Borehole dimensions needed to fit the Brazilian inventory of disused radioactive sealed sources from lightning rods and smoke detectors

Abstract

Brazil has a large inventory of disused sealed radioactive sources (DSRS), accounting over two hundred thousands sources being safely stored at Brazilian National Nuclear Energy Commission institutes. The majority of them can not be disposed of in near surface repository due to long lived alpha emitters, mainly Ra 226 needles formerly used in brachytherapy and Am 241 sources from smoke detectors and lightning rods, which correspond to more than 90% of the total number of the sources currently stored. Therefore, an alternative strategy to dispose of these classes of DSRS is to adopt the IAEA Borehole Disposal Concept (BDC); in this sense, due to the usually small diameter of the disposal container, knowledge of the volume of each of these sources is needed. All Ra 226 needles and Am 241 sources from lightning rods do not have significant variations in geometry and size, but Am 241 sources from smoke detectors have more than 20 different sizes covering a range from a few millimeters to about sixty millimeters in length. Of course, this fact will strongly affect the borehole design. The main objective of this paper was to determine accurately the capacity of the BDC disposal capsules required to condition these sources as a function of the different diameters of the capsules. Results showed that the borehole linear extension necessary to fit the Brazilian inventory of Am 241 sources from lightning rods and smoke detectors can reach hundreds of meters, depending on the BDC capsule diameter chosen. These findings showed that the final destination of this class of DSRS of the Brazilian inventory should be carefully examinated.