Development and construction of a tomography assay system for routine operation to quality control radioactive waste drums

Governmental regulations require to specify the radioisotopic inventory of radioactive waste containers to be disposed of in a repository. Therefore, a need emerges to thoroughly examine the containers' contents. This paper describes the work performed at the Forschungszentrum Juelich (KFA) to quality control radioactive waste drums by applying tomogrpahy techniques. In particular, a tomography assay system is built which is designed to analyze Low-Level-Waste (LLW) contained in 200 liter drums of a maximum surface dose rate of 2 mSv/h. Prior to determining the nuclide inventory of a radioactive waste drum, the measurement of the density distribution by a technique called Transmission Computerized Tomography (TCT) is applied. In TCT, the drum's attenuation of a γ-beam radiated by an external source is measured by one or more detectors. The density throughout an observed plane of the drum is then visualized by a density image. Generally, the internal attenuation of the emitted radiation by the sources contained in the drum, cannot be neglected. For this reason, the attenuation coefficients obtained form the transmission measurements, are applied to correct the emission data of the internal sources. Finally, the analysis and visualization of the activity distribution within the drum is carried out by applying the technique of Emission Computerized Tomography (ECT). (orig./SR)