Deriving Site-Specific DCGLs: One Approach to Regulatory Acceptance

H.T. Downey and J.W. Lively (MACTEC)

The Nuclear Regulatory Commission (NRC) has established screening values for common radionuclides in surface soils that may be used to demonstrate compliance with the requirements for license termination. Depending upon the radionuclide and level of residual radioactivity, these values may be satisfactory or site-specific values can be derived using computer modeling with the RESidual RADioactivity (RESRAD) code. RESRAD allows for pathway modeling of radionuclides through the environment and calculates potential doses to a hypothetical receptor in various exposure scenarios. The challenge of deriving site-specific derived concentration guideline levels (DCGLs) is to establish parameters in RESRAD that are realistic and acceptable to the regulatory agencies. An example of this process is a former nuclear fuel manufacturing site that is undergoing decommissioning and recently had Site-specific surface soil DCGLs for enriched uranium and byproduct radionuclides approved by the NRC. Two RESRAD parameters that were evaluated as part of this process are presented in detail: distribution coefficient (Kd) and building shielding factor (SHF1). In addition, the approach used to combine three uranium isotopes into a total uranium DCGL and to reduce the insignificant radionuclides from a potential list of twenty-two to only one (Co-60) following the guidance in NUREG-1727 and NUREG-1757 is discussed. These DCGLs were derived and accepted at 20.6 Bq/g (557 pCi/g) total uranium and 0.18 Bq/g (5 pCi/g) Co-60, which represent a potential dose of 0.19 mSv (19 mrem) per year from the resident farmer scenario. The discussion of this process provides insight to the regulatory review of DCGLs, the level of effort necessary to allow for the use of site-specific values for RESRAD parameters, and an approach to reducing insignificant radionuclide DCGLs.

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