Fishing for a way to assess mixing behavior in treatment tanks for radioactive waste, ecologists at Pacific Northwest National Laboratory came up with an innovative use of radio frequency technology previously used to track migrating fish.
But rather than swimming out to sea implanted in young steelhead and salmon, thousands of passive integrated transponder, or PIT, tags were added to a clay simulant and then whipped around in tests of air sparger and pulse jet mixer equipment in large test tanks and scaled prototypes.
This novel application of the PIT tags provided a means of assessing fluid motion without sampling. Performance results from the tests led to equipment configurations adopted for implementation, said Dean Kurath, an engineer with PNNL's radiochemical engineering group. The Hanford Waste Treatment Plant, currently under construction at the Department of Energy's Hanford Site in Washington state, will be the world's largest facility for treating highly radioactive waste. The Hanford Site stores 53 million gallons of the waste from past production of plutonium for the nation's nuclear weapons program in 177 underground tanks.
Waste slurries of various compositions and thicknesses will be mixed in several different tanks in preparation for immobilizing the radioactive waste in glass through a process of vitrification or "glassifying." Mixing the materials will maintain homogeneity in process vessels, limit solids settling and stratification, improve heat transfer and mix in various process solutions. Mixing will also provide for the controlled release of flammable gases generated by the breakdown of organic materials in the waste slurries.
Bechtel National Inc., which is designing and building the Waste Treatment Plant for DOE, enlisted PNNL to help determine the best designs and technologies for mixing the mudlike waste that will be present in some of the approximately 20,000-gallon tanks. Technologies under consideration included pulse jet mixers, air spargers and steady jets generated by recirculation pumps.
In one test of an air sparger, 6,000 PIT tags were added to a tank filled with opaque simulant. Not much bigger than a grain of rice, the PIT tags contain an integrated circuit and an antenna encapsulated in glass. The tag is activated when it passes within range of an antenna that generates an electromagnetic signal. The signal alerts the tag to transmit its unique digital code back to the reader. The same principle is at work in antitheft devices attached to retail merchandise in department stores and in subcutaneous ID tags for pets. But according to PNNL researcher Rich Brown, this is likely the first time PIT tags have been associated with mixing simulated radioactive waste.
As the sparger went to work mixing waste, the tags were detected using custom-made antennas housed in four vertical wells of PVC pipe placed in the tank around the central sparger. A remote-controlled motorized system moved the antennas up and down within the wells to detect passing PIT tags at varying depths in the tank.
The tags have a signal range of 3 to 4 inches. Movement of the slurry was determined by identifying individual PIT tags during multiple antenna passes. In other tests, the PIT tag antennas were placed around the outside of the vessel.