S. Korean & Japanese Nuclear Submarine Propulsion & Weapons

Hi Tri-ring and Anonymous (your August 11, 2020 comments on Japanese and South Korean submarines nuclear propulsion possibilities).

1.  Tri-ring I think it much more likely Japan and South Korea will further develop what they have already achieved that is Pressurized Water Reactors (PWRs) that are small enough to fit on small ships. The fusion and other advanced engines (you mention) are for the US and China in coming decades.

Anonymous, I agree with you that South Korea will eventually move to nuclear propulsion, but suspect it will build SSBNs before SSNs.

As a technical and political EVOLUTIONARY process South Korea has been at looking at the issue of a nuclear or conventionally propelled ballistic missile launching First and Second Strike platform for more than 10 years. South Korea would have been looking at the nuclear issues even earlier: since 2003 when North Korea withdrew from the NPT and since 2006 North Korea's first nuclear weapon test.  

Japan, in 1972, completed the Mutsu small nuclear propelled ship, working nuclear reactor and all. 

South Korea has had a less visible marine reactor program with a view to using a modified civilian SMART reactor (note mention of an even smaller "SMART-P(SMART pilot plant, 1/5 scale)") 

or equivalent since 1997. At the DSME website is Nuclear Propulsion Ship. Meanwhile South Korea likely has a parallel program with plans for a smaller reactor for a submarine. 

Since these major advances in reactor miniaturisation for Japan (48 years ago) and South Korea (23 years ago) these advanced civilian reactor countries will have made substantial gains in knowledge on how to further miniaturise.

Also Japan and South Korea have overt and covert intelligence programs to gain knowledge from existing submarine reactor countries (US, UK, France, Russia, China, India and Brazil) on how to further miniaturise.

2.  Meanwhile South Korea is technically and politically evolving submarines as First and Second Strike weapons against discreet targets in North Korea's cities and nuclear weapons facilities. South Korea is developing its KSS-III (3,360+ ton) submarines to each eventually vertically fire 10 missiles (cruise and eventually ballistic). The 9 x KSS-IIIs planned have air independent propulsion (AIP) and from the Batch II subclass onwards they may have Lithium-ion Batteries (LIBs). AIP + LIBs may give a KSS-III the ability to lie in wait, sitting on the seafloor, for 6 weeks without surfacing. 

KSS-IIIs being commissioned up to the mid 2030s may be a credible deterrent against North Korea but not against China. North Korea, would generally assume that 500+kg warhead incoming missiles may well have nuclear warheads. North Korea would therefore deliver a nuclear response. South Korea going to the trouble of building whole submarines with only 10 relatively small (500kg) warhead missiles is only explicable if these missiles eventually have nuclear warheads.

A South Korean nuclear propelled ballistic missile class (lets call them "KSS-BNs") being launched from the late 2030s may be sufficient against China and Russia (and against India, the US and Japan for that matter). 

Can South Korea afford nuclear weapons and nuclear propelled submarines? When you look at nominal GDP for South Korea its GDP is equal to or greater than Russia's. South Korea's nuclear military aspirations have been held back by US promises of a nuclear umbrella against North Korea and China. This umbrella and US, Russian and Chinese political power has not stopped North Korea from nuclear arming. South Korea will feel even more desperate to build a true deterrent in 12 years time when North Korea will have fleshed out its nuclear triad.