Ecologists Spawn New Use For PIT Tags: Radio Frequency Technology Tracks Mixer Efficiency

Butrather than swimming out to sea implanted in young steelhead andsalmon, thousands of passive integrated transponder, or PIT, tags wereadded to a clay simulant and then whipped around in tests of airsparger and pulse jet mixer equipment in large test tanks and scaledprototypes.

This novel application of the PIT tags provided ameans of assessing fluid motion without sampling. Performance resultsfrom the tests led to equipment configurations adopted forimplementation, said Dean Kurath, an engineer with PNNL's radiochemicalengineering group.

The Hanford Waste Treatment Plant, currentlyunder construction at the Department of Energy's Hanford Site inWashington state, will be the world's largest facility for treatinghighly radioactive waste. The Hanford Site stores 53 million gallons ofthe waste from past production of plutonium for the nation's nuclearweapons program in 177 underground tanks.

Waste slurries ofvarious compositions and thicknesses will be mixed in several differenttanks in preparation for immobilizing the radioactive waste in glassthrough a process of vitrification or "glassifying." Mixing thematerials will maintain homogeneity in process vessels, limit solidssettling and stratification, improve heat transfer and mix in variousprocess solutions. Mixing will also provide for the controlled releaseof flammable gases generated by the breakdown of organic materials inthe waste slurries.

Bechtel National Inc., which is designing andbuilding the Waste Treatment Plant for DOE, enlisted PNNL to helpdetermine the best designs and technologies for mixing the mudlikewaste that will be present in some of the approximately 20,000-gallontanks. Technologies under consideration included pulse jet mixers, airspargers and steady jets generated by recirculation pumps.

In onetest of an air sparger, 6,000 PIT tags were added to a tank filled withopaque simulant. Not much bigger than a grain of rice, the PIT tagscontain an integrated circuit and an antenna encapsulated in glass. Thetag is activated when it passes within range of an antenna thatgenerates an electromagnetic signal. The signal alerts the tag totransmit its unique digital code back to the reader.

The sameprinciple is at work in antitheft devices attached to retailmerchandise in department stores and in subcutaneous ID tags for pets.But according to PNNL researcher Rich Brown, this is likely the firsttime PIT tags have been associated with mixing simulated radioactivewaste.

As the sparger went to work mixing waste, the tags weredetected using custom-made antennas housed in four vertical wells ofPVC pipe placed in the tank around the central sparger. Aremote-controlled motorized system moved the antennas up and downwithin the wells to detect passing PIT tags at varying depths in thetank.

The tags have a signal range of 3 to 4 inches. Movement ofthe slurry was determined by identifying individual PIT tags duringmultiple antenna passes. In other tests, the PIT tag antennas wereplaced around the outside of the vessel.

PNNL is a DOE Office ofScience laboratory that solves complex problems in energy, nationalsecurity, the environment and life sciences by advancing theunderstanding of physics, chemistry, biology and computation. PNNLemploys 4,000 staff, has a $700 million annual budget, and has beenmanaged by Ohio-based Battelle since the lab's inception in 1965.