But HMX is also a "dual use" substance powerful enough to ignite the fissile material in an atomic bomb and set off a nuclear chain reaction.
The fissile material does not "ignite". The explosives drive together or compress the material to the point where fission occurs. Obtaining suitable explosives to make this happen is one of the easiest steps in actually constructing a nuclear bomb.
The "dual use" nature of these explosives is a non-issue.
Actually the process you are talking about is called fusion not fission. The compacting of material until it combines on a molecular level is nuclear fusion.
fission is the spiting of the molecules which is a entirely different process and is in essence "ignited"