South Korean Nuclear Powered Subs by the 2030s: Predicted 2015-2021

Pete's earliest detailed comments on South Korea and its SSN ambitions were April 16, 2015's "South Korean Submarines, 3,000+ ton KSS-III, Nuclear Potential" at https://gentleseas.blogspot.com/2015/04/south-korean-submarines-3000-ton-kss.html 15 comments, 10,155 pageviews.

Pete wrote "S Korean nuke sub Reactor likely to be French K15 Variant" on December 16 2021 at https://gentleseas.blogspot.com/2021/12/s-korean-built-nuke-sub-reactor-likely.html 4 comments, 2,220 pageviews.
Please see text below: 

"...SUBMARINE MATTERS BACKGROUND AND COMMENT [written in 2021]

South Korea is leaning toward a [Low Enriched Uranium (LEU)] reactor solution perhaps initially on a South Korea SSBN - which will be based around the ballistic missile submarine KSS-III SSB. It is not surprising South Korea has long been in talks with France (since 2017 if not earlier) about South Korea building a variant of France's K15 LEU reactor. The K15 is on France's Triomphant class SSBNs and, in slightly smaller form, on the France's Barracuda-Suffren class SSNs. 

In Submarine Matters’ “South Korea seeks Submarine Reactors from US and RUSSIA” of October 22, 2019 South Korea may have concluded it is better to buy an existing submarine reactor or at least a ship reactor than totally reinvent a submarine reactor. 

Since 2017 (if not earlier) South Korea has been considering France’s new Barracuda SSN with its K15 (aka K 15 aka K-15) reactor. With North Korea's buildup of nuclear weapon and missile capabilities South Korea, in October 2019, has been testing any increased US willingness for South Korea to explore nuclear propulsion options. 

The Triomphant-class's 2(?) x K15s and Barracuda's single K15 reactor stands for 150 MWt  power which translates to a total of 30 MWe electrical power (for the French Barracuda’s non-propulsive electrical services hotel load + propulsion).

"In October 2017, the [South Korean] Navy commissioned the Seoul-based Korea Defense Network to conduct a five month study on the feasibility of developing an indigenous nuclear-powered attack submarine. The think tank reported in March 2018 the results to the Navy, suggesting the service build a nuclear attack submarine along the lines of the French 5,300-ton Barracuda-class sub. The French sub is fuelled by low-enriched uranium."

Now that the US and UK have given the green light to supplying Australia with a reactor under the AUKUS deal it is conceivable that France may want [to] supply just the Barracuda’s K15 reactor or transfer technology (including a reactor design) for South Korea to incorporate in its 4,000+ tonne KSS III Batch III building program. 

The Barracuda’s K15 reactor has the:

-  political/regulatory advantage of using LEU ie. less than 20% U-235 which South Korea sees as not being restricted by the NPT or nuclear Safeguards Agreements. This is compared to the political sensitivities and anti-proliferation regulatory restrictions (up until September 2021's AUKUS deal) placed on exporting/importing US and UK HEU submarine reactors. The UK/US reactors use weapons’ grade HEU of 93-97 percent (see p. 20). 

-  technical advantage of being built for a submarine of just over 5,000 tons, ie. in South Korea's KSS III Bach III weight bracket. While the KSS III Batch II with 10 x VLS silos  has diesel-electric (also AIP) propulsion the proposed KSS III Batch III may eventuall have 12 to 16 VLS silos requiring an unusually large propulsion solution. Nuclear has been the solution for SSBNs built by the US, Russia, UK, France, China and India. 

-  Also Naval Group's assistance to Brazil to create a SSN, known as Alvaro Alberto [~5,900 tonnes submerged?], is highly relevant to the South Korean SSN (aka KSS-N) project, Alvaro Alberto's projected launch year (2029) and commissioning (2032-2034) may occur in the same timeframe as the South Korean SSN project.  At Submarine Matter’s 2015 article see the subheading "Brazilian Nuclear Submarine (SN-BR)” “There currently appears to be expectations that SN-BR will be around 100m long and 9m wide. This coincides with France's Barracuda SSN dimensions of: 99.5m long and beam: 8.8m.” 

-  So a A K15 variant first mounted on a South Korean SSBN by the early 2030s may be followed by mounting a K15 reactor variant on a South Korean SSN by the mid 2030s. In terms of vertically launched cruise or ballistic missiles the KSS Batch I features 6 VLS tubes, each with one missile. The KSS III Batch II [now features 10] and the KSS III Batch III perhaps 12 to 16. 

South Korea's neighbour Japan (a part strategic competitor of South Korea) is observing South Korea's interest in nuclear propulsion and ever larger submarines.

India, already having SSBNs, is interested in building 6 x Project 75 Alpha class SSNs with specialised SSN reactors. India is naturally talking to France's Naval Group. Naval Group is already  helping India build the 6 Kalvari-class Scorpenes, and bidding for India's 6 x AIP diesel-electric Project-75I SSK competition. Naval Group produces all of France's SSNs and SSBNs. So India can have far ranging discussions, on many topics, with Naval Group."

SubMatts article ends.

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Very recently, in July 2026, Bill Seney and I have been discussing the above topic:

"Bill Seney [said]

I have seen news reports:
https://www.japantimes.co.jp/news/2026/05/26/asia-pacific/south-korea-nuclear-submarine-2030s/

that South Korea [SK] is looking at a nuclear powered variant of the KSS-III. Given that US nuclear subs seem to be getting further and further away and that UK subs are at least 2 decades away is it worth Australia's while to look at a SK option?

Given that [SK] has yet to build a nuclear submarine I could see the project slipping or even facing cancelation, so it is high risk. On the other hand, we know SK can build diesel boats. Perhaps a mixed fleet, with initial deliveries of conventional KSS-III followed by a nuclear variant when available? If the nuclear option is cancelled Australia would still end up with a powerful fleet of conventional submarines.

7/15/2026 10:41 AM
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Pete2 [said]

Thanks Bill [I agree with your 7/15/2026 10:41 AM comment]]

I'll develop your idea as an article probably tomorrow.

I've long theorised here at SubMatts that SK would look to France's K15 LEU reactor for technology transfer. Also that the last KSS-III Batch 2 or Batch 3s would be nuclear powered.

Cheers Pete

7/15/2026 10:53 AM

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Bill Seney [said]

Thanks Pete

Something I forgot to mention in my earlier comment is that the KSS-III is about the same size as the American Skipjack class SSN, so a nuclear KSS-III variant should be technically feasible.
[Pete comment - Yes the Skipjack-class SSN (1956-1990) was indeed KSS-III sized, at ~ 3,075t (surfaced) & ~3,513t (submerged) and the
French K15 using, Suffren/Barracuda SSN is ~4,765t-5,300t] 

Also, if Australia wanted to partner with South Korea on SSNs we can provide not just uranium [Australia has the 2nd largest Uranium reserves worldwide] but enrichment technology [Australian invented SILEX] as well, something the Koreans may be interested in.

7/15/2026 10:53 PM
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Pete Comment

Whether the US and its changeable Trump card can play a useful part in SK's (at least 11 year old) nuclear powered submarine program follows in a week. 

Certainly the US has an international law "123 Agreement" hold over SK that transactional Trump will maintain unless SK pays the US the price Trump seeks, that may vary from week to week. 

Indonesia Unlikely to Buy/Build SSNs

Ghalib's response to my article Australia's post-Election Nuclear Proliferation Concerns of May 11, 2022 was overwhelmingly oppositary. 

So it is in the spirit of intellectual honesty and détente that I quote his arguments in full.

On May 11, 2022, Ghalib Kabir said:

“Indonesia has its own 9 dash line related issues with China. It flies US made jets, has ordered French Rafales and is a customer for German-Korean [Type 209] subs... 

It might come down to how Indonesia is handled diplomatically in the larger Indo-Pacific picture me thinks....compared to the kind of animosity that hostile neighbours have such as in the in the Indian sub-continent or Eastern Europe, Australia and Indonesia have much less reasons for being mutually tetchy and concerned about their aims and abilities. 

Putin is capable of risk taking, however, I have serious questions about him leasing an Akula-class sub to the Indonesian Navy on 3 counts: 

1.  China - It is almost 100% guaranteed go down very very badly in Beijing...even the idea might not be entertained by a Beijing with big leverage over a post-Ukraine Russia. China is very likely to veto such a thought in the Russian echelons. 

2.  Indonesia's nuclear infrastructure is non-existent and I think this cannot be stressed enough. Nuclear engineers and technicians don't grow on trees and building domestic nuclear ecosystems from scratch is no joke, both money wise and effort wise. 

I posit that Indonesia has nowhere close to the manufacturing base required to even contemplate a nuclear propulsion based submarine. I am willing to bung an Australian  Monash (100 $) bill on the table as a wager with you mate on this one. 

3.  Indonesia cannot expect support even for an SN-BR type project with the help of France with the ostensible figleaf of a LEU based SN-BR SSN to counter NPT issues. France will not cross the US and proceed...It cannot even it wanted to as the Indonesians are decades behind even the much criticized Brazilians in the nuclear infrastructure sphere. 

Lastly, even leap frogging Australia GDP wise won't help jump over these barriers. (I seriously think Indonesia will not be able to the foot the bill even for a 2 SSN fleet). 

Indonesia and Australia and the larger Quad might all be best served if Indonesia's SSK fleet acted as a complement to the other partners SSK-SSN fleets. 

ASEAN, led by Malaysia and Indonesia, will not be helped one bit if they keep flapping around like wet hens throwing a hissy fit [over AUKUS] ...Throwing their lot with India-Quad-Ozzie land is better.

On May 12, 2022 Ghalib continued:

Of course, anything can happen over time. What I talk about is the possibility of such an occurrence, namely Indonesia getting an SSN. 

Australia has many advantages in terms of: 1. bloc membership, 2. geopolitical currents, 3. economic strength and 4. technological capabilities. 

What chance does Indonesia have to cover all those 4 bases? 

Bloc membership and economic strength is gettable with effort and indeed Indonesia is getting there. 

Geopolitical currents and technology abilities - highly unlikely anything similar to Australia will happen any time soon. 

PS: Mountains of 'facts at that time' changed as Australia and friends were inclined to make it change and all [AUKUS] factors were 'alignable' and hence they aligned. 

Tomorrow Indonesia can announce a national security policy shift to include SSNs and given its limitations cited above, we can be quite sure and add a red lobster $20 note on top of the Monash Note to up the wager that in 5-10 years Indonesia will exactly be at the same spot.

Pete Comment

I would add:

Even if Indonesia has a GDP higher than Australia in 10 years not much of Indonesia’s national budget would be available for SSNs.

Much of Indonesia’s GDP, for more than a decade, will be diverted to the mega-project of moving Indonesia’s capital from Jakarta to “Nusantara” on the island of Kalimantan (once called Borneo) if that project gets final approval.

Indonesia’s military has always been heavily orientated to internal security (keeping its disparate island peoples together) followed by protection of its energy resources from “friendly” ASEAN neighbours.

Indonesia is a democracy of relatively impoverished people who would not look kindly at the odd $100 Billion being blown on SSNs - money better spent to lift their living standards. 

Slow Brazilian SSN Program due to No Argentine Threat

As I mentioned (between the lines) before, Brazil did the right thing in advising the FBI of the approach by the Toebbe couple. This is partly due to Brazil's good relations with the US.

In response to Gessler’s comment of March 18, 2022.

Brazil’s nuclear propelled submarine Alvaro Alberto SN-BR program is moving so slowly in large part because a strategic nuclear threat from Argentina no longer exists. Argentina is no longer contemplating a nuclear weapons program. Decades ago part of the Brazilian nuclear submarine program had a Strategic Rationale which included:

“The Brazilian Navy modernization program plans the development and construction of six SSN submarines.^[27] In the Brazilian doctrine, the raison d'etre of the national defence strategy is to develop deterrence capability against a possible hostile force to the national territory.^[28] The country understands that with its future nuclear fleet, at least some of its weapons will be able to survive the first strike of an enemy and prevent further attempts at aggression.^[29]"

This rationale included a nuclear weapons development arms between Brazil and Argentina race extending back to the 1950s see:

"In the 1970s and 1980s, during the military regime, Brazil had a secret program intended to develop nuclear weapons.^{[1][2][3][4][5][6][7]} The program was dismantled in 1990, five years after the military regime ended, and Brazil is considered free of weapons of mass destruction.^[8]

In the 1950s, President Getúlio Vargas encouraged the development of independent national nuclear capabilities.^[2] At that time, the United States worked actively to prevent Brazil from acquiring the centrifuge technology that could be used to produce high-enriched uranium for nuclear weapons.^[13]

 …Brazil pursued a covert nuclear weapons program known as the "Parallel Program",^[2] with enrichment facilities (including small scale centrifuge enrichment plants, a limited reprocessing capability, and a missile program).^[14] Brazil also reportedly bought highly enriched uranium from China in the 1980s.

… [But] In 1990, President Fernando Collor de Mello symbolically closed the Cachimbo test site, in Pará, and exposed the military’s secret plan to develop a nuclear weapon.^[14]

Then–U.S. Secretary of State Colin Powell stated in 2004 that he was sure that Brazil had no plans to develop nuclear weapons.^[23]"

Under the military dictatorship, Argentina began a nuclear weapons program in the early 1980s, but this was abolished when democracy was restored in 1983.

"During the 1980s, the Alacrán (English: Scorpion) and Cóndor 1 (English: Condor) missiles were developed.^[1] The Cóndor 2, with a range of around 1,000 kilometres,^[2] was intended to be developed with assistance from Egypt and Ba'athist Iraq. However, the project was condemned by the United States and the Missile Technology Control Regime.^[3] It was reportedly scrapped during the Menem administration under pressure from the United States government and due to a lack of funds in 1990.^[3][4]

Argentina conducted a nuclear weapon research program during the [early 1980s military dictatorship], in part because of a similar Brazilian program assisted by West Germany.^[3] International concern over the possibility of an Argentine nuclear weapons program magnified after the Falklands War in 1982, when the U.S. intelligence community estimated that Argentina could build a nuclear bomb from its civilian nuclear program.^[9] Government officials at the time confirmed, in November 1983, that research carried out at the Balseiro Institute's research reactor had yielded the capacity for weapons-grade uranium enrichment.^[10] 

The program was abandoned, however, shortly after the return of democracy, on December 10, 1983. President Raúl Alfonsín placed the nuclear program back under civilian control.^[3] The program was also abandoned because Argentina did not have bad relations with Brazil, and because Brazil was wealthier than Argentina and thus more advantaged in an arms race.^[11]

After the Brazilian transition to democracy, Argentina and Brazil began cooperating on nuclear non-proliferation.^[11] In 1991 the National Congresses of Argentina and Brazil ratified a bilateral inspection agreement that created the Brazilian-Argentine Agency for Accounting and Control of Nuclear Materials (ABACC) to verify both countries' pledges to use nuclear energy only for peaceful purposes. 

[However] In 2010, the [Argentine] government announced that it would start working in the creation of a nuclear submarine.^[12] …The announcement was highly criticized by politicians from opposing parties.^[13]"

Pete Comment

The end of any Brazilian-Argentine nuclear weapons arms race has meant a Brazilian nuclear submarine has a lower, slower, priority. It also appears lack of Argentine political support and lack of money has slowed or ended an Argentine nuclear propelled submarine program. 

See earlier Submarine Matters' articles on "SN-BR" all the way back to 2014.

S Korean nuke sub Reactor likely to be French K15 Variant

India's Bharat Express News December 14, 2021 carries the New York Times (NYT) December 13, 2021 article "South Korea has long wanted nuclear submarines. A new reactor could open a door." without readers having to subscribe to NYT. NYT's article is reproduced by Bharat Express here https://www.thebharatexpressnews.com/south-korea-has-long-wanted-nuclear-submarines-a-new-reactor-could-open-a-door/

SUBMARINE MATTERS BACKGROUND AND COMMENT

South Korea is leaning toward a LEU reactor solution perhaps initially on a South Korea SSBN - which will be based around the ballistic missile submarine KSS-III SSB. It is not surprising South Korea has long been in talks with France (since 2017 if not earlier) about South Korea building a variant of France's K15 LEU reactor. The K15 is on France's Triomphant class SSBNs and, in slightly smaller form, on the France's Barracuda-Suffren class SSNs. 

In Submarine Matters’ “South Korea seeks Submarine Reactors from US and RUSSIA” of October 22, 2019 South Korea may have concluded it is better to buy an existing submarine reactor or at least a ship reactor than totally reinvent a submarine reactor. 

Since 2017 (if not earlier) South Korea has been considering France’s new Barracuda SSN with its K15 (aka K 15 aka K-15) reactor. With North Korea's buildup of nuclear weapon and missile capabilities South Korea, in October 2019, has been testing any increased US willingness for South Korea to explore nuclear propulsion options. 

The Triomphant-class's 2(?) x K15s and Barracuda's single K15 reactor stands for 150 MWt  power which translates to a total of 30 MWe electrical power (for the French Barracuda’s non-propulsive electrical services hotel load + propulsion).

"In October 2017, the [South Korean] Navy commissioned the Seoul-based Korea Defense Network to conduct a five month study on the feasibility of developing an indigenous nuclear-powered attack submarine. The think tank reported in March 2018 the results to the Navy, suggesting the service build a nuclear attack submarine along the lines of the French 5,300-ton Barracuda-class sub. The French sub is fueled by low-enriched uranium."

Now that the US and UK have given the green light to supplying Australia with a reactor under the AUKUS deal it is conceivable that France may want supply just the Barracuda’s K15 reactor or transfer technology (including a reactor design) for South Korea to incorporate in its 4,000+ tonne KSS III Batch III building program. 

The Barracuda’s K15 reactor has the:

-  political/regulatory advantage of using LEU ie. less than 20% U-235 which South Korea sees as not being restricted by the NPT or nuclear Safeguards Agreements. This is compared to the political sensitivities and anti-proliferation regulatory restrictions (up until September 2021's AUKUS deal) placed on exporting/importing US and UK HEU submarine reactors. The UK/US reactors use weapons’ grade HEU of 93-97 percent (see p. 20). 

-  technical advantage of being built for a submarine of just over 5,000 tons, ie. in South Korea's KSS III Bach III weight bracket. While the KSS III Batch II with 10 x VLS silos  has diesel-electric (also AIP) propulsion the proposed KSS III Batch III may eventuall have 12 to 16 VLS silos requiring an unusually large propulsion solution. Nuclear has been the solution for SSBNs built by the US, Russia, UK, France, China and India. 

-  Also Naval Group's assistance to Brazil to create a SSN, known as Alvaro Alberto, is highly relevant to the South Korean SSN (aka KSS-N) project, Alvaro Alberto's projected launch year (2029) and commissioning (2032-2034) may occur in the same timeframe as the South Korean SSN project.  At Submarine Matter’s 2015 article see the subheading "Brazilian Nuclear Submarine (SN-BR)” “There currently appears to be expectations that SN-BR will be around 100m long and 9m wide. This coincides with France's Barracuda SSN dimensions of: 99.5m long and beam: 8.8m.” 

-  So a A K15 variant first mounted on a South Korean SSBN by the early 2030s may be followed by mounting a K15 reactor variant on a South Korean SSN by the mid 2030s. In terms of vertically launched cruise or ballistic missiles the KSS Batch I features 6 VLS tubes, each with one missile. The KSS III Batch II may feature 10 and the KSS III Batch III perhaps 12 to 16. 

South Korea's neighbour Japan (a part strategic competitor of South Korea) is observing South Korea's interest in nuclear propulsion and ever larger submarines.

India, already having SSBNs, is interested in building 6 x Project 75 Alpha class SSNs with specialised SSN reactors. India is naturally talking to France's Naval Group. Naval Group is already  helping India build the 6 Kalvari-class Scorpenes, and bidding for India's 6 x AIP diesel-electric Project-75I SSK competition. Naval Group produces all of France's SSNs and SSBNs. So India can have far ranging discussions, on many topics, with Naval Group.

Gessler: Future Indian SSNs & SSBNs: Comment 1.

Following India's 6 Future Alpha SSNs - SSBN Protectors of April 7, 2021 Gessler made some learned and extensive comments on April 9, 2021. Pete has added some comments in [...] brackets. Here is the first of 3 Gessler instalments:

“@Pete. I'm sceptical about the figures of India’s future indigenous Project 75 Alpha SSN tonnage/displacement floating around in the media. I suspect those displacement figures (not only mentioned by Hindustan Times but many others as well) are from the same erroneous sources that also claimed that the S-3 boat (INS Arighat) the sister of S-2 (INS Arihant) would be considerably bigger and carry 8 missile silos. 

The photo in question, showing INS Arihant and Arighant, at right, berthed alongside each other and the pier. (Photo reproduced from idrw . com December 18, 2020.)
---

But satellite images (like this) of the Ship Building Centre (SBC) harbour at Visakhapatnam (Visak, most commonly pronounced & spelled Vizag) prove that both of the Arihant-class INS Arihant and INS Arighat are the same eg. Arihant class displacement 6,000 tonnes (surfaced, confirmed), length (116m) and size (Beam 11m, Draught 15m), and carry the same number of missile tubes, ie. 4. [See "Future of Indian Navy - Nuclear Submarines"]

Now don't get me wrong - I still am inclined to believe the 4th nuclear boat to be built by SBC (known in the media as the S-4* or S-4 Star) [see "Ships in class"] would indeed be considerably bigger than the Arihant-class, satellite images of a new submarine dockyard shelter constructed at Vizag which is up to 40m longer than Arihant's shelter substantiate the theory that a new, bigger boat is around the corner - and it definitely can't be the S-5 yet. So it has to be the rumoured S-4* SSBN, with perhaps 8 silos.

Nuclear expert Hans M. Kristensen's observation of the same summarized in his tweet, though he speculates this to be intended for the S-4 itself (3rd nuclear boat), not the S-4*. Personally, I would think all three Arihant-class SSBNs will be of the same dimensions, it would not make any sense to have the last boat in class to be substantially different than the others.

The S-4* on the other hand might make sense as both a test platform for new, larger equipment that may eventually find use on the still larger S-5 class down the line, once the systems mature after at-sea testing on this boat. The Arihants would have to be at sea (or in refit), delivering on the deterrence role, you can't ask them to come off their vitally important duties to function as test platforms for new stuff - the S-4* might make a lot of sense in that respect. 

Another reason to be sceptical about the Alpha's displacement figures would be that pretty much all available sources (unless I'm misremembering) quote the Arihant-class boats' surfaced displacement as approx. 6,000 tonnes. 

So if the Alpha indeed turns out to 'Arihant without silos' then there's no way it'll also be 6,000 tonnes surfaced. The Alphas will have to be 5,000 tonnes [surfaced], but even that is speculation.

So personally I'll hold my horses for now regarding the displacement figures."

[For comparison see the 6,000 tonne (likely submerged) 100m long, Brazilian future SSN Alvaro Alberto (SNB or SN-BR). Implicitly its displacement may also be around 5,000 tonnes surfaced.] 

On Saturday April 17 Gessler's 2nd informative Comment will appear. 

GhalibKabir on Future Indian Alpha SSN issues

Following India's 6 Future Alpha SSNs - SSBN Protectors of April 7, 2021 came the blessing of a veritable avalanche of good comments from Anonymous, GhalibKabir, Gessler and Arpit Kanodia. Rather than attempt to summarise each comment (obscuring meaning and context). Or try to respond point by point (which would fill 10 pages/5 articles) I'll republish the comments. I'll also add some lengthened weapon designations,  comments in [...] brackets and extra links.

I'll start combining GhalibKabir’s comments of Apr 8, 2021:

“Pete, I think we discussed this way back in 2019, similar to the Chinese Type 091 (NATO (Chinese Dynasty) reporting name "Han" class) SSN. The future Indian Project 75 Alpha SSN will likely (initially) use a slightly uprated Arihant 40% HEU PWR (uprated from 83 MWt to 100 MWt) implying electrical power uprates to 16.7 MWe from a maximum of 13.8 MW presently) (the rule of 1/6 for naval reactors vs 1/3 for land based). 

Peter Lobner got it right with his 100 MWt prediction I think. [See Peter Lobner "Marine Nuclear Power 1939-2018: Part 5, China, India, Japan & other nations" (July, 2018) page 159 (Table) and page 163 at https://lynceans.org/wp-content/uploads/2018/07/Marine-Nuclear-Power-1939-2018_Part-5_China-India-Japan-Others.pdf about 50 MB, 294 pages] 

The S-5 class future full size Indian SSBN could see a single Soviet/Russian OK-650 reactor [which India may have designated the CLWR-B2 Compact LWR - see right sidebar] derivative as the bigger SSBN will need 25 MWe or more capacity to be available implying a 160-170 MWt OK-650 [even 190 MW and see also ] class reactor.

However, SSN reactor dynamics are likely to be different as the rapid 'power up power down' variation demand on the SSN reactor for 'sprint chases' will be much higher than an SSBN. The Arihant class' or even the S-5's likely speed requirements could stay between 22-24 knots while the Alpha SSN will need burst speed capability of 35 knots and the ability to do 28-30 knots frequently and consistently... (like China's current Type 093 Shang class SSN for instance). 

Also I think INS Chakra II elements with Arihant base is the most likely in SSN Alpha and not Barracuda or the Alvero Alberto [aka "SN-BR"] Brazilian Scorpene SSN (though some learnings will cross over in welding, silencing, reduction gear design etc.) [Also the future Chakra III might influence the Alpha SSN. "India and Russia, March 7, 2019, signed $3Billion deal to lease another Akula SSN, dubbed Chakra III, should be delivered to Indian Navy 2025].

I do hope India is able to leverage [France's] Thales and [Israel's] Rafael under water suites expertise to equip the SSN with good sonar suites and combat capabilities."

[The Russia-India lease deal means] INS Chakra II cannot be used in offensive war. It cannot be armed with any missile ranging > 290 km i.e. original MTCR restrictions apply (unless I am gravely mistaken)...it can help in case of open war, but will be limited to defending with torpedoes and such short range missiles as to be of very limited utility indeed....plus the sonars and combat suites are very old as well....

I think a vertical launch system (VLS) [on the Alpha SSN? Within a 6,000 tonne (submeged) displacement limit?] combo with Horizontal launch capability could be better... 24 Tri-packed Nirbhay sub-sonic SLCMs in 8 VLS with 20+ combination of Varunastra Torpedoes and BrahMos SLCMs are the most likely combos..

Also while a propulsor pumpjet is desirable and is being tried, I think a conventional 7 blade screw is likely for the first 2 Alpha SSNs at least. Like the Chinese Typo 091 SSN in 1975, the focus first should be to get a couple of Alphas out and test them. Then we can see and make real progress, instead of an unhealthy obsession with 'perfect profile' product launches.

The Type 091 used a 60 MWt inefficient PWR that delivered 8 MWe and was very radiation leak prone. The key difference being the PWR was a LEU (thanks to the Soviets screwing the Chinese over [Soviets were worried Mao was prepared to risk dragging the Soviets into nuclear war against US and Taiwan]. China was left with minimal uranium supplies and also a loss of nuclear and submarine expertise after China's Cultural Revolution). India has been a HEU navy.

Good that [India's Chief of Defence Staff, General] Bipin Rawat, Doval and co were able to prevail on the SSNs after 3 years of wrangle at North Block" [ie. wrangling at the Ministry of Finance, New Delhi].