Japan wants nuclear submarines, but can it overcome the obstacles?

The Liberal Democratic Party began talks this week with its coalition partner, the Japan Innovation Party, on revisions to Japan’s defense strategy. Among the many issues, a central topic will be the potential acquisition of nuclear-powered submarines for the Maritime Self-Defense Force.

Under the coalition agreement between the ruling parties, the JIP has called for the introduction of submarines powered by “next-generation propulsion systems,” with the junior partner advocating a shift from Japan’s diesel-powered vessels to those equipped with nuclear power plants. The idea also has considerable support within the LDP.

If the coalition reaches an agreement, the parties are expected to clarify the need for these advanced weapon systems in the strategy documents and include them in the acquisition plans outlined in the Defense Buildup Program, which is part of the three defense documents to be revised by the year-end. The other two documents are the National Security Strategy and National Defense Strategy.

The push comes as Japan seeks to keep pace with both security partners and emerging threats.

Six countries currently operate nuclear-powered submarines: the United States, United Kingdom, France, China, Russia and India. Australia and South Korea are also actively pursuing the capability.

Nuclear-powered submarines offer several advantages over their diesel-powered counterparts. They are faster, can generate more electricity to support advanced onboard systems and can remain submerged longer. Their time underway is limited primarily by the requirement to resupply food and other life support items for their crews.

There are also political arguments for Japan to acquire the capability.

Pursuing nuclear-powered submarines could strengthen Japan’s case for deeper integration into the AUKUS framework ​between Australia, the United Kingdom and the United States. The first pillar of the security partnership focuses on Australia’s acquisition of nuclear-powered submarines. Japan currently participates in Pillar II, but investment in nuclear propulsion could bolster its case for greater cooperation with Australia, the United Kingdom and the United States, including potential access to related technologies.

Acquiring the technology would also satisfy a longstanding desire among some policymakers to ensure Japan maintains the same capabilities as other top-tier military powers.

A model submarine displayed during an event at in Maryland in the U.S. last month. Anduril Industries is investing $3.7 billion to build a massive new shipbuilding facility named Arsenal 2 in Maryland, a move that will enhance the U.S. Navy's nuclear-powered submarine production. | Bloomberg

This echoes the debate over aircraft carriers that preceded the previous revision of the three defense documents in 2022. Before the 2022 versions were adopted, Japanese lawmakers and security commentators argued that Japan was the only major maritime power without aircraft carriers. The country has since begun converting its large helicopter destroyers into aircraft carriers.

But the operational and political benefits come at a cost.

From a practical standpoint, nuclear-powered submarines require considerably more resources than their diesel-powered counterparts.

They are more expensive to build and maintain, and Japan currently lacks the domestic infrastructure needed to develop and support this. Although Japan is a world leader in conventional submarine technology, decades have passed since it last seriously explored nuclear propulsion for vessels.

Part of the challenge stems from the 1974 nuclear accident involving the nuclear-powered ship, Mutsu. Japan had succeeded in launching what was the world’s fourth boat with nuclear propulsion, but a radiation leak discovered during sea trials scuttled subsequent efforts to use nuclear propulsion.

Personnel present another major obstacle. Nuclear-powered submarines require larger crews than conventional vessels because they need additional nuclear engineers, reactor maintainers and other specialized technicians. There are also the many on-shore training, inspection and support personnel necessary to sustain this capability.

Japan’s aging population and ongoing recruitment difficulties in the Self-Defense Forces present major obstacles to meeting these staffing requirements. The ruling coalition is already considering a proposal to extend the mandatory retirement age for SDF personnel to 65. How the government would meet the additional personnel demands of a nuclear-powered submarine fleet remains unclear.

If the ruling coalition agrees to incorporate nuclear-powered submarines into the three defense documents following its working level talks, the proposal is likely to face three major challenges in subsequent parliamentary deliberations.

The first concerns the missions for the nuclear submarines. If Japan’s defense documents identify China, North Korea and Russia as its primary security threats, it may be difficult to justify acquiring submarines whose greatest advantages are most apparent during extended, long-range deployments.

Former Constitutional Democratic Party of Japan lawmaker Ichiro Ozawa has already challenged this rationale, asking: “What missions cannot be carried out with the conventionally powered submarines it currently has? To casually call for their introduction without even explaining this is utterly unacceptable.”

Others may question why Japan is not investing more heavily in its domestic submarine industry while accelerating the development of unmanned underwater vehicles. This argument is particularly relevant given Tokyo’s effort to incorporate lessons from Ukraine’s defense against Russian aggression, including the importance of domestic production capacity and the growing role of uncrewed technologies.

A further issue concerns the civilian-use restrictions governing nuclear energy in Japan. Under the current interpretation of the Atomic Energy Basic Act, the use of nuclear power plants for non-civilian purposes is prohibited, meaning that the administration would need to address that legislative issue before moving forward with procurement.

Finally, there is the issue of what is known as Japan “nuclear allergy,” encompassing both opposition to nuclear-related military capabilities and public anxiety over the risk of nuclear accidents. Both are likely to fuel politically charged debates over the proposal.

In September, Defense Minister Shinjiro Koizumi challenged the notion that the prospect of opposition should make such discussions taboo.

“Japan has been overly optimized for peacetime,” he said. “The fact that even mere discussions on nuclear weapons/nuclear-powered submarines are not tolerated is out of sync with my sense of crisis.”

Given the trajectory of the ruling coalition’s agreement on “next-generation propulsion systems,” such discussions now appear increasingly likely – along with the political, legal, and practical issues that come with them.