Measuring radioactive waste streams with a wide range of activity from very low to high activity with associated high dose-rates
Increasingly, waste measurement systems are required to measure radioactive materials that range from high to low activity, with associated high and low dose rates and contained in drums with a wide density range.
ANTECH was approached by customers in Finland and Australia, each of which had a similar requirement to measure waste in drums that included high activity samples as well as waste containers with very low or no activity. For waste containers with very low or no activity the systems were required to demonstrate that the waste was below relevant regulatory thresholds. In both customer applications low background areas were available in which to locate the instruments.
To meet the challenges posed by these requirements, ANTECH developed the Wide Range Segmented Gamma Scanner (WR-SGS). In order to measure drums with density approaching and beyond than 2 g/cc, ANTECH have employed a strong transmission source based on the use of a 30 mCi 152Eu source. The ANTECH solution involved three innovative elements:
1. REDUCING BACKGROUND – Shielded Transmission Source Safe
To make low activity measurements it is necessary to reduce background radiation. A significant component of the background can come from the transmission source. Conventional shutters are only partially successful in reducing the background arising from a strong transmission source. In order to solve this problem, ANTECH designed a shielded source safe where the transmission source is placed when not in use. The design is such that the highest energy line from the transmission source (1,408 keV) cannot be detected in the germanium detector when the source is in its safe position.
2. VARIABLE DETECTION SENSITIVITY – Variable Aperture Collimator
In order to address the need for variable detection sensitivity to cover the range of source activities to be measured, ANTECH have developed an innovative variable aperture collimator (VAC) for the Wide Range Segmented Gamma Scanner. Although a separate calibration is needed for each aperture, different apertures can be chosen for emission and transmission measurements. For example, a large aperture is employed for measuring a low activity high-density drum (for both emission and transmission). However, for a low activity, low-density drum a small aperture is used for the transmission measurement and a large aperture is used for the emission measurement. Similar flexibility in the choice of collimator aperture is available for other waste items, depending on density and activity given the use of a high activity transmission source.
3. HELICAL SCANNING TO REDUCE MEASUREMENT TIMES
Traditional Segmented Gamma Scanner (SGS) measurements involve moving the detector with a fixed collimator to specific vertical positions, delineating each of the vertical segments into which the drum is divided. The WR-SGS can use either discrete or continuous helical scanning. In the case of continuous helical scanning, the segment height is determined by three factors, (i) the scan speed, (ii) collimator aperture opening, and (iii) the start and end times of data acquisition for the segment.
Two WR-SGS instruments were designed and built and are currently in service. A third unit incorporating Tomographic Gamma Scanning (TGS) is under development. These systems have meet all of the requirements of our customers.
The first system included manual drum loading and semi-automatic operation. The second system operates in fully automatic mode. It is capable of unattended operation and measures batches of drums that are typically of 20 drums or more. Both systems have been in service for a number of months and the performance of both systems have exceeded customer expectations.
J. A. Mason, M. R. Looman, R. A. Price, A. C. N. Towner, R. Kvarnström and H. Lampen, “Design and Operation of a Wide Range Segmented Gamma Ray Scanning Assay Instrument for the Measurement of both Low and Intermediate Level Waste”, Proceedings of INMM11, Palm Springs, CA, July 2011. (11-A-424-INMM)
John A. Mason, Marc R. Looman, Antony C. N. Towner, Kapila Fernando, Roland Wong, Robert A. Price, Bret Taylor and Paul Anderson, “Calibration and Validation of a Wide Range Segmented Gamma ray Scanning Instrument for the Measurement of Low and Intermediate Level Waste”, Proceedings of ICEM11, Reims, Champagne Congres, France, September 2011. (ICEM11-59304)
Ma13a J. A. Mason, M. R. Looman, L. V. Odell, A. J. Poundall, A. C. N. Towner, D. Hong, W. Jang, K. Kwak, S. Seo and M. Piotrowski, “Design and Testing of a Novel Wide Range – Segmented Gamma Scanner Incorporating Tomographic Gamma Scanning for Measuring both Low and Intermediate Level Waste in Drums-13470”, WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona, USA. (WM13-13470)