Japanese Nuclear Propulsion 1 - The Mutsu Episode

Mutsu photo (Courtesy http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/tayu/ACT95E/06/0601.htm)

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Mutsu Diagram (Courtesy) 

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S has located a chapter in Japan's history that is surprising. That Japan developed and utilised "dual-use" nuclear reactor propulsion to power the ship Mutsu (see Wikipedia and IAEA) in the 1970s to 1992.

In Comments in mid June 2016 S identified Japan’s one public example of a nuclear propulsion for a ship.

Japan's Atomic Energy Agency (JAEA) built a nuclear propelled ship, the Mutsu, nearly 50 years (1968-1970). Mutsu's had a 36 MW Mitsubishi pressurised water reactor (PWR). The reactor was completed in August 1972. Low enriched (less than 20% U235) Uranium was loaded into the reactor in September 1972.

Minor radiation (neutrons and gamma rays) emanated from Mutsu's reactor during workup assessments, at sea, in September 1974. This was misinterpreted by the Japanese press as a leak of radioactive substances. The ray "leakage" was only equivalent to twice as much as the usual "radiation" from an old-style television picture tube.

With this press coverage the Mutsu's future became a high profile anti-nuclar issue. Local fisherman blocked Mutsu's return to her home port for more than 50 days.

“The Japanese Government finally came to an agreement with the local government and fishermen. The Mutsu was allowed back to port on condition that it was to find a new home port, and the ship returned to Ōminato on 15 October.

In Sasebo, between 1978 and 1982, various modifications were made to the reactor shield of the Mutsu, and its home port was moved to Sekinehama in 1983. Following an overhaul, the Mutsu was completed in February 1991. She then completed her original design objective of travelling 82,000 kilometres in testing, and was decommissioned in 1992. Over 25 years the programme had cost (about US$ 1.2 billion).” [quite a modest sum for reactor development.]

But Mutsu had a second life - after its reactor was removed in 1995 it was renamed the (non-nuclear) Mirai.

During and after the Mutsu experience Japan's Atomic Energy Agency has not planned, built or purchased a nuclear propelled ship.

S Comments

Building nuclear submarines is technically possible for Japan and many in the Japanese Navy (JMSDF) would be happy having Japanese nuclear propelled submarines. But public and political anti-nuclear sensitivities in general and memories of the Mutsu, have blocked any serious thought of Japanese nuclear propelled submarines. Even if the Japanese Navy built nuclear submarines, no local governor of a Japanese prefecture would permit such a submarine to enter a port. 

Pete's Comment

This is also noting that Japan's major submarine bases are Kure (in Hiroshima Prefecture) and Yokosuka (near extremely populated Yokohama-Tokyo). It is odd that US nuclear propelled submarines and the US Seventh Fleet supercarrier share the Yokosuka base - but that is a nuclear political paradox.

An isolated naval base in Japan would probably need to be built to permanently base Japanese nuclear submarines. Australia has a similar problem in that Fleet Base East (Sydney Harbour) could not serve as the number two base (including emergency porting) for any future Australian nuclear submarines.

The military value of a Japanese civilian ship reactor is that it represents a dual-use technological testbed for future miniaturisation and increase in power output (100 MW would be a submarine minimum). This is a necessary preliminary for a mature submarine reactor. Though the French Rubis class SSN had a 48 MW reactor - that was for an unusually small 2,400 tonne SSN with only 14 x heavyweight weapon shots.

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Next week S and Pete will complete Japanese Nuclear Propulsion 2 - further down the technological track towards a Japanese submarine reactor capability.

S and Pete

Japan Probably Has Nuclear Submarine and Weapons Know-How

	First Thermonuclear Test	First Submarine Reactors (from when submarine launched)	1st Ship reactor (built or went critical)
United States	1952 Ivy Mike	1954 USS Nautilus	1961 USS Enterprise 1962 Savannah
Soviet/Russia	1953 Joe 4	1957 November class K-3 Leninsky Komsomol	1959 Lenin
Britain	1956-58 Grapple	1960 HMS Dreadnought
France	1968 Canopus	1967 Redoutable
China	1967 Test No. 6	1970 Type 091 (401)
India	1968 Shakti I  (H-Bomb?)	2009 INS Arihant
Israel	1968 Canopus (shared France-Israel test assumed)	None. Dolphin SSKs used.
Germany			1968 Otto Hahn
Japan			1972 Mutsu

In the table above there is a pattern. Nuclear weapons powers achieved thermonuclear (H-Bomb) tests first, then they develop a submarine reactor. This is with the exception of Israel which is too small to build nuclear subs. 

This may reveal the higher priority given to nuclear weapons. Also it is of little use launching an SSBN if it has no compact thermonuclear warheads to place on ballistic missiles.

The ability of Germany and Japan to develop ship reactors may indicate they have sufficient nuclear know-how to develop submarine reactors and thermonuclear weapons. 

Brazil has not yet launched a submarine reactor but is considered capable of building nuclear weapons fairly quickly. 

Various types of information sharing has been known to happen between the 5 official nuclear powers.

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JAPANESE AND SOUTH KOREAN NUCLEAR SHIPS - NO OFFICIAL PROGRESS

The Japanese Atomic Energy Agency (JAEA) has published reports on propulsion reactors but has not built any since the Mutsu nuclear propelled cargo ship in the 1970s. Japan is not currently planning development of a nuclear submarine.

Plans exist for a very small nuclear reactor known as Deep sea Reactor X (DRX) for a Japanese deep submergence vehicle - see https://www.tandfonline.com/doi/abs/10.13182/NT94-A34996  . 

A Japan Atomic Energy Research Institute (JAERI) reactor known as "MRX" that was intended to be 3 times more powerful than the Mutsu's reactor. MRX was being researched, from the 1990s for use in a far larger ship. 

[Pete tracked down "ADVANCED MARINE REACTOR MRX AND ITS APPLICATION FOR ELECTRICITY AND HEAT CO-GENERATION" at http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/31/058/31058473.pdf published in  2000. It was envisaged:

"The basic concept of an innovative advanced marine reactor MRX has been established by design study toward the goals of light-weightiness, compactness, and safety and reliability improvement with adoption of several new technologies. The MRX is the integral-type PWR aimed for use of ship propulsion. Adoption of a water-filled containment makes the reactor light-weighted and compact greatly. The total weight and volume of the reactor are 1600 tons and 1210 m3, which are equivalent to [half the weight of Mutsu's 36 MWth reactor?] , although the reactor power of MRX is three times greater."

With power output of MRX calculated as 300 MWth (again see http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/31/058/31058473.pdf )

PETE COMMENT

Once MRX is steadily miniaturised its electrical output of 30 MWe-100 MWe may be well suited  to a Japanese SSBN or SSN.]

Factors that may cause Japan to research (or perhaps re-research) submarine reactors may be competition with "frenemy" South Korea or greater strategic threats from China, Russia and North Korea. Also US isolationism may encourage more Japanese research.

Set against Japanese submarine reactor development:

-  Japan's strategic threats are close (just a land based MRBM flight across the Sea of Japan) making the marginal utility low for fast moving, long range, Japanese nuclear propelled submarines.  

-  Japan may place a higher priority on developing the necessary dual (civilian-military) use precursor technology for nuclear weapons for quick conversion to a nuclear weapon capability.. 

Russia already has highly developed submarine reactors but China apparently has not built fully satisfactory, efficient and quiet submarine reactors. North Korea probably cannot afford submarine reactors but North Korea is continually surprising.

While South Korea is not a strategic threat to Japan there is "frenemy" rivalry. South Korea studied (probably still studying) the possibility of submarine reactors under various names like KSS-N or KSS-III (nuclear) for decades.  

If South Korean  Hanwha Ocean's dual-use "Nuclear Propulsion Ship" concept.  

South Korea's - Korea Atomic Energy Research Institute (KAERI) might modify its licensed 100 MWe  "SMART" small PWR reactor which has ample power output for a nuclear sub.

Possible declining support from the US (in the shape of another Trump Presidency) may instil a feeling and fact of strategic isolation in Japan and South Korea. This may prompt Japan and South Korea to look at nuclear options, including submarine propulsion and nuclear weapons.

Pete and S

Retrofitting Reactors into SSKs Unlikely: Molten Salt Out

Hi Shawn re your June 1, 2025 comment:

South Korea's (SK's) KSS-III Batch III might turn out to be nuclear powered - a true SSBN or SSN-SSBN hybrid. Various countries, like France with its K-15 reactor could help SK. SK has also experimented and built reactors for decades and considered responses to Japan's Mutsu nuclear powered ship https://gentleseas.blogspot.com/2016/06/japanese-nuclear-propulsion-1-mutsu.html

Judging by public sources and Fleet Base West's Indian Ocean positioning Australia's SSKs rarely visit NZ or South West Pacific islands. Further north of the Sydney base may be more likely.

Due to differing SSK vs SSN electrical fitouts, differing balance/buoyancies and in fact far differing internal arrangements retrofitting even a small reactor into an operational SSK has never occurred, to my knowledge. I think a French Navy non-operational, Daphne-class SSK was used as a reactor testbed (?) in the 1960s-1970s prior to the launch of the first Rubis class SSN in 1979. https://www.lynceans.org/wp-content/uploads/2015/09/Part-4_UK-France-Others-60-yrs-of-marine-nuc-power.pdf page 56 onwards, may provide clues.

Retrofitting an intentionally small, hence low power, small modular reactor (SMR) makes little sense particularly relying on an SMR with molten salt coolant.

The US experimented with an earlier USS Seawolf (SSN-575) (built in the 1950s) being the only US submarine built with a sodium-cooled reactor. That Seawolf's S2G reactor was a liquid metal (sodium) cooled, beryllium-moderated nuclear reactor. The Seawolf's sodium-cooled reactor faced challenges, including leaks in the steam generators and performance issues. Its was later replaced with a standard PWR. See https://en.wikipedia.org/wiki/USS_Seawolf_(SSN-575)#Comparison_to_Nautilus

The USSR also attempted molten salt in its Alfa-class SSNs https://en.wikipedia.org/wiki/Alfa-class_submarine#Propulsion with negative results. In emergency shut downs molten salt reactors freeze - preventing restarts - requiring reactor replacements. Pier side heating was essential.

I regrettably reckon there is little SSK to SSN retrofit or alternative reactor coolant flexibility. Otherwise someone would have tried retrofitting SSK to SSN.

Too little is known about the possible Wuhan mini-SSN to draw conclusions.
https://www.orfonline.org/expert-speak/subsurface-setbacks-china-s-submarine-accident-in-wuhan

Cheers Pete

Heightened US ITAR Regs Blocking AUKUS: Also SK, NK, Japan, Canadian & Singaporean nuclear issues

In response to Shawn C’s comment of March 10, 2025, I (Pete) say:

Very true and disturbing about the possibility the US may retrospectively heighten ITAR regulations to block Australian access to US and US (via the UK) AUKUS technology. That may see the US reneging on AUKUS Pillar One (Virginia SSNs) and even Pillar Two "AI, cybersecurity and Quantum computing" including US Anduril and Australia DoD/RAN developed GHOST SHARK XLUUVs. 

Also the US could force the UK to block Australian access to US content/inventions the US passed to the UK eg. in UK reactors for SSN-AUKUS submarines intended for Australian in the late 2030s to 2050s.

I've been looking at the possibility of South Korea (SK) developing a nuclear powered submarine (variously called KSS-N, KSSN and KSSX-N) in several articles since 2012. See https://gentleseas.blogspot.com/search?q=kss-n  which yields:

"South Korean...Nuclear KSSX-N Option"
of Feb 22, 2012 at https://gentleseas.blogspot.com/2009/09/south-korea-has-bought-six-more-u-214.html

and

"South Korean Submarines, 3,000+ ton KSS-III, Nuclear Potential"
of 16 April 2015 at
https://gentleseas.blogspot.com/2015/04/south-korean-submarines-3000-ton-kss.html

What I suspect is currently an NK “nuclear submarine” mock-up or animation at http://www.hisutton.com/North-Korea-Nuclear-Submarine-OSINT.html  may partly be NK responding one better to South Kora’s https://www.world-nuclear-news.org/articles/korean-smr-powered-container-ship-design-revealed

The US has always actively prevented Canada from obtaining SSNs – see https://en.wikipedia.org/wiki/Canada-class_submarine#American_opposition . This is due to US-Canadian national competition regarding Arctic sea lanes and resources and the US desire to retain its nuclear submarine ownership monopoly in “America’s Western Hemisphere”. Warming of arctic waters, freeing up shipping and submarine access, will only increase US determination to be the SSN monopolist of the Western Hemisphere.

Delays in SK being a potential SSN supplier to Australia includes the 20-40 years SK might take to produce a stealthy SSN design. It has taken the super or great nuclear powers decades to develop quiet SSNs.

Also SK (if unprotected by the unreliable US) is extremely vulnerable to land invasion or "nuking" by neighboring NK, China or Russia. Unlike France, SK has no nuclear deterrent to defend its future SSN shipyard and no current second generation Suffren class SSN already in the water.

Without the US protecting SK, SK's nuclear armed neighbours might take extreme steps to stop SK developing nuclear weapons, SSBNs or SSNs. The same goes for Japan. Japan modifying its 1970s-90s Mutsu nuclear ship reactor into  a submarine reactor https://gentleseas.blogspot.com/search?q=mutsu might be violently stopped by Japan’s authoritarian neighbours.

Interesting about Singapore's SMR studies. https://www.newcivilengineer.com/latest/case-made-for-smrs-to-be-built-underground-to-protect-from-military-and-natural-threats-25-11-2024/ of Nov 12, 2024:

“Small modular reactors (SMRs) should be built underground, including in city centres, to protect them from military attacks, seismic activity and other natural hazards, according to a new academic study. The recommendations come from a paper written by Nanyang Technological University (NTU), Singapore, S. Rajaratnam School of International Studies (RSIS) senior fellow Alvin Chew. Academy of Engineering Singapore fellow and International Society for Rock Mechanics and Engineering (ISRM) fellow Zhou Yingxin co-authored the paper.”

South Korean and Japanese Nuclear Submarine Propulsion

South Korea (SK) has carried out research on building a nuclear powered submarine (aka KSS-N, KSSX-N and SSX) since the 2003 (codename) “362 initiative” if not longer ago. But that 362 initiative was officially (perhaps not actually) terminated in 2004 once it became public - under ( likely) US, Chinese, Japanese and Russian, pressure.

The lead time and technical hurdles for SK to develop a submarine reactor, and specialised submarine to use it, are huge. Only the US, Russia, China and France (with much larger defense budgets than SK) have fully designed and produced submarine reactors. To avoid the huge cost even a big player (the UK) used/(still assisted by) US designs and India uses Russian.

Early speculation that SK would shrink and modify its existing small land-based SMART reactor seems misplaced. SMART is unnecessarily large (for SK proposed 4,000 tonne submarine) at 350 MWt (thermal), which if developed for submarine would translate to 70 MWe (electrical). More significantly SMART uses low power/weight ratio 4.8% LEU fuel with an overly frequent 3 year refueling cycle.

Unless SK is conducting an extremely well hidden indigenous reactor for submarine effort one preliminary step might be to produce a reactor powered ship. SK’s DSME has had hints, but no details, for years. Go to the DSME technology website > hover on R&D and you will see “Nuclear Propulsion Ship” > Click on Nuclear Propulsion Ship and you will see Arctic Drill Ship. Such a ship could in future use a reactor for its ice-breaker propulsion function and for its drilling function. Neither SK nor DSME seem to provide further details on nuclear ship plans.

Instead SK has approached other countries for technology transfer assistance for a reactor and a specialized submarine to use it (the subject of my next article).

JAPAN

Japan's nuclear propelled ship Mutsu with Reactor and auxiliary machinery compartments shaded. (From Submarine Matters' June 17, 2016 article).

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Japan built and operated a nuclear propelled ship – the Mutsu (diagram above) – from 1968 to 1992 with a reactor developed by what is now the Japan Atomic Energy Agency (JAEA). From that experience Japan may have quietly conducted further design research on submarine reactors within the JAEA under the guidance of the Japanese Ministry of Defence (MoD). The most likely countries to assist Japan are the US and France.

Pete

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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See more.

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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.

Japan Long Term Planning ICBM as well as SSGN Capabilities? Hypersonic Missiles

On October 20, 2025, Japan’s ruling Liberal Democratic Party (LDP) and Japan Restoration Party signed a coalition Agreement "Liberal Democratic Party-Japan Restoration Party Coalition Government Agreement".

This Agreement brings together the long-dominant LDP and the reform-minded Japan Restoration Party under a shared vision of national revival. It establishes policy cooperation across a wide range of areas, including revising the constitution, defence build-up, fiscal and social security reform, and governance restructuring, to build a "strong and independent Japan" capable of responding to intensifying domestic and international challenges.

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wispywood2344, a Japanese expert on submarines, missiles, other strategic issues and physics, who has reported reliably on Submarine Matters for many years. He commented in part at 10/26/2025 2:09 AM : 

"Meanwhile, Japan recently saw a realignment of political parties and the formation of a new coalition government. The agreement between the ruling parties included the phrase “long-endurance submarines equipped with next-generation power plants and VLS and stand-off missiles”"  

It is the 5th page of "Liberal Democratic Party-Japan Restoration Party Coalition Government Agreement" at https://o-ishin.jp/news/2025/images/624de5f22900f6e88e892abb49d3fc70ef3cac92.pdf#page=5 
which includes in Japanese:

lわが国の抑止力の大幅な強化を行うため0スタンド・オフ防衛能力の整備を加速化

する観点から0反撃能力を持つ長射程ミサイル等の整備及び陸上展開先の着実な進

展を行うと同時に0長射程のミサイルを搭載し長距離・長期間の移動や潜航を可能

とする次世代の動力を活用したVLS 搭載潜水艦の保有に係る政策を推進するu 

That translated into English states: 

"In order to significantly strengthen Japan's deterrent power, we will accelerate the development of stand-off defense capabilities. From this perspective, we will steadily develop long-range missiles with counterattack capabilities and develop land-based deployments.  

At the same time, we will promote a policy to possess VLS-equipped submarines equipped with long-range missiles and utilizing next-generation power sources that enable long-distance and long-term travel and submerged operation." 

wispywood2344 comments that "this is widely seen as referring to SSGNs. In the distant future, “JUKUS-SSGNs” similar to “AUKUS-SSNs” may become a reality." 

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Pete Comment

When the Agreement on page 5 mentions "Japan's deterrent power [and] counterattack capabilities and develop land-based deployments." I can interpret the drafters as meaning a long term strategic option of nuclear deterrence using ICBMs.

ICBM? Japan has been developing  the dual-use Epsilon long range rocket/missile capability since 2007. (See image below) Epsilon is a 3 to 4 stage solid fuel rocket with no strap-on boosters. This Epsilons ideal for deployment in protective silos. 

More see my article "Japan's Epsilon Rocket, An ICBM in Waiting" of December 22, 2013 at https://gentleseas.blogspot.com/2013/12/japans-new-military-buildup-seen-as.html

Japanese SSGN? Japan developed the nuclear powered ship Mutsu in 1972. This means Japan's prior development of a miniature marine reactor, more than 50 years ago, gives it the beginnings of a submarine reactor capability.  

In the US context SSGNs (with Tomahawk missiles) are used for land attack. Land attack may be the main function envisaged for Japanese future SSGNs although anti-ship may be a secondary capability.

Japanese Hypersonic Missiles? Japan is developing a Hyper Velocity Gliding Projectile, (also see this and this video commentary here) with a booster it would be a hypersonic missile. A submarine launched variant is under consideration. Japan is also developing a Hypersonic Cruise Missile (HCM) (see page 22 of this US document). Japan's opponents in Northeast Asia would probably have great difficulty shooting them down.

SSN Proliferation & Australia Fears English Reactor Disease

Jun's, on July 19, 2026's comments and links of July 19, 2026 spawned a wide variety of interesting topics and issues. South Korea (SK's) bid for US help not only relates to SSN construction (KSS-N) but an SK desire to build SSBNs (SK-SSBNs) to deter nuclear armed North Korea.

SK's program is also a trigger "justifying" Japan building its own Mutsu-class nuclear  submarines. AUKUS (from good US, UK etc intelligence) can be seen as anticipating the above - justifying Australia's wants - AUKUS Pillar 1.

Australia is relying on UK and US assistance, at great cost. SK is relying on US assistance and international legal permission to use reactors for military/naval purposes. SK would also have liaised with France for 40+ years due to France's long experience with lower proliferation sensitivity-safer-cheaper LEU submarine reactors.

Of major value is Jun’s provision of an unofficial translation of Seong-Chang CHEONG's, Vice President, Sejong Institute's softpower@sejong.org Strategy and Roadmap for ROK-France Cooperation on Nuclear-Powered Submarines: EU-Based Naval Integration and a Mutually Beneficial Partnership Model at SEJONG FOCUS, February 10, 2026 at https://sejong.org/web/boad/1/egofiledn.php?conf_seq=22&bd_seq=12692&file_seq=41163

The Strategy and Roadmap is important for what it says and eventually says. Particularly on page 4: 

"France is precisely the only Western nation to have accumulated more than 40 years of experience in all three of these areas [including LEU reactors] since the commissioning of its first Rubis-class attack submarine in 1983. France has designed, built, and operated a total of 17 nuclear-powered vessels: 6 Rubis-class SSNs, 6 Suffren-class SSNs (including those under construction), 4 Triomphant-class SSBNs, and the aircraft carrier Charles de Gaulle…” 

The above statement is so far incorrect, as it omits the not usual 4 SSBNs but 6 French Redoutable-class SSBNs that each used up to 90% HEU reactors. 

Only further down on page 4 of Strategy and Roadmap is France's use of HEU remembered: 

"France also has a history of using highly enriched uranium (HEU) in its early nuclear submarine development. However, it has now fully transitioned to LEU. The early variants of the Le Redoutable-class SSBNs, France’s first independently developed strategic nuclear submarines in the 1970s, were equipped with reactors using weapons-grade HEU. In the 1960s, while enrichment capabilities were limited, highly enriched fuel was essential to increase the power output of compact submarine reactors. Weapons-grade uranium produced at the Pierrelatte military enrichment facility was used for this purpose."

Some Background and Issues on France's Wise Choice to Stick with LEU

Noting Submarine Matter's articles France’s Future SSBN “SNLE 3G” Starts Production of March 28, 2024; France's New K22 reactor for Carrier & 3G SSBN of December 27, 2023; and French LEU Reactor & SSN Implications of March 24, 2023: 

The USN, and partly the UK RN, rely on the US' massive HEU naval reactor industrial base to develop new submarine reactor technology and to create the 94+% HEU fuel.

If HEU submarine reactors and piping go drastically wrong - as with the UK's deeply flawed PWR2 reactors - HEU problems seem much harder and more expensive to rectify than regularly fixed LEU reactors.

Perhaps the engineering and potentially toxic health problems of accessing the PWR2's HEU assembly is the the problem that has "beached" all of the UK RN's Astute SSNs simultaneously and, I suspect, HMS Vanguard SSBN (withdrawn under inspection and maintenance for 10 years 2012 to 2022).

If the UK's future HEU PWR3 and/or PWR3+/PWR4 reactors fail to resolve the UK's PWR2 reactor technical disaster one hopes it doesn't impact Australia's SSN-AUKUS program. This is a hugely expensive issue people worry about in Canberra.

To avoid the English reactor disease should Australia plead with the US to sell us (at even greater expense) not 3, not 5, but all 8 SSNs Australia is seeking under AUKUS? Unlike the UK the US apparently makes reliable reactors.

I think the UK and US would prevent Australia turning to French LEU technology. One reason is the RAN officers/crew and shipyard workers currently receiving Top Secret nuclear training in the US and UK would not be trusted by the US and UK NOT to pass this Anglosphere SSN and reactor knowledge onto France.

Also how many countries can we throw Australian taxpayer's money at in our SSN quest? Not only to the US and UK, but to France and maybe even to South Korea's (world's most efficient) submarine building sector?

Submarine Reactors for South Korea and Japan?

In response to Anonymous from France’s December 30, 2023 comments:

If South Korea (SK) experiences great difficulties developing its own submarine reactors SK money may make France more willing to supply K22 or K15 reactor technology:

-  a changing reactor tech proliferation environment, led by the US and UK’s AUKUS reactor offers to Australia, has probably weakened the Western world's reactor proliferation taboo.

-  again SK and even Japan may be willing to offer high prices, making French naval reactor technology a highly profitable defence export item.

This may initially be for SK and Japanese SSBN (rather than SSN) reactors - such reactors need not be as miniaturised and operate on gradual acceleration and deceleration compared to SSN reactors. Their SSBNs may be bastion protected if in the near seas. Bastion protection may compensate for the more extended time for SK and Japan to develop more difficult SSN technology.

Your claim may well be accurate that the LEU K22 is probably very similar to the civilian NUWARD SMR proposed by Électricité de France (EDF) .

-  Yes SK, with its Small Modular Reactor (SMR) designs for the KEPCO BANDI-60 (60 MWe) PWR and KAERI SMART (100MWe) PWR, might eventually develop an efficient submarine reactor. 

Equally Japan’s JAERI MRX (30-100MWe) PWR may result in a submarine reactor. Also Japan’s operation of the Mutsu 36MWth nuclear powered civilian ship has given Japan some propulsion experience.

However SK or Japanese adoption of French, US or UK reactor tech could minimize extended indigenous development times and cost. This is why the UK pressed for a US S5W reactor installed in HMS Dreadnought (S101) in the early 1960s. Subsequently some US S9G reactor tech has reportedly gone into the UK’s latest reactor, the PWR3.

Some Major Japanese Politicians Talking Nuke Propelled Subs

The AUKUS agreement effectively removes the legal taboo of the US or UK exporting 93+% weapons grade nuclear reactor HEU to Australia. By this taboo removal this trends toward legitimizing nuclear submarine propulsion for Japan. 

Some senior Japanese political leaders are now talking of possible future Japanese nuclear propelled submarines quite openly.

From the late 1960s Japan completed the nuclear propelled ship Mutsu with a Japanese built miniaturized reactor. Japan has maintained a highly developed conventional sub industry and a nuclear industrial base. 

Rahul Kumar, for India's daijiworld, September 28, 2021 has written an excellent commentary on the Japanese leaders' statements: 

"After AUKUS, Japan debates if it too should have nuclear-powered submarines"

New Delhi, Sep 28: The AUKUS deal between the US, Australia and the UK to provide Australia with nuclear-powered submarines to take on China in the Indo-Pacific has caused rumblings in Japan.

Japanese politicians are debating whether Japan should also possess one. Japan goes to elections in November [2021] and the nuclear submarine for Australia, coupled with Chinese aggression in the region, has become a political issue.

Taro Kono - Japan's administrative reform minister, who is likely to lead the ruling party - Liberal Democratic Party (LDP), and may become the Prime Minister after Yoshihide Suga, is in favour of acquiring the submarines. In a TV debate on Sunday, he said: "As a capability, it is very important for Japan to have nuclear submarines".

Nikkei Asia reports that former internal affairs minister Sanae Takaichi endorsed the idea. Talking about the positives of owning a nuclear-powered submarine, she said that Japan would be able to travel longer with such a machine. These submarines are preferred because these are quieter, have a longer range and remain underwater for long.

Not everyone within the party endorses the idea.

Fumio Kishida, former policy chief of the LDP [and on Sept 29, 2021 appointed Leader of the LDP] feels that the country's security needs are not such that it needs to go far out from Tokyo's shores. He, therefore, does not feel the need for the Japanese Self Defence Forces to acquire a nuclear-powered submarine.

The fast-track geo-political changes in the seas around Japan has given fuel to much debate within the country.

China's rise as an aggressive economic and military power is making Tokyo uncomfortable. With frequent war cries about annexing Taiwan, Beijing has given much stress to Japan which feels that China might invade Japanese islands as well. The China factor has forced a worried Japan to rethink its military strategy.

The slow but continuous process through which China has usurped a number of reefs and islands in the South China Sea has made Japan and other countries nervous.

In the last few months Japan has been regularly highlighting Chinese aggression in the region. A white paper on defence accepted Taiwan as a separate territory from China. Tokyo has gone to the extent of saying that it will join forces with Taiwan against Chinese aggression."

Pete Comment

With Fumio Kishida likely to become Japan's next Prime Minister in November 2021 Japan may not "go nuclear sub" in the short-medium term. But AUKUS' lifting of the nuke propelled sub taboo, now appears to be permitting a trend. Previously unthinkable pro-nuclear sub statements are now being made by major Japanese politicians.