Assessment of New Calculation Method for Toxicological Sums-of-Fractions for Hanford Tank Wastes

Abstract

The toxicological source terms used for potential accident assessment in the Tank Farms DSA are based on toxicological sums-of-fractions (SOFs) that were calculated in fiscal years 2002 and 2003 based on the Best Basis Inventory (BBI) from May 2002, using the method described by Cowley et al. (2003). The present report describes a modified SOF-calculation method that is to be used in future toxicological updates and assessments and compares its results (for the 2002 BBI) to those of the old method. The new method generally calculated different (usually larger) SOFs than the old. The dominant reason was the more conservative way in which the new method represents concentration variability, in that it uses the waste layer with the maximum SOF to represent the tank SOF. The old method had used a tank-average waste composition and SOF. Differences between thermodynamically modeled and BBI solubilities were the next most common reason for differences between old (modeled) and new (BBI) SOFs, particularly in the liquid phase. The solubility-related changes in SOF were roughly equally distributed between increases and decreases. Changes in the effective toxicities of TOC and lead, which resulted from changes in the compounds in which these analytes were considered to be present, were the third most common reason. These toxicity changes increased SOFs and therefore were in a conservative direction.