In the case where the wall thickness is used to resist internal pressure, the wall thickness is minimized if a hemispherical shape is used. Of course, the true optimization of any large plant is to minimize cost ( while maintaining reliability and safety objectives), and it likely follows that the minimum wall thickness yields the lowest cost.
Pressurized Water Reactors PWR containment is a pressure vessel based upon a design basis accident DBA
That DBA is a primary or secondary coolant weld breaks which vents the coolant which converts to steam in a very short time. The containment is designed to hold that steam and water mixture. Then cool the entire system to a safe state.
The shape is the same shape as a gas bottle for the same reasons.
The demonstration reactors (<100 mWe) actually had sphere shaped containment structures for the same reason.
