How would North Korea develop its nuclear arsenal? The fact that they continue nuclear testing suggests that they started from a relatively primitive design that they are trying to improve.
The greatest constraint on that development is the amount of fissile material they have. In 2008, they declared about 38.5 kg of weapons-grade plutonium. Since then, they have run their plutonium-producing reactor and may have refined additional plutonium. They also have a uranium enrichment program which can be assumed to have supplied highly enriched uranium (HEU). Last September, Siegfried S. Hecker estimated that they have 32 to 54 kilograms of plutonium and perhaps 300 to 400 kilograms of HEU. He assumes that 5-7 kilograms of plutonium or 25 kilograms of HEU is sufficient for a bomb.
Does this mean that North Korea has 20-30 nuclear weapons on the shelf? Probably not. The limited amounts of material that have been available constrain how they develop their nuclear arsenal.
North Korea has done five nuclear tests. One of their objectives is likely to be to minimize the amount of fissile material used, in order to fit their weapon onto their missiles and to use their limited supply effectively.
Under Hecker’s scenario, they have enough fissile material for 6-8 plutonium bombs or 12-16 HEU bombs. But there are complications. Plutonium bombs may have uranium components, and pits combining plutonium and HEU (composite pits) are another possibility. It’s also likely that the design North Korea started with requires more of either than Hecker’s estimates. Using a large part of their fissile material supply to build weapons of that design would be a poor idea. Existing nuclear weapons can be reworked as the design improves, but material is lost in the reworking.
It’s also a poor idea to build a number of weapons of an untested or partly tested design.
On the other hand, North Korea would probably want to have one or a few nuclear weapons. The United States bluffed with two in 1945; North Korea might have a few that can’t be delivered by missile. The rest of the fissile material would be in relatively unprocessed form, perhaps metal.
Their testing program is likely aimed at a design that can be carried on a missile, which implies lesser amounts of fissile material. North Korean rhetoric and a recent attempt to sell lithium-6 imply that they are also working toward a thermonuclear component in their design. Development requires testing, but testing uses up part of their fissile material. They may also be doing hydrodynamic tests, which would not be as noticeable. Fissile material from those tests could be reused.
Tests must be planned carefully to acquire the most data to advance the design. For the objectives of reducing fissile material and introducing a thermonuclear component, too much change in design can lead to failed tests. So the North Korean weapons designers are walking a fine line. Given the apparent similarities in yield of the last several tests, their changes are likely incremental rather than fundamental.
Until a design is felt to be reliable, it is unlikely that weapons would be manufactured for the stockpile. Hence a desire to increase the weapons stockpile must be balanced with proving reliability.
The current North Korea nuclear weapons stockpile is thus likely to be fewer than a half-dozen weapons.
Activity at the North Korean test site has slowed down after a flurry of concern that another nuclear test was imminent. Another test would use up some of the fissile material supply and would indicate that North Korea is not now ready to manufacture a full weapon stockpile. Or it could be the proof test of an approved design.
Photo from 38 North.