Management of solid wastes during decommissioning of research reactors. Evaluation of gross clearance levels and mathematical simulation of solid waste assay techniques

Full text: Decommissioning of nuclear facilities constitute a challenge mainly due to the huge and complex nature of radioactive waste generated during this process. In the context of management and disposal of waste and reuse/recycle of usable materials during decommissioning of reactors, clearance levels for relevant radio nuclides are of vital importance. Radionuclide specific clearance levels are developed by IAEA and such levels allow the facility for free release of materials to the environment without further regulatory consideration. An effort has been made in this paper to establish clearance levels for radionuclides associated with various system and structural components of a research reactor and rather than radionuclide specific clearance levels, these values are derived for gross activity concentration, which is more practical for radioactive waste categorization, disposal and reuse or recycle of usable materials. The first step towards the derivation of clearance levels is the calculation of annual doses relating to unit activity concentration for each nuclide using various enveloping scenarios. After the estimation of doses, the limiting enveloping scenario (the one that gives the highest dose) is identified. The clearance levels are then derived by dividing the reference dose level (10 μSv/y) by the annual dose calculated per unit activity concentration for the limiting enveloping scenario The clearance level for gross beta-gamma activity concentration is then evaluated as the product of the limiting clearance level and the number of radionuclides characterized for the structural components. Simulation studies were also carried out for the design of a monitoring system for estimation of activity concentration of the decommissioned materials, especially rubbles/ concrete, using mathematical models. Conventional solid waste assay techniques would not suffice to the requirement of decommissioning waste categorization since very low level activity estimation is warranted to satisfy the clearance level values. Several standard designs were studied using mathematical simulation and it was observed that for estimation of such low levels of activity concentration, detection system using the principle of Computed Tomography is the best suitable method. By detecting and measuring the attenuated gamma-ray intensity levels at specific energies, using an external gamma source, a map of the attenuation coefficient (a function of material density and atomic number) of the waste drum and its contents can be arrived. These maps are reconstructed to depict a drum's waste matrix attenuation per volume element and energy. Several measurements of the gamma activity emitted from a source inside different volume elements of the drum are simulated using a combination of several detectors. The attenuation, caused by material between the isotope and the detector, is corrected by suitable algorithms using the attenuation map derived. This correction leads to a far more accurate assay of radioactivity within the drum thereby reducing the error in measurements to acceptable values as desired.(author)