| RADIATION MONITORING & ALARA
assay times are longer since two scans are performed of each package. SGS systems are increasingly being delivered with the TS option.
A method which should fully account for both activity and matrix density inhomogeneities, at least in theory, is a tomographic gamma scan (TGS). Its development began in the 1990s, in response to an increasing demand to assay materials exhibiting arbitrary distributions of activity in a matrix of extreme heterogeneity. By taking an SGS-TC system and adding a transversal motion to the drum, a 3D matrix density map can be constructed and used to correct the activity evaluation. A 3D activity distribution map is produced as a bonus. Assay times are significantly longer than SGS-TC, forcing
a tradeoff between throughput and the activity uncertainty achievable under real conditions. The level of density and activity map detail is currently not comparable to medical tomography. TGS systems are commercially available from two vendors and there are only a handful of units in the field.
One common characteristic of all NDA systems is that during measurement the waste package, the detector and the transmission source are moving in a pre-defined manner. The package rotates, continuously or in increments. The detector moves vertically and so does the transmission source, if present. In addition, some systems feature variable drum-detector distance and/or collimator aperture and a package conveyor. Auxiliary facilities may include dose rate measurement, bar code or RFID scanning, surface swipes, package marking etc. The number of independent motion axes can be anywhere from one in IGS to 10 or more in a high-end TGS. Mechanically, the SGS of today is not very different
from the early units produced fifty years ago. During the same period, the HPGe detectors and electronics have experienced significant improvement including, for instance, electrical cooling and digital signal processing, respectively.
The design of more advanced NDA systems has simply added a motor for each new motion axis. For example, if a variable measurement geometry SGS was to be produced, two motors, one for the drum-detector distance adjustment and one for the collimator aperture control, would be bolted onto an existing standard SGS design. This approach is increasingly cumbersome for complex systems, since some options are incompatible with others, necessitating a major system redesign. Larger field upgrades are essentially impossible, eg adding the capability to measure 400l drums to an NDA system designed for 200l drums. Consider the long lead time and high price of these single-purpose units as well as high vulnerability in rough industrial settings and the urgent need for a new paradigm emerges. In our opinion, that new paradigm is a switch to industrial robots. The first industrial robot was installed on a General
Motors production line in 1961. Since then, most manufacturing and many non-manufacturing sectors have introduced robots, in some cases eliminating humans completely. The fact that the nuclear D&D industry has not followed suit is remarkable. Robots have relatively recently been used, most prominently in the aftermath of the Fukushima Daiichi accident. However, this and other similar interventions in hostile environments were performed by purpose-designed robotic devices, not industrial robots.
The first D&D facility to use industrial robots appears to be Sellafield, where they are employed for waste sorting, size reduction, waste container manipulation, filling, lidding, swabbing etc. Identical or similar tasks have been performed by industrial robots in other industries, so it is not surprising that a nuclear application would take its cues from them.
Use of industrial robots for NDA lacks a direct industrial
counterpart. To the best of our knowledge, the RoboCount™ 2020 system described below (and pictured, left) is the first of its kind. It was developed in 2018 by DuAl GmbH, Austria, for Amec Foster Wheeler Nuclear Slovakia (now Jacobs Slovakia) and its Jaslovske Bohunice decommissioning project.
The initial specification called for simple NDA of waste
pallets and flexible intermediate bulk containers (FIBCs) in open geometry using a cart-mounted HPGe spectrometer. The cart would have to be manually positioned around the waste package for each of up to four measurements. Concerns about reproducibility of the measurement geometry, the practical impossibility of adjusting the measurement parameters based on package dose rate, low throughput, and noncompliance with the ALARA principle have resulted in an agreement to replace the original setup with a prototype of a robotic NDA system which at that point existed only on a drawing board. This illustrates the key role of early adopters in fostering technological innovation. RoboCount™
2020 is based on an industrial six-axis
articulating robot carrying a HPGe spectrometer module, combined with a dedicated waste package platform and auxiliary components. The robot is a Comau model NJ-220-2.7 with a CG5
control unit and TP5 teach pendant. The maximum wrist payload of 220kg is significantly higher than the mass of the spectrometer module it is carrying. This is intentional, because higher payload corresponds to a larger working area (5400mm in diameter), enables the use of additional detectors and equipment as needed, and minimises wear and maintenance. Even if operated at maximum load, the wear and maintenance are expected to be minimal, since the robot performs a few relatively leisurely motions during a typical assay compared to hundreds of high-speed cycles per day in the manufacturing operations it was designed for. The robot is mounted on a massive steel pedestal which increases its effective maximum horizontal reach. The waste package platform is an optional system
component. However, it significantly expands the system functionality, as it can weigh and position by rotation, continuously or in steps. The rotation motion is integrated into the robot control architecture as a seventh axis. The built-in balance has a maximum load of 2500 ± 1kg. The spectrometer module is a Mirion Technologies, liquid-nitrogen cooled, HPGe detector model GC2020, connected to a Lynx®
by a 5cm modular ISOCS™
multi-channel analyser, and protected lead shield. The front shield
segment has a collimating aperture defining the detector field of view. High-activity packages could cause the detector incoming
count rate to exceed the maximum throughput of the spectrometry chain, leading to increased detector dead time and underestimating activity. A fast pre-measurement is therefore performed with a 90o
conical opening
collimator. If the detector dead time preset is not exceeded, a regular measurement is performed. In the opposite U
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