Study of a mixture of soil and brazilian bentonite using a protocol for clay characterization for near-surface radioactive waste repository

Abstract

Aiming to implement the Brazilian repository for disposal of low and intermediate level of radioactive wastes, the “Project RBMN” is being coordinated by CNEN. The repository is made of multiple barriers in order to prevent or retain the migration of radionuclides throughout these paths to the environment. Clays are used as backfill and coverage layers, which are the repository natural barriers. Recently, in Nuclear Technology Development Center (CDTN) a protocol for clay characterization was elaborated, and it was used the Brazilian bentonite as a reference. In this protocol are established some physical and chemical tests and their reference values that are essential for the characterization of materials to be used as backfill and coverage layers in the surface repository. This research intended to evaluate the applicability of this protocol to characterize soil without and with bentonite. The bentonite content added to the soil was 30%. The results were compared with the reference values of the protocol and literature for materials with similar mineralogical features. The results showed an increase of the specific surface and of the cation exchange capacity, as the result of the bentonite addition. The hydraulic conductivity is significant decreased by the addition of bentonite.

Characterization of bentonite, vermiculite and soil as potential materials to compose the natural barriers in a near-surface repository

“…The soil/bentonite ratio of the mixture was arbitrarily defined, since the definition of an optimal value for this ratio will be established in a future time, for the CENTENA site. The objective was to analyze the influence of the addition of bentonite on the soil properties [7]. Studies of the mineralogical, physical and chemical properties of BN, VM, SL and SBN have been carried out to provide a database for CENTENA and optimize the choice of natural materials to be used as backfill and coverage layers in the surface repository.…”

“…As the local, where the CENTENA will be built, was not selected yet, and due to the impossibility of studying the large variety of soils scattered throughout the Brazilian territory, it was decided to collect the soil sample in the CDTN site. This decision also took into account technical and logistic needs to sampling the material any time (Figure 1) [7]. Source: Barroso, 2021.…”

“…For the preparation of the soil/bentonite mixture an additional step was made, after the disaggregation, the soil was mixed with the bentonite, and the samples of 1 kg were prepared as previously. This preparation was made to obtain the bulk samples (lab samples) [7].…”

“…All characterization was conducted using bulk samples of BN, VM, SL and SBN. The exception was the PSD of the SL, in which the soil in natura was used [5][6][7].…”

“…The purpose of the article is to show the research performed with the clays for use in natural barriers, beginning with the creation of a characterization Protocol [5] using a sodium bentonite as reference material. Then it was applied to characterize an expanded vermiculite [6], a soil sample collected in the Centro de Desenvolvimento da Tecnologia Nuclear (CDTN) and a mixture of this soil with sodium bentonite [7].…”