Current site remediation plans call for most of the waste currently stored in tanks at the Oak Ridge and Hanford sites to be pumped out for processing, but removing tank waste presents a number of challenges. Clogged transfer lines is one of the foremost problems.

Thickened or solidified tank wastes may be thinned to a liquid or slurry that can be pumped through a pipeline, but dissolved chemical constituents in the slurry may precipitate or gel under certain conditions. Such events are presumed to be the reason four of Hanford's six slurry transfer lines are now permanently plugged. In addition, tank heel retrieval projects such as the Hanford Tanks Initiative (HTI) and the Gunite and Associated Tanks (GAAT) retrieval project at Oak Ridge National Laboratory may inject large particles into slurry transfer systems. If too many large particles are injected, the solids could bridge across the pipe and block the transfer system.

To reduce the likelihood of pipeline blockages during waste transport operations, the Slurry Monitoring Accelerated Site Technology Deployment (ASTD) project is committed to deploying in-line slurry monitors at ORNL and the Hanford Site. These slurry monitors will measure important process parameters such as slurry density, velocity, and temperature and will help warn process engineers of changing conditions that could result in clogged pipes. When measured properties begin to deviate from safe operating windows, operators can stop the slurry flow and flush the pipeline with hot water.

Under the ASTD program, HTI was awarded $630,000 to deploy three new instruments: a Lasentec M600 particle size monitor, a Red Valve Series 48 pressure sensor, and an ultrasonic densimeter developed at the Pacific Northwest National Laboratory (PNNL). Eric Daymo, PNNL senior development engineer, applauds ASTD for reducing barriers to technology implementation and linking sites towards a common goal: “ASTD has provided an opportunity for close cooperation with Oak Ridge. By working with Oak Ridge, we are learning how to streamline deployment at Hanford.”

Lasentec In-Line Particle Size and Population Monitor
The Lasentec M600 in-line particle size and population monitor uses a technique called focused-beam reflectance measurement to measure the size distributions of slurry particles that flow by the probe window. It was selected for monitoring at ORNL and Hanford after extensive testing in FY98 with physical simulated wastes. More recently, preliminary testing with radioactive supernatant (the clear, pumpable liquid that generally floats above a layer of settled solid) has been completed at ORNL. Further testing with radioactive slurries is scheduled for the very near future. The Lasentec particle size analyzer will be installed initially in the ORNL GAAT Waste Conditioning System, primarily to reduce the number of laboratory samples needed for the GAAT retrieval project.

At Hanford, the Lasentec in-line particle size analyzer will be used to monitor slurry particle size distribution in real time. HTI is required to safely transfer hardpan slurry from Tank 241-C-106 to 241-AY-102. Most likely, this hardpan will be retrieved by a confined sluicing technique that could produce a large number of particles larger than 1 millimeter in diameter. If the Lasentec analyzer detects too many large particles in the slurry, the solids concentration can be lowered by diluting with Tank AY-102 supernatant, or the slurry can be recycled to Tank C-106.

Red Valve Pressure Sensor
A Red Valve pressure sensor is targeted for installation at Hanford Tank C-106. This sensor will be configured to measure pressure drop over time. Information about the status of a slurry transfer can be inferred from a measured pressure drop. For example, as long as solids form a moving bed at the bottom of a pipe, it is unlikely that the onset of settling will be signaled by a pressure difference. However, if a stationary bed starts to build in a pipe due to gel formation or to some other increase in slurry viscosity, a gradual increase in the pressure difference (at a constant flow rate) would be observed. In addition to being installed at the Tank C-106 pump pit, the Red Valve pressure sensor is used at the ORNL GAAT facility.

Ultrasonic Densimeter
In FY98, with funding from the Characterization, Monitoring, and Sensor Technology crosscutting program, a prototype ultrasonic density sensor was developed by PNNL and constructed by Sigma Transducers. The instrument underwent validation tests with waste simulants at ORNL in 1997 and was approved in October 1998 for deployment at Hanford in FY00. Daymo is excited about this success and attributes it to ASTD for “allowing technologies such as the ultrasonic densimeter to be deployed that otherwise would not have the opportunity.”

In the ultrasonic densimeter, six transducers are mounted on a “plastic wedge” (see diagram). Five of these transducers (A, B, C, D, and E) produce longitudinal waves, and the sixth (S) is a shear wave transducer. When the ultrasonic waves from transducer A strike the wedge-liquid interface, part of the energy is reflected back to A and the voltage is recorded; part of the energy is transmitted into the liquid. Similarly, transducer B produces ultrasound that strikes the wedge-liquid interface at an angle, and some of the energy is reflected to transducer C. The voltage in C is recorded. Transducers D and E operate similarly. The voltage measurements are used to determine reflection coefficients (ratio of energy transmitted to energy reflected) at several angles. The density of the liquid can be determined since the reflection coefficients depend upon the density of the liquid, the speed of sound in the liquid, and known wedge properties. These reflection coefficients are analyzed in real time to provide continuously updated density measurements.

The pipe spoolpiece that houses the sensor is compact and insensitive to vibration and radiation fields. Because of these operational advantages, the density monitor is expected to be successfully deployed close to a large pump in the Tank C-106 pump pit, which has limited space and a significant radiation field, and provide useful information to operators regarding slurry density.

The economic penalties for pipeline blockages are steep: DOE may be required to pay millions of dollars each day that tank waste cannot be delivered to the privately owned vitrification facility at the Hanford Site. The deployment of these sensors at Hanford not only will diminish the risk of such penalties, but could save up to $7 million in transfer line replacement and operating costs.

Penny Colton, PNNL project manager, provided most of the information in this article. She can be reached at (509) 375-2983.

Also check out:

• Hanford Tanks Initiative Web site
• ORNL Gunite and Associated Tanks Web site

Readers interested in this project should take care not to miss "The science of sludge."