Or, why are they dome-shaped?Thank you, to those who will answer my question =]
It's primarily an issue of strength. The dome shape is very strong because it evenly distributes pressure forces over the whole surface. A roof with peaks or ridges would result in weak spots forming at those peaks or ridges. A silo must be strong to contain the huge amount of grain it contains. A nuclear power plant containment building must be strong to contain the potentially large amount of pressure that can build up in the event of a certain accidents such as the Three Mile Island accident in 1979.
The containment buildings of nuclear reactors are dome-shaped to control the direction of outward pressure from steam. See this diagram:
