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Tuesday, April 16, 2002

Corrosion monitoring technology makes noise in the radioactive liquid waste world

The project is part of an effort funded by DOE's Office of Environmental Management.

Past weapons production and nuclear fuel reprocessing activities, in support of national defense, created millions of gallons of radioactively contaminated waste. Much of this liquid waste is stored in underground tanks that face the potential of corrosion, a destructive electrochemical process that eats away at metallic surfaces, eventually destroying the integrity of the material if not addressed.

Until this waste can be removed from the storage tanks and stabilized for long-term storage and disposal, it must be safely stored and monitored.

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or by evaluating tank material test strips, called coupons, immersed in the waste for long periods of time and withdrawn for analysis.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 – sometimes $100,000 per sample – and presents exposure risks to workers. Also, neither sample nor coupon exposure methods provide rapid results that are needed to initiate corrective measures quickly.

A new monitoring technique using electrochemical noise, or EN, provides real-time corrosion information and significantly reduces the potential for exposing workers to radiation. In addition, the enhanced corrosion data can lower operating costs through improved tank-waste management.

EN 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 is connected between the working electrode and a pseudo-reference electrode to measure the voltage.

The magnitude and polarity of the signals, as well as the relationship of the timed signal traces to each other, 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 are similar, whatever happens on the electrodes, after some period of gaining equilibrium, is the same as what's happening on the tank walls," says project manager Mike Terry of Nuclear Systems Design (D-10).

Using electrochemical noise to detect corrosion is not new. It was used in the oil and gas industry for 10 years before a test program was initiated in 1995 by the DOE's Tanks Focus Area to evaluate EN'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' involvement in the Tanks Focus Area as safety technology integration manager.

Because of the range of tank construction materials, tank designs and varied waste chemistries encountered, different probe/ electrode designs are being tested to confirm meaningful signatures are being generated and that measurements can be accurately interpreted.

The primary focus is to determine if pitting or stress corrosion cracking occurs and to what extent. If corrosion is detected, then inhibitors can be added to the waste to halt the corrosion.

In addition to Los Alamos, Tanks Focus Area collaborators on the corrosion-monitoring project includes researchers, technology developers and managers from the Hanford and Savannah River sites, Oak Ridge National Laboratory, Idaho National Engineering and Environmental Laboratory, HiLine Engineering and Fabricators of Richland, Wash., EIC Laboratories of Norwood, Mass., and AEA Technology based in the United Kingdom.

Tanks Focus Area launched an initial deployment of EN detection probes at Hanford Site in Washington to develop EN technology for use in tanks of different types and with different waste contents.

With successful development of a corrosion monitoring system for Hanford, TFA funded development efforts for similar systems at Hanford, Oak Ridge and Idaho National Engineering and Environmental labs.

The EN method is intended to provide needed data to DOE to help assure that the integrity of the tanks holding the liquid radioactive waste can be safely maintained until the contents can be properly and safely disposed of. The Tanks Focus Area receives funding through the Office of Science and Technology whose mission is to deliver science and technology solutions in support of the Office of Environmental Management.

"This is just a piece of the puzzle, to protect the world from this waste until we can complete our mission to clean it up," Terry said. "The potential to save personal exposure and cleanup dollars, in addition to giving us a better understanding of what's really happening in those tanks, is very valuable to the sites. It's part of the single most challenging cleanup issue in the United States, figuring out safer, faster, and cheaper ways to get rid of our legacy waste."

--John Bass

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