IQ3 Automated Low Level Waste System

Features

• ²³⁹Pu detection levels <1 mg container sizes up to 300 L (85 gal) six large area LEGe detectors.
• GWAS quantitative assay software.
• MGA/MGAU U/Pu isotopics software.
• Transmission correction for matrix density.
• Fully automated system.
• Non Uniformity Correction Software.
• 15 cm (6 in.) low background steel shield.
• LN₂ autofill system.

Introduction

The IQ3 is CANBERRA's Automated Low Level Waste Assay System. The system is designed to quantify plutonium and uranium in 200 L drums at levels below 10 nCi/g and at the same time perform plutonium isotopics measurements on drums containing less than 10 mg of plutonium. At this level low level waste drums can easily be segregated from TRU drums.

For TRU drums the IQ3 system utilizes several techniques to provide accurate quantification of the fissile content of the drum. These include transmission correction to determine the drum matrix density, lump detection using multi energy assays, and non uniformity correction using CANBERRA's Non Uniformity Correction Software (NUCS).

Since most TRU waste is also quantified using neutron assay techniques, the IQ3 system utilizes the Multi-Group Analysis (MGA) software to determine the plutonium isotopics as well as the U/Pu ratio and the Pu/²⁴¹Am ratio. MGAU provides uranium isotopics values for materials which do not contain plutonium. These numbers are critical for obtaining a good assay result from the neutron system.

The system is designed to be operated as an automated counting system which can process batches of drums, or can be incorporated into a complete NDA process line.

The following sections describe the various parts of the IQ3 system hardware, software and indicate typical performance characteristics.

Description

IQ3 Shield

The IQ3 shield provides a 4p, 15 cm thick, low background steel shield as a measurement chamber for the drums. The shield is one of the key features in enabling the low detection limits to be achieved. Special steel is used to eliminate the typical ⁶⁰Co contamination which is found in most steels, and other construction materials are carefully selected to minimize those which might contain naturally occurring gamma emitters. This provides an assay chamber which eliminates outside radiation levels, and does not contribute radiation from contamination in the construction materials.

Inside the shield, the drum is placed on a rotator minimizing the effects of source and matrix non uniformities. An index signal at the start of rotation resets the indexing on the Dual Multi Spectrum Scaling (DMSS) module so that incremental spectra can be collected as the drum is rotating.

An automated absorber can be provided to extend the dynamic range of the assay system so that high fissile gram content drums can be assayed.

Loading System

The loading system is a fork mechanism on a vertical drive which is mounted onto the inside front of the shield door. The forks rest between the conveyor rollers at the beginning of a load sequence. Once the drum is positioned, the forks lift the drum from the conveyor, the door closes, and the forks lower the drum onto the shield rotator.

The loading mechanism is designed for a working weight of up to 400 kg (880 lb) with a safety factor of 1.5. The mechanism can handle drum sizes up to 320 L (85 gal).

As a part of the loading system, a special load conveyor section is provided in front of the shield. This is used to stop the conveyor in the correct position for loading and to ensure that the drum is properly centered. This section is typically mounted on a set of load cells to automatically measure the drum weight during the loading sequence.

Transmission Source Mechanism

To the left side of the shield opposite one of the sets of detectors, is the transmission source shield and actuator mechanism. This mechanism houses three 5 cm long line sources which are located at the same heights as the germanium detectors. This provides an accurate determination of the average density of the matrix material in the drum near the top, center, and bottom.

The sources are interconnected by a rod and exposed by being pulled vertically up into the collimator slots. This design ensures that the sources will drop into the shielded position in the event of a power failure. Sensors verify the full open and full closed positions of the transmission sources and a red warning light indicates when the transmission source is open.

The sources which will be used for this system are ¹³³Ba sources. These have been chosen to provide a source with a reasonable half-life and gamma energies close to the key plutonium lines.

Detection System

All nuclides which must be quantified by the system have key gamma lines which are low to medium energy.

LEGe Detectors

Two sets of three high purity germanium detectors are provided for the data collection. These detectors are optimized to provide the necessary high resolution required for the MGA analysis while providing high efficiency for the quantitative analysis. One set of detectors will provide at least 2800 mm² surface area and 20 mm depletion depth. The detectors will be matched to within 10% for resolution and efficiency. The second set of detectors are typically 20% coaxial detectors to optimize the quantification of the high energy gamma emitters.

The detectors are mounted in Model 7935-7 multi-attitude cryostats. These cryostats are configured with CANBERRA's exclusive Remote Detector Chamber (RDC) which permits back shielding to be placed around the detector end cap. This helps to further improve the low background counting capability of the system.

The cryostats are filled with liquid nitrogen and typically have a five day holding time. Because of the number of detectors and the frequent filling required, the IQ3 system comes with an automatic liquid nitrogen filling system. This permits the detectors to be filled on a scheduled basis after hours when the system is not in use. The autofill system must be interfaced to either a 240 L portable storage Dewar or a fixed facility storage Dewar as an LN₂ supply.

Electronics

ICB NIM

All of the electronics specified for this system are based on CANBERRA's Instrument Control Bus (ICB) line of instrumentation. These modules eliminate all of the front panel controls so that all set up and control of the modules is handled through the application software. Routine setup and status checks by the software ensure that the software is properly set up and turned on.

The following is a list of the modules which are normally used for this system, along with a brief description of each of the modules.

2100 NIM Bin

9615 ICB Programmable Spectroscopy Amplifier

9645 ICB Programmable High Voltage Power Supply

9633 ICB Programmable ADC

DMSS
The DMSS module is a Dual Multi-Spectrum Scaling Module. This is used to collect incremental spectra on the drum as it is rotating. This data is used by the NUDS and NUCS software.

1614 Pulser
The Model 1614 pulser is a four channel pulser which can be used as a reference pulser to correct for counting losses which are not handled by the standard live time correction such as random summing correction during high count rate measurements.

556 AIM Module
Data collection is into CANBERRA's Model 556 AIM Module. This module interfaces to the application computer over an Ethernet connection. The AIM module is required for communication to the ICB modules and to provide adequate spectral data storage (32K channels per detector) for the DMSS spectra.

All of the NIM modules are mounted into a locking air conditioned industrial cabinet.

Computer Requirements

The computer is typically a rack mounted industrial computer from the Industrial Computer Source. The computer has the following minimum characteristics:

• 166 MHz Pentium* processor
• 24 MB of RAM memory
• 1.0 GB hard disk drive
• 1.44 MB floppy drive
• Ethernet interface card
• 14 in. SVGA monitor

PLC Control System

The PLC control system will handle the loading, and unloading of the drum from the shielded assay system. In addition it controls the transmission source actuator, the rotator, and the attenuator.

The PLC used in the system is a GE/Fanuc Series 90 PLC. This is mounted into an environmental cabinet. All of the loading functions are powered by ac motors.