Gamma ray spectrometry is a passive assay technique commonly used for identification and quantification of the radionuclides present in huge complex objects such as nuclear waste packages, waste drums or glove boxes. The spectra treatment is performed in two steps: 1. The first step is the extraction of the raw data spectra (energies and net photoelectric peak areas), 2. The second step is the absolute efficiency calibration coefficient calculation. The absolute efficiency calibration coefficient is essential to quantify the activity of nuclear materials. Each nuclear waste package is single, complex and non reproducible. Waste packages and waste drums configuration change for each one of them. Therefore the absolute efficiency calibration coefficient can not be determined using an empirical calibration with a standard. An other process to obtain absolute efficiency calibration coefficient is achieved by a numerical modelling of the measured scene. The absolute efficiency calibration coefficient is modeled with all informations known on package or drum composition. However few data regarding package or waste drum composition are available. These informations needed are density, materials, screens, geometric shape, matrix composition, matrix and source distribution. The absolute efficiency calibration coefficient obtained by numerical modelling is strongly dependent on the package data knowledge and operator feedback. Consequently activity and uncertainties calculation depend on the absolute efficiency calibration. Atomic Energy Commission (CEA) has developed a methodology to quantify nuclear materials in waste packages without operator adjustment and package configuration knowledge. This methodology uses multi gamma ray emission radionuclides. A simple numerical modelling is automatically adjusted to determinate the absolute efficiency calibration coefficient. This adjustment methodology is only performed by the global mass and volume of the package. Knowledge or data about package such as density cartography, matrix composition and geometric shape is not used in this methodology. This methodology has been tested experimentally to quantify the activity from different configurations of standard sources, standard waste packages and asymmetric contamination of a steel plate. Activity uncertainties are achieved by the adjustment methodology. Validity domain is associated with the adjustment methodology. Moreover this adjustment is fast (about ten minutes) to achieve automatically activities and uncertainties calculation.
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