Laboratory Characterization of Backfill-Buffer Bentonite Blocks Under Hydration Aided by X-Ray CT
Authors: Lee G. W. , Yun S. H. , Diaz M. B. , Kim K. Y. , Kim M. S. , Kim J. S. , Lee C. S. 
Published: August 2024

ABSTRACT:
We aimed to analyze the physical changes resulting from the hydration process of buffer-backfill composite materials, used in nuclear waste repositories. To achieve this, we established experimental setups capable of simultaneous hydration and X-ray CT imaging and conducted experiments using four composite samples. The experiments were divided into three swelling stages for analysis, with individual experiments conducted for each sample lasting approximately 100 to 170 days, corresponding to the appropriate stages. X-ray CT imaging was performed simultaneously to analyze the reduction in porosity, changes in heterogeneity, and localized density variation of the samples caused by swelling. For samples with distinct particle structures, the Madogram function was employed to understand the heterogeneity patterns, while the Full Width at Half Maximum (FWHM) was utilized to analyze the overall density variations of individual hydrated samples. By comprehensively examining the behavior patterns of swelling pressure according to the backfill material and the overall density variations of the entire samples, the density and heterogeneity changes were satisfactorily assessed.

1. INTRODUCTION
Recently, considerable research has been conducted on the construction of high-level waste repositories, which are essential for the safe isolation of waste generated from nuclear power plants and other sources for extended periods. Engineering barrier systems typically consist of disposal containers containing nuclear fuel, backfill, buffer, gap-filling materials, etc. The materials considered for buffer in engineering barrier systems should be suitable for maintaining safety against internal radionuclide release and external groundwater influences. The most common material for buffer is bentonite due to its high thermal conductivity and low hydraulic conductivity. Regarding backfill materials, a mixture of bentonite blocks, pellets, and rock fragments are considered as candidate materials. In the case of the KRS+ disposal facility model suggested in South Korea, the use of backfill materials composed of rock fragments and compressed bentonite blocks is being considered (Lee et al., 2020). Consideration of long-term performance maintenance is crucial in the construction of engineering barrier systems, with dry density maintenance of backfill and buffer materials being one of the essential factors (Sellin and Leupin, 2013). It is impractical to install only backfill materials of a certain density in engineering barrier construction due to the necessity of creating gaps between the installed barrier and the natural barriers.
Consequently, the installation of engineering barrier systems using combined gap-filling and backfill materials is inevitable. However, given the conditions for these barriers to be permanently sealed, heterogeneities arising from local fissures or variances in material composition could result in critical failures (Meils et al., 2020).