Japan Has Considered Nuclear-Powered Submarines Since 1958. Why Has It Never Built One?

Por Takahashi Kosuke — The Diplomat

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From a 1958 design study to reported approaches to the U.S. Navy, Japan has repeatedly examined nuclear-powered submarines without making an acquisition decision.

JS Sogei, the sixth Taigei-class diesel-electric submarine, entered the water in a ceremony held at KHI’s shipyard in Kobe City, Japan, Oct. 14, 2025.

Japan is once again considering acquiring nuclear-powered submarines . As Prime Minister Takaichi Sanae’s government prepares to revise its three key national security documents by the end of 2026, proposals have emerged to include further research into their acquisition. On October 14, Takaichi told parliament that her government would deepen its “concrete and realistic consideration” of the issue. Defense Minister Koizumi Shinjiro has said no options should be ruled out.

Yet Japan has been here before. Parliamentary records and contemporary newspaper reports reveal that nuclear-powered submarines have repeatedly been considered for nearly 70 years, without Japan ever acquiring one.

The earliest evidence of Japan’s consideration dates to May 1958, less than four years after the USS Nautilus, the world’s first nuclear-powered submarine, entered service. A preliminary design document bearing the name Kawasaki Heavy Industries examined a nuclear propulsion system for a 2,500-ton submarine.  The 1958 study concluded that the submarine concept was technically feasible but acknowledged that Japan could not independently manufacture its reactor and would need extensive U.S. technology.

The document’s existence was revealed in February 1974, when Japan Socialist Party lawmaker Narazaki Yanosuke questioned the government in parliament. The government and Kawasaki maintained that the document represented an individual engineer’s work rather than an official corporate development project. Narazaki challenged that explanation, but institutional involvement was never conclusively established.

In June 1964, a government-industry advisory group concluded that nuclear-powered submarines would provide the most effective means of countering foreign nuclear submarines’ superior underwater speed and endurance. However, legal obstacles remained.

In April 1965, then-Science and Technology Agency chief Aichi Kiichi stated that using nuclear propulsion aboard Self-Defense Forces vessels was impermissible under Japan’s Atomic Energy Basic Act because nuclear propulsion had not become generally established for civilian maritime use. At the same hearing, then-Defense Agency chief Koizumi Junya declared that Japan had no intention of building nuclear-powered submarines “at the present stage.”

On November 14, 2025, the Takaichi Cabinet reaffirmed that longstanding legal interpretation in a formal written response to a parliamentary question

The issue resurfaced in 1986. The Mainichi Shimbun reported that the Maritime Self-Defense Force had informally approached the U.S. Navy about acquiring nuclear-powered attack submarines. Japanese naval officers reportedly believed conventional submarines could no longer match U.S. nuclear submarines in speed and endurance.

According to a 2011 investigative report by the Sankei Shimbun, Defense Agency officials also secretly examined the possibility of developing nuclear-powered submarines domestically or acquiring them from the United States while preparing Japan’s 2004 defense guidelines. The idea was abandoned as premature.

Across these episodes, recurring obstacles included legal restrictions, technological challenges, substantial costs, and public opposition shaped partly by the 1974 radiation leak aboard Japan’s experimental nuclear-powered ship Mutsu.

Today’s strategic environment is markedly different.

China is expanding its naval presence across the western Pacific. The U.S. Department of Defense’s 2025 report on China’s military power assessed that Beijing aims to field nine aircraft carriers by 2035. China is already building a fourth carrier that is widely believed to be nuclear-powered.

North Korea has announced construction of a nuclear-powered submarine, while South Korea is pursuing its own acquisition plans .

Japan, meanwhile, is studying a next-generation submarine equipped with vertical launch systems (VLS) for long-range missiles. Incorporating VLS would require design modifications, potentially including a larger hull. Advanced sonar systems and unmanned underwater vehicles (UUVs) would further increase power demands. Conventional submarines, however, face limits on the electrical energy available while submerged, particularly because of their finite battery capacity.

Nevertheless, VLS does not necessarily require nuclear propulsion: South Korea already operates conventionally powered submarines equipped with such systems.

Nuclear propulsion could enable Japanese submarines to remain submerged for extended periods and maintain high underwater speeds, strengthening their ability to monitor Chinese naval movements and support deterrence.

But nuclear-powered submarines are no panacea. They entail major financial and technical burdens, while conventional submarines can offer acoustic advantages under certain conditions. Japan must also address the need for nuclear safeguards, specialized crew training, maintenance infrastructure, and public acceptance.

At a September government advisory meeting, one expert urged Tokyo to commit to acquisition, warning that reactor development and crew training would take a decade or more. Others argued that it was too early to decide.

In 1965, Koizumi Junya said Japan had no intention of building nuclear-powered submarines at that stage. Sixty-one years later, his grandson, Defense Minister Koizumi Shinjiro, says no options should be ruled out. But keeping an option open is not the same as making a decision. Another open-ended study risks repeating a familiar pattern: identifying the advantages, recognizing the obstacles, and postponing a decision.

Long development timelines justify timely decisions. They do not, however, establish which decision is correct. Japan needs a clear timetable, defined responsibilities, and a rigorous assessment of the legal, technological, financial, and operational questions. Deciding against acquisition after such an assessment would itself constitute a decision.

Nearly seven decades after the first documented study, Japan’s forthcoming security strategy revision will test whether Tokyo is finally prepared to move beyond perpetual consideration.

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Japan is once again considering acquiring nuclear-powered submarines . As Prime Minister Takaichi Sanae’s government prepares to revise its three key national security documents by the end of 2026, proposals have emerged to include further research into their acquisition. On October 14, Takaichi told parliament that her government would deepen its “concrete and realistic consideration” of the issue. Defense Minister Koizumi Shinjiro has said no options should be ruled out.

Yet Japan has been here before. Parliamentary records and contemporary newspaper reports reveal that nuclear-powered submarines have repeatedly been considered for nearly 70 years, without Japan ever acquiring one.

The earliest evidence of Japan’s consideration dates to May 1958, less than four years after the USS Nautilus, the world’s first nuclear-powered submarine, entered service. A preliminary design document bearing the name Kawasaki Heavy Industries examined a nuclear propulsion system for a 2,500-ton submarine.  The 1958 study concluded that the submarine concept was technically feasible but acknowledged that Japan could not independently manufacture its reactor and would need extensive U.S. technology.

The document’s existence was revealed in February 1974, when Japan Socialist Party lawmaker Narazaki Yanosuke questioned the government in parliament. The government and Kawasaki maintained that the document represented an individual engineer’s work rather than an official corporate development project. Narazaki challenged that explanation, but institutional involvement was never conclusively established.

In June 1964, a government-industry advisory group concluded that nuclear-powered submarines would provide the most effective means of countering foreign nuclear submarines’ superior underwater speed and endurance. However, legal obstacles remained.

In April 1965, then-Science and Technology Agency chief Aichi Kiichi stated that using nuclear propulsion aboard Self-Defense Forces vessels was impermissible under Japan’s Atomic Energy Basic Act because nuclear propulsion had not become generally established for civilian maritime use. At the same hearing, then-Defense Agency chief Koizumi Junya declared that Japan had no intention of building nuclear-powered submarines “at the present stage.”

On November 14, 2025, the Takaichi Cabinet reaffirmed that longstanding legal interpretation in a formal written response to a parliamentary question

The issue resurfaced in 1986. The Mainichi Shimbun reported that the Maritime Self-Defense Force had informally approached the U.S. Navy about acquiring nuclear-powered attack submarines. Japanese naval officers reportedly believed conventional submarines could no longer match U.S. nuclear submarines in speed and endurance.

According to a 2011 investigative report by the Sankei Shimbun, Defense Agency officials also secretly examined the possibility of developing nuclear-powered submarines domestically or acquiring them from the United States while preparing Japan’s 2004 defense guidelines. The idea was abandoned as premature.

Across these episodes, recurring obstacles included legal restrictions, technological challenges, substantial costs, and public opposition shaped partly by the 1974 radiation leak aboard Japan’s experimental nuclear-powered ship Mutsu.

Today’s strategic environment is markedly different.

China is expanding its naval presence across the western Pacific. The U.S. Department of Defense’s 2025 report on China’s military power assessed that Beijing aims to field nine aircraft carriers by 2035. China is already building a fourth carrier that is widely believed to be nuclear-powered.

North Korea has announced construction of a nuclear-powered submarine, while South Korea is pursuing its own acquisition plans .

Japan, meanwhile, is studying a next-generation submarine equipped with vertical launch systems (VLS) for long-range missiles. Incorporating VLS would require design modifications, potentially including a larger hull. Advanced sonar systems and unmanned underwater vehicles (UUVs) would further increase power demands. Conventional submarines, however, face limits on the electrical energy available while submerged, particularly because of their finite battery capacity.

Nevertheless, VLS does not necessarily require nuclear propulsion: South Korea already operates conventionally powered submarines equipped with such systems.

Nuclear propulsion could enable Japanese submarines to remain submerged for extended periods and maintain high underwater speeds, strengthening their ability to monitor Chinese naval movements and support deterrence.

But nuclear-powered submarines are no panacea. They entail major financial and technical burdens, while conventional submarines can offer acoustic advantages under certain conditions. Japan must also address the need for nuclear safeguards, specialized crew training, maintenance infrastructure, and public acceptance.

At a September government advisory meeting, one expert urged Tokyo to commit to acquisition, warning that reactor development and crew training would take a decade or more. Others argued that it was too early to decide.

In 1965, Koizumi Junya said Japan had no intention of building nuclear-powered submarines at that stage. Sixty-one years later, his grandson, Defense Minister Koizumi Shinjiro, says no options should be ruled out. But keeping an option open is not the same as making a decision. Another open-ended study risks repeating a familiar pattern: identifying the advantages, recognizing the obstacles, and postponing a decision.

Long development timelines justify timely decisions. They do not, however, establish which decision is correct. Japan needs a clear timetable, defined responsibilities, and a rigorous assessment of the legal, technological, financial, and operational questions. Deciding against acquisition after such an assessment would itself constitute a decision.

Nearly seven decades after the first documented study, Japan’s forthcoming security strategy revision will test whether Tokyo is finally prepared to move beyond perpetual consideration.

Takahashi Kosuke is Tokyo Correspondent for The Diplomat.

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Fonte: The Diplomat