Quantification and localisation of actinides in concrete waste packages using photofission and delayed neutron counting

Radioactive-waste management is an issue in every country with a nuclear program. The development of sensitive, practical, non-destructive assay techniques for quantification of low level actinide activities is vital for safety and long-term waste management. In addition it is important to define the correct category of nuclear waste in order to minimize the storage cost. Photofission associated with delayed-neutrons measurement can be used as a non-destructive method to give an answer. Since the beginning of 1990's, our team has been working on the development of this method associated with Monte Carlo simulations. The IPAA (instrumental photon activation analysis) consists in irradiating the fissile nuclei present in the wastes with high-energy photons (Bremsstrahlung radiation) and thus producing photofission reactions. Delayed-neutrons counting, following photofission reactions, allows to assay and locate the mass of actinides. This paper describes this method and the results obtained in the SAPHIR facility (Irradiation and Photon Activation System), a device allowing experiments for research and development programs. Several campaigns of measurements have been performed in order to determine the sensitivity and the detection limits in the case of several real containers and a concrete mock-up. The results are presented and the mass detection limits are determined for three different types of real containers, in terms of nature and geometry. The first package is a 200 liters barrel, 130 kg heavy, filled with low-density technological wastes from a radioactive waste package. The second is a 200 liters barrel, 270 kg heavy, surrounded with concrete. The third real container is composed by four compacted drums and surrounded by concrete. Its mass is 1.2 ton. The mass detection limits vary from 30 mg to 600 mg, depending on the containers. The results of measurements performed with a concrete mock-up of 870 liters are also given. This mock-up is representative of a large concrete package, with total weight of 1.78 ton. In addition, since one year, a new program is being developed at CEA Saclay aiming to improve the current method. It consists among others in optimizing the photons energy and the interrogation geometry. From the present mass detection limit of 10 g/t of nuclear waste, the introduced improvement should allow reaching a 1 g/t detection limit. (authors)