51312-01543-Through-Tank Ionic Conductivity Measurement Feasibility Study

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Product Number: 51312-01543-SG

ISBN: 01543 2012 CP

Author: Dong Liang

Publication Date: 2012

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High level radioactive waste from defense operations is being stored in carbon steel tanks at the Hanford Site in the State of Washington. The first storage tanks Single Shell Tanks (SSTs) were reinforced concrete with a welded steel liner. Of the 177 tanks as many as 67 of the SSTs could have leaks that resulted in the loss of approximately 1 million gallons of waste into the ground. Part or all of the leaks could have been associated with corrosion and stress corrosion cracking of the steel caused by high temperature along with the presence of a low pH and high nitrate ion concentration liquid waste. The liquid waste in the SSTs has been pumped out and the remaining solids will eventually be removed by jet sluicing. It is of interest to have knowledge of the existence of any through-liner pathway under the current waste level and to have a sensor to detect such a pathway above the current waste level if sluicing will increase the waste level. The chemical and mobile radioactive species are present in the waste in the form of ions so a pathway that could lead to radioactive leakage would be one that exhibited ionic conductivity. The goal of this project is to investigate the use of Electrochemical Impedance Spectroscopy (EIS) to detect the presence of ionic-conductive pathways in the SST steel liners. EIS works by applying a potential sine wave of varying frequency to an electrode and measuring the current. The response of a system with an ionic short through the liner should be different than that of a system having a sound liner with electrochemical interfaces on either side. This difference would be true if the time constants R x C of the polarization resistances and double layer capacitances associated with the liner interfaces were vastly different than the time constant of Metal 1 in the sludge. These different time constants would provide an EIS spectrum of very different nature than if there were an ionic path directly from the surface of Metal 1 to the soil. Equivalent circuit modeling was first performed to have some basic understanding of the system. Laboratory experiments were performed with small (1 liter) and large (10 gallons) cans buried in soil. The experimental results were modeled using equivalent circuit model again to compare with the initial results. The work was supported by Washington River Protection Solutions.