White House: 'The clock has now run out' on North Korean nuclear program

[longship]

19. Yes, the Little Boy gun was certain to work from theory.

But it still had an initiator in two parts, one on the end of the bullet part and one inside the target. When bullet hits the target, at the instant of assembly, the initiator parts collide to provide a slow neutron or two to start the chain reaction. Without that element, the assembly blows itself apart before a chain reaction can start and the bomb is a fizzle. It does not go off (with the exception of the high explosives used to fast assemble the bullet with the target).

In the Fat Man the initiator was at the center of the plutonium implosion core. It gets crushed by the imploding core to provide the necessary slow neutrons at the precise instant of assembly. Again, without the initiator, the core just blows apart before it detonates.

No initiator. No nuclear detonation.

It really is simple.