2004 Initial Assessments of Closure for the S-SX Tank Farm: Numerical Simulations

Zhang, Z. F. ; Freedman, Vicky L. ; Waichler, Scott R. ; White, Mark D.

Pacific Northwest National Laboratory (U.S.)

关键词: Technetium 99;    Numerical Simulation;    Storage Facilities;    Ground Water;    Radionuclide Migration;   

DOI  :  10.2172/860080
RP-ID  :  PNNL-14604
RP-ID  :  AC05-76RL01830
RP-ID  :  860080

美国|英语

来源: UNT Digital Library

【 摘 要 】

In support of CH2M HILL Hanford Group, Inc.'s (CHG) preparation of a Field Investigative Report (FIR) for the closure of the Hanford Site Single-Shell Tank (SST) Waste Management Area (WMA) tank farms, a set of numerical simulations of flow and solute transport was executed to investigate different potential contaminant source scenarios that may pose long-term risks to groundwater from the closure of the S-SX Tank Farm. This report documents the simulation of 7 cases (plus two verification) involving two-dimensional cross sections through the S Tank Farm (Tanks S-101, S102, and S-103) and the simulation of one case involving three-dimensional domain of the S Tank Farm. Using a unit release scenario at Tank S-103, three different types of leaks were simulated. These simulations assessed the effect of leaks during retrieval as well as residual wastes and ancillary equipment after closure. Two transported solutes were considered: uranium-238 (U-238) and technetium-99 (Tc 99). To evaluate the effect of sorption on contaminant transport, six different sorption coefficients were simulated for U 238. Overall, simulations results for the S Tank Farm showed that only a small fraction (< 0.4%) of the U-238 with sorption coefficients  0.6 mL/g migrated from the vadose zone in all of the cases. For the conservative solute, Tc-99, results showed that the simulations investigating leaks during retrieval demonstrated the highest peak concentrations and the earliest arrival times due to the high infiltration rate before water was added and surface barriers installed. Residual leaks were investigated with different release rate models, including uniform release, advection-dominated, diffusion-dominated, and saltcake (solubility-controlled) release models. Of the four models, peak concentrations were lowest and arrival times later for the uniform release model due to the lower release rate of the residual tank waste solids; similar high peak concentrations occurred for the advection-dominated and the salt cake models due to the higher release rate. For the tank ancillary equipment leak case, the diffusion-dominated release rate model yielded peak concentrations and arrival times that were similar to the majority of the past leak cases for residual tank wastes. Comparison between the results of the two-dimensional and those of the three-dimensional simulations show that the two-dimensional simulation significantly overestimated the peak concentrations of the contaminants by a factor of about 41 for Tc-99 and 37 for U-238 with sorption coefficient of 0.03 mL/g.