The Sleeper Has Awakened: Six Key Takeaways From the Rollout of North Korea’s “Tactical Nuclear Attack Submarine”

On September 8, North Korean media announced that “a Korean-style tactical nuclear attack submarine” had been rolled out of a construction hall at the Sinpho South Shipyard into the water two days before. Associated photos revealed substantial modifications made to an existing old ROMEO-class conventionally powered submarine to launch “tactical nuclear weapons.” The text of a speech Kim Jong Un reportedly gave on the occasion provided further insight into the origin and purpose of the new sub, and North Korean plans for a nuclearized Navy, including further such conversions and a renewed commitment to building nuclear-powered submarines.

There are six key takeaways from the rollout of the “new” nuclear-armed conventionally powered ballistic missile submarine (SSB):

• This is most likely the same ROMEO that was being modified when Kim Jong Un visited in July 2019, which had probably been in the construction hall since 2014. At that stage, the new submarine was expected to be configured to carry three submarine-launched ballistic missile (SLBM) launch tubes. However, extensive modifications appear to have been made after that site visit.
• The most significant remodifications focused on accommodating more missiles. The missile section is much longer, containing four launch tubes for SLBMs of about the same diameter as the 1,250-kilometer (KM) range Pukguksong-1 or 1,900-km range and Pukguksong-3 (which could also accommodate the smaller-diameter KN-23 short-range ballistic missile [SRBM]), and six launch tubes most likely for the 0.5-0.6-meter (m) diameter, 2,000-km-range Hwasal-2 land-attack cruise missile (LACM). The larger tubes could not accommodate the North’s newer, larger Pukguksong-4, -5 and probable -6 SLBMs.
• This shift ’in an SSB’s mission from strategic to “tactical” is consistent with North Korea’s emphasis on “tactical nukes” over the past few years for propaganda and deterrent purposes. The Pukguksong-1, -3, and LACM could cover all of South Korea and Japan, and US bases there, from North Korean territorial waters; the KN-23 would be largely limited to South Korea. Deployment close to North Korea will also be the best way to mitigate the extremely high vulnerability of the old-tech, very noisy sub—increased by the sub’s various modifications—to allied anti-submarine warfare (ASW).
• Kim apparently also intends to convert “all” of the North’s remaining ROMEOs (up to 19) to the new configuration, raising the prospect of a future force carrying up to 80 SLBMs and 120 LACMs. This combination of more subs carrying more missiles is probably North Korea’s best option to obtain a large enough sea-based deterrent to be militarily significant. But it remains to be seen how many ROMEOs are actually converted into missile subs and how long that might take, given North Korea’s limited shipbuilding capacity. Work on future conversions is likely to be slow-going—probably at least five years per boat. Deploying additional road-mobile missiles will almost certainly remain a more cost-effective and more survivable way for North Korea to add to its nuclear strike capability.
• Kim’s vision for the future of the Navy is heavily focused on it “going nuclear,” including explicitly for reasons of cost-effectiveness. This is strangely reminiscent of the Eisenhower Administration’s “New Look” strategy from the mid-1950s that relied on nuclear weapons as a less economically costly alternative to large conventional forces.
• That vision also still includes developing nuclear-powered submarines, to which Kim said, “…we should give greater impetus.” But the speech seems to recognize such a capability will be a long time coming, barring substantial Chinese and/or Russian technical assistance.

The bottom line is that, while the rollout of the larger-capacity SSB and prospect of further conversions provides a credible path for a future (albeit theater-focused) sub-launched missile force, such a force will almost certainly continue to play second fiddle to the much larger, still growing, and much more survivable land-based ballistic and cruise missile force.

A Reconfigured Old Submarine

The newly rolled-out sub is a large-scale modification of one of North Korea’s 20 Soviet-pattern 1950s ROMEO-class diesel/electric submarines, as was expected based on the photographs released by North Korea in July 2019 of a modified ROMEO under construction inside this same construction hall at Sinpho South Shipyard. Based on those photos, analysts expected to see a ROMEO with a sail elongated to accommodate three launch tubes for SLBMs about the size of the North’s Pukguksong-1 (first revealed in 2015) or Pukguksong-3 (first flight tested in October 2019).

The rollout, however, revealed much more extensive modifications. The modified sub is about 10 meters longer than the original ROMEO, with the addition of a much longer missile section than expected based on the July 2019 photos (now some 22.4 m, including the sail), a roughly 30 percent reduction in the length of the hull forward of the sail, and a bow reconfigured to a rounded, bulbous shape.[1]

Some of the modifications are different than those seen in the July 2019 North Korean photos (others were obscured in those photos), raising the possibility that the submarine seen in 2019 and the one rolled out in 2023 are two different boats. Most likely, however, the same sub seen in July 2019 was remodified into the current configuration since then—a possibility previously flagged in 38 North. Not only do other prominent analysts seem to agree with this, but there has been no open-source reporting of a “missing” second ROMEO, another ROMEO being added to the construction hall, or the switching out of the one seen in 2019, which has not been seen outside the hall until now.[2] Remodification of the original modified ROMEO also seems to be most consistent with the lengthy period the submarine spent under modification (since about June 2014); North Korea’s original July 2019 claims that the sub’s “operational deployment is near at hand,” and Kim saying in his latest speech that in 2019 he “came here [to Sinpho South Shipyard] and gave the task of introducing advanced power systems in the existing medium-sized submarines and improving their overall underwater operation capabilities.”

A Surprising Missile Load-out

Most importantly, the longer missile section incorporates hatch-covered launch tubes for 10 missiles rather than the originally expected three. Four of the 10 hatches appear large enough for missiles of about 1.5 m in diameter,the size of the Pukgukong-1 and -3. (Such tubes also could accommodate the 0.95-m diameter KN-23 SRBM.) The other six hatches are substantially smaller, most likely covering launch tubes for the 0.5-0.6 m-diameter Hwasal-2 land-attack cruise missile (LACM)—a possibility also foreshadowed in 38 North.

• The Pukguksong-1 has an estimated maximum range of about 1,250 km but has not been flight tested from a submarine since 2016, and only then to a range of about 500 km. Its land-based version, the Pukguksong-2, was last flight tested in a lofted trajectory in 2017, but reportedly has been operationally deployed since about 2019.
• The Pukguksong-3 has an estimated maximum range of about 1,900 km and has only been flight tested once in October 2019, from a submersible test platform.
• The Hwasal-2 has been reported by North Korea to have a 2,000 km range. It has reportedly been flown on the order of 17 times, mostly from road-mobile land launchers. But it was reportedly launched twice from a submerged submarine in March 2023 (apparently from its torpedo tubes) and once in August 2023 from a corvette surface combatant ship.
• The KN-23 has demonstrated a maximum range of 800 km but has generally flown to ranges of 400-630 km that maximize its maneuver capability against missile defenses. It was launched submerged from the single-tube GORAE/SINPO test sub in October 2021.

Given the long time since the Pukguksong-1 and -3 were last launched, it is unclear whether North Korea would conduct further launches before deploying one of them on the new missile sub or if it would first conduct them from the submersible test platform or the single-tube GORAE/SINPO test sub. The Hwasal-2 has not been launched submerged from a vertical launch tube like that on the new missile sub, but it is not clear whether the North would test-launch it in such a configuration before loading it onto the new sub. It might regard the previous underwater torpedo-tube testing, in conjunction with the large number of other launches, as enough to be confident in Hwasal launches from the new sub. The KN-23 presumably would be good to go on the new SSB. (Interestingly, by September 8, the submersible test platform had been moved near where the new submarine is currently berthed.)

A Different Mission

Based on the July 2019 North Korean photos, the missile sub was generally expected to have a “strategic” mission, albeit within the confines of the range of the missiles it would carry. North Korea has since displayed, but has not yet flight tested, three progressively larger and longer-ranged SLBMs. It was generally expected that the launch tubes would have been modified over time to carry one of these, thus increasing the sub’s threat range and “strategic” utility, although at least the third and most recent SLBM was probably too large for that sub.

It is now clear that the new missile sub will not be carrying any of the longer-range SLBMs, which exceed two m in diameter and thus are too large for its launch tubes. Kim Jong Un has specifically called the new SSB a “tactical nuclear submarine,” carrying “tactical nukes,” that will “stand in different parts of our territorial waters.” The refocusing of the sub on “tactical nukes” is consistent with North Korea’s emphasis on such weapons over the past few years for propaganda and deterrent purposes. An additional possible motivation behind the 2019 remodification of the sub may have been to devise a political counter to South Korean plans that the North could have become aware of in 2015 or 2016 to deploy short-range ballistic missiles (SRBMs), albeit with conventional warheads, on its next generation of submarine.

From North Korean territorial waters, the new sub could cover all of South Korea and Japan, and US bases there, with the Pukguksong-1, -3, and LACM. The longer-ranged Pukguksong-3 and Hwasal-2 would have such coverage from any part of North Korea’s east coast. The KN-23, however, would be largely limited to South Korea. Deployment close to North Korea will be the best way to mitigate the extremely high vulnerability of the old-tech, very noisy ROMEO-based missile sub to allied anti-submarine warfare (ASW) assets—a vulnerability increased by the speed and maneuverability penalties and probably reduced battery capacity (and likely increased noise) caused by its various modifications.

A Larger Sub-launched Missile Force

Not only does the new sub carry more missiles than previously expected, but Kim also noted his “intention to turn all the other existing medium-sized submarines into attack ones like this one,” which he termed “a revolutionary step for ensuring maximum efficiency,” yielding “a rapid improvement of our national defence capabilities.” This presumably means Kim intends to convert all the remaining 19 ROMEOs, which would yield an ultimate force of 80 SLBMs and 120 sub-launched LACMs. This combination of more conversions of existing subs carrying more missiles would help address a key shortcoming of the apparent three-SLBM track North Korea had been on in 2019: the difficulty of obtaining a large enough SLBM force to be militarily significant in anything resembling a cost-effective way compared to simply deploying more land-mobile missiles.

But it remains to be seen how many ROMEOs are actually converted into missile subs (some boats may be too decrepit at this point, and conversion will sacrifice the missions those boats currently fulfill), how long that process will take, and whether Kim will stay the course or change his mind at some point about continuing with the conversions or about converting boats to the current configuration. Although Kim spoke in terms of “five or ten years” to “usher in an era when our Navy changes,” North Korea has limited capacity for this kind of work. The construction hall at Sinpho South that modified this sub can accommodate two boats at once, but currently appears to be empty.[3] The even larger construction hall nearby could also be used for this purpose, but may be intended for future, larger submarines—possibly including future nuclear-powered submarines if North Korea is able to achieve them (see below). We also do not know the rate at which it could manufacture all the new submarine hull sections required to convert 19 more boats.

It apparently took five years to get the current sub from ROMEO to first-iteration missile sub, and another four years to get it to its final configuration. Now that the North has apparently settled on the design for what Kim called “the standard type of tactical nuclear submarine,” and has experience with the first conversion, future production can presumably be faster and more efficient than nine years per boat—but five years each would seem a reasonable minimum.

Furthermore, the construction and operation of each additional submarine will almost certainly remain a less cost-effective way of adding nuclear strike capability than producing and deploying four more road-mobile launchers for medium-range missiles and two more five-tube road-mobile LACM launchers. The survivability advantage of North Korea’s field-deployed land-mobile missiles over ROMEO-based SLBMs is even greater, given the further reduction in survivability from the missile subs’ modifications.

A “New Look” Navy

One of the most remarkable things about Kim’s speech was his focus on “go[ing] nuclear “as “the most important thing” in strengthening the Navy, calling it “a rapid improvement of our national defence capabilities with the nuclear deterrence as the core.” Kim’s citing of the nuclear sub build-up as a direct alternative to “the past” where “we, in developing the submarine industry, focused on building many small and fast submarines,” and terming the nuclear build-up “a revolutionary step for ensuring maximum efficiency,” is strangely reminiscent of the Eisenhower administration’s “New Look” strategy from the mid-1950s that relied on nuclear weapons as a less economically costly alternative to large conventional forces. The nuclearization of the Navy is also evident in the deployment of LACMs on a surface ship and the launching of a KN-23 SRBM from the GORAE/SINPO test sub.

A Renewed Commitment to Nuclear-powered Subs

Kim’s current vision for a nuclearized Navy also includes nuclear-powered submarines. He noted “our development-oriented, prospective plan for building nuclear submarines” in conjunction with the nuclear-armed, conventionally-powered modified ROMEO missile subs as “a ‘low-cost, hi-tech strategy’” combination. Kim avers that he gave the Sinpho South Shipyard the task of “commissioning” nuclear-powered subs “to the combat ranks” four years ago.

But it seems clear that, although Kim said, “…we should give greater impetus to the building of nuclear-powered submarine[s],” he recognizes such subs will be a long time coming. This may help explain why he also said that “whether we launch powerful nuclear submarines sooner or later,” it is “how quickly our Navy is equipped with nuclear weapons… [that] will have a critical bearing on our state’s destiny.” Indeed, he claimed that the rollout of the new ROMEO-based missile sub “will be as burdensome to our opponents as is our building a new-type nuclear-powered submarine.” A nuclear-powered submarine is highly unlikely to be part of Kim’s inventory “when we, in five or ten years, usher in an era when our Navy changes” unless North Korea receives substantial technical and material assistance from China and/or Russia.

The Bottom Line

The long-awaited rollout of the modified ROMEO missile sub makes good on an effort underway since at least 2014. The apparent remodification of the sub to carry a larger number of missiles for a “tactical nuclear” role against South Korea and Japan (and US bases there), and the newly-announced plan to convert more and perhaps all other ROMEOs to this configuration, is probably Kim’s best near-term option for a militarily meaningful sea-based nuclear deterrent. Kim may well also see his “New Look” nuclearized Navy as the best way to get combat value out of an old, technically lagging fleet. But converting all 19 of the other ROMEOs is likely to take much longer than five-10 years, and a nuclear-powered submarine, however much additional impetus it is going to receive, is unlikely to be part of the picture for a long time. The missile sub force will almost certainly continue to play second fiddle to the much larger, still growing, and much more survivable land-based ballistic and cruise missile force.

1. [1]

   See H.I. Sutton, “North Korea’s New Missile Submarine: Hero Kim Gun-ok,” Covert Shores, September 8, 2023, http://www.hisutton.com/North-Korea-Submarine-Hero-Kim-Gun-ok.html; and PBfirefly. Twitter Post, September 8, 2023, 1:31 a.m., https://twitter.com/peacebread1/status/1700018883531546967.

2. [2]

   For example, see Joseph Dempsey. Twitter Post, September 7, 2023, 7:21 p.m., https://twitter.com/JosephHDempsey/status/1699925968750604582; and Ankit Panda, “North Korea’s nuclear ambitions reach new stage with ballistic missile submarine,” NK Pro, September 8, 2023, https://www.nknews.org/pro/north-koreas-nuclear-ambitions-reach-new-stage-with-ballistic-missile-submarine.

3. [3]

   See Nathan J Hunt. Twitter Post, September 8, 2023, 2:04 a.m., https://twitter.com/ISNJH/status/1700027215839424514; and Tarao Goo. Twitter Post, September 8, 2023, 10:58 a.m., https://twitter.com/GreatPoppo/status/1700161655504507001. The latter notes that the presence of another boat cannot be ruled out.