Modern Sorters for Soil Segregation on Large Scale Remediation Projects
- Shonka Research Associates, Inc., 4939 Lower Roswell Road, Suite 106, Marietta, Georgia 3006 (United States)
In the mid-1940's, Dr. C. Lapointe developed a Geiger tube based uranium ore scanner and picker to replace hand-cobbing. In the 1990's, a modern version of the Lapointe Picker for soil sorting was developed around the need to clean the Johnston Atoll of plutonium. It worked well with sand, but these systems are ineffective with soil, especially with wet conditions. Additionally, several other constraints limited throughput. Slow moving belts and thin layers of material on the belt coupled with the use of multiple small detectors and small sorting gates make these systems ineffective for high throughput. Soil sorting of clay-bearing soils and building debris requires a new look at both the material handling equipment, and the radiation detection methodology. A new class of Super-Sorters has attained throughput of one hundred times that of the old designs. Higher throughput means shorter schedules which reduce costs substantially. The planning, cost, implementation, and other site considerations for these new Super-Sorters are discussed. Modern soil segregation was developed by Ed Bramlitt of the Defense Nuclear Agency for clean up at Johnston Atoll. The process eventually became the Segmented Gate System (SGS). This system uses an array of small sodium iodide (NaI) detectors, each viewing a small volume (segment), that control a gate. The volume in the gate is approximately one kg. This system works well when the material to be processed is sand; however, when the material is wet and sticky (soils with clays) the system has difficulty moving the material through the gates. Super-Sorters are a new class of machine designed to take advantage of high throughput aggregate processing conveyors, large acquisition volumes, and large NaI detectors using gamma spectroscopy. By using commercially available material handling equipment, the system can attain processing rates of up to 400 metric tons/hr with spectrum acquisition approximately every 0.5 sec, so the acquisition volume is 50 kilograms or less. Smaller sorting volumes can be obtained with lower throughput or by re-sorting the diverted material. This equipment can also handle large objects. The use of spectroscopy systems allows several regions of- interest to be set. Super-Sorters can bring waste processing charges down to less than $30/ metric ton on smaller jobs and can save hundreds of dollars per metric ton in disposal charges. The largest effect on the overall project cost occurs during planning and implementation. The overall goal is reduction of the length of the project, which dictates the most efficient soil processing. With all sorting systems the parameters that need to be accounted for are matrix type, soil feed rate, soil pre-processing, site conditions, and regulatory issues. The soil matrix and its ability to flow are extremely crucial to operations. It is also important to consider that as conditions change (i.e., moisture), the flowability of the soil matrix will change. Many soil parameters have to be considered: cohesive strength, internal and wall friction, permeability, and bulk density as a function of consolidating pressure. Clay bearing soils have very low permeability and high cohesive strength which makes them difficult to process, especially when wet. Soil feed speed is dependent on the equipment present and the ability to move the soil in the Super-Sorter processing area. When a Super-Sorter is running at 400 metric tons per hour it is difficult to feed the system. As an example, front-end loaders with large buckets would move approximately 5-10 metric tons of material, and 400 metric tons per hour would require 50-100 bucket-loads per hour to attain. Because the flowability of the soil matrix is important, poor material is often pre-processed before it is added to the feed hopper of the 'survey' conveyor. This pre-processing can consist of a 'grizzly' to remove large objects from the soil matrix, followed screening plant to prepare the soil so that it feeds well. Hydrated lime can be added to improve material properties. Site conditions (site area, typical weather conditions, etc.) also play a large part in project planning. Downtime lengthens project schedule and costs. The system must be configured to handle weather conditions or other variables that affect throughput. The largest single factor that plays into the project design is the regulatory environment. Before a sorter can be utilized, an averaging mass must be established by the regulator(s). There currently are no standards or guidelines in this area. The differences between acquisition mass and averaging mass are very important. The acquisition mass is defined based on the acquisition time and the geometry of the detectors. The averaging mass can then be as small as the acquisition mass or as large as several hundred tons (the averaging mass is simply the sum of a number of acquisitions). It is important to define volumetric limits and any required point-source limits. Super-Sorters handle both of these types of limits simultaneously. The minimum detectable activity for Super- Sorters is a function of speed. The chart below illustrates the detection confidence level for a 0.1 {mu}Ci point source of Ra-226 vs alarm point for three different sorter process rates. The minimal detection activity and diversion volume for a Super-Sorter is also a function of the acquisition mass. The curves were collected using a 0-15 kg acquisition mass. Diversion volumes ranged from 20-30 kg for a point source diversion. Soil Super-Sorters should be considered for every D and D project where it is desirable to reduce the waste stream. A volume reduction of 1:1000 can be gained for each pass through a modern sorter, resulting in significant savings in disposal costs.
- Research Organization:
- American Nuclear Society, 555 North Kensington Avenue, La Grange Park, Illinois 60526 (United States)
- OSTI ID:
- 21144258
- Resource Relation:
- Conference: DD and R 2007: ANS Topical Meeting on Decommissioning, Decontamination, and Reutilization 2007, Chattanooga, TN (United States), 16-19 Sep 2007; Other Information: Country of input: France; 3 refs; Related Information: In: Proceedings of the 2007 ANS Topical Meeting on Decommissioning, Decontamination, and Reutilization - DD and R 2007, 336 pages.
- Country of Publication:
- United States
- Language:
- English
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