Detection of Shielded Sealed Radioactive Sources in Radioactive Waste by Non-Destructive Assay Techniques

Abstract

Experiences dealing are seldom reported on the detection of sealed radioactive sources (SRS) or its shielding in waste by non-destructive assay (NDA) methods in literature. Further the knowledge and experience in this field of waste characterization will vary from specialist till basic. The main aim of this paper is to give some guidance with the aid of an overview or scheme with which the possibility can be assessed of the detection and of a shielded SRS in (historic) waste packages. This aim could be reached by simulating the gamma flux emitted by a SRS at the outline of a standard 220 litres drum. The simulations have been performed with the Monte Carlo Neutron Photon transport code (MCNP). The results, visualized by means of iso-plots, are then be used for assessing the detection probability of the SRS by available NDA techniques. The following conclusions could be made: • “Heavy and dens” objects (e.g. shielded sealed radioactive source) can be detected and an indication of its location can be obtained by a sudden significant decrease of the gamma photon flux (cold-spot) at the outline of the drum. This cold-spot is caused by the “heavy or dens” object itself. A cold-spot can be detected the best by analyzing and visualizing the gamma photon flux from the backscatter area (e.g. range 100 keV up to 200 keV) and second best by analyzing the total gamma flux. • With relative simple radiological equipment (e.g. dose rate meter) the possible presence and an indication of the location of a shielded SRS can be estimated. • The detection possibility of detecting a shielded SRS depends not only on the applied NDA technique, but also on the applied effort (e.g. number of measurements, measuring time, applied measurement grid), the properties of the waste packages (e.g. density of the waste matrix and uniformity of the inside dispersed radioactivity), the location and the properties of the shielding of the SRS (e.g. geometry). • For confirmation and the determining of the exact location advanced NDA techniques (e.g. transmission computer tomography or other NDA that techniques gives three dimensional results) have to be applied.