On March 8, North Korean (Democratic People’s Republic of Korea or DPRK) media reported that Kim Jong Un, during undated visits to major shipyards, “learned about the building of a nuclear-powered strategic guided missile submarine, which is being pushed forward according to the decisions of the Eighth Congress of the WPK [Workers Party of Korea].”[1] Associated photos showed Kim walking past a portion of the hull of a new-type submarine (sub) under construction inside a building.
The key components of a nuclear missile sub are the hull, the missile system it carries, and the nuclear reactor wrapped within it. The current status of all three is uncertain.
The photos indicate the new sub’s hull will be much larger than what is used for North Korea’s current subs, and could be about the size of the first US nuclear-powered ballistic missile submarine (SSBN). Without knowing how much of the sub has already been completed, or its final intended configuration, there is really no reliable way of assessing when construction might be complete. In any case, the nuclear reactor likely is the pacing item.
The DPRK calling the new sub “strategic,” as opposed to the conventionally-powered “tactical nuclear attack submarine” launched in 2023, suggests the new sub is intended to be targeted on the US. It is most likely intended to carry a ballistic missile able to reach the US from defended waters near North Korea. The best current candidate is a submarine-launched missile first paraded in April 2022 but not yet flight-tested, presumably designated the Pukguksong-6. Based on past DPRK practice, such a missile could be sub-deployable in a few years.
There is no open-source reporting that North Korea has yet built or tested a submarine nuclear reactor. However, the history of its weapons programs indicates that a fully indigenous “good enough for the DPRK” sub reactor cannot be ruled out. How long that reactor would take, and how far along the North is in developing one, is quite unclear, but its track record with other reactors suggests it could take at least several years. Russian aid (for which there is no current evidence) could potentially shave a few years off this.
The time required to complete all three of these components, plus fitting out and sea trials, means there is probably still a good amount of time, and likely quite a few observables, before the hull seen in the construction hall turns into an actual threat. But a single SSBN provides little additional strategic capability; an SSBN force of at least several boats will be required, taking yet more time. Even then, this force would almost certainly remain significantly less capable and survivable than North Korea’s much larger (and probably still growing) force of land-based road-mobile intercontinental ballistic missiles (ICBMs).
The Mystery: Nature and Progress of the Submarine Hull
It is unclear from the North Korean photos how much of the new submarine has been built (the bow and stern are not depicted, for example, nor the uppermost portions), or its final intended size and configuration. Extrapolating from Kim Jong Un’s height leads analysts to estimate the new sub’s diameter at 11.5 to 12.5 meters, much larger than North Korea’s previously widest indigenously-produced submarine, the Gorae-class one-tube missile test sub (6.7 m diameter). This diameter has led analysts to suggest the new sub will have a weight (displacement) of some 5,000 to 8,000 tons,[2] much larger than the 3,000 tons ascribed to the Sinpo-C, the North’s new conventionally-powered missile submarine (SSB), converted from a Soviet-designed Romeo-class sub, which was rolled out of its construction hall in September 2023. Although the full hull has not been seen, one source puts the new sub’s length at “at least 117 m,” perhaps based on the length of the building at the Pongdae Submarine Factory in Sinpho South Shipyard—where the sub is being constructed—being about 195 m long.
In comparison, the first US nuclear-powered ballistic missile submarine (SSBN), the USS George Washington, launched in 1959, was 116.36 m long, 10.06 m in diameter, and displaced 5,900 tons surfaced.
It is unclear how long the new North Korean sub has been under construction, or when it may be complete. As is often the case with North Korea, publicly available information is sparse and sporadic, complicated by the indoor construction of the submarine and likely DPRK concealment practices.
The construction hall at the Pongdae Submarine Factory, where the new submarine is being built was refurbished and extended to accommodate a longer boat between about July 2014 and November 2015, based on commercial satellite imagery.[3]
Imagery from September 2016 shows a jig or submarine hull section 11 meters in diameter near the Sinpho South Shipyard’s fabrication hall.[4] This diameter is consistent with the new sub, but too large for the 6.7 m diameter Sinpo-C SSB also being built at the time in a different building.
Imagery from March 2019 shows probable welding jigs measuring 11 meters in diameter in the component staging area near the extended construction hall.[5]
Between April 2019 and November 2022, components were generally absent from the staging area. This might reflect a construction hiatus due to North Korea’s COVID-19 shutdown.[6]
The first public indication that the DPRK was committed to building a nuclear submarine was in January 2021, when Kim Jong Un reported to the Eighth Party Congress that “the design of new nuclear-powered submarine [sic] was researched and was in the stage of final examination” and that “the tasks were brought up to… possess a nuclear-powered submarine and an underwater-launch nuclear strategic weapon.”
In September 2023, Kim stated that “we should give greater impetus to the building of nuclear-powered submarine [sic],” while also stating that the conventionally-powered Sinpo-C SSB “will be as burdensome to our opponents as is our building a new-type nuclear-powered submarine,” and “when equipped with nuclear weapons, it is precisely a nuclear submarine.”[7]
In January 2024, while “guiding” the test of a submarine-launched cruise missile, Kim reportedly “learned in detail about the building of a nuclear submarine… discussed the issues related to the building of a nuclear-powered submarine… indicated the immediate tasks to be carried out by relevant sectors and state measures to be taken, and made an important conclusion on the ways to implement them.”[8]
Imagery from February 18, 2024, revealed an approximately 11-meter component (consistent with the diameter of the new sub) being prepared for movement into the construction hall assessed to be used for the nuclear submarine, and additional 11-meter components or their jigs were seen on imagery from March 2024.[9]
In October 2024, a South Korean parliamentarian reported that the ROK Defense Intelligence Agency had detected “partial signs that appear to be the start of the submarine’s construction,” but that because “construction is still in its early stages, further confirmation is needed on whether it is nuclear powered,” although the sub was larger than existing vessels.
Putting these pieces together, the overall nuclear submarine program may have started as early as 2014 (given when extension of the construction hall started). The hull of the new sub appears to have been under construction for some eight years, although construction may have been suspended for about three of those years due to COVID-19 and may have lagged from late 2022 until early 2024.
Without knowing how much of the sub has already been completed, its final intended configuration, or the status of any nuclear reactor for it (see below), there is really no reliable way of assessing when construction might be complete. Other sources have estimated the new sub could be launched in one to two years, or two to three years. Even if launched, however, it remains to be seen whether a suitable missile or especially a nuclear reactor is ready at the same time.
The Riddle: What Missile(s) the New Sub Will Carry
The available information does not permit associating the new submarine with a specific missile system; the new photos do not show the top of the sub where the missile hatches would be, and it is not even clear whether the missile section of the sub has been built yet. (One analyst nonetheless opined that the sub can carry “around 10” missiles, perhaps keying off the number carried by the Sinpo-C SSB.) Many outside analysts interpret the North Korean description “strategic guided missile submarine” as indicating the new sub will carry nuclear-armed (thus “strategic” missiles).[10] One analyst apparently has interpreted the North’s use of “guided missile submarine” rather than “ballistic missile” to indicate that the new sub “will carry cruise missiles but not ballistic ones,” making it a nuclear-powered cruise missile sub (SSGN). Most other analysts assume the sub will carry submarine-launched ballistic missiles (SLBMs), making it an SSBN.
The most interesting thing about North Korea’s characterization of the sub as being “strategic” is how that distinguishes the new sub from the Sinpo-C SSB launched in 2023, which Kim Jong Un called a “tactical nuclear attack submarine” and is apparently to be armed with a mix of short- or medium-range ballistic missiles and vertically launched land-attack cruise missiles (LACMs). This suggests the new sub is intended to carry longer-range missiles than the SSB, and to have a mission other than theater strike.
The new sub is most likely intended to carry a longer-range SLBM, perhaps the “solid-fuel engine-propelled inter-continental underwater ballistic rocket” mentioned by Kim Jong Un in his January 2021 report as being in development. Use of an SLBM on the new sub technically would not be inconsistent with the North’s “guided missile submarine” characterization, since both cruise and ballistic missiles (like any “missile”) are “guided.” A sufficiently long-range SLBM also would allow deploying SSBNs close to North Korea so they can be defended by DPRK naval, air, and air defense forces against the substantial anti-submarine warfare (ASW) capabilities of the US and its allies. Using long-range SLBMs in this way would be the most sensible way for the North to maintain the most survivable, credible, sea-based deterrent against the US.[11]
A LACM payload, or a mixed SLBM/LACM payload like the Sinpo-C, cannot be ruled out. But to strike the US with LACMs a DPRK nuclear-powered submarine would have to make a long transit through Allied submarine detection networks to locations within missile range—North Korea’s have thus far demonstrated ranges of 1,500-2,000 km[12]—leaving the sub highly vulnerable to US and Allied ASW forces. This would be particularly true if, as is commonly assessed, a North Korean-made nuclear sub (and a first-generation one in particular) will be very noisy and thus quite susceptible to acoustic detection.
The best current candidate for deployment on the SSBN is the largest SLBM thus far revealed by North Korea, presumably designated the Pukguksong-6. It was first paraded in April 2022 but not yet flight-tested. Based on its size (about 13 meters long and 2.2 meters in diameter), this missile, in theory, could be capable of a range of upwards of about 12,000 km (comparable to the similar-size 1990 US Trident-II/D5 SLBM)—enough to strike all of the continental US from waters close to North Korea. (The North probably would use a taller “turtleback” missile section to accommodate a 13-meters-long missile and its ejection and launch gear in a sub with an 11.5-12.5 meter- diameter hull.)
If North Korea intends to deploy a longer-range SLBM on the new sub, we can expect it to conduct ground (static) testing of the missile’s main stage motors, to modify land-based test facilities and/or one of its test barges to permit “pop-up” ejection tests of the larger new missile, and begin missile flight-tests from land and/or a barge and then from the Gorae-class test sub. This entire sequence could take a few years, although it is possible that some of the early steps might have already taken place without open-source indications and Kim Jong Un could mandate a faster timeline. As of now, however, there would appear to be plenty of time for SLBM development to occur before a new sub with an operating reactor was ready to put to sea.
The Enigma: Status of a Submarine Nuclear Power Plant[13]
The biggest apparent gap in realizing a North Korean nuclear-powered submarine is the small nuclear reactor that would power it, and without which there is no nuclear sub.[14] Unlike the sub and its candidate missiles, North Korea has made no mention of a naval reactor program distinct from the above references to “nuclear-powered submarines;” no associated photos or videos of a small reactor have been released either. That said, the above analysis suggesting the nuclear submarine program may have started as recently as 2014 would also imply that work on a suitable reactor started at least that recently.
There is no open-source reporting that North Korea has yet built or tested such a reactor, although it cannot be ruled out that concealed development work has been going on. Based on other countries’ practice, the North would be highly likely to build and test a land-based prototype of a submarine reactor prior to installing it in a sub (although this is not strictly required), but no test reactor has thus far been identified. Pyongyang’s construction and initial operation of a pressurized water reactor (PWR) known as the EWLR (Experimental Light Water Reactor), would give it basic familiarity with building a PWR for a sub. But the industrial-sized EWLR using low-enriched uranium is a far cry from the very small and compact, higher-pressure reactor probably using highly-enriched uranium (HEU) needed for a submarine. Moreover, a sub reactor would need to be engineered to remain operational and reasonably reliable at depth in a moving submarine with no outside support under combat conditions, and provided with heavy shielding to protect its crew from radiation at least long enough to complete a potentially long deployment.
Although a number of US nuclear experts and former submariners have opined that North Korea would be unable to build a submarine nuclear reactor or a nuclear submarine on its own without foreign assistance, the history of North Korean weapons programs indicates that a fully indigenous “good enough for the DPRK” nuclear-powered submarine cannot be ruled out. It may take a long time (or may have taken a long time thus far), and may not be very good, but an eventual indigenous nuclear-powered submarine is within the realm of possibility. How long the reactor for such a sub would take, and how far along the North is in developing one, is anybody’s guess. But the EWLR began construction in November 2010 and did not begin operating until October 2023, suggesting a sub reactor could take at least several years.
Clearly, the most feasible way for North Korea to get a reasonable submarine nuclear power plant soon would be with foreign assistance. The degree to which Pyongyang has or will receive such aid is unknown. Russia would be the most likely governmental source of such assistance given the strengthened Kim-Putin relationship and North Korea’s provision of assistance to Russia’s war in Ukraine. Western governments clearly are concerned about Russia providing nuclear sub technology,[15] but there is no open-source evidence of this apart from a few claims by Ukrainian officials[16] that may at least in part serve propaganda purposes. Assistance from China or Chinese entities also cannot be ruled out, although there has been no evidence of this kind of cooperation either.
If substantial Russian nuclear submarine assistance is being provided to North Korea, it presumably started in the wake of the September 2023 Kim visit to Russia. That does not provide a great deal of time until now for any assistance to have been assembled and provided by the Russians, and received, assimilated, and incorporated into hardware by the North Koreans. The new submarine structure apparently was already under construction in September 2023. The sub’s reactor presumably is much less far along, and so there is more opportunity for Russian assistance to be useful and have a tangible effect, and North Korea probably would benefit more from reactor assistance in any case.
That said, even extensive Russian nuclear submarine assistance does not necessarily translate into quick results. India started receiving comprehensive aid from the Soviet Union and then Russia in the 1980s, including leasing and operating actual nuclear submarines, extensive assistance in submarine reactor design (after India’s indigenous efforts proved unsuccessful), design and construction assistance for nuclear submarines, Russian equipment to help fit out the actual constructed submarines, and SLBM technology.[17] Even so, its Advanced Technology Development program, which started in 1983, did not result in a commissioned SSBN until 2016. North Korea might not take as long to build a sub reactor with Russian assistance, but it might only shave a few years off what the North Koreans could do on their own.
Implications
We are probably at least several years away from a fully complete, reactor-equipped North Korean SSBN with adequately tested SLBMs. Even then, as with the first Sinpo-C SSB that has not yet begin sea trials, a year or two of fitting out probably would be required before an SSBN rolled out of the construction hall was ready for sea trials, with additional time required to reach operational status. This means there is likely still a good amount of time, and likely quite a few observables, before the hull seen in the construction hall turns into an actual threat.
Moreover, a single SSBN provides little additional strategic capability compared to North Korea’s established force of road-mobile ICBMs that presumably will still be growing. An SSBN force of at least several boats will be required—more depending on each sub’s missile and warhead load—to begin to make a substantial contribution to Pyongyang’s strategic forces, taking yet more time to eventuate. But even this force would almost certainly remain significantly less capable and survivable than the much larger mobile ICBM force.
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