From North Korea’s Nuclear Weapons: Technical Issues, a February, 2012 report (PDF here) by Mary Beth Nikitin, for the U.S. Congressional Research Service. Nikitin is a nuclear proliferation analyst:
Acquiring fissile material—plutonium-239 or highly enriched uranium (HEU)—is the key hurdle in nuclear weapons development.
Producing these two materials is technically challenging; in comparison, many experts believe weaponization to be relatively easy.
North Korea has 8 industrial-scale uranium mining and plants for milling, refining, and converting uranium; it also has a fuel fabrication plant, a nuclear reactor, and a reprocessing plant—in short, everything needed to produce Pu-239.
In its nuclear reactor, North Korea uses magnox fuel—natural uranium (>99%U-238) metal, wrapped in magnesium-alloy cladding. About 8,000 fuel rods constitute a fuel core for the reactor. When irradiated in a reactor, natural uranium fuel absorbs a neutron and then decays into plutonium (Pu-239).
Fuel that remains in the reactor for a long time becomes contaminated by the isotope Pu-240, which can “poison” the functioning of a nuclear weapon. Spent or irradiated fuel, which poses radiological hazards, must cool after removal from the reactor. The cooling phase, estimated by some at five months, is proportional to the fuel burn-up.
Reprocessing to separate plutonium from waste products and uranium is the next step. North Korea uses a PUREX separation process, like the United States. After shearing off the fuel cladding, the fuel is dissolved in nitric acid. Components (plutonium, uranium, waste) of the fuel are separated into different streams using organic solvents. In small quantities, separation can be done in hot cells, but larger quantities require significant shielding to prevent deadly exposure to radiation.
North Korea appears to have mastered the engineering requirements of plutonium production.
