An integrated Tomographic Gamma Scanning system for non-destructive assay of radioactive waste

Abstract

The Tomographic Gamma Scanning (TGS) technique is a relatively new method in the field of non-destructive assay (NDA) of radioactive waste. The TGS technique combines High Resolution Gamma Spectrometry (HRGS) with Three-Dimensional (3-D) low spatial resolution transmission and emission image reconstruction techniques to achieve assay goals. When compared to the traditional methods such as Segmented Gamma Scanning (SGS), the TGS technique can yield better accuracies for cases where the radionuclide is distributed non-uniformly in a heterogeneous matrix. The TGS technique is ideally suited for low-to-moderate density waste matrices, say 1.0 g cm −3 or below for 55 US gal. drums, although it can be extended to higher densities by using alternative approaches to the design or analyses. Recently Canberra Industries designed, built and characterized four such TGS systems for nuclear power plant applications. Many of the design features and the end application itself set these TGS systems apart from the others that had been built previously. The four TGS systems are the first commercial grade systems that could quantify radionuclides contained in nuclear power plant waste, using the TGS technique. The TGS systems featured two different combinations of collimator and source–detector distance; a “near” geometry with a collimator aperture of 50.8 mm and a “Far” geometry with a narrower collimator aperture of 40.6 mm. For assaying drums with matrix densities greater than 1.0 g cm −3 and/or dose rates greater than 6 mSv h −1 the system could be configured as a SGS. In the SGS mode, five different assay geometries could be configured using different collimator, source-to-detector distance and absorber combinations. During operation, the appropriate assay geometry was selected automatically based on the drum weight (density) and dose rate measurements. The characterization and performance of the one of the TGS systems are discussed in detail for both TGS and SGS modes of operation. Quantitative results are presented for point source and rod source nuclide distributions. Transmission and emission images obtained in the TGS assays will be presented.