South Korean nuclear submarines: Post APEC, ergo propter APEC

The 2025 meeting of APEC (Asia Pacific Economic Cooperation) has ended. Gyeongju served as a charming backdrop for discussion among leaders from 21 economies representing 35 percent of the global population, 50 percent of global trade and 60 percent of global GDP. Pomp and pageantry burnished South Korean President Lee Jae Myung’s reputation domestically and abroad. South Korea, as host of this year’s APEC, did a creditable job finding consensus on a joint declaration that advocates open trade, underlines the economic importance of cultural and creative industries, and focuses attention on artificial intelligence and demographic challenges.

But make no mistake — the multilateralism of APEC proper was overshadowed by the bilateral meetings that took place before and after. Trump met Xi. Trump met Lee. Xi met Lee. Lee met Takaichi. Takaichi met Xi. And so on. The world was relieved that Trump and Xi seemingly put a floor — even if tenuous — on the U.S.-China relationship. The U.S. and South Korea agreed to some headline details on the investment part of their new trade arrangement. Seoul managed to negotiate down its upfront cash investment into the U.S. (on average, $20 billion annually for ten years), get automobile tariffs at the same rate as Japan and Europe (15 percent), and reduce chip tariffs to the same level as Taiwan.

Those outcomes are meaningful, but what has seized imaginations is the agreement by President Trump that South Korea can have nuclear-powered submarines. This is a big deal, but questions abound.

First, how deeply and comprehensively was this agreement discussed prior to the Trump-Lee bilateral? This is not something that would need to be asked of a normal U.S. administration, but Trump 2.0 is deeply abnormal. In fact, I suspect that Seoul-Washington discussions of this issue were nugatory, which means that there is a huge amount of uncertainty regarding the prospects for establishing functional South Korean nuclear naval propulsion with the U.S.' blessing. Depending on the details, laws and regulations governing nuclear security would have to be passed, changed, or amended in the U.S. The watchword here for some commenters has been the U.S.-South Korea 123 Agreement, but that's only part of a much broader legal framework. There are many obstacles — both legal and political — that could easily undermine or invalidate Washington’s approval for these submarines. For Seoul's part, it would also have to create a legal structure for this endeavor, an area in which it has no experience.

Second, does South Korea really want or need nuclear-powered submarines, or is this a Trojan horse to get U.S. permission to close the nuclear fuel cycle (i.e., acquire nuclear fuel reprocessing capabilities to produce plutonium) and enrich uranium? The lead-up to the Gyeongju meeting between Trump and Lee featured cryptic mention of revising the U.S.-South Korea civil nuclear cooperation agreement, but nuclear-powered submarines did not seem to be on the agenda. South Korea has occasionally argued for nuclear-powered submarines for decades, and even done some relevant research, but the military logic is unclear and the cost (and thus trade-offs) substantial. Perhaps the Lee administration used the argument that South Korea armed with nuclear-powered submarines would be an asset to U.S. strategic interests in a regional security (i.e., China-related) crisis, and therefore needs (to fuel the boats) a closed fuel cycle and uranium enrichment capabilities, which the U.S. would otherwise be unwilling to grant. If the nuclear submarines were indeed such a Trojan horse, that would further open questions about how seriously South Korea has thought through the implications of this potential deal.

Third, there are numerous technical questions about the potential South Korean nuclear submarine acquisition.

What kind(s) of submarines does South Korea intend to have? Attack submarines? Ballistic missile submarines? Both? How many submarines? Over what timeline? To what strategic end does it want nuclear-powered submarines? Countering North Korea in the proximate waters of Northeast Asia? The relative military value of nuclear-powered submarines in this context is questionable. Countering China in the broader western Pacific (e.g., the East and South China Seas)? Nuclear-powered submarines would be undoubtedly militarily valuable for such missions, but is this a wise strategic choice?

Arguably, the trickiest technical-legal questions involve fundamental procurement decisions. What type of reactor and fuel would drive the submarine? Highly enriched uranium (HEU), as is the case with the U.S.? The reactors don’t require refueling (lowering proliferation risk), but also use HEU, which is a proliferation risk. Low enriched uranium (LEU), as employed by France? The reactor has to be refueled periodically, but LEU poses less proliferation risk per se.

The "what reactor and fuel" question may ultimately depend on the question of whose reactor and fuel would be used, and where the boat would be built. If the deal is primarily technology transfer, with a boat using HEU in a U.S.-designed reactor, then presumably production would be in the U.S. Trump has already indicated this. This would be a sort of Korean-style AUKUS, but U.S. shipyards are already maxed out, and this option would be less likely to secure reprocessing or uranium enrichment, as Seoul seemingly wants. South Korea also wants to maximize its part of development and production, including both the reactor and fuel type. The likely path forward for that involves construction partially taking place in the U.S. and partially in South Korea. Would this arrangement intertwine U.S. and South Korean defense-industrial interests sufficiently to ensure project sustainability?

The answer to all of the above questions is opaque at the moment. Only time will tell if APEC 2025 was the start of a new nuclear era for South Korea.

Mason Richey is a professor of international politics at Hankuk University of Foreign Studies, president of the Korea International Studies Association and editor-in-chief of the Journal of East Asian Affairs.