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Staff Accomplishments

PIT tags go from salmon tracking to waste tank mixer testing

August 2005
A radio frequency technology that Battelle-Pacific Northwest Division has used extensively to track migrating Northwest salmon has taken the plunge into a very different kind of sea. PNWD researchers are using these passive integrated transponder tags or PIT tags in tanks full of simulated radioactive waste to test mixing technologies. PIT tags are small identification tags not much bigger than a grain of rice. Each tag contains an integrated circuit and an antenna encapsulated in glass, but no battery. The tag is powered when it passes within the range of an antenna that generates an electromagnetic signal which activates the tag causing it to transmit its unique digital code back to the reader. The same principle is at work in the RF tags used as anti-theft devices in retail merchandise and internal ID tags for pet dogs and cats. But according to PNWD ecology researcher Rich Brown this is the first time PIT tags have been associated with radioactive tank waste. Rich and other PNWD ecologists have been working with PNWD's Radiochemical Engineering Team to test waste mixing technologies for the Hanford Waste Treatment Plant. The plant, which will be the world's largest facility for treating radioactive waste, is being built at the Department of Energy's Hanford Site in Washington State to glassify nuclear waste remaining from Hanford's plutonium production days. Plant processes will involve transporting, processing, and mixing waste slurries of various compositions and thicknesses in several different tanks. Mixing of the materials in the tanks is needed to maintain a reasonable degree of homogeneity in process vessels, limit solids settling and stratification, improve heat transfer, and mix in various process solutions. Mixing will also provide for the safe, controlled release of flammable gases generated in the waste slurries. The plant contractor enlisted PNWD to help determine the best technologies and designs for mixing the thick, mud-like waste that will be present in some of the tanks. Mixing technologies that PNWD tested included pulse jet mixers, air sparging, and steady jets generated by recirculation pumps. These mixing technologies were selected because they lack moving mechanical parts that might require more frequent retrieval for maintenance in the radioactive environment of the tanks. PIT tags were used for tests of the single sparger and tests of the pulse jet mixer in large tanks and scaled prototypes. The PIT tags were one of several methods used by PNWD to test mixing technologies. Other testing methods PNWD used included adding dye, fluid sampling, and ultrasonic Doppler-shift-based velocity sensor probes. In one test of an air sparger, 6,000 PIT tags were tossed into a tank that was filled with a sludgy clay simulant designed to have the same consistency as actual radioactive waste. As the air sparger went to work mixing waste, PIT tags were detected in the opaque simulant using custom-made transceivers or readers. The readers were protected inside four vertical "wells" of PVC pipe located in the tank around the central sparger. A remotely controlled motorized system moved the readers up and down within these wells to detect and record passing PIT tags at various depths in the tank. The tags have a signal range of up to 9 inches. Movement of the slurry was determined by identifying individual PIT tags during multiple antenna passes. In other tests, the PIT tag readers were placed around the outside of the vessel instead of in PVC wells within the tank. "The use of PIT tags was an innovative application for assessing mixing behavior in the opaque simulants used in our pulse jet mixer hybrid mixing tests. This novel application of PIT tags provided a relatively noninvasive method for assessing fluid motion that didn't require sampling. The results of these tests provided performance results that ultimately led to mixing equipment configurations that have been adopted by the Waste Treatment Plant contractor for implementation," said Dean Kurath, PNWD task manager for the Pulse Jet Mixer Program. For more information, please contact Walter Weimer, Process and Measurement Technology Product Line Manager.

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