Why do silos and nuclear power plants have curved roofs?
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: