Automatic Conveyor Monitor for Soil and Debris

Features

• Automatically assays soil, rubble, and debris, and sorts it by activity
• Saves both time and money by making site decontamination a continuous, rather than batch, process
• Conveyor speed and material depth controls allow processing rates from 1 to 50 tons per hour
• Dual Germanium (Ge) Detectors provide a typical sensitivity of 1 pCi/g for Ra or K in soil, and 0.1 pCi/g for ¹³⁷Cs and ⁶⁰Co in normal soil
• 15 cm low background steel shielding used throughout
• Adjustable shield shutters allow material depths from 5 to 30 cm for flat bed conveyors
• Trailer mounted for easy transportation to, from, and around a job site
• Conveyor/Detector designed for operation outside
• Separate environmentally controlled operator/instrument control room

Description

The Automatic Soil and Debris Conveyor Monitor can easily reduce by half the time, effort, and expense required to clean up a contaminated site. To see how, compare the procedures that must be performed using the traditional approach to this new method of contamination remediation.

The Traditional Approach

Traditional soil decontamination can be a very slow and expensive process, for it requires multiple samples to be taken and analyzed, and all of the soil to be handled at least twice:

1. First you need a detailed site survey and excavation grid plan.
2. Next, each block in the grid must be sampled and analyzed to determine if it needs to be excavated. This is commonly done at various levels for each grid point, making it very expensive and requiring several months to get the results.
3. Now each contaminated grid must be excavated and the soil temporarily stored.
4. Each stored batch is then sampled and analyzed to determine its activity level, and then moved for proper disposition.
5. For the soil remaining under each block that was excavated, start again at step 1 to insure that sufficient soil was removed.

The New Method

Extensive detailed sampling is not required, and the soil and debris need only be handled once.

1. Using high resolution, high sensitivity Ge detectors, perform an in situ survey of the top layer only, using a coarse grid. CANBERRA*s in situ system is pre-calibrated, and gives instant results.
2. For each contaminated section, excavate a layer of soil, and segregate it by radioactivity level for disposition in real time. Since the counter is automatic, excavation of some extra "clean" material in the process does add more disposal volume. Perform another coarse grid in situ survey, and remove another layer if necessary.
3. Perform a final in situ survey to insure that all contaminated material was removed.

The net result is a significant reduction in the time and effort required to clean up a site. And the material is separated into 2-3 different streams to minimize the ultimate disposal costs. Unlike statistical sampling techniques where only a small sample is assayed, 100% of the soil is assayed, and with the same high quality gamma spectroscopy tools that would have been used in the laboratory. This greatly improves the quality of the data for non-uniform contamination.

THE CONVEYOR, SHIELD, AND DETECTORS

The Conveyor, Shield, and Detector Assembly for a typical system are shown above.

Input Material Processing

The soil to be counted is generally screened to remove large items such as rocks and vegetation, although these can be processed and counted if desired. This capability can be supplied by CANBERRA or provided by the customer.

The Counting Conveyor

The typical conveyor used here has a 76 cm wide flat belt unit under computer control. By controlling the depth of the material on the conveyor as well as its speed, the throughput may be set to any desired rate from 1 to 50 tons per hour. Special techniques are used to clean the belts, to minimize dust, and to monitor for residual contamination in the belt.

The flat belt design is preferred, for it allows the entrance and exit shutters of the counting chamber shield to be positioned more closely to the soil and debris as it is being transported. In the usual conveyor configuration, a diverter mechanism on the conveyor*s output is used to steer the assayed material to cold, low level, and high level containers or trucks.

The Shield

The shield supplied with the counter is made of 15 cm thick low background steel free from ⁶⁰Co, and weighs approximately 10 000 kg. At the soil entrance and exit adjustable shutters are used to bring the shield down to just above the level of the material. These shutters are adjustable for material depths from 5 to 30 cm.

The Detectors

In its standard configuration the system is supplied with two Germanium detectors in a vibration isolation mounting. The height of the mount is adjustable to compensate for differing soil depths and activities. For a typical installation these detectors yield a sensitivity of 1 pCi/g for Ra and Th, and 0.1 pCi/g for ¹³⁷Cs and ⁶⁰Co in normal soil, assuming a one minute count time.

Environmental protection for the detectors is provided by a plastic dust shield located between the detectors and the soil, the use of LN₂ vent gas as a dry air purge for the detector chamber, and a moisture resistant covering on the preamplifiers. Then, the entire electronics are enclosed in an environmental enclosure.

The system is also available with dual large NaI(Tl) detectors for those situations where the lower resolution of these detectors is not a problem.

Output Conveyors

After leaving the counting conveyor, the material is sent to the diverter gate. When the analysis is complete, and when the leading edge of the sample reaches the diverter gate, the computer instructs the diverter to send the material to one of three pathways. For example, high level material can be sent off site for disposal or placed in a high integrity on-site disposal cell. Medium level material can be sent to a less expensive off-site location, or perhaps used for the top of the disposal cell. Low level material can be used in an unrestricted manner anywhere.

SYSTEM ELECTRONICS

Signal Processing

Computer-controlled ICB NIM are used as the signal processing electronics for the detectors. They connect to the system*s Host PC via an Ethernet link and provide the ability to manage all signal processing remotely from the Host.

Programmable Logic Controller

A programmable logic controller (PLC) handles all conveyor and diverter control and monitoring. The operator*s control station also ties into the PLC, and a communications link is provided between the PLC and the Host PC.

SYSTEM SOFTWARE

The powerful Genie basic spectroscopy package serves as the base for the system*s software. The application software which runs on top of this base utilizes CANBERRA waste management analysis algorithms and techniques that have proven their effectiveness in scores of installations.

The results from the assay software are used to control the output diverter of the conveyor via the PLC. In addition, all results are stored in a relational database to insure that a defensible record of all of the work is maintained.