Asia Defense | Security | East Asia

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

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........

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