Study of the sorption and modelling of cesium by a Brazilian bentonite using PHREEQC
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.57
Study of the sorption and modelling of cesium by a Brazilian bentonite using PHREEQC Clédola C. O. de Tello1, Daisy M. M dos Santos1 and Thais B. Teixeira 1 1
Centro de Desenvolvimento da Tecnologia Nuclear, CDTN, Belo Horizonte, Brazil
ABSTRACT
To estimate the cesium sorption by the bentonite and to obtain the isotherms, some batch-adsorption experiments are being carried out, being the K d (retardation coefficient) calculated from these isotherms. One-dimensional flow cell was constructed to measure the bentonite permeability regarding to a cesium solution, which results will be used to evaluate the diffusion coefficient – D. It will be used the PHREEQC software to model the transport of the cesium radionuclide through this bentonite with the Kd and D data. The modelling of radionuclide transport in the Brazilian materials will contribute to evaluate the efficiency of multi-barriers system of the national repository, because it is one of its safety criteria. INTRODUCTION The management of radioactive waste aims to protect the environment and humans. The final step of this management is the disposal in the repository. Low-andintermediate level radioactive wastes (LILW) are generally disposed in near-surface disposal facility using a multi-barrier system which comprises the natural geological barrier (NGB) and an engineered barrier system (EBS) (see figure 1). These barriers act in order to avoid or to limit the release of radionuclides into the environment until their decay at a safe level of radiation to humans and the environment [1, 2, 3, 4, 5, 6, 7, 8,9]. In Brazil, nuclear energy is used in the different areas, including the production of electricity through two nuclear power plants - NPPs - Angra 1 and Angra 2. The largest amount of radioactive wastes generated are classified as low- and intermediatelevel. These wastes will be disposed in a near-surface repository with a multi-barrier system, which conceptual design and site selection are in progress. This multi-barrier system NGB and EBS is similar to the French and Spanish repositories, L’Aube and El Cabril, respectively, and it is in accordance of Brazilian regulation [9]. 245
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Figure 1: Schematic of multibarrier system (Adapted from [4]).
The release and migration of radionuclides through the repository multi-barriers depend on the chemical and physical interactions with their materials. The mechanisms of interaction between the radionuclide and the soil considered important are precipitation/dissolution and adsorption/desorption [10, 11]. Sorption values are generally determined in batch-adsorption experiments, generating an isothermal adsorption equation, being the Freundlich and Langmuir isotherms are the ones most commonly used [12]. The water passage t
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