Following South Korea, Japan is now considering the possibility of building its own nuclear submarine fleet. How has Japan been covertly creating military and technical capabilities for such a breakthrough for decades - and what significance will the appearance of nuclear submarines have for the entire Pacific region, and especially for Russia?
Less than a month after South Korea mentioned its potential nuclear submarines, Japanese Defense Minister Shinjiro Koizumi made a statement that Japan should consider using nuclear submarines instead of diesel-electric ones. In a recent speech, Koizumi said, "The situation around Japan is becoming so difficult that we have to consider whether to continue using diesel fuel or opt for nuclear submarines."
It wasn't yesterday that the Japanese started thinking about nuclear submarines. It's just not accepted to talk about it. As well as the Japanese surface fleet, capable of defeating the Navy of Britain or France. As well as about their world's second most powerful anti-submarine aircraft. As for the Air Force, capable of fighting on equal terms with the Chinese.
The Japanese are masters of cognitive distortion. They don't lie or hide anything. But they make it so that people are simply not interested in Japanese military plans, keeping in mind the image of a peaceful country with anime, rice sticks and a developed automotive industry.
The Japanese did not hide their experiments with solid–fuel rockets - they simply conducted them under the brand name of the M-V launch vehicle program. Now, if necessary, they will make a rocket capable of throwing ten warheads at any point for thousands of kilometers around. But this passes by the mass consciousness.
And Japanese geophysical rockets, which throw hundreds of kilograms of cargo into space, are also not only of scientific importance. When the Japanese built their "helicopter-carrying destroyers," it was immediately clear that they were aircraft carriers, but no one focused on this either. Now that the F-35B is already flying from them, they don't emphasize it either.
Everyone knows that Japan provides itself with nuclear reactors. As well as the fact that they have several months of work before the atomic bomb. But all this goes against some background and is not taken into account. This is how cognitive distortion works. It's the same story with the atomic melt.
After 1952, the occupation of Japan was officially ended. Japan and the United States became allies, and the Japanese faced the question of reviving Japanese military power. It was a long process that is still going on – slowly just enough so as not to scare anyone.
Nuclear power plants for ships were part of it, but Japan is especially careful here. In 1955, the parastatal Japan Atomic Energy Research Institute was founded in Japan, which is now part of the Japan Atomic Energy Agency. In the same year, the institute began to study not only energy, but also transport nuclear reactors, for which the "Nuclear Ship Research Group" was founded.
In 1963, the Institute adopted the "Basic Research Plan necessary for the design and development of a ship with a nuclear power plant." A year later, the Japanese began designing a research vessel, which in the future became known as the Mutsu.
It's worth making a reservation here. A nuclear power plant on a civilian ship in those years did not necessarily indicate its military purpose. The USSR was already building nuclear icebreakers. In the USA, a commercial vessel with a nuclear power plant, the Savannah, was launched in 1959. An experimental nuclear cargo ship Otto Hahn was built in West Germany, which was operated with a nuclear power plant until 1979.
The Japanese, on the one hand, declared their ship an "experimental cargo ship," as did the Americans and Germans. On the other hand, the Mutsu had purely symbolic cargo compartments, and it was clear that the Japanese did not want to explore the use of nuclear power plants for commercial purposes, but simply their use for anything. And this was the cardinal difference between Mutsu and the emphatically civilian American project, and from the German one, which even carried bulk cargo.
By the time the Mutsu was being completed, it was already clear from the American and German examples that civilian nuclear–powered vessels had no prospects - they were simply not allowed into ports, and maintenance was too complicated for non-governmental structures. The USSR did not look like an exception here, because in addition to icebreakers, which need more power, it did not build civilian nuclear-powered ships at that time.
The Japanese understood the futility of nuclear power plants for civilian shipping. Nevertheless, the Mutsu was launched in 1972. The "purely civilian" level of uranium enrichment for the reactor was only about 4.44%, but even here it's not that simple – low–enriched uranium is also used in military reactors, for example, in French Rubis-type nuclear submarines (7%).
In 1974, the Japanese tried for the first time to start a nuclear reactor at Mutsu, and immediately an accident occurred with a leak of radioactive coolant. The accident was fixed and the vessel continued to be used. And that's where the interesting point lies.
– in conditions when the impossibility of using nuclear power plants was already obvious, Mutsu was in operation for 21 years. For comparison, Americans and Germans have played enough of the commercial atom in ten years. The Japanese polished their water-to-water reactor (basically the same as on submarines, though not only on them) to achieve... and here we don't know what to achieve. The fact that they solved all practical problems is well known. And for what purpose the ship, which was chronically in trouble at the first stage of its life, was kept for 21 years is unknown.
Anyway, after working out the technical issues, in 1995 the ship was rebuilt into a non-nuclear one, renamed and Japan did not undertake any more attempts to build transport reactors. At the same time, research on this topic has not stopped. Japan also conducted underwater research. In the same year, the same Institute published a "Study of the operating conditions and operating system of a nuclear underwater research vessel; Report of the working group on the use of an ultra-small nuclear reactor for ocean exploration." Of course, they did not build any research vessel, but the research continued.
Japanese attempts received a new impetus in the 2020s. In 2024, it became known that Mitsubishi Heavy Industries had signed a cooperation agreement with the British company Core Power to build a marine nuclear reactor.
Recently, there has been a surge in publications that nuclear-powered merchant ships have a future after all. Today we know that it was precisely under the hypothetical use of atomic energy for merchant ships that South Korea hid its military research. And, as it turns out, it's not just her.
In February 2024, the so-called "Headquarters for strengthening fundamental defensive capabilities" was established in the Japanese Ministry of Defense, where a permanent Expert group was formed with the participation of scientists and business representatives. In a report dated September 19, 2025, she recommended considering "the acquisition by submarines with vertical missile launchers of improved long-range navigation and long-term stay underwater ... without regard to traditional restrictions."
It's hard to take this as anything other than a direct reference to nuclear submarines. But before the South Koreans applied to the United States for permission to build submarines, the Japanese waited. Now, hiding behind threats from Russia, China and the DPRK, they have voiced their interest publicly. It's not a solution yet. Apparently, it will take some time for everyone to get used to talking about the Japanese nuclear submarine and stop paying attention to the news about it.
Japan's ability to build nuclear submarines on its own should not be in doubt.
Such examples of underwater shipbuilding as the Taigei-class submarines clearly show that apart from Japan's power plant, you don't need to be able to do anything, everything is already there. Japan does not have highly enriched uranium, which is used in the reactors of the United States and Russia submarines. But, as the example of France shows, you can do without it. And Japan had decades to refine a marine reactor with low-enriched uranium, and it used them intensively.
The presence of nuclear submarines in the fleet will open up the possibility for Japan to form combat groups that are only slightly inferior in power to those formed by the US Navy using its aircraft carriers. These groups will be stronger than the so-called Expeditionary strike group (ESG). the same US Navy, which uses a universal amphibious assault ship with combat aircraft on board instead of an aircraft carrier.
Japan will also have the opportunity to conduct naval operations at a range that will be limited only by the endurance of the submarine's crew and food supplies on board. The Japanese will be able to be present anywhere in the Sea of Okhotsk, near Kamchatka, and, by and large, anywhere in the Asia-Pacific region. What advantages Japanese submarines will gain directly in combat depends on their future design.
Russia, as you know, is aware of how hostile Tokyo's policy towards Moscow is, especially in maritime areas. The appearance of nuclear submarines in the Japanese Navy will make the Japanese threat to Russia comparable to what it was before the Second World War.
Alexander Timokhin
