Measurements of radionuclide in Par Pond sediments with an underwater HPGe detector

Savannah River Site (SRS) effluent gamma emitting radionuclides in Par Pond sediment were examined in situ with an underwater HPGe detector prior to and following a 19 ft drawdown of the pond in 1991 to address dam repairs. These measurements provide a map of the 137Cs concentrations of the pond sediment, indicating that 9.4 ± 1.5 Ci is exposed by the drawdown and that 46.6 ± 7.2 Ci is the entire pond inventory. The highest individual 137Cs concentration was 25 μCi/m2 for the exposed sediment and 50 μCi/m2 for the entire pond. The results are consistent with parallel studies conducted by SREL, as well as historical data. Aside from 137Cs, the only other SRS-produced isotope observed was 60Co, with activity of only about 1% of that for 137Cs. This observation was also confirmed in grab samples of pond sediment and vegetation, which were returned to the laboratory for ultra-low-level gamma spectrometry analysis. A special effort was required to calibrate the underwater HPGe detector, where both measurements and calculational models were used. The effects of sediment depth profiles for density and 137Cs concentration were addressed in the calibration. Calibration factors for sediment surface concentrations (μCi/m2/cpm) and sediment mass concentrations (pCi/kg/cpm) were obtained. In general, the μCi/m2/cpm factor is recommended, as the pCi/kg/cpm factor depends on the depth location of the sediment of interest. However, a pCi/kg/cpm factor, which is dependent on the depth within the sediment is presented to address dose calculations that require it