N. Korea’s Nuclear Detterence

North Korea remains one of the most opaque nuclear states in the world, offering little information about its nuclear fuel cycle (NFC) and weapons programme. Yet, the structure and scope of its programme have significant implications for global stability. There is a need for a robust analytical methodology that can derive insights from limited data to assess North Korea’s NFC and nuclear strategy. Such analysis is crucial for the international community, particularly should diplomatic avenues reopen, providing a rare opportunity to curtail or reverse North Korea’s nuclear ambitions.

This paper builds on work conducted by the Verification Research, Training and Information Centre (VERTIC) to model North Korea’s NFC. It integrates VERTIC’s estimates of North Korea’s production of weapons-usable fissile material – specifically weapons-grade plutonium and highly enriched uranium (HEU) – into a comprehensive framework that assesses Pyongyang’s nuclear strategy. By examining the trade-offs in arsenal structure amid fissile-material constraints, the paper offers a nuanced exploration of North Korea’s approach to deterrence.

North Korea’s nuclear arsenal serves two purposes: to deter distant nuclear powers such as the US; and to counter nearby non-nuclear adversaries such as South Korea, United States Forces Korea and Japan. Should deterrence fail, North Korea aims to contain and win any conflict on the Korean Peninsula, ensuring the regime’s survival. To meet these objectives, North Korea appears to be developing both a strategic and more versatile arsenal – featuring smaller-yield weapons and varied delivery platforms – for regional and battlefield use.

This paper delves into the various strategies North Korea might employ to optimise its arsenal, given its fissile-material constraints. Based on the analysis of force-structure requirements to achieve credible deterrence, it is assessed that North Korea might aim to develop 25–35 strategic thermonuclear warheads and 80–200 short- and medium-range single-stage warheads.

Given its fissile-material inventory at the end of 2023, as analysed by VERTIC, North Korea might have already developed approximately 21–23 composite (plutonium and HEU) thermonuclear warheads. With its remaining HEU inventory, the state could additionally have manufactured 51–165 single-stage nuclear warheads, supporting its regional deterrence goals (via tactical and battlefield deployment). Although North Korea is still likely constrained by its plutonium supply, evidence suggests that its nuclear material production goals may have been met or are nearing sufficiency.

Consequently, North Korea may choose to work within its existing fissile-material resources, deeming its arsenal adequate. Alternatively, it might ramp up production to exceed current needs, maintain a surplus stockpile or continue enhancing its arsenal’s capabilities to align with the availability of its fissile material.

While the future trajectory of North Korea’s nuclear strategy remains uncertain, this paper provides an analytical framework that enriches understanding of North Korea’s nuclear force structure given its fissile-material production. By offering these insights, the paper seeks to advance a more informed international dialogue, recognising the critical need for preparedness in the face of limited information.

Introduction

North Korea provides very little information to the international community about its nuclear fuel cycle (NFC) and nuclear weapons programme. However, the structure and extent of its programme impacts geopolitical stability. An analytical methodology that can work with only limited data to assess both North Korea’s NFC and its nuclear arsenal strategy is therefore crucial. Such a methodology would help to prepare the international community should negotiations restart with North Korea and an opportunity present itself to restrain or roll back its nuclear programme.

To address this issue, the Verification Research, Training and Information Centre (VERTIC), the James Martin Center for Nonproliferation Studies (CNS) and RUSI, with funding from Global Affairs Canada, embarked on a multi-year project to assess the entirety of North Korea’s WMD programme. For the nuclear programme, the whole NFC was modelled – from uranium mining to weapons-grade material production – using software provided by the UK’s National Nuclear Laboratory (a process hereafter referred to as the “VERTIC model”), with the help of open source information, including extensive satellite imagery analysis. The result of this work was a range of estimates of North Korea’s inventory of fissile material, given different plutonium reprocessing and uranium enrichment scenarios.

The purpose of this analysis, conducted by the Open Nuclear Network (ONN), a programme of PAX sapiens, is to take the fissile-material inventory as it stood at the end of 2023 and assess North Korea’s nuclear force structure. It specifically aims to address two research questions:

• What key fissile-material demands are derived from various strategies that North Korea might adopt to structure its nuclear arsenal?

• How might North Korea’s nuclear arsenal structure have been affected by fissile-material constraints?

The paper has four chapters. Chapter I explores North Korea’s nuclear strategy, Chapter II examines its potential nuclear arsenal size and Chapter III addresses the potential designs of its nuclear weapons. Chapter IV then explores the fissile-material demands and constraints that might affect each of those areas. The paper concludes with overall observations and future recommended work.

Methodology

Complete answers to the above research questions can only be provided by a handful of North Korean leaders and nuclear decision-makers. Nonetheless, ONN provides here an analytical framework with which to evaluate North Korea’s nuclear weapons strategy, including examining trade-offs that the country must consider when deciding how to structure its arsenal.

To address the first research question, ONN analysed the composition of North Korea’s nuclear arsenal. This encompassed a comprehensive literature review of North Korea’s nuclear strategy, arsenal size and weapons designs. This aided in evaluating North Korea’s declared nuclear strategy and identifying the types of weapons developed, quantities of weapons required for different deterrence strategies and the corresponding amount of fissile material. Further evaluation was made of the nuclear weapon varieties and potential designs. This primarily entailed measurements based on accessible open source images of North Korean nuclear armaments.

To address the second research question, ONN took the cumulative fissile-material inventory estimates – as they stood at the end of 2023 – from the VERTIC model, assessed trade-offs North Korea might make under fissile-material constraints and analysed other areas that may potentially constrain the programme (such as tritium production and high-explosives testing capabilities).

From this picture of North Korea’s fissile-material demands and constraints, several conclusions were drawn about what North Korea’s current nuclear weapons arsenal may look like and how the state might be hoping to develop its nuclear weapons programme in the future. While the fissile-material models ran to the end of 2023, the analysis was completed by March 2024. The methodology presented here can be adapted if assumptions change or new information is revealed. This allows changes in the arsenal and fissile-material demands/constraints to be seen.

There are clear bounding constraints or design imperatives deriving from the development of delivery systems that are relevant to fissile-material requirements. However, this study does not consider in detail the implications for weapons design of the development of specific North Korean delivery systems. This research also focused primarily on analysing the present situation; historical situations are considered only to provide clarity on the present. These two aspects, in addition to other recommended follow-on work, are discussed further in Conclusions at the end of this paper.

I. Nuclear Strategy

Based on a state publication from 2013, North Korea classifies weapon size according to yield (with “super-large” above 1 megatonne, all the way down to “super-miniaturised” at less than 1 kilotonne). North Korea reportedly further classifies its nuclear arsenal by use: tactical (to destroy personnel and equipment on the frontline or operational tactical depth zone via tactical ballistic missiles or artillery shells); battlefield (to strike regional targets via medium-range missiles); and strategic (to strike large cities, industrial centres and other strategic targets via intercontinental or submarine-launched ballistic missiles).

According to state media, North Korea has begun to gradually shift its nuclear strategy from solely intending nuclear weapons for strategic deterrence to including potential regional and battlefield use. Additionally, North Korea is looking to diversify its nuclear forces to operate in a variety of environments and with a variety of delivery means and ranges, including greater emphasis on the nuclearisation of North Korea’s navy. This means developing nuclear weapons with a range of yields that can be delivered by a wide range of platforms

In 2022, North Korea released its codified nuclear doctrine, the Law of the Supreme People’s Assembly of the DPRK on the State Policy on the Nuclear Forces. The new law is a departure from its 2013 Law on Consolidating Position of Nuclear Weapons State, primarily in that it provides more details on North Korea’s nuclear command and control. It also specifies the following conditions for nuclear weapons use, which were absent from the 2013 law:

• “In case an attack by nuclear weapons or other weapons of mass destruction was launched or drew near is judged.

• In case a nuclear or non-nuclear attack by hostile forces on the state leadership and the command organization of the state’s nuclear forces was launched or drew near is judged.

• In case a fatal military attack against important strategic objects of the state was launched or drew near is judged.

• In case the need for operation for preventing the expansion and protraction of a war and taking the initiative in the war in contingency is inevitably raised.

• In other case an inevitable situation in which it is compelled to correspond with catastrophic crisis to the existence of the state and safety of the people by only nuclear weapons is created.”

Supreme Leader Kim Jong-un also stated that North Korea’s “status as a nuclear weapons state has now become irreversible” and there would be no more denuclearisation talks.

Further attempting to solidify its position as a nuclear state, in September 2023 North Korea amended its constitution to read:

The Democratic People’s Republic of Korea relies on the all-people, nationwide defence system. The Democratic People’s Republic of Korea, as a committed nuclear state, shall accelerate the development of nuclear weapons to ensure its rights to existence and development, deter war and safeguard peace and stability of the region and the rest of the world.

With this amendment, North Korea is now the only nuclear-armed state to have any reference to nuclear weapons in its constitution.

Since October 2023, changes in the political landscape may have implications for North Korea’s nuclear strategy. One such change is the reclassification of South Korea as the “principal enemy”, departing from a previous relationship aimed at unification. This shift followed North Korea’s complete suspension in November 2023 of the Comprehensive Military Agreement, accompanied by the remilitarisation of former buffer zones. Subsequently, North Korea’s rhetoric towards South Korea has become more confrontational, evidenced by tests and inspections of close-combat weaponry, and strategic maps of Seoul. These developments may lead to greater emphasis on the development of tactical and battlefield nuclear warheads. Reported and observed technical advancements also indicate a continued expansion of medium- and long-range missiles and the development of space capabilities. Notable examples include the testing of a new solid-fuelled intermediate-range ballistic missile equipped with a manoeuvrable re-entry vehicle payload, another ICBM test and North Korea’s first successful spy satellite launch.

These statements and actions, taken together, suggest that North Korea is looking to deter both a large, distant nuclear adversary (that is, the US) and a range of regional conventionally superior, non-nuclear adversaries (that is, South Korea, United States Forces Korea (USFK) and Japan) by making its arsenal useable (with smaller yields, a greater variety of platforms and an acknowledgement of battlefield use). This approach is consistent with its stated goals of being able to deter, contain and win a conflict on the Korean Peninsula in order to preserve the state.

II. Nuclear Arsenal Size

This chapter explores North Korea’s potential nuclear arsenal size. First, a literature review is presented of external open source estimates of North Korea’s fissile material and nuclear weapons amounts. Following this, a potential numerical force structure is developed through analyses of likely defence needs and analogous nuclear weapon programmes.

Literature Review

External evaluations from the time of North Korea’s last reprocessing campaign in 2022 to the analysis cut-off period of March 2024 provide a range of assessments about the amounts and types of nuclear material and weapons that North Korea possesses (Table 1). These align closely with the outputs from the VERTIC model and ONN’s subsequent analysis (discussed later).

▲ Table 1: External Open Source Estimates of North Korea’s Fissile-Material and Nuclear Weapons Amounts from 2022 to March 2024. Source: The authors. Note: Additional open source estimates have been released since the end date of this analysis (March 2024) and are included in a separate literature review.

Potential Numerical Force Structure

Based on North Korea’s own definitions, the country seems to divide its arsenal into strategic (long-range), battlefield (medium-range) and tactical (short-range) nuclear weapons. ONN has used these categories in its own analysis of the potential number of weapons that North Korea may be seeking to fulfil its goals.

Strategic Weaponry

The strategic weaponry of North Korea’s arsenal is seemingly aimed at deterring the US from invading or attacking the country – such an invasion would present an existential risk to the regime. Given this overarching goal, it is plausible to assume that North Korea views the US as minimally deterred if North Korea can, with a strategic nuclear weapon, hold at least one target on the continental US at risk. To do this, North Korea might seek a quantitative capacity that would be significantly likely to breach US missile defences working at optimal performance levels. This assumes a conservative level of North Korean risk tolerance. If North Korea is prepared to take on more risk, the country might require a smaller strategic arsenal to accomplish its desired level of deterrence.

The US ICBM defence architecture contains 44 ground-based interceptors (GBIs), based in Alaska and California. At least two GBIs would be fired per incoming re-entry vehicle (RV), meaning these 44 GBIs might intercept 22 RVs, assuming the capacity for simultaneous launch. An additional 20 GBIs will reportedly be added by 2026, meaning eventually 32 RVs might be intercepted.

To be reasonably confident that at least one nuclear weapon would breach US missile defences and strike a target on the mainland, North Korea would need to launch at least 23 weapons loaded on separate ICBM RVs (33 by 2026). For the purposes of this analysis, 25–35 weapons is the range considered, given that at least two weapons will likely be undergoing maintenance or be in transport at any one time.

This “worst-case” estimate – from North Korea’s perspective – assumes perfect operation of US missile defence and that North Korea does not take evasive measures (for example, dummy RVs). As mentioned above, modified assumptions may mean that North Korea might be satisfied with a smaller strategic arsenal. On the other hand, varying the assumptions might end up with North Korea requiring a larger strategic arsenal; for example, to account for the potential for missile or warhead failures, or the loss of nuclear weapons due to an attack before they can be used.

Tactical and Battlefield Weaponry

The tactical and battlefield weaponry of the North Korean arsenal is aimed at deterring large-scale attacks or invasion from South Korea, USFK and Japan by establishing the threat that North Korea has the capability and willingness to use nuclear weapons in the region (either on the frontlines or at regional targets on the Korean Peninsula, Japan or surrounding areas). The number of weapons North Korea might be building for this portion of its nuclear arsenal is more difficult to assess. What constitutes “enough” weapons for warfighting or regional use varies state to state and person to person.

One possible insight into North Korea’s tactical strategy was provided in the March 2023 reveal of the Hwasan-31, a new type of nuclear warhead that was depicted as part of eight different delivery systems. While this may or may not be an accurate representation of North Korea’s current capabilities or actual intent, it does suggest one way North Korea may decide to shape its arsenal: with nuclear weapons on a variety of delivery systems.

The next step is to assess how many weapons per delivery system North Korea might be aiming to deploy. These could be all the same design but mated to specific, different delivery systems. Or they could be different designs made specifically for deployment on one type of delivery system. The implications for fissile-material amounts are negligible. There is no universally accepted way to assess this, as every state has vastly different strategies when deciding how many warheads should be deployed on each delivery system. Additionally, there are even fewer states that actively deploy tactical or battlefield nuclear weapons.

In this case, the probable compositions of the Indian and Pakistani nuclear arsenals are helpful guides as each of these states is trying to hold many targets at risk in their neighbour’s territory. Hans M Kristensen and Matt Korda assessed that for these states there are 10–25 warheads mated to each delivery vehicle type. This range can be used to consider the North Korean arsenal. Parts of the Indian and Pakistani nuclear arsenals are for similar uses – on short- or medium-range ballistic missiles – and are relatively small nuclear programmes.

If 10–25 warheads are similarly mated to each of North Korea’s eight notional delivery systems, North Korea could aim to have 80–200 tactical weapons deployed. This would presumably be “enough” tactical and battlefield nuclear weapons for warfighting, as it would allow the state to hold at least 80 targets at risk, both on the Korean Peninsula and further afield in the Northeast Asia region. As discussed earlier, there are many potential targets in the region for North Korea’s tactical and battlefield nuclear weapons, and this number provides flexibility and extensive coverage for the country’s targeting plans.

As with the strategic arsenal, varying any of these underlying assumptions might mean a larger tactical and battlefield arsenal (for instance, to account for greater caution on the potential for missile or warhead failures, the loss of nuclear weapons due to an attack before they can be used, a larger set of identified targets, or more warheads per system) or a smaller one (such as to account for a higher level of North Korean risk tolerance, or fewer delivery systems or warheads per system).

III. Nuclear Weapons Designs

This chapter analyses potential nuclear weapon designs North Korea may have incorporated into its arsenal. First, a literature review is presented, followed by a summary of imagery analysis conducted on North Korea’s displayed nuclear devices. This is followed by a discussion on design element trade-offs, and concludes with a list of designs North Korea may have developed.

Literature Review

External experts and analysts have suggested that North Korea may have tested or may be developing the following possible nuclear weapons designs. Not all listed designs are considered plausible (either by the original authors or ONN).

Single-stage device:

• A 15-kt device, with a diameter of 40–60 cm.

• An HEU device with a total warhead weight of 500 kg.

• An unboosted 250-kt HEU device.

• A tactical warhead (Hwasan-31) with a levitated pit capable of being used in different delivery vehicles, and with a diameter of 40–45 cm and yield of 10 kt.

• A 15-kt composite core made of a 2-kg plutonium sphere with 10–15 kg of HEU shells.

• A thermonuclear “layer cake” device with a 3.5-kg plutonium core, 80 kg of uranium (HEU, low-enriched uranium and natural uranium), and lithium-6 deuteride and lithium-6 deuterium-tritium shells.

Two-stage device:

• A thermonuclear device with a 4-kg plutonium primary and a 37–40-kg HEU secondary.

Other analysts have been less specific in suggesting possible designs, although several have assumed when conducting their calculations that an average of 4 kg of plutonium or 12–20 kg of HEU is used per weapon, depending on whether the weapon uses only plutonium or only HEU.

Imagery Analysis

Estimates of the size of each device shown by North Korea can be used to obtain bounding constraints on the possible designs of North Korea’s devices. The amount of fissile material and designs do not correlate exactly to size. Nonetheless, it is an important aspect to assess, as North Korea releases relatively little data and only a handful of images of these devices exist.

ONN independently conducted an analysis of open source images of nuclear devices, released by North Korea, to generate the size estimates. The results are published separately and are summarised below. These estimates of maximum device diameter are a guide to the potential sizes of North Korean nuclear devices and show the country’s desire to move towards more complex and compact designs. While the devices shown may be fake, their very release at the least indicates some sort of drive or desire for these types of weapons.

▲ Figure 1: Pre-2012 Image of Kim Jong-il Inspecting a Suspected Implosion Device Shown on the Wall During a December 2017 Meeting to Celebrate Nuclear Programme Achievements. Source: KCTV, “자위적국방력강화의 력사에 특기할 승리와 영광의 대회 경애하는 최고령도자 김정은동지를 모시고 제8차 군수공업대회 성대히 개막” (“Meeting of Victory and Glory That Will Go Down in History of Strengthening Self-Defensive National Defense Capabilities: The Eighth Munitions Industry Meeting Successfully Opens with Respected and Beloved Supreme Leader Comrade Kim Jong Un”), Supersuhui, YouTube, 13 December 2017. Note: Based on ONN’s mensuration analysis, the approximate diameter is 600 mm.

▲ Figure 2: Kim Jong-un Inspects a Reported Implosion Device, 8 March 2016. Source: KCNA, “북한 핵탄두와 KN-08 ICBM 격납고 내부 최초 공개” (“North Korean Nuclear Warheads and Interior of KN-08 ICBM Hangar Revealed for the First Time”), 9 March 2016. Note: Based on ONN’s mensuration analysis, the approximate diameter is 578–615 mm.

▲ Figure 3: Kim Jong-un Inspects a Reported Two-stage Thermonuclear Device, 3 September 2017. Source: KCTV, “金正恩同志核兵器兵器化事業を現地指導” (“Kim Jong Un Gives Guidance on Weaponization of Nuclear Weapons”), 3 September 2017, DPRKnow, YouTube. Note: Based on ONN’s mensuration analysis, the approximate diameter is 581–642 mm for the primary and 436–81 mm for the secondary.

▲ Figure 4: Kim Jong-un Inspects the Hwasan-31 Warhead, 27 March 2023. Source: KCTV, “敬愛する金正恩同志が核武器兵器化事業を指導された” (“Respected Comrade Kim Jong Un Guides Weaponization of Nuclear Weapons”), 28 March 2023, DPRKnow, YouTube. Note: Based on ONN’s mensuration analysis, the approximate diameter is 450–63 mm.

Specific Design Capabilities and Resources

Composite Pit

During its third test, some assessed that North Korea used an HEU or composite pit. If it did, this would mirror the Soviet Union’s third test, which used a composite plutonium-HEU pit. As a late mover, North Korea has a relative technological advantage, lending weight to the argument that the country has diversified its fissile-material use.

Gas Boosting

While gas boosting would increase yield and allow for more advanced miniaturisation, it is unlikely that North Korea has incorporated this design into its arsenal, although it is technically possible. To do so, North Korea’s programme, as with earlier programmes of other states, would have to overcome several issues, such as gaseous tritium/deuterium storage and handling, tritium production and integration of such materials into the weapons themselves.

Gaseous Tritium/Deuterium

Gaseous tritium/deuterium, which can be used for boosting the primary, creates challenges in handling and storing within a warhead system. There is potential leakage under very high pressure, which increases as tritium decays. This is exacerbated by the radioactive decay specifically to helium, which accelerates cracking in imperfections in the metal lattice of the containment vessel.

Even in storage, gaseous tritium “leaks” from containers, and will cause pressure increases in its containers due to radioactive decay. Tritium stored in a solid matrix, such as in uranium or palladium metal, is easier to handle and ship; however, the process to recover the tritium can be difficult depending on the metal used.

As tritium has a half-life of 12.26 years, 5.7% decays per year, requiring any nuclear-armed state incorporating gas boosting to manufacture tritium close to the date of a test or use in any weapon, to prevent as much decay as possible and lower the risk of storage incidents.

Tritium Production

Based on data from the VERTIC model, North Korea’s NFC can produce 3–17 g of tritium per year depending on how much of its plutonium and HEU production North Korea is willing to sacrifice; there are trade-offs between tritium production on the one hand, and plutonium and HEU production on the other.

Weapons Integration

In the past, gas boosting has required significant testing to ensure its reliability. As North Korea has only conducted six tests, it is unlikely to have mastered this technology, which eluded other nuclear-armed states until they were well into their testing programmes. Confidence in a boosted system requires significant testing, especially as it is a potential single point of failure for the system.

Additionally, as mentioned above, tritium’s short half-life requires that it is regularly changed out of the weapon, thus increasing maintenance time and cost.

Given such difficulties, the lack of technical necessity or security imperative and the lack of announcements from North Korea’s government, ONN assesses that North Korea is unlikely to be employing gas boosting in its nuclear weapons. North Korea might instead be opting for the more abundant lithium-deuteride fusion fuel, which would remove some logistical burden in handling tritium directly, eliminate the half-life issues and allow for production in situ when the secondary is initiated.

With all these considerations, North Korea has likely decided to use more proven designs that can still produce considerable yield without sacrificing reliability and safety. This area should be monitored for further developments as this is likely a technology North Korea is eager to master, allowing for greater yield at lower warhead weights.

Thermonuclear

ONN assesses that North Korea already has a fielded thermonuclear weapon, likely demonstrated in its most recent test. This is consistent with, for example, China’s programme, which designed and tested a two-stage thermonuclear device within five nuclear tests.

Lithium

Lithium-6 can be used to breed tritium (which is used in fusion reactions), either directly in a thermonuclear weapon or in a nuclear reactor via neutron activation. Open source information indicates only a few kilograms are used per weapon. According to the available literature, it appears that North Korea has access to millions of tonnes of natural lithium (both through domestic reserves and dual-use imports), and the capability to enrich to higher concentrations of lithium-6 at its Hungnam Chemical Complex.

Based on these external assessments, for the purposes of this analysis, it is assumed that North Korea has enough lithium, and in particular lithium-6, for use in any thermonuclear arsenal it has built or plans to build.

Beryllium

Beryllium can be used in nuclear weapons as a neutron reflector to increase the fissioning efficiency of the nuclear material, thus requiring less fissile material to produce a critical mass. North Korea has known reserves of beryllium and, although little is known about the exact amount, based on geological data from the 1950s, they are assumed to amount to at least thousands, if not millions, of tonnes. Additionally, there is data suggesting that North Korea sent beryl for processing to the Soviet Union in the 1950s. It is unknown if there are any current processing capabilities in North Korea.

While there are uncertainties, this analysis assumes – based on the limited evidence available – that beryllium is not a constraint on North Korea’s weaponisation strategy. This is especially the case as other materials can be substituted for beryllium in nuclear weapons applications. Should additional information come to light, this assumption should be revisited.

High Explosives

High explosives are used to achieve criticality in nuclear devices, while also having numerous non-nuclear military applications. There is an abundance of raw materials and production techniques known to exist in North Korea for high-explosives manufacturing, and a lack of any indication that the country is having issues with supplies.

North Korea conducted high-explosives tests at the Yongbyon Nuclear Complex in the 1980s and 1990s. Since the 1990s, media and research articles have repeatedly associated Yongdoktong (approximately 40 km northwest of Yongbyon) with the country’s nuclear weapons programme, specifically in connection to high-explosive tests and storage, and the storage of nuclear weapons. According to ONN’s analysis of available satellite images, the Yongdoktong site has a number of locations with features similar to other high-explosives testing sites (such as enclosed structures to avoid blast effects and a monitoring area). The site has underground facilities with multiple entrances, and activities related to excavation and construction have been seen. North Korea has demonstrated its ability to manufacture high explosives for its six nuclear tests and numerous non-nuclear tests. Thus, it is assessed that high explosives are not a constraint on nuclear weapons manufacturing in North Korea.

Modelling and Simulation

To verify nuclear weapons design information and gain insight from nuclear weapons tests, some modelling and simulation capabilities are necessary. Of course, testing is a form of design verification, but the more modelling capability a state possesses, the better it can interpret the results.

North Korea has clearly developed expertise with weaponisation-relevant codes (receiving valuable early assistance on nuclear weapons designs from A Q Khan), as demonstrated by its training of others on these codes. In 2011, it was reported that North Korea had provided Iran with a US-origin computer program. This program simulated neutron flows and would help with nuclear weapons development. North Korea also trained Iranian specialists to establish and maintain proficiency with the software. Furthermore, North Korea was linked to participation in the provision of “mathematical formulas and codes for theoretical design work” in Iran. It is unknown how much of this expertise was indigenously developed or learned from external sources.

This analysis assumes that North Korea has sufficient competencies to model and simulate nuclear devices. Further in-depth analysis of this aspect of North Korea’s weaponisation programme is needed for additional clarity and insight.

Other Weaponisation-Relevant Equipment and Technologies

In some instances, it can be difficult to assess the availability of weaponisation-relevant technologies (such as specialised machining equipment, high-speed cameras or other monitoring equipment used for nuclear weapons testing) because of the lack of open source information, including international trade and domestic commercial data. Due to the focus of this paper, an in-depth analysis of these issues was not conducted, but could be explored in further analysis. Difficulties in procuring such equipment and technology could constitute a significant complication for North Korea’s nuclear strategy. However, as North Korea has demonstrated its ability to conduct nuclear tests despite these limitations, it can be assumed for this study that other weaponisation-relevant technologies are not a major constraint.

Potentially Incorporated Designs

Following the discussion presented in this chapter, ONN has assessed that the six nuclear devices tested and incorporated into North Korea’s arsenal are likely one of the following designs:

• Implosion-based single-stage device with a levitated pit made of plutonium, HEU or a composite of plutonium and HEU.

• Two-stage thermonuclear device that incorporates a version of its single-stage device as the primary.

North Korea has likely explored several designs that could be considered more reliable (that is, those that will produce the designed yield when detonated) and, conversely, more technically sophisticated (that is, miniaturised weapons). The state has displayed what was indicated to be a thermonuclear weapon and a tactical weapon, designed to be deployed on a number of delivery vehicle types. Therefore, to understand the full range of North Korean force structure strategies and provide a fully exhaustive assessment, it is assumed that the country has built both single-stage implosion devices and a two-stage thermonuclear device. These devices would satisfy the two purposes of North Korea’s nuclear arsenal and allow the state to diversify its nuclear force.

The most likely design selection for the strategic weaponry is a two-stage thermonuclear device, given that North Korea defines its strategic weapons as ones that will strike large cities, industrial centres and other strategic targets. It is certainly possible that North Korea has single-stage implosion devices in its strategic arsenal. However, based on the country’s own definition, this analysis assesses that “strategic” means two-stage thermonuclear. Should this assumption be modified, the analysis could be adapted to account for another composition of the strategic arsenal.

The most likely warhead design for the tactical/battlefield part of North Korea’s arsenal is a single-stage implosion device, as it would be used in proximity to North Korea’s own forces and homeland. Tactical/battlefield nuclear weapons could be two-stage thermonuclear weapons, and thermonuclear weapons could be designed with lower yields to prevent harm to friendly troops. However, based solely on North Korea’s own definition, ONN assesses that tactical/battlefield is a lower-yield, single-stage device. The most likely candidate for this is the Hwasan-31, as it is the only reported tactical weapon North Korea has shown. If any of these assumptions were modified, the analysis could be adapted to account for another composition of the tactical/battlefield arsenal.

IV. Fissile-Material Demands and Constraints

This chapter examines the intersection between fissile-material demands and constraints, and how this interplay could affect North Korea’s nuclear weapons programme.

Fissile-Material Demands from Potential Force Structure and Designs

Given all the considerations presented, North Korea has many choices for how to structure its arsenal to optimise its fissile-material use while also accomplishing its geopolitical and security objectives. For this analysis, ONN has identified plausible arsenal configurations and derived the associated fissile-material demands of these arsenals.

Strategic Arsenal Demands

The following assessments about the fissile materials potentially used in North Korea’s strategic arsenal are made based on nuclear physics principles and assumptions made in open sources. The figures shown in Table 2 represent the final amounts of fissile materials in each two-stage thermonuclear device (losses due to the weaponisation process are discussed later).

• All plutonium: 4 kg of weapons-grade plutonium per device.

• All HEU: 12–15 kg of HEU per device.

• Mixed-material: 2–4 kg of weapons-grade plutonium and 10 kg of HEU per device.

▲ Table 2: March 2024 Estimates of Strategic Arsenal Fissile-Material Demands. Source: The authors; Kenneth S Krane, Introductory Nuclear Physics (New York, NY: John Wiley & Sons, 1988); Hecker with Serbin, Hinge Points; Sublette, “Nuclear Weapons Frequently Asked Questions”.

Tactical/Battlefield Arsenal Demands

The following assessments about the fissile materials potentially used in North Korea’s tactical and battlefield nuclear warheads are made based on nuclear physics principles and assumptions made in open sources. The figures shown in Table 3 represent the final amounts of fissile materials in each single-stage nuclear device (losses due to the weaponisation process are discussed later).

• All-plutonium pit: 4–6 kg of weapons-grade plutonium per device.

• All-HEU pit: 10–12 kg of HEU per device.

• Composite pit: 2 kg of weapons-grade plutonium and 8–10 kg of HEU per device.

▲ Table 3: March 2024 Estimates of Tactical/Battlefield Nuclear Weaponry Fissile-Material Demands. Source: The authors. Krane, Introductory Nuclear Physics; Hecker with Serbin, Hinge Points; Sublette, “Nuclear Weapons Frequently Asked Questions”; Thomas B Cochran and Christopher E Paine, “The Amount of Plutonium and Highly-Enriched Uranium Needed for Pure Fission Nuclear Weapons”, Nuclear Weapons Databook, Natural Resources Defense Council, 13 April 1995.

Summary of Fissile-Material Demands

Should North Korea be fielding an all-plutonium arsenal, based on the options presented, it would need 420–1,340 kg of plutonium. For an all-HEU arsenal, it would require 1,100–2,925 kg of HEU. For an arsenal made of composite pits, it would require 210–540 kg of plutonium and 890–2,350 kg of HEU. There are of course ranges in between, should there be different mixes of weapon types (such as ½ plutonium, ½ HEU; ⅓ plutonium, ⅓ HEU, ⅓ composite); the estimates here represent the extremes of fissile-material demands.

The figures in Table 4 represent the nuclear material actually used in the device (losses from machining and fabrication are discussed later).

▲ Table 4: March 2024 Estimates of Total Nuclear Weaponry Fissile-Material Demands in Direct Warhead Use for Four Different Arsenal Configurations. Source: The authors. Note: Production and testing losses are not accounted for.

Fissile-Material Production

Uranium is a naturally occurring element. In its natural form, it mostly consists of the isotope U-238 with very small (less than 1%) amounts of the isotope U-235. To manufacture most fuel for nuclear reactors, uranium needs to be enriched to approximately 3–5% U-235 to create the proper fission dynamics. For nuclear weapons, uranium needs to be highly enriched, at over 20% U-235, although the ideal enrichment level is over 85%.

Various isotopes of plutonium are produced via fissioning of uranium in a nuclear reactor, typically consisting of approximately 24% Pu-240. For use in nuclear weapons, the amount of Pu-240 needed for practical purposes is less than 7% of the total amount of plutonium, which corresponds to 93% Pu-239. This can be accomplished by configuring the nuclear reactor fuel in specific ways to produce this “weapons-grade plutonium” and thus optimise the amount of Pu-239. However, the plutonium products must be separated from the rest of the spent reactor fuel via reprocessing, and then further refined into a form usable in nuclear weapons.

VERTIC ran three different fissile-material production scenarios (as of 2023) in its NFC model for ONN to produce cumulative totals of fissile material produced in North Korea. All scenarios assume no plutonium production prior to 1991 and no HEU production prior to 2003, and only significantly vary in the HEU production estimates.

Scenario One

The first scenario is based on analyses from Siegfried S Hecker at Stanford University’s Center for International Security and Cooperation:

There is a second large clandestine centrifuge facility that may be Kangson or some facility not yet identified in open sources … Hecker, under this scenario, assumes a pilot plant with two cascades (660 centrifuges) and a SWU/centrifuge of 4. The UEW [uranium enrichment workshop at Yongbyon] then has 8000 SWU/year from 2010-2015 and 16,000 SWU/year from 2016 onwards. A clandestine facility is constructed in 2013-2014 and expanded in 2017-2018 for a total 35,000 SWU/year. Hecker’s analysis is based on North Korea’s annual production capability of rotors, with two flow-forming machines between them capable of producing 1500 P2 rotors/year.

Scenario Two

The second scenario is based on analyses from David Albright at the Institute for Science and International Security (ISIS):

In this scenario, the UEW [Uranium Enrichment Workshop] at Yongbyon is the largest centrifuge facility and only a pilot plant exists alongside this, responsible for all pre-2010 evidence of enrichment. In this scenario, Albright takes Hecker’s estimate of 2000 P2-type centrifuges observed in the UEW and assumes a four-stage enrichment process as observed in Libya … and operational challenges with the centrifuges that reduces a 5 SWU/centrifuge effectively by a third for a total SWU/year for the plant of 6500 from 2010 to 2015 and 10,000-13,000 from 2016.

Scenario Three

The third scenario is also based on analyses from David Albright at ISIS:

In this scenario, there is a second large clandestine centrifuge facility that may be Kangson or some facility not yet identified in open sources. Albright in this scenario assumes the clandestine facility becomes operational from 2005-2010 with a comparable size to the UEW for total SWU/year of 26,000.

Outputs

As shown in Table 5, all three scenarios generally agree that North Korea has produced 64–65 kg of plutonium enriched to weapons grade (98% Pu-239). The three scenarios, however, indicate vastly different HEU (88–89% U-235) accumulations, ranging from 868 kg to 2,111 kg. This large difference in uranium largely stems from the external community’s uncertainty about North Korea’s uranium enrichment programme, as detailed above.

▲ Table 5: North Korea NFC Fissile-Material Cumulative Outputs Calculated by the VERTIC Model. Source: The authors.

Potential Fissile-Material Losses

From Production

ONN assumes that losses due to production and experimentation, and keeping some material in storage for excess, should account for around 10% of total fissile-material availability. Estimations of fissile-material losses are difficult to assess due to secrecy surrounding the manufacturing process. An initial estimate of 10% was chosen, as material losses from the NFC are already accounted for within the VERTIC model. This is therefore a reasonable starting point for the losses due to weaponisation. See Table 7 for the adjusted fissile-material availability numbers as of 2023.

From Testing

Based on the information that is both known and assumed about North Korea’s nuclear tests, the likely fissile material available at the time of the test, and its nuclear strategy, the assessment in Table 6 was made about the devices tested by the country thus far.

Table 6: North Korean Nuclear Tests and Assumed Devices. Source: The authors.

Adjusted Fissile-Material Availability

Based on the assessments of the fissile material consumed via losses (see above) and testing (presented in the “ONN Device Assessment” column in Table 6), the total fissile-material inventory available for North Korea’s nuclear weapons arsenal was adjusted to the amounts shown in Table 7.

Table 7: Calculated Cumulative Fissile-Material Inventory, Including 10% Losses and Testing Consumption. Source: The authors. Note: Based on fissile-material estimates from 2023.

Plutonium Versus Uranium Weapons

Nuclear weaponry using plutonium is aligned with North Korea’s overarching nuclear strategy aimed at deterring the US. This approach allows lighter-weight strategic weapons to fit on ICBMs. As its nuclear doctrine evolves, North Korea is incorporating more tactical and battlefield weapons into its nuclear approach, facilitated by its accumulated stockpile of HEU. The accessibility of HEU provides the state with increased flexibility in crafting a variety of tactical/battlefield nuclear weapons, stemming from the ability to design devices with various arrangements of fissile materials.

However, it is also possible that HEU is used in North Korea’s strategic nuclear weapons in combination with plutonium in the primary, as this configuration would allow the state’s limited stockpile of plutonium to be stretched much further. Additionally, HEU was likely first produced in 2003, before North Korea’s first test in 2006, which means it is highly likely that HEU is in at least some of the test devices (to confirm reliability), including the strategic weapon tested in 2017.

Comparison with Demands

Based on the analysis of North Korea’s demands, it was concluded that the state may be aiming to develop at least 25–35 strategic thermonuclear warheads and 80–200 tactical and battlefield fission devices to meet its geopolitical and security objectives. Given the fissile-materials constraints presented here, it is likely that North Korea has not yet accomplished this goal – although it could be very close – as it has not yet produced sufficient quantities of nuclear material.

Should North Korea intend to produce 25 composite thermonuclear weapons, it would, at a minimum, need 2 kg of plutonium and 10 kg of HEU per weapon, leading to a demand of 50 kg of plutonium and 250 kg of HEU. Given the material constraints presented in Table 7, North Korea would have been able to produce 21–23 devices of this type, using at least 42–46 kg of plutonium and 210–30 kg of HEU. This leaves a deficit of 2–4 strategic thermonuclear weapons.

As the tactical and battlefield mission of its nuclear arsenal was added after its strategic mission, it is assumed that North Korea would work to accomplish its strategic nuclear requirements before its tactical ones. The use of nuclear materials for strategic weapons described above would leave minimal plutonium (0 or 0.5 kg) but large quantities of HEU (513–1,652 kg) remaining for the tactical and battlefield mission, which could be accomplished with only HEU. As assumed before, North Korea would need at least 10 kg of HEU per tactical and battlefield weapon for a single-stage, all-HEU device. This indicates that North Korea could have 51–165 weapons after production losses and testing, which could already or nearly fulfil its tactical and battlefield deterrence objectives.

Should any of the underlying assumptions in this analysis be modified, this final weapons deficit could end up being larger or even result in a surplus. Additionally, any further data that would require adjustments to the VERTIC model or underlying assumptions made there would require adjustments to the figures derived in this analysis. As North Korea reveals so little information, any further information that confirms or changes these inputs and assumptions will greatly increase the precision and accuracy of these analytical outputs.

Conclusions

North Korea is likely diversifying its nuclear force to operate in a variety of environments and with a variety of delivery means. Relatedly, the country looks to be continuing to move towards a nuclear strategy with two purposes: to deter both a large nuclear adversary (that is, the US); and a range of regional conventionally superior, non-nuclear adversaries (that is, South Korea, USFK and Japan) by making its arsenal useable. Based on this potential dual focus of North Korea’s arsenal, the state is likely developing at least three different categories of weaponry: strategic; battlefield; and tactical.

There are many options for how North Korea can structure its arsenal to optimise its fissile-material use and demands while also accomplishing its deterrence objectives. To meet its objective in deterring the US, one possibility is that North Korea could strive for at least 25 strategic thermonuclear warheads, with the eventual goal of 35 based on the 2026 target delivery date for the expansion of US missile defences. Based on calculations of North Korea’s potential available fissile materials as of 2023, the state could have sufficient material to enable it to have already developed 21–23 thermonuclear warheads. After developing its strategic thermonuclear devices, North Korea would have enough HEU remaining to manufacture 51–165 single-stage nuclear warheads, thereby also potentially addressing its regional objectives (which could be 80–200 warheads, although if underlying assumptions change, this goal could change as well).

While it is clear that North Korea is still operating under some fissile-material constraints, especially regarding plutonium, both the strategic and tactical/battlefield objectives have either been met or are approaching nuclear material sufficiency. The state could therefore decide to work within the current fissile-material inventory and determine that its weapons demands have been sufficiently met at this point, without any further expansion of nuclear material production. Alternatively, North Korea could continue producing fissile material at a quick pace to either: catch up to full current demands if they have not yet been met; exceed its defence demands and maintain an excess stockpile of fissile material without expanding the weapons arsenal; or restructure its nuclear strategy to continue to add defence capabilities to fit its fissile-material supply. However, as North Korea moves from an early-stage programme a more established one, like other nuclear states it will likely continue to make nuclear material in excess of defence needs.

This paper presents a handful of arsenal options that North Korea may be considering. Without more information, it is not possible to know precisely what North Korea is trying to achieve with its nuclear weapons programme. This analysis provides an analytical framework and points of discussion to stimulate a transparent and detailed conversation about the types of choices and trade-offs that North Korea is making on nuclear force structure and fissile-material production.

Future Recommended Research

There are several follow-on analyses that would be beneficial to better understand some underexplored areas that ONN identified while developing this paper.

Analysis of North Korea’s nuclear strategy further into the future (that is, 15–20 years) is already underway to consider how the state might expand and structure its NFC to meet its long-term fissile-material demands.

While some history of North Korea’s nuclear weapons programme was considered to provide clarity on present realities, a more in-depth analysis of the entire history of the programme using the analytical methodology presented here (combining demand- and supply-side analyses) would provide additional clarity on present and future North Korean nuclear arsenals.

An additional area for exploration is the interaction between nuclear-warhead development and delivery-system development. While some consideration of delivery vehicles were factored into this paper’s analysis as context, a more detailed look would aid in discovering further details about North Korea’s current and planned nuclear weapons programmes.

In both the nuclear and missile programmes, it would be advantageous to analyse potential external programme assistance, as any assistance could affect the assumptions of this analysis.

Finally, additional investigation into the other potential weaponisation constraints is needed. While there is some published research on North Korea’s lithium and tritium production capabilities, further research is needed into its: beryllium supply and refinement capabilities; high-explosives manufacturing and testing facilities; weapons-relevant modelling and simulation codes; and additional weaponisation-relevant technologies. Further investigating North Korea’s capabilities in all NFC-relevant material, equipment and technologies, especially those related to uranium enrichment and plutonium reprocessing, would provide a fully comprehensive picture of the state of North Korea’s civilian and military nuclear programmes, and would be a much-needed information source for external analysts.

Further research in these areas could help to clarify some of the uncertainties on the issues presented in this paper and could help raise the global community’s awareness and understanding of additional aspects of North Korea’s nuclear weapons programme.

Sarah Laderman is a Senior Analyst for ONN, where she conducts open source and technical analyses of nuclear programmes and leads research projects.

Nikita Degtyarev is a Research and Engagement Assistant for ONN, where he focuses on using open source information and remote-sensing techniques to research countries’ nuclear programmes and nuclear risk reduction. His research interests include nuclear risk reduction, nuclear non-proliferation and Russian and NATO nuclear policy.

Tianran Xu is an Analyst for ONN. He focuses on Northeast Asian security and missile systems. Tianran uses photo mensuration to measure the size, range and capacity of missiles. He also analyses photos, videos and satellite imagery to understand nuclear and missile programmes. With a background in media and journalism, Tianran is a regular contributor to Chinese and English open source and science publications.

Elin Bergner is a Research Assistant with ONN, where she focuses primarily on political analysis of the Korean Peninsula and developing research on the non-traditional nuclear risk nexus in the Indo-Pacific. In addition, she provides support on nuclear/radiological disinformation. Elin is a social anthropologist and political scientist by training.

Marcy R Fowler is the Research and Analysis Manager for ONN, leading its team of analysts and strengthening their use of innovative analytical methodologies and technologies in support of nuclear risk reduction. Marcy is an expert in using information analysis to understand issues of international security, strategic stability, nuclear non-proliferation, arms control and disarmament, and to recommend appropriate policy and technical interventions to reduce risk and verify treaty compliance.