Photofission tomography of nuclear waste packages

Quantifying the mass of actinides in large concrete waste packages using non-destructive methods is a major challenge for the management and the storage of packages in appropriate facilities. Since the beginning of 1990s, our team in CEA has been working on the development of a method based on interrogation with high-energy photons to assay actinides in large concrete waste package. The method consists in using photons of high energy (bremsstrahlung radiation) in order to induce photofission reactions on the fissile nuclei present in the wastes. The measurement of the delayed neutrons emitted by fission products allows us to quantify the actinides present in the wastes. The accuracy of the method can be deeply improved by carrying out a tomography, i.e. computing the three-dimensional spatial distribution of the actinide mass inside the nuclear waste package. This paper presents the first experimental results of a tomography carried out on a 1.2 t real concrete waste package. Measurements associated with reconstruction algorithms and Monte Carlo simulations have allowed to locate an equivalent mass of 690 mg of uranium 238 centered in a disc of 20–25 cm of diameter at 75 cm height. These measurements have been performed in the SAPHIR irradiation facility at CEA/Saclay. This facility houses a pulsed linear electron accelerator (energy range from 15 to 30 MeV, pulse duration of 2.5 μs, peak current of 130 mA). The located mass has been subsequently confirmed by a destructive analysis of the package.