A High Throughput Segmented Gamma Scanning System for Automatic Waste Assay
|
Introduction
As
a result of increasing customer demand for automated handling of waste
containers, Canberra has designed, manufactured, and installed a very
high throughput low level waste assay scanner. Among the performance
specifications were:
- Automatic drum handling, weighing, rotation, and scanning for drums weighing up to 400 kg each.
- Surface dose rate monitoring and level checking.
- Drum content analysis sensitivity of < 30 kBq for a homogeneous content density 0.2 g/cc.
- Report generation and data base updating.
- Minimum operator interaction.
- Do all of the above at a rate 20 drums per hour.
In short, improve the throughput rating of a traditional waste assay scanner by a factor of 10 with no loss of functionality or sensitivity!
To do this required both creative systems engineering and the application of a unique multiprocessor control and assay system. In this note we'll be looking at the methods and technologies that were used to achieve those performance levels.
Functional Overview
As you can see in Figure 1, the final objective of the customer's waste processing system is to compact the drums of waste to minimize handling and disposal costs. A "Super Compactor" capable of reducing a 55 gallon drum to a height of about 15 cm (6 in.) is used to minimize the size of the drums.
Figure 1.
Overall Waste Processing System
The compactor is capable of minimizing one drum every three minutes; this became the throughput goal for the overall system, and established the 20 drums per hour throughput rating for the assay system.
Feeding such a system keeps the operating crew very busy, so operator input had to be kept to a minimum. For routine operation all that is required is a scan of the barrel ID bar code and the pressing of single button; all drum handling and assay operations are automated from that point on.
Once the assay is completed, the drum is either sent on to the compactor or rejected due to excess activity. Should a rejection occur, the drum is returned to the input feeder and an indicator activated to let the operator know that intervention is required. In either case a full assay report is generated, and everything - from drum loading to report printing - takes place within the three minute per drum limit.
Operational Overview
A functional block diagram of the assay system can be found in Figure 2, and should be referred to as we follow a drum through the system in the paragraphs which follow.
Figure 2.
Assay System Block Diagram
Click for enlarged view. To return to this page, click the "Back" button.
The assay of a drum is initiated by the operator placing it on the conveyor and scanning the barcode on the Drum ID label. The Master CPU then verifies the ID and flashes a green indicator located on the top of the Control Cabinet to let the operator know that all is well.
After receiving the "Good Read" feedback the operator pushes the Start button to begin the assay process, which causes the PLC (Programmable Logic Controller) to start the drum moving toward the counting station. Once it is in place the PLC informs the Master CPU that an assay can begin.
The Master CPU then records the weight of the drum, and, via a Thin Wire Ethernet Network, instructs the Process CPU to begin the assay. While the assay scan is being performed by the Process CPU, the Master CPU performs a surface dosimetry scan via four ionization chamber dose-rate meters equipped with RS-232C computer interfaces.
Once the Process CPU and its four System 100 MCAs has completed the data acquisition for the scan, a complete radionuclide assay is per-formed on each of the four spectra that were collected. These assays are then used as the basis for the analysis of the drum contents, and the results of that analysis are used to determine if the drum can be compacted or should be sent back to the operator for further handling. The compact/reprocess deci-sion plus the assay results are then sent via the network to the Master CPU.
Upon receiving the completed assay, the Master CPU instructs the PLC on how to route the drum and prints the assay report.
Performing all of the above within the three minute per drum limit is significant in itself, but that's just the tip of the iceberg. This is a real-time system, so all of these operations are going on simultaneously on multiple drums.
That is, while Drum A is being compacted, Drum B is being assayed, Drum C is waiting in line on the conveyor to be assayed, and Drum D is being loaded onto the conveyor and having its label scanned. In parallel with all of this is the report generation and database updating needed for all of the record keeping. And it all runs at a 20 drum per hour rate!
In the sections which follow we'll take a closer look at the various subsystems that were used to achieve this level of performance.
The Detection Subsystem
Figure 3.
Overall View of the Detection Subsystem
Since the detection subsystem is the heart of any assay system, that's where we'll begin.
Figure 3 provides an overall view of the detection system. The transmission sources and surface dosimeters can be seen to the right of the drum, and the Germanium detectors to the left; the large box below the Ge detectors is one of the two Cryolectric refrigerators used to cool the detectors.
Note that there are four of everything associated with radiation detection. This means that vertical scanning is eliminated, allowing the system to complete its data acquisition cycle in only one minute. This short counting time was required because, when the system is running at its maximum throughput rate, only one minute can be allocated to the actual data collection process; the remaining time is used for analysis and drum handling.
The Surface Dosimeters
Located just to the left of the Transmission Sources are the Victoreen Model 450 Dosimeters used for exterior dosimetry of the drums (see Figure 4). Each has an RS-232C interface for transmitting its readings to the Master PC, and can measure surface contamination to levels as low as 0.1 mR/Hr during the one-minute scan of the drum.
Figure 4.
Details of the Transmission Sources and Surface Dosimeters
The Transmission Sources
In Figure 4 you can also see the details of the 3.7 x 105 kBq 152Eu transmission sources. Each is housed in it's own shield, and the shutters are controlled by the PLC. A gravity operated fail-safe is provided; if the power should fail the shutters will automatically drop into the closed position.
The Collimators and Ge Detectors
As you can see in Figure 5, each of the germanium detectors has a collimator to restrict its view of the drum to just the segment which it is assaying. Because of the space restrictions imposed by placing four detectors within the height of a single drum, Cryolectric cooling was used rather than the traditional LN Dewar.
Figure 5.
The Lead Collimators and Germanium Detectors
The size of the detectors is also non-traditional; to insure sufficient sensitivity in a one minute scan, all are 20% relative efficiency rather than the 12% to 15% size normally found in standard segmented gamma scanners.
The Electronics Subsystem
Figure 6.
Overview of the Electronics Subsystems
In Figure 6 you can see the cabinets which house the electronics of the system. We'll look at each section in detail, beginning with the NIM modules, which are shown in Figure 7.
Figure 7.
High Speed NIM Signal Processing
The NIM Electronics
Each of the four Ge detectors has its own NIM signal processing chain, and to minimize the loss of any events due to pulse pileup only units with the highest throughput ratings were used. Specifically, in addition to its HVPS, each detector has its own Model 8077 450 MHz ADC and Model 2024 Fast Amp/Gated Integrator.
In addition to speed, the NIM system also had to be extremely stable under the widely fluctuating environmental conditions of an industrial facility, so each counting chain also has its own Model 8232 Digital Stabilizer to provide both zero and gain stabilization.
The MCAs
Each of the NIM front end channels feeds one of the four System 100 MCA boards that reside in the Process CPU chassis. While a single System 100 could have been shared among the four front ends, the use of separate boards insures minimum data storage dead time; when you only have one minute to collect a spectrum you want to make sure you acquire every single event you can!
The PLC Control Logic
While the System 100 and the Process CPU are performing the assay, a Programmable Logic Controller, shown in Figure 8, is controlling the conveyor and the rotation platform for the drum which is being assayed. The PLC also performs all needed motion safety interlocking and activates the operator feedback indicators and audible alarms.
Figure 8.
The System Control Cabinet and PLC
The System Processors
Two 486-based PCs, shown in Figure 9, handle all of the computing chores for the system. A 25 MHz system is used for the System Master and a 33 MHz system for the Process CPU, with a high speed Thin Wire Ethernet link tying the two together. While each has its own display and keyboard, the operator only uses the Master Console under normal system operation; the console on the Process system is used only for spectral display and System 100 maintenance.
Figure 9.
Two Networked DELL 386 Based PCs Perform Overall System Control and
Data Analysis Functions
The System Software
While the hardware may actually do all of the physical work, it is the software in the two 486 processors, shown functionally in Figure 10, that ties the various bits and pieces together into an integrated system.
Figure 10.
System Software Block Diagram
VM386 for Multitasking
The first layer of software you'll find is a package called VM386. Produced by IGC, it is a commercially available program that allows multiple copies of DOS to be run simultaneously on a single system. As you can see in Figure 10, it is used in both the Master and the Process CPU to give each a multitasking capability.
DOS and Direct Access
Next comes DOS, one copy for each of the main tasks in each processor. Since each application program has its own copy of DOS, each thinks it has a complete system to itself; task switching is handled by VM386, and is completely invisible to the application.
While DOS is necessary to all of the application programs, it is not the most user-friendly system interface for an industrial environment. To minimize the operator interactions with DOS, a shell program called Direct Access by Delta Technologies is used.
In Figure 11 you can see how Direct Access replaces the DOS C:> prompt with a simple menu; the operator need only press the A, B, C, etc. key to choose the operation that is to be performed.
Figure 11.
Typical Direct Access Control Screen
Operator Interface Task
Under normal production operation this is the operator's only contact with the system. Once an assay run is started, this is the task that handles the scanning of the labels on the drums, and, after verifying that all is well, tells the Assay Control Task to proceed.
Assay Control Task
This is the heart of the real-time operations of the system, for it is this task that communicates with the PLC which controls drum motion.
After telling the PLC to position a drum, the task waits to receive a "Drum Ready to Scan" signal from the PLC. When it's received, the Assay Control task weighs the drum, starts the external dosimetry, and, via the Thin Wire Ethernet Link, tells the Process CPU to begin an assay.
Communications Task
Since all Process CPU operations
are initiated by commands from the Master, a separate Communications Task is used to monitor the network link. When the Communications Task receives the "OK to Assay" message, it alerts the MCA Control and Assay Task that it's time to get to work.
MCA Control, Assay, and Data Base Task
As its name implies, this is the task which collects the spectral data, performs the radionuclide assay, and updates the data base in which the results are stored. The assay itself is based upon the field-proven SPECTRAN-AT analysis package, so fast, accurate results are assured.
When the assay is complete, the results - which are based upon the analysis of four separate germanium gamma spectra - together with a pass/fail flag are sent to the Master CPU via the Communications task.
Upon the receipt of the assay results the Master tells the PLC how to route the drum - to the Super Compactor or back for further handling - and the report is printed.
That, in a nutshell, is how the various tasks in the two CPUs interact to assay a drum. But keep in mind this is a real-time system; while the activities related to any one drum are serial in nature, several drums are in different stages of the assay process at the same time, so all tasks are really running parallel.
System Setup and Calibration
In addition to controlling the system and performing the actual assays, the software also includes provisions for system setup and calibration. Since these operations can critically impact the operation of the system, they are all password protected, as shown in Figure 12.
Figure 12.
All Critical Operations are Password Protected
Many of the operations performed under Setup and Calibration are what you would expect in a Germanium gamma assay system: Energy Calibration, Efficiency Calibration, Transmission Source Attenuation, Isotope Library Editing, and so forth. To these basics are added the drum characteristics such as size and weight that are needed to complete the assay.
Lotus Symphony for Parameter Entry
While the need for these added parameters is far from unique, the way in which they are handled and stored definitely is: They all exist as entries in a Lotus Symphony database, and use Symphony macros for input processing and limit checking. The resulting "spreadsheets" are then used as parameter tables for all of the tasks which require them, yielding a straightforward table-driven software system with an easy to use input editor for building the tables.
Conclusion
With this discussion of the software our overview of what we believe is one of the highest throughput waste assay systems ever produced is complete. While the system is admittedly unique, and may very well not be the optimum solution to your particular needs, we believe it does serve as an effective demonstration of the ability of our Applied Systems Group to bring together a wide range of technologies to solving unique application problems. For further information on our capabilities, or to discuss the specifics of your system requirements, please feel free to give your local Sales Engineer or one our Applications Specialists a call.
|
| Top of Page |