In a move that marks a major shift in both U.S. defense cooperation and South Korea’s military ambitions, President Donald Trump announced on October 29 that the U.S. government has granted approval for South Korea to construct nuclear-powered submarines on American soil. The decision was revealed after Trump’s meeting with South Korean President Lee Jae-Myung and other Asian leaders in Gyeongju, South Korea.
According to posts made later by President Trump on his Truth Social account, the submarines will be built at the historic Philadelphia Shipyards, which were acquired by South Korea’s Hanwha Group in late 2024. Hanwha, one of South Korea’s leading defense and aerospace conglomerates, is expected to spearhead the modernization and production of these advanced submarines—an unprecedented venture for both nations.
“Our military alliance is stronger than ever before, and based on that, I have given them approval to build a nuclear-powered submarine,” Trump wrote.
The announcement represents a turning point for South Korea’s long-standing ambition to develop nuclear-powered submarines. Until now, the Republic of Korea Navy (ROKN) has relied solely on conventionally powered submarines, including the KSS-I, KSS-II, and KSS-III classes. Efforts to develop nuclear propulsion had been stalled for years due to Washington’s reluctance to approve such projects—largely out of concern for regional proliferation risks and the U.S.-ROK nuclear cooperation framework.
Trump’s decision effectively ends that deadlock. Yet, significant logistical and technical hurdles remain. The Philadelphia Shipyards, which have primarily produced commercial vessels, are not currently equipped for nuclear submarine construction. In anticipation of this challenge, Hanwha has reportedly committed an additional $5 billion to upgrade the facilities to meet stringent nuclear safety and production standards.
Despite the bold announcement, reports indicate that there is still no finalized, signed agreement between Washington and Seoul outlining the precise framework for construction, technology transfer, or operational collaboration.
South Korea’s push for nuclear-powered submarines stems from strategic necessity as much as technological ambition. Nuclear propulsion provides virtually unlimited underwater endurance, allowing submarines to remain submerged for months without surfacing—a crucial capability for maintaining deterrence and persistent surveillance amid rising regional tensions.
North Korea’s growing submarine-launched ballistic missile (SLBM) program, combined with expanding Chinese and Russian naval activity in the East China Sea and Sea of Japan, has added urgency to Seoul’s pursuit.
At a recent National Assembly Defense Committee hearing on October 30, Chief of Naval Operations Adm. Kang Dong-gil stated:
“The start date for the nuclear-powered submarine program has not yet been decided, but once it begins, it will take more than 10 years. Its displacement is expected to exceed 5,000 tons.”
Similarly, Defense Acquisition Program Administration (DAPA) Commissioner Seok Jong-geon noted that, based on international experience, the construction of a nuclear-powered submarine typically takes about a decade. However, he suggested that South Korea’s industrial base could shorten this timeline through collaboration with U.S. shipyards and expertise.
Defense Minister Ahn Kyu-baek added that the eventual acquisition would likely involve at least four submarines, though the final number would be determined through further consultation with the Navy and budgetary assessments.
Currently, the Republic of Korea Navy operates three distinct classes of conventionally powered submarines:
KSS-I (Jang Bogo-class) – Based on the German Type 209 design, with nine boats in service.
KSS-II (Sohn Wonyil-class) – Derived from the German Type 214, also with nine boats in service.
KSS-III (Dosan Ahn Changho-class) – The first fully indigenous design, featuring advanced combat systems, greater displacement, and vertical launch systems for cruise missiles.
The KSS-III represents a major technological leap for South Korea. Batch I submarines displace around 3,750 tons submerged and carry six K-VLS (Korean Vertical Launch System) cells for land-attack cruise missiles. The newly launched Batch II variant will feature ten VLS cells and enhanced propulsion and acoustic performance.
Naval analysts have long speculated that the KSS-III design could serve as a foundation for a nuclear-powered derivative. Such a platform would offer longer range, faster sustained speeds, and greater power generation for advanced sonar and weapons systems—qualities that would significantly boost South Korea’s undersea deterrent capabilities.
The decision to allow construction on U.S. soil underscores the evolving nature of the U.S.-South Korea alliance, blending industrial cooperation with strategic deterrence in a way that benefits both sides. For Washington, the move strengthens its Indo-Pacific posture amid mounting great-power competition. For Seoul, it represents a historic step toward self-reliant defense and parity with other advanced naval powers such as the United States, the United Kingdom, and France.
Still, the plan faces hurdles—ranging from political opposition in both countries to regulatory and nonproliferation concerns. Nuclear submarine construction remains among the most complex and secretive undertakings in defense technology.
