EURAD - D7.12 Final report on influence of temperature on clays

Result code in IS VaVaI

<a href="https://www.isvavai.cz/riv?ss=detail&h=RIV%2F68407700%3A21110%2F24%3A00382574" target="_blank" >RIV/68407700:21110/24:00382574 - isvavai.cz</a>

Result on the web

<a href="https://www.ejp-eurad.eu/publications/eurad-d712-final-report-influence-temperature-clays" target="_blank" >https://www.ejp-eurad.eu/publications/eurad-d712-final-report-influence-temperature-clays</a>

DOI - Digital Object Identifier

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Result language

angličtina

Original language name

EURAD - D7.12 Final report on influence of temperature on clays

Original language description

Most Safety Cases for spent nuclear disposal facility limit maximum disposal container surface temperatures to 100°C to protect undesirable evolution. Higher temperature limits could have significant advantages such as allow disposal of higher enrichment/burn-up spent fuels and shorter interim storage/cooling requirements. EURAD HITEC WP aimed to improve Thermo-Hydro-Mechanical (THM) description of clay-based materials at elevated temperatures. The host rock clays were studied under saturated conditions under 120°C, while buffer bentonites were studied both in saturated and unsaturated conditions under 150°C. In clay host rock, the overpressure generated by the thermal expansion of pore water and the solid rock skeleton may have deleterious consequences. In far field, this could induce rock damage and reactivate fractures/faults. In near field, this could induce fracture opening or propagation in this fractured zone, altering the permeability. Our major observations for near field were following. Higher calcite content decreases self-sealing. Sample orientation shows no clear effect. Self-sealing is faster for tighter cracks. No significant temperature effect. For far field, short-term and long-term (creep) compression tests were performed. The initial heating before the short-term compression tests on the COx induced transitory pore water overpressure (due to thermal expansion) and then microcracks parallel to the bedding planes. Temperature has globally a negative impact on the peak strength of the COx claystone until 100 °C. The decrease in the peak strength is the most significant for the parallel to bedding samples under uniaxial test conditions. These microcracks were also closed when the axial stress was increased during the compression tests performed perpendicular to bedding. According to the experimental results, temperature has a likely, but small under confining pressure, negative impact on the short-term resistance to failure of the COx claystone. For bentonite buffer, proving that higher temperatures than presently accepted are suitable is very relevant even for current concepts. It increases safety margin and gives greater credibility to the design (e.g., if it is proven to work for 130°C then for 100°C it is likely to be safe). Also, this type of optimisation could be used to increase thermal limits on the bentonite buffer, reducing the footprint of the facility. So potentially significant cost savings and improved environmental sustainability. No new processes were identified for bentonite, but swelling, swelling stress formation and water conductivity were studied in detail over 100° degrees C temperatures. Relationships delineated within Task 3.2 include: (i) the observation of swelling pressure and permeability as a function of temperature for various dry densities, swelling strains, chemical states and conditions and (ii) water retention curves, as function of temperature. For the materials and conditions tested, an influence of elevated temperature on water retention capacity has been observed. Multiple test programmes, in both Ca- and Na-bentonite have also found evidence that, whilst changes to hydraulic permeability are not very significant, swelling pressure can be substantially impacted by elevated temperatures under certain conditions. Further work to investigate and consider the mechanisms and consequences of this behaviour for repository design are recommended as a result. The experiments and modelling at higher temperatures has required much development work, which was carried out successfully. Key results are new experimental observations and models for both materials studied in HITEC: clay host rock and buffer bentonite.

Czech name

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Czech description

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Type

O - Miscellaneous

CEP classification

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OECD FORD branch

20101 - Civil engineering

Project

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Continuities

R - Projekt Ramcoveho programu EK

Publication year

2024

Confidentiality

S - Úplné a pravdivé údaje o projektu nepodléhají ochraně podle zvláštních právních předpisů