Development of Authorized Limits for the Radiological Release of Portions of the Hanford Reach National Monument
B.A. Napier1 and W.M. Glines2 (1Pacific Northwest National Laboratory; 2Department of Energy - Richland Operations Office)
In June 2000, a Presidential Proclamation created the Hanford Reach National Monument (HRNM) within the boundaries of the Department of Energy (DOE) Hanford Site. Approximately 143,000 acres of land, including the Wahluke Slope, Saddle Mountain, Fitzner/Eberhardt Arid Lands Ecology Reserve, and McGee Ranch/Riverland areas, within the HRNM are planned to be transferred to the Department of Interior. The transfer of these lands would constitute a release from DOE control. Therefore, the DOE Richland Operations Office evaluated the potential for residual radioactive contamination on these lands, and demonstrated compliance with the requirements of DOE Order 5400.5, Radiation Protection of the Public and the Environment, by developing radiological release criteria, i.e., authorized limits. Since generic guidelines have not been established for volumetric residual radioactivity for the radionuclides of concern for these HRNM lands, the authorized limits, in units of pCi/g in soil, were established to ensure that the public dose limit of 100 mrem per year is not exceeded. These authorized limits were developed using "realistic" yet conservative, radiation dose analyses based on the "likely use" and "worst use" scenarios. The expected end-use, i.e., likely use scenario, for these HRNM lands is recreational use. A dose constraint of 25 mrem per year is applied to this likely use scenario in developing Authorized Limits. The worst use scenario is considered to be a subsistence farmer, for which the primary 100 mrem per year public dose limit is applied. The RESRAD Version 6.21 computer program was used as the calculational model for translating these dose values into surface soil concentrations for each radionuclide of concern, for each of the exposure scenarios, for several geographical units of the HRNM. The final Authorized Limits were determined as the most limiting (smallest) soil concentrations for each radionuclide, across the scenarios and HRNM locations.