25 October 2001
Electrochemical noise tells the story
by Bob Felton
Corrosion rates inferred from current and voltage fluctuations.
Sensors can now monitor corrosion of nuclear waste storage tanks by checking current and voltage changes in radioactive waste, said engineers at the U.S. Department of Energyís Hanford nuclear waste site in Washington state. Additionally, data from the sensor provides guidance for addition of chemicals used to inhibit corrosion.
Manhattan Project engineers began construction of a "hazardous manufacturing area" at Hanford in March 1943 and completed 554 permanent structures before World War II ended, including three nuclear reactors, three processing plants, and 64 underground waste storage tanks. The federal government then built two additional reactors, the Plutonium Finishing Plant, and 42 more waste storage tanks between 1947 and 1949. Yet more growth followed the Soviet Unionís successful detonation of an atomic bomb and Mao Tse-Tungís defeat of the Nationalist forces in China.
Today, the site hosts 177 underground storage tanks, some of them almost 60 years old, containing 253 million liters (8.94 million cubic feet) of radioactive waste with liquid, solid, and sludge consistencies. Twenty-eight of the tanks are double-shell, welded-steel construction; the remainder are single-shell, welded-steel construction.
Try and try again
"A number of measurement techniques have been tried with limited success," the engineers said, "to determine the best monitoring approaches to measure and track corrosion rates. One technique involved inserting and removing steel coupons into the tanks for periodic measurements. This approach was less than satisfactory because it was difficult to duplicate conditions for establishing accurate corrosion ratese.g., the effects of time and placement of coupons in various positions, including below the waste surface." Engineers also tried recovering samples of the waste, chemically analyzing them, and then estimating corrosion rates. They were dissatisfied with both of these approaches.
Engineers then designed a probe that measures the current and voltage, or electrochemical noise, generated by electrochemical reactions inside the tank. "Time-dependent fluctuations in corrosion current and corrosion potential between the electrodes are analyzed and accurately correlated to the rates of corrosion," according to a report issued by Hanfordís engineers.
Following fabrication, installation, and testing of a prototype probe, the designers assembled and installed permanent, operational probes in 1997. The probe has eight sets of corrosion electrode arrays positioned along the length of the probe assembly for better measurement of conditions at different elevations within the tank.
Further, engineers use the system to determine how much corrosion inhibitor should be added to the tanks.
"In addition to detecting the onset of dangerous corrosion conditions," they reported, "the new system can be used to optimize corrosion inhibitor addition. Currently, sodium hydroxide is added to achieve a certain waste chemistry demanded by operating specifications. The specification is conservative and may result in excess sodium hydroxide addition. Electrochemical noise-based corrosion monitoring during hydroxide addition could prevent excess sodium addition, resulting in immediate plus future cost savings."
The savings could be substantial: "Future cost to treat sodium in the vitrification waste disposal process has been estimated at $750,000 to $1.2 million per ton of sodium in the waste."
The Hanford engineers have adapted the design to the online world, collecting data continuously and making updates available via the Web. Visitors to www.hanford.gov/twrs/corrosion/data.htm can view near-real-time data from each of the eight sensor elevations in one of the tanks, along with descriptive material that identifies the type of material surrounding the probe and the sort of corrosion associated with that elevation.
Additionally, visitors can download at www.hanford.gov/twrs/corrosion/772web/wmti772.htm a paper that sets forth a comprehensive explanation of the technical theory underlying the probe design and the results of the prototype tests. IT
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