2017 South Korea sought Submarine Reactors from US, RUSSIA.

In Submarine Matters’ “South Korean and Japanese Nuclear Submarine Propulsion” October 21, 2019 Pete assessed that South Korea saw its land based SMART nuclear reactor as a technological dead-end for submarine use.

Building submarine reactors is the most expensive and technologically challenging part of building nuclear submarines. Hence better to buy than reinvent. 

South Korean politicians, officials and scientists appear prepared to cross longstanding alliance lines in seeking reactors from Russia. 

US

Professor Frank von Hippel provides details in his excellent academic paper titled “Mitigating the Threat of Nuclear-Weapon Proliferation via Nuclear-Submarine Programs” Pages 133-150, Published June 27, 2019 at Taylor Francis Online https://www.tandfonline.com/doi/full/10.1080/25751654.2019.1625504. Here is a paragraph with references:

[von Hippel] “South Korea’s president, Moon Jae-in, has declared an interest in building or buying nuclear attack submarines. During US President Donald Trump’s November 2017 visit to Seoul, the two leaders reportedly discussed the possibility of South Korea purchasing a US nuclear attack submarine.”

Further details see “Nuclear-Powered Subs, JSTARS Make South Korea’s Wish List” Defense News, November 8, 2017 at https://www.defensenews.com/global/asia-pacific/2017/11/08/nuclear-powered-subs-jstars-make-south-koreas-wish-list/ 

"Moon had indicated an interest in nuclear-powered submarines even before his election in May, and the topic was reportedly discussed during his meeting with Trump. However, details of those discussions were not revealed, and the Trump administration has not indicated that it is open to that idea.

South Korea’s interest in nuclear-powered submarines has been stoked by North Korea’s development of ballistic missile submarines capable of firing the KN-11, also known as the Pukguksong-1, submarine-launched ballistic missile, which has undergone testing during the North’s recent spate of missile tests.

... Should South Korea acquire nuclear-powered attack submarines, either through an unlikely direct purchase from the U.S. or more likely through the development and production of an indigenous design with U.S. technical assistance, it would represent a significant shift in the country’s defense posture and stoke concerns with nuclear proliferation on the Korean Peninsula.

But significant obstacles to these ambitions remain. South Korea is a signatory to the Treaty on the Non-Proliferation of Nuclear Weapons, and the Asian nation is also prohibited from enriching uranium and reprocessing spent fuel for military purposes under a deal signed with the U.S. in 2015. The U.S. has also not sold nuclear-powered submarines and has not transferred such technology even to allies [Pete - Except to the UK for decades – see https://en.wikipedia.org/wiki/1958_US%E2%80%93UK_Mutual_Defence_Agreement] and it is likely to loath doing so out of nonproliferation concerns.

[Pete Comment - US submarine reactors use “Bomb Grade” 93% to 97% U-235, another reason for the sensitivity of their design and export. See page 20 of https://www.hsfk.de/fileadmin/HSFK/hsfk_downloads/prif124.pdf]

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RUSSIA

[von Hippel continues] Russia’s premier designer of naval propulsion reactors, OKBM Afrikantov, has made public the fact that, in 2017, it had discussions with the Korea Atomic Energy Research Institute on the possibility of providing the design of a new Russian icebreaker reactor as a “reference design” for a South Korean “maritime propulsion” reactor.”
[endnote 9 "An email communication to the author [Frank von Hippel] from a South Korean government official on 9 August 2018 stated that the Moon administration was no longer actively pursuing the idea of a nuclear-powered attack submarine."]

Further details see “Russia May Help South Korea to Build Nuclear Reactor for Maritime Vessels.” Sputnik International, August 7, 2018 at https://sputniknews.com/asia/201808071067008274-russia-south-korea-nuclear-reactor/ 

MOSCOW (Sputnik) - Russia may help South Korea to build a ready-to-operate nuclear power plant for maritime propulsion using the RITM-200 reactor as a basis for its design, JSC Afrikantov OKBM, a subsidiary of Rosatom nuclear corporation, said in a 2017 annual report made public on Tuesday.

"In November 2017, during a meeting in Nizhny Novgorod [Russia], representatives from the Korean Atomic Energy Research Institute [KAERI] said that the Korea is considering the possibility of developing a concept for a new reactor with a capacity of about 150-200 MW for sea-faring vessels and is viewing the RITM-200 reactor as a possible reference design," the report says.

According to the report, Moscow expressed readiness to cooperate with Seoul in the production of a turnkey reactor plant after receiving an official request for cooperation from South Korea and obtaining appropriate approval from the Rosatom State Nuclear Corporation.

JSC Afrikantov OKBM is the main developer of nuclear reactor cores for Russian naval vessels."

[von Hippel continues] “The Russian icebreaker reactor, the RITM-200, was designed to use LEU fuel,”

Further details see IAEA. 2016a 180 “Advances in Small Modular Reactor Technology Developments, A Supplement To: IAEA Advanced Reactors Information System (ARIS).” at https://aris.iaea.org/Publications/SMR-Book_2016.pdf.

[von Hippel continues] “but apparently will use HEU fuel.”

Further details see “New Russian Icebreakers Will Use HEU Fuel.” International Panel on Fissile Materials, November 29, 2017 http://fissilematerials.org/blog/2017/11/new_russian_icebreakers_w.html.

Despite the earlier plans to use LEU fuel in reactors that will power its new icebreakers, Russia has apparently decided to develop HEU fuel for these reactors. The "integrated propulsion unit" with a nuclear reactor, known as RITM-200 [new link added by Pete] was developed by the Afrikantov OKBM design bureau in Nizhniy Novgorod. The unit will be installed on icebreakers of the Project 22220 series (referred to as UAL, Universal Atomic Icebreaker, in Rosatom documents). The lead ship of this class, Arktika, is expected to enter service in 2018, followed by two more ships, Sibir and Ural...”

Frank von Hippel [another reference] is a Senior Research Physicist and Professor of Public and International Affairs emeritus at Princeton University’s Program on Science and Global Security. He was a founding co-chair of the Program and of the International Panel on Fissile Materials. During 1993–94, he served as Assistant Director for National Security in the White House Office of Science and Technology Policy.