[phkahler]

>> Fat Man used explosives to squeeze together two hemispheres of plutonium.

I thought the plutonium core was one piece and the explosives compressed it, making the same mass critical due to increased density.

[nradov]

The plutonium cores were machined as hemispheres because they had to put an initiator in the center.

[gonesilent]

Also to prevent criticality problems when moving them.

[ChrisTheBore]

And to prevent criticality problems while manufacturing them.

You're machining a perfectly spherical plutonium core out of a LARGER chunk of plutonium.

[Maursault]

> I thought the plutonium core was one piece...

While other responses have explained it was not, I thought I should point out that any sphere, even if a single piece, will always have at least two hemispheres, iow, a sphere having two hemispheres does not imply it is more than one piece. When you leaned of the Earth's northern and southern hemispheres, did you wonder if it was more than one piece?