Advanced Approach for Calibration of the Segmented Gamma Scanner for the Radioassay of Drummed Waste

Fast and complete waste characterization is essential for waste handling and environmental protection. Nondestructive assay by gamma ray scanning can provide a fast, convenient and reliable way of measuring many radionuclides in closed items. Segmented gamma scanner (SGS) is designed to perform accurate quantitative assays on gamma emitting nuclides found in fission product, activation product, and transuranic (TRU) waste. In this paper we describe a challenging waste measurement need resulting in the development of a spectroscopy system with a wide operational dynamic range. The instrument described represents an advance in the state of the practice of segmenting gamma scanning technology. It combines in a single device the ability to accurately measure drummed contact handled (CH) radioactive waste with high sensitivity and also the items of high activity. In the former case a fairly conventional transmission corrected approach is followed with a traditional slot collimator. In the latter case an attenuator with three narrow slits is used to define a representative field of view of the segments and the gross matrix correction based on the average drum density. The calibration of the system is also novel because whereas experimental measurements in the factory are practical in the sensitive mode it is not possible to construct and use simulated waste forms representative of the high activity waste. To overcome this limitation the response with the slit collimator was estimated using the ISOCS numerical simulation approach benchmarked to the experimental data in the sensitive mode. Numerical simulation for waste assay is an emerging technology. It is gaining acceptance especially for difficult to measure situations. Standards bodies have recognized this trend and are working toward formulating best practice guides. The present work provides a successful case history on this important development.