Decommissioning a nuclear facility is subject to various constraints including regulatory safety requirements, but also the obligation to limit the waste volume [1] and toxicity. To meet these requirements the activity level in each component must be known at each stage of decommissioning, from the preliminary studies to the final release of the premises. This document describes a set of methods used to determine the radiological state of a spent fuel reprocessing plant. This approach begins with a bibliographical survey covering the nature of the chemical processes, the operational phases, and the radiological assessments during the plant operating period. In this phase it is also very important to analyze incidents and waste management practices. All available media should be examined, including photos and videos which can provide valuable data and must not be disregarded. At the end of this phase, any items requiring verification or additional data are reviewed to define further investigations. Although it is not unusual at this point to carry out an additional bibliographical survey, the essential task is to carry out in situ measurements. The second phase thus consists in performing in situ measurement campaigns involving essentially components containing significant activity levels. The most routinely used methods combine the results of elementary measurements such as the dose rate [2] [3] or more sophisticated measurements such as gamma spectrometry using CdZnTe detectors [4] and gamma imaging [2] to estimate and localize the radioactivity. Each instrument provides part of the answer (location of a contamination hot spot, standard spectrum, activity). The results are combined and verified through the use of calculation codes: Mercure [5], Visiplan [6] and Microshield [7].

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