Could Japan be gearing up to equip its already formidable submarines with long-range strike systems and cutting-edge power sources?
That's what an expert panel has recommended to help boost deterrence as security tensions in Asia run high and Tokyo’s neighbors rapidly upgrade their military capabilities.
Established in February last year to review the progress of revisions to Japan’s key security documents, the panel has used a recent report to call for the development of submarines armed with “long-range missiles” that can be launched vertically.
Submitted to the Defense Ministry last month, the report also states that Japanese submarines should be capable of operating over long distances and for longer periods of time, including while submerged. The additional power required should come from “next-generation” sources — without being “bound by conventional approaches” — phrasing that has raised speculation Tokyo is considering nuclear power or another energy source for propulsion.
While these weren’t the only recommendations made by the expert panel, the suggested submarine capability upgrades have drawn considerable attention.
They indicate that Japan’s defense activities will no longer be confined to its national borders as Tokyo shifts toward a more regional-focused defense posture.
But perhaps most notably, the recommendations can also be seen as moves to further loosen the postwar limits on Japan’s military.
Hurdles for implementing nuclear propulsion aren't just technical or financial — first and foremost, they are legal and political. Indeed, possessing nuclear-powered subs would, on paper, contravene the country’s Atomic Energy Basic Law, which limits nuclear power research and use to peaceful goals.
"If we follow the current interpretation (of the law), it would be difficult for Japan to possess nuclear submarines,” Chief Cabinet Secretary Yoshimasa Hayashi, the government’s top spokesperson, told a news conference in September last year.
Sanae Takaichi, the newly elected leader of the ruling party and Japan’s presumptive prime minister, has in the past said she views the idea of possessing nuclear subs favorably — something she said is not unconstitutional, while emphasizing "a need to sort things out."
Japan has a pacifist Constitution and has long adhered to its “three nonnuclear principles” of not possessing, producing or allowing nukes to be introduced to Japan.
As for the technical aspects, the expert panel’s first proposed capability upgrade, the installation of a vertical launch system (VLS) for standoff weapons, is something both the Defense Ministry and the Maritime Self-Defense Force (MSDF) have been considering for some time.
In 2023, Kawasaki Heavy Industries proposed a new sub design featuring a VLS fitted between the sail and the bow. The company, which at the time was participating in a study commissioned by the ministry, presented this concept as a potential successor to the current Taigei class, the last of which is set to be built in fiscal 2027.
While some subs’ torpedo tubes can launch cruise missiles, such as the Harpoon anti-ship missile deployed by the Taigei class, there are limitations as to how quickly these can be reloaded, partially because the tubes are usually shared by torpedoes and strike weapons.
“A VLS would enable Japan’s next-gen subs to fire both more, larger (and longer-range) weapons than they can now, turning them into more lethal strike platforms,” said Jeffrey Hornung, a Japan defense expert at the Rand Corp.
Among the missiles reportedly being considered is a variant of the Type 12, which would put both ship and land targets at risk from far away. Another possibility would be using U.S.-made Tomahawk cruise missiles, which Japan is already planning to deploy to some destroyers.
But introducing a VLS would require the new subs to be larger and equipped with more robust power sources.
“A larger hull tends to reduce acoustic stealth,” said Masashi Murano, a Japan defense expert at the Hudson Institute think tank.
And if propulsion power is inadequate, this will also make the sub slower, which is why the MSDF is working to find a technological solution for future VLS-equipped subs, he added.
The expert panel’s call for submarine capability upgrades comes as China equips its surface ships and submarines with a considerable number of standoff weapons. This means they can threaten Japanese and U.S. forces without having to approach or transit choke points where MSDF subs would normally try to ambush adversaries.
Because of this, deterring or countering the Chinese Navy will require not only traditional tactics, Murano said, but also long-range weapons and the capability to conduct offensive anti-submarine operations closer to them.
Conventional diesel-electric and air-independent propulsion submarines use a lot of battery energy in transit, so they are not well suited for prolonged sailings.
The expert report’s vagueness about the envisaged propulsion system is likely intentional.
“I suspect this reflects not so much political delicacy as a genuine lack of consensus among committee members about whether to pursue nuclear-powered subs (SSNs),” Murano said.
“Within the Defense Ministry and the SDF, there is still no consensus on SSN acquisition,” he added, noting that proponents are not always clear whether they primarily seek improved submarine capabilities or a more credible ability to launch retaliatory strikes.
One thing is clear, though, Murano said: “The quickest way to offset these disadvantages is to increase power via nuclear propulsion.”
Hornung noted that Japan has mastered civilian nuclear technology, even building a nuclear-powered civilian ship in the late 1960s, “so it has the ability to do this.”
“I don’t think it is crazy that Tokyo would consider nuclear-powered subs,” he added.
However, as Australia has shown, acquiring nuclear subs is easier said than done, with Murano highlighting three major hurdles for Japan: development and operational costs, a lack of qualified personnel and the time required to field the vessels.
Many believe that the most dangerous moment for the growing Sino-U.S. military rivalry will be from the late 2020s through the early 2030s. But it is unlikely that Japan would be able to acquire a nuclear sub within that time frame, according to Murano.
Indigenous building, even with U.S. technical support, would be the biggest hurdle, he said, noting that putting a nuclear sub into service would likely take around a decade.
Leasing the subs would also be problematic, as the U.S. shipbuilding industry is highly strained, and a portion of its capacity is already slated to build boats for Australia in the early 2030s.
“So even if Japan sought to lease these boats, deliveries would likely be pushed into the mid-to-late-2030s,” Murano said.
Compounding this is a personnel shortage, with the Self-Defense Forces already facing serious workforce problems and submarine crews being among the hardest posts to fill.
Given Japan’s demographic trends, introducing nuclear subs would only exacerbate the personnel problem. For example, a nuclear-powered Virginia-class requires roughly twice the crew, about 140, of a Taigei boat, which has a crew of around 70.
“From a purely operational perspective, SSNs are a natural option,” Murano said. “But they don't help mitigate the broader problems facing the SDF.”
That said, there are alternatives.
The Taigei class is powered by a mix of diesel engines and lithium-ion batteries, rather than the more commonplace lead-acid ones. This raises the possibility that the next-generation subs could feature more advanced sources such as solid-state batteries and fuel cells.
The South Korean Navy, for example, has fielded VLS-equipped ballistic missile subs that feature a mix of diesel engines and advanced lithium-ion batteries in addition to an air-independent propulsion system.
“I would think that if powerful advanced batteries do become realistic alternatives that enable MSDF subs to remain submerged for long periods of time, then politicians will likely lean in that direction,” Hornung said.
Another option, Murano noted, would be to develop a hybrid variant combining conventional diesel-electric propulsion with small modular nuclear reactors similar to those developed by Mitsubishi Heavy Industries. This, however, might require changes to the nuclear regulations.
Alternatively, Tokyo could consider retaining conventional submarines and complementing them with large and extra-large unmanned underwater drones such as Australia’s Ghost Shark or America’s Orca, said Murano.
“This would allow Japan to preserve the conventional submarine force size while extending operations closer to Chinese forces and boosting its capability to launch cruise missiles underwater.”
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