Nuclear-powered submarines would improve Japan’s national security enough to justify their costs

What's this about?

People disagree about whether submarines powered by nuclear energy would make Japan much safer.

The question asks if those gains would justify the money, risks, and political problems.

What supporters say

• The world’s main nuclear arms treaty does not fully ban nuclear energy for ships.
• Nuclear power lets submarines travel farther and stay underwater much longer.
• These submarines could watch seas, protect trade routes, and help Japan’s allies.
• Their speed and long trips could make enemy forces struggle to find them.

What critics say

• Japan would need strict safety rules for nuclear fuel, creating a hard example for other countries.
• The program would cost huge sums for boats, bases, workers, training, repairs, and shutdowns.
• Japan’s nearby seas may already give regular submarines enough useful places to work.
• Nuclear submarines could push nearby countries to build stronger forces too.

How to read this

The number of points on each side does not show who is right; the strength of the proof matters.

The bottom line

Nuclear submarines would give Japan real military gains, especially longer trips and more time underwater.

But the evidence does not show that these gains would outweigh the very large costs and risks.

Japan is considering whether nuclear-powered, conventionally armed submarines would provide enough extra security to justify their high financial, political and strategic costs. The evidence shows real military advantages, but it does not establish that those advantages would outweigh the burdens for Japan.

The case for

The strongest argument is that nuclear propulsion would give Japan greater reach and staying power under water. Unlike conventional submarines, nuclear-powered boats can remain submerged for long periods, travel quickly to distant operating areas and avoid frequent snorkeling. That could matter if Japan needed to monitor or challenge activity across the wider Western Pacific rather than mainly around its home waters. (see Figure 1) 1

Nuclear-powered attack submarines, or SSNs, could remain on station longer for surveillance, anti-submarine warfare, sea-lane protection and allied missions. Their mobility and endurance could make it harder for an adversary to find or predict Japanese and allied submarine movements. That could strengthen deterrence even if the submarines carried only conventional weapons. 2

There is also a legal argument in favor, although it is limited. The Nuclear Non-Proliferation Treaty does not categorically ban non-nuclear states from using nuclear material for naval propulsion. International safeguards materials identify possible arrangements for naval propulsion, meaning treaty membership alone would not make a Japanese program illegal. The central legal issue would instead be whether Japan could design credible safeguards while protecting military secrets. 3

The case against

The main objection is cost. A nuclear-submarine fleet would require much more than buying the boats. Japan would need specialized shipyards, reactor facilities, bases, training systems, skilled workers, maintenance arrangements and eventual decommissioning plans. These costs would continue for decades and compete with spending on missiles, aircraft, surface ships, unmanned systems, sensors and conventional submarines. (see Figure 2) Comparable programs also show that the industrial burden would be large, requiring sustained investment in suppliers and technical staff. (see Figure 3) 4

Japan’s geography may also limit the extra value of nuclear propulsion. Its main missions involve nearby waters and regional chokepoints, where submarines can operate from close bases. Japan’s Taigei-class conventional submarines already use lithium-ion batteries to improve submerged endurance and quiet operations. In some conditions, conventional submarines may be quieter than nuclear-powered boats, so nuclear propulsion is not a universal stealth advantage. 5

A Japanese program could create serious safeguards and regional-security problems. Naval propulsion may fit within an NPT framework, but the necessary arrangements could allow nuclear material to leave normal inspection procedures for operational reasons. That could make it difficult to verify that the material was not diverted and might weaken confidence in the global safeguards system. It could also give other countries a model for developing a “latent” nuclear capability. 6

The regional effects are uncertain. Japanese SSNs might strengthen deterrence in some situations, but they could also increase arms-race pressures, hedging and crisis instability involving China, North Korea and South Korea. Domestic opposition, Japan’s pacifist constitutional framework and its Three Non-Nuclear Principles would add political complications. Cooperation with the United States, Britain or Australia could help with training, technology and safeguards, but the AUKUS experience shows that such projects can create dependence on foreign shipyards, technology approvals, skilled workers and allied political commitments. 7

The bottom line

The evidence slightly favors caution, but only with low confidence. Nuclear-powered submarines would provide a genuine and potentially important advantage for sustained operations far from Japan. However, the available evidence does not show that this advantage would be large or consistently necessary across Japan’s missions.

The key comparison has not been quantified: Japan lacks a detailed costed plan showing the security gains of SSNs against the conventional, aerial, missile, sensor and unmanned capabilities they might displace. Cost evidence from Australia and the United States shows the likely scale and types of burden, but does not predict Japan’s exact bill, timetable or industrial prospects. The likely regional response is also unclear.

If Japan’s priority is nearby sea denial, homeland defense and distributed surveillance, advanced conventional submarines may offer better value. If it expects sustained operations across the wider Pacific, nuclear-powered boats would offer capabilities conventional submarines cannot fully match. On the present record, however, the claim that those benefits justify the full program is balanced but not established.