High performance explosives trigger the chain reaction. 20-30 cm thick layers of nuclear trigger surround the Plutonium ball. The most critical requirement is that the trigger explosion is uniformly directed to the center of the bomb simultaneously. High performance explosives must be shaped just right to achieve the critical temperature and pressure for nuclear explosion. Experimental data are of course the key to making bombs and the trigger is the most critical component.
Advanced nations use super-computer simulation models to design triggers.
North Korea has no super-computer and has to rely on actual tests. It should be noted that trigger tests are the final step in nuclear bomb tests. Plutonium extraction and trigger development are the two key elements of nuclear bomb development. Trigger development is the last step in making nukes and North Korea can test a nuclear bomb in 6 months, if the DIA report is correct. The power struggle between the DIA and the State Dept. on North Korea's nuclear capability ended in the DIA's favor by the US Congress in August 1998.