The DIA concluded that the North Korean test that generated 2,000-degree heats was a test of high explosive lenses for nuclear bombs. A nuclear bomb is shaped like a basketball and has three distinct layers. At the center of the bomb is a baseball-size mass of Plutonium. The Plutonium ball is surrounded by an implosion layer which is surrounded by high explosives. Chain reactions require temperatures of 2000-3000 degrees and high pressures.
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.