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An excellent report on NORK production of Lithium 6 is at "North Korea's Lithium 6 Production for Nuclear Weapons" by the Institute for Science and International Security, March 2017. Lithium 6 is a critical raw material needed for the production of single-stage thermonuclear and boosted fission weapons. Based primarily on procurement data, North Korea appears to have built a lithium 6 production plant near the city of Hamhung on the east coast of North Korea. The existence of this plant adds credibility to North Korea’s claims that it has been working on thermonuclear weapons for several years.

Production of Lithium 6
Lithium 6 is a stable isotope that exists in nature. However, it has a relatively low natural abundance of 7.56 percent in natural lithium, for the bulk of lithium is lithium 7. For use in nuclear weapons or tritium production, the lithium 6 fraction must be typically increased to 40-95 percent of the lithium via a chemical enrichment process. If the lithium 6 is for tritium production in a reactor, the fraction of the lithium 6 in the targets placed in the reactor is limited to about 40 percent to prevent failure of the targets. For use in nuclear weapons, a 95 percent fraction of lithium 6 is desirable.
Based on a 2012 procurement order, according to government sources, North Korea is assessed to have procured the wherewithal to build a lithium 6 enrichment plant based on mercury-based lithium exchange, shortened here to mercury exchange. This method of enrichment has been used by nuclear weapon states, including the United States. Almost all lithium 6 produced by the United States was enriched by the mercury-based column exchange process (COLEX) from 1954 to 1963.5 South Africa also built a pilot plant using this method to make lithium 6 for its nuclear weapons program in the 1970s.
The mercury exchange process involves an amalgam of lithium and mercury which is made via electrolysis. Subsequently, the mercury lithium amalgam is mixed with an aqueous lithium hydroxide (LiOH) solution. Lithium 6 has a slightly higher affinity to mercury than lithium 7, so lithium 7 diffuses out of the amalgam more quickly than lithium 6, leaving the amalgam with increased, or enriched, concentration of lithium 6. The lithium 6 can be separated from the amalgam. The tailings fraction of Lithium 7 is electrolyzed from the aqueous solution of lithium hydroxide. Afterwards, the lithium hydroxide can be reused.
What was procured? Most of the procurements were for industrial-scale equipment. Included in the order was metric tonne quantities of mercury and tens of kilograms of lithium hydroxide.
Given the procurement of these two materials and the replacement of mercury with less harmful metals in many industrial processes, the purchase of mercury in combination with lithium hydroxide is a strong indicator that North Korea is using the chemicals in a mercury-dependent lithium 6 production process.
The procurement evidence is reportedly contained in a 2012 North Korean contract to arrange the purchase of a wide range of industrial and lab-scale equipment and materials abroad in China. The purpose of the contract is not included but the list of goods, including the mercury and lithium hydroxide, implies they are for a lithium 6 enrichment plant using the mercury exchange method.
The most likely site for the lithium 6 plant is the Hungnam Fertilizer Complex near the city of Hamhung in North Korea. The procurement contract had handwritten notes stating that the goods were needed urgently and the procurements involved the Hungnam complex. The Hungnam complex makes sense for a lithium 6 production plant. This site is involved in ammonia processing and fertilizer production, and has electrolysis facilities.

I fear the situation is far, far worse than the government has been telling us.