Instrumental Photon Activation and Applications in a Nuclear-Waste Inspection Purpose

The development of non-destructive methods to inspect nuclear-waste containers is important for radioactive-waste management and non-proliferation purposes. This paper will present studies and results carried out by a method based on photon interrogation (photofission) which allows the determination of the actinide quantity contained in the waste. High-energy photons (produced by an electron accelerator associated with a Bremsstrahlung tungsten target) will induce photofission reactions on the actinides. Then the flux of delayed neutrons, which is directly proportional to the amount of actinides, is measured with ³He detectors. Since the beginning of 1990’s, our team in CEA has been working on the development of this method and the improvement of the existing simulation code. The two main tools will be introduced: OPERA (tool for the simulation of photonuclear reactions) which includes photonuclear cross sections in a Monte-Carlo code based on MCNP4C, and SAPHIR (Irradiation and Photon-Activation System), a device allowing experimentations for research and development programs. The applications of these tools will be illustrated mainly with two examples: 1) The feasibility study of an inspection device for old concrete containers will be reported. Two campaigns of measurements have been performed in order to determine the sensitivity and the detection limits in the case of four different types of concrete containers, in terms of nature and geometry. 2) Nuclear-waste producers and managers have been interested by the active photon interrogation possibilities to measure actinide quantity in wastes of high activity, vitrified or compacted, with constraints like a dose rate around 400 Gy/h at 27 cm from the container. The simulation-code improvement has allowed some calculations, based on the SAPHIR facility, which have shown a good linearity between the actinide mass and the number of detected neutrons, in spite of a very high passive noise and the presence of a lead protection. Several R&D programs will be also presented. On one hand, measurements are performed on real wastes, chosen for parameter which could define a limitation of the measurements, in order to improve the method and to evaluate the detection limits. For instance, tomography can be performed with this experimental device: quantity and position of actinides in the waste can be calculated. On the other hand, a new method is studied, using the delayed-gamma flux in order to quantify and to identify the different actinide isotopes contained in the waste. These methods and device offer a large panel of results in terms of measurements and simulations. Our team is now involved in several prospecting and R&D programs in order to improve the current method and to find some new applications for nuclear-waste management.