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
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.