The conventional explosives (TNT and RDX) in the bomb are used to start a chain reaction in the plutonium. Specifically, Physicist Seth Neddermeyer at Los Ala

The conventional explosives (TNT and RDX) in the bomb are used to start a chain reaction in the plutonium. Specifically, Physicist Seth Neddermeyer at Los Alamos constructed a design for the plutonium bomb that used conventional explosives around a central plutonium mass to quickly squeeze and consolidate the plutonium, increasing the pressure and density of the substance. An increased density allowed the plutonium to reach its critical mass, firing neutrons and allowing the fission chain reaction to proceed. To detonate the bomb, the explosives were ignited, releasing a shock wave that compressed the inner plutonium and led to its explosion.[1] [1] http://www.atomicheritage.org/history/science-behind-atom-bomb http://www.atomicheritage.org/history/science-behind-atom-bo...