A nuclear plant contains a large amount of radio-active material which could be a potential threat to public health. The plant is therefore designed, built and operated so that the risk to the public is low. Careful design of the normal reactor systems is the first line of defense. These systems are highly resistant to an accident happening in the first place, and can also be effective in stopping it if it does happen. The normal systems are backed with special independent and redundant safety systems. Their sole purpose is to minimize the effects of an accident, or to stop it completely. They include shutdown systems, emergency core cooling systems, and containment systems.