In order to minimize the number of analyses per sample, numerous in-line analyzers are used at the Marcoule plant for reprocessing irradiated fuel. These analyzers, which determine Plutonium concentration by alpha counting, use a horizontal drum that rotates inside a tank containing the process liquid. The drum transfers a thin, uniform film to a position opposite a ZnS scintillator. The variation coefficient (standard deviation divided by the mean value) is 3.1 to 3.2% for in-line counters used in the aqueous phase and 4.6% for those used in the solvent phase.