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As a condition to the Department of Energy (DOE) Low Level Waste Disposal Federal Facility Review Group (LFRG) review team approving the Savannah River Site (SRS) Composite Analysis (CA), SRS agreed to follow up on a secondary issue, which consisted of the consolidation of several observations that the team concluded, when evaluated collectively, could potentially impact the integration of the CA results. This report addresses secondary issue observations 4 and 21, which identify the need to improve the CA sensitivity and uncertainty analysis specifically by improving the CA inventory and the estimate of its uncertainty. The purpose of the work described herein was to be responsive to these secondary issue observations by re-examining the radionuclide inventories of the Integrator Operable Units (IOUs), as documented in ERD 2001 and Hiergesell, et. al. 2008. The LFRG concern has been partially addressed already for the Lower Three Runs (LTR) IOU (Hiergesell and Phifer, 2012). The work described in this investigation is a continuation of the effort to address the LFRG concerns by re-examining the radionuclide inventories associated with Fourmile Branch (FMB) IOU, Pen Branch (PB) IOU and Steel Creek (SC) IOU. The overall approach to computing radionuclide inventories for each of the IOUsmore » involved the following components: • Defining contaminated reaches of sediments along the IOU waterways • Identifying separate segments within each IOU waterway to evaluate individually • Computing the volume and mass of contaminated soil associated with each segment, or “compartment” • Obtaining the available and appropriate Sediment and Sediment/Soil analytical results associated with each IOU • Standardizing all radionuclide activity by decay-correcting all sample analytical results from sample date to the current point in time, • Computing representative concentrations for all radionuclides associated with each compartment in each of the IOUs • Computing the radionuclide inventory of each DOE-added radionuclide for the compartments of each IOU by applying the representative, central value concentration to the mass of contaminated soil • Totaling the inventory for all compartments associated with each of the IOUs Using this approach the 2013 radionuclide inventories for each sub-compartment associated with each of the three IOUs were computed, by radionuclide. The inventories from all IOU compartments were then rolled-up into a total inventory for each IOU. To put the computed estimate of radionuclide activities within FMB, PB, and SC IOUs into context, attention was drawn to Cs-137, which was the radionuclide with the largest contributor to the calculated dose to a member of the public at the perimeter of SRS within the 2010 SRS CA (SRNL 2010). The total Cs-137 activity in each of the IOUs was calculated to be 9.13, 1.5, and 17.4 Ci for FMB, PB, and SC IOUs, respectively. Another objective of this investigation was to address the degree of uncertainty associated with the estimated residual radionuclide activity that is calculated for the FMB, PB, and SC IOUs. Two primary contributing factors to overall uncertainty of inventory estimates were identified and evaluated. The first related to the computation of the mass of contaminated material in a particular IOU compartment and the second to the uncertainty associated with analytical counting errors. The error ranges for the mass of contaminated material in each IOU compartment were all calculated to be approximately +/- 9.6%, or a nominal +/-10%. This nominal value was added to the uncertainty associated with the analytical counting errors that were associated with each radionuclide, individually. This total uncertainty was then used to calculate a maximum and minimum estimated radionuclide inventories for each IOU.« less
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Technical Report
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United States