12.154. Among the factors considered in evaluating a proposed site for a nuclear power plant are certain radiological criteria which serve as guide­lines. These guidelines, as stated in 10 CFR Part 100, are calculated ra­diation doses to various population groups resulting from a hypothetical accident associated with a substantial release of fission products from the reactor core, followed by leakage from the containment vessel. The con­ditions postulated for these site acceptability calculations are such that they could be realized only in the highly improbable circumstances of a major LOCA accompanied by failure of essentially all the components of the ECCS.

12.155. In considering possible radiation exposures to population, it has been traditional for siting purposes to designate certain areas or zones surrounding the reactor plant following the philosophy that only a low population density would be acceptable close to the reactor where the radiation exposure from an accident might be significant. Immediately surrounding the plant is an exclusion area, a region in which the plant management has control of all activities.

12.156. The low-population zone is the region just outside the exclusion area; the total number and density of residents (if any) in this zone should be such that appropriate protective action, e. g., taking shelter or evacu­ation, would be possible in the event of a major accident. Radiation ex­posure limits have been prescribed for the outer boundaries of each zone. For example, at the outer boundary of the exclusion area, a limiting whole — body dose to a person of 25 rem from all sources for a 2-hour period has been designated. A whole-body dose limit of 25 rem at the outer edge of the low population zone was based on the person remaining in place during the entire period of passage of the radioactive cloud resulting from the accident. The philosophy of the low population zone is incorporated in the requirement for an Emergency Planning Zone (§12.171).

12.157. Another siting consideration is the distance of the plant site from population centers. Recognizing that in the event of a serious acci­dent, the societal risk from delayed cancers may be significant up to a distance of 80 km (50 miles) from the plant site, a remote location is favored by the NRC. Candidate locations are evaluated on a case-by-case basis with the Emergency Response Planning requirement (§12.171) applicable.

12.158. To test the radiological acceptability of a given site, a procedure described in NRC Regulatory Guides 1.3, 1.4, and 1.23 is followed. An accident in which substantial core meltdown occurs is assumed, such as a LOCA with complete failure of all emergency core cooling systems. Fol­lowing the accident, highly conservative (worst-case) assumptions are pre­scribed for the fractions of the core radionuclide equilibrium inventory that would be immediately available for leakage from the containment using maximum leak rate design specifications. Allowance may be made for radioactive decay during holdup in the containment vessel and material removed by containment vessel sprays and filters. However, source term studies (§12.104) show that these site evaluation radionuclide release as­sumptions are unrealistically conservative. Therefore, it is important not to confuse these site evaluation procedures, which are prescribed to assure a consistent treatment, with risk assessment studies to be described later (§12.231).

12.159. In our procedure, the containment leak rate acts as a source of a plume that is dispersed by atmospheric conditions, leading to a radioactive concentration downwind which is ingested by a person, say at the exclusion area boundary. With the aid of further assumptions in our model, a thyroid dose commitment at this point can be derived. For this purpose a simple Gaussian plume dispersion model is assumed [18]. More sophisticated (and realistic) models are used in risk assessment studies.