That orb in the middle of the apparatus is The Demon Core, a piece of plutonium produced during the Manhattan Project, for a third nuke which was never needed. So it was used for criticality experiments, which is where those hemispheres come in.

Anyway, in those experiments it was key in a few accidents, which caused the deaths by radiation of several researchers. After the later bout of experiments, the core was scrapped.

A little more reading in case you’re interested