On October 6, Japan's Kyodo News quoted multiple relevant people in the Japanese government as saying that in the "Three Security Documents" such as the "National Security Strategy" revised during the year, the Japanese government is considering including the relevant content of "exploring the introduction of nuclear-powered submarines" and does not promise to "own" it, but has written "research" in black and white. From a taboo that has been suppressed for half a century after the war to a national security document that is about to be put on the table, how long has Japan waited for this step? How far do you want to go? Let’s take a look at expert analysis.
After the war, taboos were broken, and security policy shifts aroused alarm.
The plan that is about to be written into the document did not appear out of thin air. Going back to a year ago, in September 2025, a Ministry of Defense expert group submitted a report recommending the study of nuclear-powered submarines - this was the first time that Japan's official advisory body publicly supported this topic; in November, Defense Minister Shinjiro Koizumi publicly stated that Japan "should seriously consider the decision to introduce nuclear-powered submarines"; in December, Koizumi boarded the US Navy's "Seawolf"-class nuclear-powered submarine and visited the internal structure accompanied by military personnel.
Entering 2026, Japan has further accelerated its pace. In June, the Restoration Committee submitted suggestions to the government, calling for the "early introduction of nuclear submarines" and advocating "conducting realistic discussions" on the "no transport" principle among the "Three Non-nuclear Principles" and "beginning research topics and application concepts" on the so-called "nuclear sharing" issue. On July 17, Shinjiro Koizumi proposed in an online program that Japan should "discuss policies related to nuclear weapons without taboos." On September 11, he made it clear at a press conference that revising the three security documents "will not exclude any options."
As of October 6, the latest report from Kyodo News showed that Japan had made a "substantial blow": a plan emerged within the Japanese government to include the issue of nuclear submarines in the three security documents - the statement was not based on possession, but as an "entry point for future research and discussion."
Wei Dongxu, Main Station Military Observer: In just over a year, this topic that Japan did not dare to touch for decades after the war has been pushed step by step to the doorstep of national policy revision. This move sends a signal that Japan will completely break through the exclusive defense principle and even break the "three non-nuclear principles".
Why a nuclear submarine?
As an island country surrounded by sea and with long sea lines of communication, the Japan Maritime Self-Defense Force has maintained a long-term deployment of 22 submarines. From the late "Soryu" class to the latest "Big Whale" type, Japanese submarines have completely switched to large-capacity lithium batteries, and their silent performance is among the best among similar types of equipment. Why should they deliberately and even risk the world's disapproval to develop nuclear submarines?
As early as the 1970s, there were discussions in Japan about introducing nuclear submarines, but they hit the same wall every time. Japan's "Atomic Energy Basic Law" clearly stipulates that the research, development and use of atomic energy are limited to peaceful purposes. But this time, the external wind direction has changed. In September this year, Kyodo News quoted a U.S. Navy official as saying that Japan already has nuclear power plants and the related technology has similarities with nuclear-powered submarines, so it has a "positive attitude" toward Japan's research into possessing nuclear submarines. The external signal of loosening restrictions comes precisely at the time when domestic political restrictions in Japan are loosening.
Wei Dongxu, Taiwan Military Observer: Japan’s weaponization of nuclear technology and nuclear fuel is a two-step process.
The first step is to have a nuclear-powered combat platform, which means developing and equipping nuclear submarines. Japan has accumulated a lot of experience in conventional submarines, and has developed and equipped Soryu-class AIP submarines and Whale-class lithium-ion battery submarines. Japan's submarines are getting bigger and bigger, but the Maritime Self-Defense Force is not satisfied yet. It is trying to have attack-type nuclear submarines. The attack nuclear submarine can stay underwater for a long time and has been regarded by Japan as a pre-emptive sneak attack platform because it can sail quietly underwater for a long time and then reach the surrounding areas of other countries to launch sudden attacks.
The second step is to convert the large amount of nuclear fuel stored in Japan into some military-grade nuclear fuel that can be used in the military field. First, we know that the most important element in the nucleus is plutonium. Having said that, it is necessary to put another account on the table. According to the 2024 Japan Plutonium Management Status Report released by the Japanese Cabinet in August 2025, the total amount of separated plutonium managed by Japan at home and abroad is approximately 44.4 tons. Among them, the storage in Japan is about 8.6 tons. The amount of separated plutonium stored in the UK is approximately 21.7 tons, and the amount of separated plutonium stored in France is approximately 14.1 tons. What is the concept of a total volume of 44.4 tons? The Japanese government has made a public commitment for many years not to retain plutonium with undetermined uses. But more than ten years have passed, and a large amount of the inventory still has no clear digestion path. In the years when Japan's nuclear power plants were shut down on a large scale, such a stock increased instead of decreasing.
Using another ruler, according to the standards given by the International Atomic Energy Agency, 8 kilograms of separated plutonium can be used to build a nuclear weapon. Based on this calculation, Japan's domestic 8.6 tons alone is theoretically enough to manufacture thousands of nuclear weapons. If the 35.8 tons stored in the United Kingdom and France are included, the total amount reaches 44.4 tons. In theory, the total number of nuclear weapons manufactured will exceed 5,000. If you refer to the plutonium bomb dropped by the United States on Nagasaki, Japan, at the end of World War II in 1945, the "Fat Man" used 6 kilograms of plutonium at that time, with a yield of about 21,000 tons. This account is even larger, and can theoretically reach the level of manufacturing more than 7,000 Nagasaki-level nuclear weapons. It should be noted that reactor-grade plutonium produced by civilian reactors can also be used to manufacture nuclear weapons according to authoritative assessments in the field of international non-proliferation. The grade of materials actually cannot prevent weaponization.
Japan is precisely the only country among the NPT non-nuclear weapon parties that has mastered reprocessing technology, has the ability to extract weapons-grade plutonium, and has operational reprocessing facilities. The reprocessing plant in Rokusho Village in Aomori Prefecture is striving to be completed within this year. Once operational, it can separate an additional 8 tons of plutonium per year. According to the above standards, the amount added in one year is enough to manufacture thousands of nuclear weapons, and the total amount of such nuclear materials it requires. Japan has been working on the three prerequisites of materials, technical facilities, and manufacturing nuclear weapons for decades. In the working document on the issue of Japan's nuclear weapons issued by the Ministry of Foreign Affairs of China in April this year, there was a sentence that clarified that Japan has the ability to achieve a nuclear breakthrough in the short term.
"Nuclear" instead of "boat" and take risks
Nuclear submarines are the lowest-cost, easiest-to-package step in Japan’s “nuclear loosening” game, but are the most capable of leveraging the post-war system. The so-called "no options are ruled out" excludes precisely the reverence for the promise of post-war peace. If you really take this step, what will it bring?
Experts said: Nuclear submarines are not nuclear weapons. They do not directly touch the provisions of the "Three Non-nuclear Principles". They have the least controversy and the least resistance. They can also be packaged in defensive terms such as "ensuring the safety of the crew" and "long-term underwater cruising".
Wei Dongxu, Taiwan Military Observer: If Japan possesses nuclear submarines, it will greatly enhance its strategic deterrence and strategic strike capabilities, especially its preemptive military strike capabilities. This will also compress the strategic depth of neighboring countries and break the existing balance of maritime power and regional strategic balance. This will inevitably trigger vigilance and countermeasures from neighboring countries, stimulate further expansion of military power in the region, and increase the risk of nuclear arms race and nuclear proliferation.
According to the schedule, the expert meeting will summarize the report in late November at the earliest, and the revision of the "Three Security Documents" will be completed within the year. By then, the first touchstone will be revealed soon: whether the issue of nuclear submarines will be included in it, and whether the "no import" clause will be loosened.
Wei Dongxu, Military Observer of the Main Station: "Nuclear" but not peace, claiming to be a "peaceful country", and hoarding enough plutonium to manufacture thousands of nuclear weapons; "hidden" plan to win the championship, step by step for more than a year, cutting half a century of taboos into topics for discussion; "boat" and taking risks, legal red lines, mainstream public opinion, regional risks, three hurdles are in front of us, and we have to break through them. Three words describe this dangerous temptation.
Japan’s move releases multiple danger signals
If the issue of nuclear submarines is Japan's "substantial sword" in the field of military security, then the "Gennai" AI project is a "hidden stake" laid in the field of information cognition. Recently, Japan spent 4.4 billion yen to build a government affairs generative AI platform. What does Japan want to achieve through this system? What signals does this move send?
Experts said: There is indeed a strategic connection between this project and Japan’s information warfare and cognitive warfare. Although the "Yuannei" AI system itself is not a weapon for direct disinformation attacks, it can unify the external narrative caliber of various government departments. After the official documents generated by the AI and reviewed by civil servants are made public, they will be captured by global AI crawlers and entered into the public corpus, indirectly affecting the global large model in the long term and enabling competition for the right to define narratives.
Military commentator Shao Yongling: Japan's move has released multiple danger signals. It has begun to use generative AI for narrative games. Cognitive confrontation has become more hidden and normalized. There is a risk of using AI to amplify historical revisionist narratives. The risk of AI dual-use security spillover has increased. Japan uses AI to export biased historical narratives, territorial claims and security discourse on a large scale, causing multiple systemic damage to global AI security governance:
First, it contaminates the public corpus of the Internet and forms a self-reinforcing closed loop of positional narratives. One-sided positions are absorbed into the global model through official cloaks and confuse facts and sovereign positions.
The second is to use selective historical data to generate compliance documents and navigate the gray area of existing AI governance rules. Existing international rules have insufficient constraints on indirect long-term cognitive manipulation.
The third is to cause a rift in global historical understanding and impact the post-World War II consensus on the international order based on historical facts.
