Geologic Nuclear Waste Storage

by Zoe Dilles

Nuclear energy has enormous potential to alleviate the energy demands of the future, but poses a challenge in its production of nuclear waste. More than 10% of the world’s electricity is generated in nuclear power reactors creating some 10,000 metric tons of radioactive heavy metal waste annually. A sought-after, safe approach to storage is within deep geological repositories but the evolution of these systems over time mechanically, thermally, and hydraulically must be carefully considered. These myriad factors make it quite the engineering feat to accommodate high-level nuclear waste. Not only must the waste be placed in a body of relatively inert rock at depth, particular consideration must be made towards the process of excavation itself. The bore-holes that function as the access points to deep strata with reservoir potential subject the surrounding rock to increased stress which can result in mechanical failure in the form of microcracks, called the excavation damage zone (EDZ). This fracturing can be pinpointed using acoustic emissions that are transmitted through the adjacent intact rock.

Zaoui et al. (2015) investigated EDZ development at the Bure Underground Research Laboratory, sometimes referred as simply URL, in France. At this locality, a layer of argillite serves as the geologic unit with the most potential to store nuclear waste as this type of sedimentary rock is not fissile and its clay mineral structure is useful. The study investigates the interaction of the contingent layered clay minerals and calcite under reservoir stress conditions to see how readily the material holds up in the short term and how viable these types of operations can be in creating stable nuclear waste management practices. Given realistic pressure values without directional constraints, the rock deformation is most pronounced in the vertical or z-direction of the mineral spacing. Zaoui et al. establish the maximum strength for this medium by testing the elastic, plastic, and brittle failure conditions. More damage occurs in clay layers with octahedral structure rather than in the tetrahedral structure of calcite. Overall, it is notable that the presence of water at the molecular interface in these tested layers acts to stabilize the mineral structures given typical storage conditions. By establishing the bounds of failure for this medium through such a nanoscale technique and thereby building understanding the viability of this system for radioactive waste, this study opens up an avenue of inquiry that can be applied to other minerologies and locations.

Zouai, A., Sekkal, W. Can clays ensure nuclear waste repositories? 2015. Scientific Reports, 5, 8815.

doi: 10.1038/srep08815

Also citing commentary article as an overview of nuclear waste issues;

Ewing, R. C., Long-term storage of spent nuclear fuel. 2015. Nature Materials, 14, 252-257.

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