In the autumn 2022 issue, Beswick and Gibb extolled the virtues of deep borehole disposal as an alternative to mined repositories for high-level nuclear waste disposal, asserting that: “geology in terms of isolation is only cosmetic.” The consensus is that geology is the ultimate barrier of the multibarrier containment system and that engineered barriers will eventually fail over the multimillennial timeframes for which the waste must remain isolated . But a fundamental question remains: can we gather sufficient data from a deep borehole repository to demonstrate long-term post-closure safety?
A key feature of mined repositories, such as Finland’s Onkalo, is that they expose the host rock in tunnel and shaft walls at a scale and level of detail that can’t be matched by coring and geophysics alone. Tunnel-scale observations allow us to test the geological and hydrogeological models assembled from borehole and geophysical data, and to validate core measurements by assessing the host rock in-situ and in-bulk, building confidence in the site characterisation. If this is unnecessary, why do we go to that trouble and expense for mined repositories?
Without tunnel-scale observations, we may miss aspects of the host rock, such as small but interconnected fractures or lateral variations in chemistry, that may affect post-closure performance. Would a deep borehole repository therefore be a “see no evil” approach to long-term safety? Without tunnel-scale host-rock exposure, how will we know we have enough data to be justifiably confident in the safety of the host rock for a deep borehole repository?
One could argue that hydrocarbon exploration routinely characterises subsurface geology using largely only boreholes and geophysics. However, there is a vast difference in the permeabilities, flows, and timeframes involved for hydrocarbon reservoirs compared to nuclear waste repositories, not to mention far lower societal risks if a reservoir is wrongly characterised compared to nuclear waste prematurely reaching the human environment.
Howard Lee is a geoscientist and science writer based in Massachusetts, USA.