1 June 2002
Monitoring radioactive waste sites
Los Alamos, N.M.After years of weapons production and nuclear fuel reprocessing activities, the U.S. created millions of gallons of radioactively contaminated waste. Because storage of much of this liquid waste is in underground stainless- and carbon-steel, radioactive, liquid-waste storage tanks that potentially face corrosion, researchers from Los Alamos National Laboratory are now developing new technologies for real-time monitoring.
Historically, corrosion monitoring has been accomplished by comparing the results of chemical analysis of waste samples drawn from the tank against established standards or by evaluating tank material test strips, called coupons, immersed in the waste for long periods of time and withdrawn for analysis. Pulling the coupons from a tank is expensive and risks worker exposure. Also, neither the sample nor the coupon exposure method provides the rapid results needed to initiate corrective measures quickly.
A new monitoring technique using electrochemical noise provides real-time corrosion data and reduces the potential for exposing workers to radiation. And the enhanced corrosion data lowers operating costs via improved tank waste management. Electrochemical noise corrosion monitoring tracks extremely small current and voltage fluctuations among three electrodes, made of material as similar as possible to the waste tank material, placed in the waste solution.
Current is measured between two electrically coupled electrodes (a working electrode and a counter electrode), while the third electrode connects between the working electrode and a pseudoreference electrode to measure the voltage.
The magnitude and polarity of the signals, as well as the relationship of the timed signal traces to one another, provide indicators of type and significance of the corrosion processes occurring in the tank. Particular types of corrosion have unique and potential "signatures" that indicate when pitting or stress corrosion cracking is occurring.
"Because the chemistry near the electrodes and tank walls is similar, whatever happens on the electrodes, after some period of gaining equilibrium, is the same as what's happening on the tank walls," said Los Alamos project manager Mike Terry.
Electrochemical noise to detect corrosion is not new. Officials used it in the oil and gas industry for 10 years before a test program started in 1995 by the DOE's Tanks Focus Area (TFA) to evaluate electrochemical noise's application in radioactive waste storage tanks. "It's not a new technology, but it's definitely an emerging technology for our application," said Terry, who represents Los Alamos's involvement in the TFA as safety technology integration manager.
Because of the range of tank construction materials, tank designs, and varied waste chemistries encountered, researchers are testing different probe/electrode designs to confirm they generate meaningful signatures and accurately interpret the measurements.
The project is part of an effort funded by DOE's Office of Environmental Management.
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