Desenvolvimento de processo para imobilização de rejeitos de usinas nucleares utilizando betumes nacionais

Abstract

The main goal of this work is the development of a process to incorporate radioactive wastes from nuclear power plants into bitumen matrice. This process aims at obtaining waste forms that are monolithic, homogeneous and mechanical and chemically stable with low leaching rates. These features are important to evaluate the waste forms according to national and international standards. The obtained waste forms should meet the standard safety criteria for disposal in order to be accepted in a repository. Two Brazilian bitumens with softening point of 71,8 o C and 91,3 o C, respectively, were evaluated. The evaporator concentrate waste was selected to be used due to the large volume of this kind of waste generated in a PWR nuclear power plant. The two bitumens were evaluated so that the Brazilian material could be used to incorporate radioactive wastes thus avoiding the import of foreign raw material specified to other nuclear power plants in Europe, for instance. The immobilization campaigns were performed at the Bituminization Pilot Plant at CDTN and the incorporated waste concentrations ranged from 30 wt% to 40 wt%, which are in accordance with data from literature and also from plants that use the bituminization process. Important properties for bitumens and waste forms were evaluated in order to assure not only the system and occupational safety but also the quality of the waste forms which are developed to be stored for a long time in a repository. The analyzed properties are the penetration, the softening point, the flash point, the water content and the leaching rate. The integrity and durability of solid radioactive waste are also related to the waste constituents leaching rates in water. The procedure to determine such leaching rates are rule established and the analysis of these data is used to investigate the leaching mechanisms (diffusion, dissolution, partition) in order to determine the waste form performance and the experiment modeling. These analyses should be carried out before the waste form disposal and are important for safety assessment purposes. This research has also contributed not only to the development of the bituminization process to incorporate waste but also to the improvement of the Bituminization Pilot Plant at CDTN and to the implementation of methodologies and techniques to characterize bitumens and waste forms. This work concludes that is feasible to use the evaluated Brazilian bitumens for immobilization of nuclear power plants wastes in Brazil, obtaining waste forms with the required properties to be disposal in repository.