With an implosion type bomb you need to make sure that the shockwave from the explosives hits the plutonium pit all around it almost simultaneously. If there wa

With an implosion type bomb you need to make sure that the shockwave from the explosives hits the plutonium pit all around it almost simultaneously. If there was a portion where the explosive shockwave was delayed then the forces on the rest of the pit are going to tend to "squish" it out towards the portion where the explosion hasn't hit it yet. This asymmetrical compression means now you don't get the ideal compressed sphere so now while it's still going to reach criticality, it's going to be blasted apart with much less of the fissile material being fissioned. A great real world example of why timing is so critical is the first North Korean nuclear bomb test. https://en.wikipedia.org/wiki/2006_North_Korean_nuclear_test#Yield_estimates_and_authenticity https://en.wikipedia.org/wiki/2006_North_Korean_nuclear_test... They had a yield of less than a kiloton. Obviously way greater than conventional explosives but it should have been an order of magnitude more powerful. In order for the explosives to form that crucial explosive lense you need detonation across numerous points on the outside simultaneously so you need a detonator that's extremely consistent on timing so a simple resistive detonator with some primary explosives packed around a filament is not going to be effective. The method of choice is an exploding bridgewire design. Basically you have a thin wire and you pass enough current through it to vaporize the wire so quickly that the wire itself generates the initial shockwave as it gets turned into plasma. The rest of the explosive lense is just two different types of explosive with different propagation velocities. The two are combined with a parabolic (or hyperbolic, I forget off the top of my head) face between the two layers so that the explosion that started from a single point goes from convex to concave so it hits the outside of the pit all at the same time. If you simply dropped a half-critical mass onto another and it was prompt critical then yes, it would explode, but with not nearly as much energy being released as a proper nuclear weapon.