South Korea Exploiting "Russia Assisting North Korean Nuclear Sub" Rumour: Japan?

South Korean news website daily Korea Joong Ang on September 17, 2025 reported here :

"Russia may have supplied North with nuclear reactor, South's military says"

South Korea’s military has obtained intelligence suggesting that Russia supplied North Korea with a nuclear reactor for use in nuclear-powered submarines, and is currently working to verify the report. If confirmed, the action will have major repercussions for Seoul-Moscow relations as well as the global security order.
 
Russia is believed to have handed over two to three nuclear submarine modules to North Korea in the first half of this year, according to multiple government officials on [September 16, 2025].

These modules include the reactor, turbine and cooling system — the core components of a nuclear propulsion unit. This raises the possibility that North Korea received an entire propulsion system, including a functioning reactor. The modules were reportedly not newly manufactured but taken from decommissioned Russian submarines..."

North Korean leader Kim Jong-un inspects what appears to be a nuclear-powered strategic missile submarine [or a very large pipe] under construction at a facility in North Korea on March 8, 2025. [Photo (or animation?) from an unknown photographer of NK's official newspaper Rodong Sinmun. See when first published at https://www.nknews.org/2025/03/north-korea-releases-first-photos-of-new-nuclear-powered-missile-submarine/ ]
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SHAWN COMMENT 

The decommissioned Russian modules will likely be used for North Korean (NK) nuke engineers to learn from, but we don’t know the full story - the modules could be unused components from the Amur shipyards, or from submarines scrapped in the Russian Far East, so NK could get a second hand parts kit and assemble something that works.

India returned the Chakra II/Nerpa in 2021, and she sat dockside for at least three years, so she could provide ‘scrap’ parts:
https://defence.in/threads/mystery-surrounds-future-of-ins-chakra-ii-as-satellite-imagery-reveals-it-sits-idle-at-russian-naval-base.11835/

South Korea now has a commercial SMR design, and could build an LNG carrier in the near future as a prototype. This has obvious implications to submarine naval reactors.

https://www.world-nuclear-news.org/articles/korean-designed-nuclear-powered-lng-carrier-certified

It took me a bit of searching, but I found that Samsung Heavy Industries signed a partnership with Seaborg to develop power station barges with 200-800 MWe power generation. https://enmobile.prnasia.com/releases/apac/korea-hydro-nuclear-power-samsung-heavy-industries-and-seaborg-technologies-form-consortium-to-develop-cmsr-based-floating-nuclear-power-plants-401006.shtml  

PETE COMMENT

I don't know whether "modules...comprising reactor cores, turbines and cooling systems removed from decommissioned Russian submarines." would truly be a modular concept - given all the electronic propulsion and hotel load control systems required, which would extend into the mid-front command centre half of a nuclear sub.

Also NK's ability to reverse engineer, retrofit, repair or upgrade decades old nuclear powered systems into an old or new build SSBN is doubtful.

However, this intelligence might be advantageous for South Korea (SK) given SK has been pressing the US for years to be permitted to develop nuclear powered SSNs and especially SSBNs. See https://gentleseas.blogspot.com/2020/10/leu-more-acceptable-for-south-korean.html

Given AUKUS now permits the US and UK to sell submarine HEU reactors to Australia (and if there is any truth in old Russian submarine reactors for NK) then France selling submarine K15 LEU reactors to SK might be increasingly equitable in proliferation terms.

Yes, there are possibilities NK could use old reactor parts.

But NK would face a new level of complexity in reverse engineering, retrofitting, repairing or upgrading decades old submarine reactors.

Thanks for https://defence.in/threads/mystery-surrounds-future-of-ins-chakra-ii-as-satellite-imagery-reveals-it-sits-idle-at-russian-naval-base.11835/ . 

Old or refurbished Russian Akula-class SSNs or Delta-class SSBNs https://en.wikipedia.org/wiki/Delta-class_submarine or their reactors are possibilities for gifting/selling to North Korea .

Maybe the transfers would come with Russian technicians and shipyard workers to help North Korea do the NK-SSBN upgrades/repairs.

Thanks for https://www.world-nuclear-news.org/articles/korean-designed-nuclear-powered-lng-carrier-certified  If the reactor they mention is 30 to 75 MW (ELECTRICAL) then that may be sufficient for a South Korea SSBN.

In 2020 I published on South Korea's small "SMART reactor", DSME's civilian "Nuclear Propulsion Ship" proposal and the SK submarine reactor concept - see https://gentleseas.blogspot.com/2020/08/s-korean-japanese-nuclear-submarine.html . The future KSS-III Batch 3 is likely already a paper and computer generated design as nuclear powered.

SK may remain interested in France’s K15 naval reactor, used on Suffren-class SSNs. The K15 is rated at 150 MW THERMAL (MWt) while its MWe is classified (maybe 150 x 0.20 = 30 MWe).

SK has competed against its "frenemy" Japan on nuclear innovations for decades. Japan operated the failed (due to anti-nuclear protests) "Mutsu" nuclear powered ship between the 1970s-90s https://gentleseas.blogspot.com/2016/06/japanese-nuclear-propulsion-1-mutsu.html

If South Korea developed a nuclear propelled nuclear missile submarine (SSBN) Japan would likely respond by developing its own SSBN within 5 years.