Methodology for determining the radiological status of a process: application to decommissioning of fission product storage tanks

This paper reports a combination of in situ measurement methods used for radiological characterization of four fission product tanks located in a cell of a fuel reprocessing research facility. The challenge was to dismantle the tanks only after the lowest possible activity level had been reached. The tanks were decontaminated by a series of chemical treatments from 1999 to 2004. The decontamination factor was evaluated using innovative in situ measurement methods combining elementary dose rate characterization with more sophisticated techniques such as CdZnTe gamma spectrometry and gamma imaging to localize the contamination and determine its activity level. The paper explains how each method contributes to the overall result in terms of localization, fingerprinting and activity levels. The results together are used as input data for calculation codes to create a 3D radiological model; both the MERCURE and VISIPLAN codes were used for this purpose. The decontamination efficiency and residual activity level were then determined. The latter ranged from a few TBq to a few GBq. These methods are now routinely used to determine the radiological conditions of a process or a portion of a facility. (author)