Quality Assurance of Bentonite - Prospects for Quality Assurance of Bentonite as a Geotechnical Barrier

The quality assurance of bentonite as a geotechnical barrier is crucial for the safe final disposal of radioactive waste. According to Paragraph 4, Section 3, No. 2 of the EndlSiAnfV [1], the geotechnical barrier, in combination with the technical barrier, forms a key component of the safety system in crystalline host rock. Together, these barriers must ensure the safe confinement of radioactive waste over an assessment period of one million years.

As outlined in Paragraph 6,  Section 4 of the EndlSiAnfV, the quality assurance process for manufacturing and constructing essential technical and geotechnical barriers must adhere to the state of the art in science and technology. The properties of the geotechnical barrier must be defined in the repository's safety concept.

The performance of bentonite as a geotechnical barrier is primarily influenced by the raw material used and its processing (e.g., mineral additives, shaping, and compaction). Quality assurance must establish criteria for each step of the production process and ensure compliance with these standards. It is advisable to define acceptable ranges for these quality criteria. For example, material parameters such as swelling pressure or sulfur mineral content, as specified in the safety concept, may vary within a permissible range. Any deviations must be addressed with corrective measures, which should be clearly defined as part of the quality assurance process.

Bentonite, as a natural mineral, is mined from various deposits worldwide [2]. These deposits can vary significantly in their mineralogical composition, such as smectite content, sulfur mineral content, and iron mineral content. Key properties of bentonite required for safe final storage include swelling pressure [3], Eh value [4], and corrosive potential [5]. Therefore, quality assurance must begin with the mining of bentonite to ensure that the necessary measures are taken at each stage of the production process to maintain the target parameters within acceptable limits.

In Germany, repository concepts consider bentonite as geotechnical barrier in the near field as well as in sealing constructions. A large quantity of bentonite will be required as a geotechnical component. The potential limited global availability of bentonite from different deposits should already be factored into the quality assurance strategy.

Literature

[1] Endlagersicherheitsanforderungsverordnung (EndlSiAnfV) vom 6. Oktober 2020 (BGBl. I S. 2094)

[2] Svensson, Daniel et. al. (2017): Developing strategies for acquisition and control of bentonite for a high level radioactive waste repository. Svensk Kärnbränslehantering AB (SKB)

[3] Dixon, David. A. (2019): Review of the T-H-M-C Properties of MX-80 Bentonite. NWMO. Toronto (NWMO-TR-2019-07)

[4] Posiva (2021): Safety Case for the Operating Licence Application - Models and Data (M&D) POSIVA 2021-04. Posiva Oy

[5] Behazin, Mehran et. al. (2021): State of Science Review of Sulfide Production in Deep Geological Repositories for Used Nuclear Fuel. Nuclear Waste Management Organization. Toronto (NWMO-TR-2021-18)