Dose Assessment for Compliance with Radiological Criteria for License Termination

Program Brief

Statement of Problem:  The Bay City thorium site was included in the NRC Action Plan to Ensure Timely Cleanup of Site Decommissioning Management Plan Sites in 1992, and the Dow Chemical Company (DCC) developed a decommissioning plan approved by NRC in 1996. Since then, several supplements to the decommissioning plan have been issued as a result of finding contaminated soil under the water table.

A revised supplement to the NRC Action Plan was submitted by DCC in 2003. NRC staff requested technical assistance for reviewing the revised supplement to the plan.

Approach and Accomplishments:  To establish the most probable scenarios for future land use, the CNWRA^® investigators visited the site and adjacent locations. In preparation for dose calculations, available site information was reviewed on land use, climate, vegetation, soils, geology and hydrology.

The source term was based on data on original radioactive material, contaminated fill material, and radiological characteristics of groundwater from wells.

The leaching rates and solubilities of the relevant radionuclides were determined by using information provided by the licensee, geochemical modeling, and data from other NRC-licensed sites.

The RESRAD 6.1 computer code was selected to perform probabilistic pathway-dose assessments (i.e., to calculate a total effective dose equivalent as a function of time, from thorium and daughters). The PHREEQC 2.8 code was used to calculate the groundwater solubility limits of thorium and radium.

Client Benefits:  In the report, the total effective dose equivalent as a function of time is calculated for a period of 1,000 years. The CNWRA calculations are supporting whether the action proposed by DCC will ensure compliance with the provisions of the Radiological Criteria for License Termination, published by NRC as Subpart E to 10 CFR Part 20.