Description

Natural analogues are important components in the development of safety cases for deep geological repositories (DGRs), and are recognized in corresponding regulatory requirements internationally. Natural analogues are supplemental to experiments performed in underground research facilities and surface laboratories. Time scales and spatial scales of natural analogues are orders of magnitude larger than available in field and laboratory experiments. Therefore a proper analogue can serve as a mimic of certain processes that may happen in the post-closure phase of a typical DGR for nuclear waste which are difficult or impossible to replicate using conventional experiments.

The Cigar Lake uranium mine is unique due to a thick halo of clay minerals that envelopes the ore deposit and provides a strong analogue to the widely used design of engineered clay barriers for nuclear waste disposal. The clay minerals were reported to be formed by hydrothermal processes dating back to the initial stage of uranium mineralization (billions of years ago). Uranium is highly enriched in the ore, averaging about 13% with a maximum around 66%, with daughter radionuclides produced by fission chain reactions. Study of transport of the in-situ produced radionuclides in the clay barriers can be analyzed using field measurements, in order to better understand the mechanism of transport, to evaluate the performance of the clay barriers, and to enhance our ability to conduct performance assessment for the DGRs.

Similarly, the Kiggavik deposit is also surrounded by a halo of massive illite, with subordinate chlorite and variable hematite, and U minerals often occur intergrown with the illite matrix at the <1 µm scale. The Kiggavik natural analogue is unique in that it has experienced multiple generations of mineralization and U mobility ranging from ~1750 to <1 Ma, whereas the mobilization appears most recent and is potentially ongoing. The U mobilization is observed to be at the centimeter scale along open fractures. This is a strong analogue to the potential influence of glacially induced fracturing that can enhance radionuclide transport in clay barriers and host rocks.

Different approaches to studying these natural analogues will be compared. From this collaborative effort, it is expected that a better understanding of the clay barrier system and near-field fractured rock will be achieved.

Experimental Data

• The isotopic concentrations of uranium and iodine from samples of vertical borehole crossing the Cigar Lake orebody have been measured by a devised analytical procedure (Mozafarishamsi 2023) and made available for further analysis. This fills the gap in the understanding of the relevance and implication of the Cigar Lake analog to the DGR for nuclear waste. A similar experimental procedure will be applied to measuring the tracer concentrations of samples from Kiggavik site. It is expected to be ready for analysis before completing Step 1 (see below).
• Variability in hydrogeological properties of the host rock and fault has been well captured in the geotechnical investigations of this site after the mine inflow and flooding happened and in the mitigations. The dataset has been reported and is available for analysis.

Approach

An incremental approach is adopted in ANALOG, with 4 steps being proposed for 2 subtasks on Cigar Lake and Kiggavik, respectively, as follows.

Step 1 focuses on the development of mathematical models for quantification of I129 source term and its transport properties in clays. The morphology and microstructure of clay samples will be considered in the conceptualization of production and transport mechanisms. The models should be able to predict the abundance (or concentration) of I129 based on concentration of Uranium, as well as its transport properties in clay as a function of porosity and/or grain size.

Step 2 consists of implementing the mathematical models developed in Step 1 to simulate I129 concentration profile in vertical borehole across the uranium orebody as measured by the University of Ottawa.

Steps 3-4 consist of adapting the model developed in Step 2 to consider the impacts of glaciation cycles and consequent impacts on tracer transport.

Participating Groups

• Canada: Canadian Nuclear Safety Commission (CNSC), Nuclear Waste Management Organisaton (NWMO)
• Germany: BGR/BASE
• Korea: Korea Atomic Energy Research Institute (KAERI)
• Netherland: Centrale Organisatie Voor Radioactief Afval (COVRA)
• Spain: Empresa Nacional de Residuos Radiactivos (Enresa)
• Switzerland: Swiss Federal Nuclear Safety Inspectorate (ENSI)
• Taiwan: Taiwan Power Company (TPC)
• USA: Department Of Energy (DOE) - Sandia National Laboratory (SNL)

Further Information

For further information, please contact the task leader, Zhenze Li.

References

1. Mozafarishamsi H (2023) In situ production and migration of 129I in the Cigar Lake natural analogue. Report to Canadian Nuclear Safety Commission (CNSC) in fulfilment of R691.2