for the ASEV.

In terms of armament, when the ASEVs first enter service, their main weaponry is expected to be a mix of variants of the Standard Missile-3 and -6 (SM-3 and SM-6). The SM-3 family is designed to engage ballistic missile threats during the mid-course portion of their flights while they are traveling outside of the Earth’s atmosphere. The SM-3 also has a demonstrated anti-satellite capability. The SM-6 is a multi-purpose missile that was originally designed with an anti-air focus. It can be used to target incoming ballistic missiles in the terminal phase of flight and also has at least a degree of capability for engaging hypersonic weapons. SM-6 can be used in a surface-to-surface mode, including against ships, as well. Japan is also a partner in the U.S.-led development of a new Glide Phase Interceptor (GPI), which is being designed specifically to intercept unpowered, but highly maneuverable hypersonic boost-glide vehicles.

SM-3, SM-6, and future GPIs will be loaded into either of the ASEV’s two Vertical Launch System (VLS) arrays, one at the bow end and another toward the stern. Each one of the ships will have a total of 128 VLS cells. For comparison, Japan’s Maya class destroyers, as well as the U.S. Navy’s Flight IIA and III Arleigh Burke class destroyers, have 96 VLS cells apiece. The U.S. Navy’s Ticonderoga class cruisers each have 122 VLS cells, and 128 is also the number of VLSs currently expected to be found on the future Trump class battleships.

At the time of writing, Japanese authorities do not appear to have explicitly mentioned plans to arm the ASEVs with other missiles when they first enter service. That being said, any VLS array capable of accommodating SM-3s and SM-6s should be able to fire a variety of other weapons, including shorter-ranged SM-2s and Evolved Sea Sparrow Missiles (ESSM), which the ship could use for more localized air and missile defense, as well as protecting the ship itself.

Renderings and models of the ASEV do show the design also armed with a five-inch naval gun in a turret on the bow, along with two Phalanx Close-in Weapon Systems (CIWS) and other automatic cannons for point defense.

As mentioned, while missile defense is the immediate focus for the ASEVs, there are already plans to significantly expand their capability to engage other ships and targets on land. Tomahawk cruise missiles and a new, extended-range version of the Type 12 anti-ship cruise missile are set to be added to the arsenal of these vessels in the 2032 timeframe, according to the latest official Japanese defense white paper.

Japanese authorities want the ASEVs to eventually be able to help fend off waves of enemy drones, too. High-energy laser-directed energy weapons are set to be integrated onto the ships down the line to help address those threats. The overall size and other general characteristics of the ships would leave the door open to the possible addition of high-power microwaves and other capabilities in the future.

TWZ has long noted that the ASEVs will bring huge new boosts in naval air and missile defense and long-strike capability to the Japan Maritime Self-Defense Forces. The vessels also reflect a significant evolution in post-World War II Japanese doctrine, toward what officials currently frame as a “stand-off defense” construct that remains within the bounds of the country’s current pacifist constitution.

Just with SM-3, SM-6, and GPI, the ASEVs will offer an important addition to the air and missile defense bubbles around Japan’s home islands and outlying territories in the face of growing geopolitical friction with China and Russia, two countries that are increasingly cooperating militarily. The vessels will also add to Japan’s deterrent against long-standing and still evolving threats posed by the regime in North Korea, which has its own close ties to the Chinese and Russian governments. All three of these nations continue to expand their arsenals of ever-more capable ballistic, cruise, and hypersonic missiles. Long-range one-way-attack drones are another growing part of the equation, with the ongoing conflict in Ukraine and fighting in the Middle East in recent years underscoring just how real a threat they pose.

With the addition of Tomahawks, the ASEVs, along with other Japanese warships, will gain the additional ability to threaten counterstrikes against targets on land (and potentially at sea) at least around 1,000 miles in any direction. As part of the broader stand-off defense strategy, the Japan Self Defense Forces is developing new land-based long-range strike capabilities, including hypersonic missiles, as well.

It is worth noting that with current plans to acquire only two ASEVs, these will be high-value but also low-density assets, which will have to be very deliberately deployed for maximum effect. They will also stand out as top-priority targets in any future conflict, and will need to be protected as such.

It is possible that the roles and missions of the ASEVs, just like their designs, could evolve further as time goes on. The threat ecosystem facing Japan will continue to evolve, likely more rapidly, in the coming years. The country is also expanding its naval capabilities in other ways, and with an eye toward more blue water operations. This includes expanded light carrier capabilities through refits of existing Izumo class ships. Anti-ship ballistic missiles and hypersonic cruise missiles, among large volumes of more traditional threats, could see the ASEVs used as escorts, even if experimentally. While just two hulls are barely enough to keep one on patrol to protect Japan, more could be built for expeditionary applications.

Regardless, the ASEVs are set to be a major addition to the Japan Maritime Self Defense Force’s fleets that will have big ramifications. From the progress that is visible at the shipyard in Nagasaki, MHI looks to be well on track to deliver the first of these ships next year or two.

Special thanks to @kensakai9310 on X, who also goes by “Sakai,” for sharing recently taken pictures of the first ASEV under construction at MHI’s yard in Nagasaki with us.

Contact the author: joe@twz.com

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