American nuclear power plants are required to create emergency planning zones within a 10-mile (16 kilometer) radius surrounding their plants. These imaginary 10- mile lines are not walls that hold back the effects of radiation, but simply a distance determined by emergency planners. In the event of an accident, the residents of the 10-mile (16 kilometer) zone might not need to be evacuated but could be advised to remain indoors with their windows closed. Nuclear plants also establish smaller zones of two and five miles (three to eight kilometers) surrounding the plants, within which the risk of radiation exposure is much greater.