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