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Technical experts and government agencies have worked to develop nuclear power plant emergency response plans for all of the nuclear power plants in the United States. As part of this planning, the areas around the plants are divided into planning zones. The Nuclear Regulatory Commission and Environmental Protection Agency have determined that an Emergency Planning Zone (EPZ) should be an area that is roughly a ten mile circle around the nuclear power plant. The Federal Emergency Management Agency (FEMA) has oversight responsibilities for emergency planning within this approximate ten-mile area.

The State of California expanded the Diablo Canyon Emergency Planning Zone so that it is much larger than the Emergency Planning Zone defined by the federal government. The expanded emergency planning zone defined by the state is divided into twelve smaller Protective Action Zones (PAZs). PAZs 1 through 5 correspond to the ten-mile area where FEMA has primary oversight responsibilities for emergency planning. The State has primary oversight responsibilities for PAZs 6-12, outside of the ten-mile area. Residents in PAZs 1 through 12 may be asked to take protective actions, such as take shelter in their homes or evacuate if there is a major emergency.

Surrounding the twelve PAZs are three zones known as Public Education Zones (PEZs). Residents in these areas are not likely to be affected by an emergency at the power plant. However, since residents in the PEZs (zones 13 through 15) are next to the Emergency Planning Zone, general information about Diablo Canyon is also provided to these residents.

Diablo Canyon has an emergency response plan that is tested regularly and evaluated by federal officials. It is unlikely that an emergency would affect the entire Emergency Planning Zone. That is why it is divided into twelve smaller Protective Action Zones (PAZs). PAZs help to organize emergency planning and emergency response actions into areas that are familiar to agencies and the public.

Two factors determine whether protective actions would be needed for any of the PAZs during an emergency at Diablo Canyon. The first is the amount of radioactive material released. The second is the speed and direction of the wind. Emergency Alert System (EAS) messages would broadcast over local radio and television stations which zones might be affected. They also would explain what residents in the affected areas should do.