Definition of Zone i (nuclear)

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Zone i (nuclear) Definition from Government Dictionaries & Glossaries
DOD Dictionary of Military Terms
A circular area determined by using minimum safe distance I as the radius and the desired ground zero as the center from which all armed forces are evacuated. If evacuation is not possible or if a commander elects a higher degree of risk, maximum protective measures will be required.
  
Source: U.S. Department of Defense, Joint Doctrine Division. ( About )