Assessment of Bentonite Saturation Impact on Swelling Pressure, Safety, and Usability in Spent Nuclear Fuel and High-Level Radioactive Waste Repository

Author: Galla Uroić, mag.ing.min.

Disposal of spent nuclear fuel (SNF) and high-level radioactive waste (HLW) is one of the most challenging engineering tasks, as evidenced by the fact that even after seventy years of their production, there is still no active repository in the world where SNF and HLW are permanently disposed of. There are several issues related to the construction of a deep geological repository for SNF and HLW (Uroić et al., 2022), such as:

• The repository must be constructed at a depth of 500 to 1000 meters in low-permeability host rock under reducing conditions.
• The repository is designed for a period of 10,000 years (HLW) to 100,000 years (SNF), while some countries (e.g., the USA) require the construction of a repository that guarantees the safety of the disposed material for a period of 1,000,000 years.
• To satisfy the above condition, it is necessary to find a geological environment that will remain stable for at least twice the duration defined as the repository's lifetime to ensure that radionuclides will not reach the biosphere and pose a threat to it.

The Republic of Croatia has an obligation to dispose of half of the SNF from the Krško Nuclear Power Plant, located in the Republic of Slovenia, but according to intergovernmental agreement, a single repository is planned to be constructed on the territory of one of the countries.

One specific problem related to repository construction is determining the type of material for the construction of engineering barriers - protective materials that will ensure the retention of radionuclides in the repository during the required period. Considering the geology of RS and RC, it is likely that the repository will be constructed in igneous (crystalline) rocks, and the KBS-3V concept (Figure 1) will be applied as the disposal concept. Based on the expected type of host rock and disposal concept, it can be concluded that the primary material for constructing engineering barriers will be bentonite clay.

Figure 1: Swedish spent nuclear fuel disposal concepts: KBS-3V (left) and KBS-3H (right) (Savage, 2012)

The research conducted for the purpose of the dissertation includes determining the characteristics of bentonite clays that will be used in the construction of engineering barriers at the future repository. To determine the behavior of the engineering barriers in the host rock, it is necessary to establish the process of saturation (moistening) of the installed bentonite clay, the intensity of its swelling, and the pressures generated by its swelling, which will be transferred to the rock and the containers holding SNF. Most of the tests will be conducted at the Geomechanics Laboratory at the Faculty of Mining, Geology, and Petroleum Engineering, University of Zagreb. The mentioned research should include: investigating the rate of bentonite saturation depending on the preparation method, testing the development of swelling pressure in bentonite, and its impact on the quality of the sealing layer/barrier. However, before conducting these investigations, it is necessary to determine the basic properties and characteristics of bentonite clay, which include the tests presented in Table 1.

Table 1: Determination of Basic Properties and Characteristics of Bentonite Clay.

The determination of permeability of cohesive soils and bentonite blankets using a triaxial cell is shown in Figure 2.

Figure 2: Determination of permeability of bentonite clay.

Additionally, in order to determine the properties of materials for constructing engineering barriers, tests on bentonite clay are conducted at the Laboratory for Geological Materials at the Faculty of Mining, Geology, and Petroleum Engineering, University of Zagreb. These tests include:

• X-ray diffraction (XRD) on original samples (Figure 3a and 3b)
• High-temperature X-ray diffraction (HT-XRD) - in-situ recording of material reaction to heating
• X-ray fluorescence (XRF) - for determining the chemical composition
• Ion chromatography (IC) - for soluble salts determination
• Cation exchange capacity (CEC) using ammonium acetate for determining released ions
• Fourier-transform infrared spectroscopy (FTIR) for determining the chemistry of the fine fraction
• Determination of surface area using methylene blue

• Particle size analysis using laser diffraction

Other basic material tests will include determination of zeta potential, electron microscopy, and differential thermal analysis.

(a)                                                                                                                           (b)

Figure 3: Preparing a sample for X-ray diffraction (XRD) (a) and XRD measurement device (b)

All the tests and their results will be verified using numerical models and simulations conducted in software packages such as Geostudio and Plaxis, where previous modeling has already been performed (Figure 4). The research results will be compared with studies on natural analogs, such as bentonite in nature and their swelling behavior.

(a)                                                                                                         (b)

Figure 4: Numerical models of stress distribution in the cross-section of the spent nuclear fuel repository created in Plaxis software (a) and Geostudio software (b).

The research will define the rate of bentonite saturation in the protective layers of the deep geological repository for HLW or SNF and determine the potential depth of saturation. The relationship between bentonite saturation and the effectiveness of the bentonite barrier in terms of permeability will be determined. Additionally, the relationship between the swelling pressure of bentonite and the structure of the protective layers made of bentonite clay and the bentonite-rock container system will be defined.

The comparison of research results with studies conducted on natural analogs will contribute to the development of a safety study and facilitate communication with stakeholders.

References:

Savage, D., Arthur, R. 2012. Exchangeability of bentonite buffer and backfill materials. STUK. Helsinki.

Uroić, G., Veinović, Ž. & Alexander, W. R. (2022): KBS-3V And Axial Canister Emplacement Of SNF - Comparison Of Disposal Concepts. Proceedings of 13th International Conference of the Croatian Nuclear Society, Zadar, Hrvatska, 2022. str. 115-1

Veinović, Ž., Vučenović, H., Uroić, G. & Rapić, A (2022): Numerical models of the deep geological repository for the spent nuclear fuel // Mathematical methods and terminology in geology 2022. Malvić, Tomislav ; Ivšinović, Josip (ur.). Zagreb: Rudarsko-geološko-naftni fakultet, 2022. str. 21-33

Galla Uroić, mag. ing. min. is PhD student at the Department of Mining and Geotehnical Engineering, Faculty of Mining, Geology, and Petroleum engineering, University of Zagreb. She enrolled in the doctoral program in Applied Geosciences, Mining, and Petroleum Engineering in 2020.

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