The Sloika and the Pyongyang Summit: Did Kim Jong-un Provide Us a Glimpse into North Korea’s Nuclear Weapons at the Fifth Inter-Korean Summit?

The week that passed witnessed the fifth inter-Korean summit between North and South Korea, this time centred upon Pyongyang. There is much to be said about the political, economic, and strategic context behind the summit. Additionally, there is much to say about the summit and its implications for the debate on US policy and North Korean denuclearisation. I would like to write about both things in a little bit more detail, however here I seek leave to briefly discuss a very narrow (possible) aspect to the summit namely Tritium and North Korea’s nuclear weapons programme.

As at Panmunjom, and Singapore, the summit led to the signing and issuing of a declaration. The Pyongyang Declaration, as it has come to be called in shorthand, contains a North Korean offer to dismantle the Yongbyon nuclear complex, long associated with production of nuclear materials for the North’s nuclear weapons programme, in return for, unspecified, US concessions

The North expressed its willingness to continue to take additional measures, such as the permanent dismantlement of the nuclear facilities in Yeongbyeon, as the United States takes corresponding measures in accordance with the spirit of the June 12 US-DPRK Joint Statement.

North Korea is reputed to have clandestine uranium enrichment plants so this would not necessarily mean that North Korea could no longer produce fissile material for its nuclear weapons programme, however the matter of Tritium is intriguing. North Korea states that it has a two-stage thermonuclear warhead, tested in September 2018, and the shape of the device tested as well as its yield adds a great deal of credibility to that claim. Furthermore, North Korea is reputed to have tested a boosted fission weapon, in January and September 2016, prior to its 6th nuclear test in 2018.

The picture we have of the evolution of North Korea’s nuclear weapons science programme is that North Korea tested a boosted fission weapon in January 2016, a version for a missile reentry vehicle in September 2016, and that in 2018 North Korea tested a two stage thermonuclear weapon with a boosted fission primary. The second stage of a thermonuclear weapon employs Lithium-6 Deuterium fusion reactions to produce Tritium for Deuterium Tritium fusion reactions, which produces considerable amounts of energy. The boosted fissile primary is injected with Tritium gas to boost the efficiency of fission.

The disco ball of death is widely reputed to be a model of a boosted fission bomb (the protruding nozzle is sometimes taken to be where the tritium gas is injected)

An earlier graphic from the era of Kim Jong-il appears to show the same type of nuclear weapon

North Korea is reputed to produce tritium at the IR-2000 research reactor at Yongbyon. Assuming the offer made in the Pyongyang Declaration refers to all the nuclear facilities at Yongbyon, we don’t know if that is the case, then the interesting question becomes what of Tritium production? This question arises because Tritium decays with a half-life of 12.3 years, and 5.5% of Tritium decays to Helium-3 per year. That means a nuclear weapons state that uses boosted fission primaries/cores needs to regularly replenish its Tritium stock. The implications for the longevity of North Korea’s bigger bombs are obvious.

The Pyongyang Declaration could mean that the consensus on North Korea’s nuclear weapons programme is not accurate. That is, North Korea might not have a boosted fission bomb or a two stage thermonuclear warhead triggered by a Tritium boosted fission primary. Both of North Korea’s big bang bombs might well produce Tritium in situ, and that’s why Kim Jong-un was able to make the offer of closing down Yongbyon without indefinitely hobbling his nuclear weapons programme. That is to assume that North Korea does not have a clandestine replica of the IR-2000 at another location beyond Yongbyon. North Korea’s production of Lithium-6 is attributed by the Institute for Science and International Security to take place at the Hungnam Chemical Complex in the proximity of Hamhung.

The Pyongyang Declaration could be telling us that North Korea has a Sloika (Russian for layer cake) boosted fission bomb and a Sloika boosted fissile primary for its thermonuclear bomb. The Sloika (or Layer Cake or Alarm cake in the US case; never tested) was a Soviet intermediate nuclear weapon, whose design is largely attributed to Andrei Sakharov, that employed alternating layers of Lithium-6-Deuterium and Uranium-238. The core of the Sloika was weapons grade Uranium-235. The Sloika was tested and had a reported yield of 400kt.

There’s much that fits neatly into this story. First of all, a Sloika with a U-235 core would be consistent with the dismantlement of Yongbyon given that North Korea is reputed to have at least two clandestine uranium enrichment plants. Secondly, Tritium can be produced in situ, as noted, for the thermonuclear reactions of the first and second stages of the two stage thermonuclear device. It fits nicely historically if this paper by Wellerstein and Geist is anything to go by. The Soviet Union tested its first two stage thermonuclear device one year after the test of the Sloika, and the Sloika was a crucial step on the path to a Soviet two stage hydrogen bomb. North Korea tested its two stage thermonuclear warhead two years after the test of its reputed boosted fission weapon. Moreover, North Korea in January 2016 stated that it tested a hydrogen bomb, and in September 2016 a version for delivery by a missile RV, a claim widely lampooned and ridiculed. When the Soviet Union tested the Sloika it too claimed to have tested a hydrogen bomb, a claim then ridiculed and that very much remains so the case today. The Sloika is not seen as a hydrogen bomb. The Wellerstein and Geist paper has a great quote on the hydrogen bomb status of the Sloika from Carson Mark, the director of Los Alamos for a good chunk of the cold war

“They managed to get 400 kilotons without going to an unreasonable or even a heavier size. And, they did it by using thermonuclear reactions. Want to call that a hydrogen bomb? Well, why not?”

When North Korea told us in January 2016 it tested a hydrogen bomb, and it could very well have only that hydrogen bomb was a Sloika.

The second North Korean test of 2016 had a yield of about 35kt (most cite a lower yield but that uses an equation to estimate yield from seismic data that does not incorporate depth. Incorporating depth leads to the high yield). That’s less than 400kt, but North Korea appears to have underdone the yield of its nuclear tests. There could be at least one reason for this, should that turn out to be the case. Underdoing the yield of its testing programme was part of the effort to conceal its clandestine uranium enrichment programme for large yields pose a risk of venting radionuclides.

The dimensions of the North Korean two stage thermonuclear warhead appears bulky compared to modern Russian and US warhead designs. The first image depicts the famous image of Kim Jong-un with some of his Sakharov’s and a model of the two stage thermonuclear device just before the September 2018 test. Note the peanut shape suggestive of a two stage device. The second image is of the device that was actually tested as depicted in a North Korean propaganda concert.

The relatively bulky nature is often attributed to the lag in North Korea’s nuclear science and technology programme but it could be, in part, a function of a Sloika primary as opposed to a Tritium gas boosted primary. That could be one reason why North Korea opted for a big Titan II like ICBM with a high volume payload fairing. It could well be that the Sloika is all North Korea has, but what we know suggests to us that North Korea really does have a two stage device. If the Sloika had a yield of 400kt one might ask, why should they have bothered to take the next step? Simple. The Sloika design couldn’t go much beyond 400kt, and a two stage thermonuclear device gives North Korea a megatonne to multimegatonne capability (recall the point about North Korea underdoing the yield of its nuclear tests).

This is of course all very speculative, and should be treated as such. But it makes sense to the extent that North Korea has no other means of producing Tritium and wants to make a diplomatic offer on its nuclear programme without indefinitely limiting it. It also makes sense historically, and with respect to Pyongyang’s claim to have tested a hydrogen bomb in January and September 2016. At the very least this is all something worth thinking about and looking further into.