Source: Emergency Planning Zones (NRC). This graphic is an illustration of the concept of EPZ. See the NRC link above for more details, including the following NRC information:
"The exact size and shape of each EPZ is a result of detailed planning which includes consideration of the specific conditions at each site, unique geographical features of the area, and demographic information. This preplanned strategy for an EPZ provides a substantial basis to support activity beyond the planning zone in the extremely unlikely event it would be needed."