In the U.S., under the Clean Air Act, Section 112(r), the U.S. EPA requires risk management plans (RMPs) be provided to the EPA by facilities possessing greater than certain threshold quantities of 140 chemicals. The list of 140 chemicals (77 toxic and 63 flammable chemicals including anhydrous ammonia) and their threshold quantities are found at 40 CFR 68.130. As part of the RMPs, facilities are required to determine the worst-case scenario release from a single chemical process, using EPA criteria and guidelines. The criteria and guidelines for determining the worst-case scenario release are found at 40 CFR 68.25. Facilities are also required to estimate the population potentially at risk from this worst-case scenario release by calculating the population that resides within a circle surrounding the facility whose radius is determined by the distance the worst-case scenario release might travel. This requirement is found at 40 CFR 68.30. The criteria for determining the distance a worst-case scenario release might travel are found at 40 CFR 68.22
With CAMEO (see http://www.epa.gov/ceppo/cameo/ ), it's easy to determine this distance yourself.