Q&A Blog, Mar. 15, 2021

Science of Hydrogeology is Crucial to Deep Geologic Disposal of Nuclear Waste

In December 2020, the peer-reviewed journal Energies published a new paper by the Deep Isolation technical and geo-science team that explores what might happen if a deep borehole repository for nuclear waste had an improperly sealed access hole connecting the disposal section of the repository to the Earth’s surface.

In a recent blog post, Deep Isolation lead hydrogeologist Stefan Finsterle summarized the results of a paper titled “Sealing of a Deep Horizontal Borehole Repository for Nuclear Waste.” This new paper is a continuation of a more extensive safety study released a year ago that examines the overall safety of deep borehole repositories.

In a public webinar set for March 30 and 31, Stefan and other Deep Isolation team members will present the results of this updated safety report and field your questions and comments.

Get to know Stefan better through this Q&A.

Stefan Finsterle Headshot
Lead Hydrogeologist, Stefan Finsterle

Q. Hydrogeology is defined as a branch of geology that deals with the distribution and movement of groundwater in the soil and rocks, including aquifers of the Earth’s crust. Why is this so critical to understand for nuclear waste disposal?

A. Groundwater is the main vehicle by which radionuclides could be transported from a breached waste canister through the rocks, aquifers and soil to the land surface, where they may find their way to people, either directly through drinking water or more indirectly through other exposure pathways. It is therefore crucial to understand how water moves through rocks, how dissolved radionuclides would migrate within the groundwater, and what natural or repository-induced driving forces may exist in the deep subsurface. Geologic layers considered suitable to host a repository for nuclear waste are very tight, that is, groundwater flows extremely slowly through the small pores of the rocks, effectively isolating the waste for very long times. I hope this clarifies why understanding hydrogeology — the interaction of water and rock — is essential when trying to find a suitable site and for assessing the safety of a nuclear waste repository.  

Q. Tell us about your background as a hydrogeologist who studies deep geologic repositories for nuclear waste. What interests you most about this work?

A. As an environmental engineer, I want to understand the natural environment, protect it, or at least help minimize or mitigate the potentially negative impacts of our intrusions. Hydrogeology has always been fascinating to me because the deep subsurface is both vast and difficult to observe, requiring innovative methods to understand and characterize it, with the response of the groundwater to our testing being the most telling messenger. Nuclear waste disposal is obviously a multifaceted challenge; the fact that answering hydrogeological questions is key to finding a viable solution is certainly a great motivation for me. Moreover, I find the idea of borehole disposal intriguing because this concept indeed minimizes the interference of the repository with its host rock and the fluids that flow through it.

Q. This latest study looks at what the consequences would be for an improperly sealed borehole containing nuclear waste. Why was this important to study?

A. There are two main reasons why it is important to analyze the safety effects of an improperly sealed borehole. First, building a repository invariably perturbs the otherwise impermeable host rock. The access hole and the disturbed rock around it are often considered the weakest elements of the repository system, as they pose a risk for radionuclide leakage. Second, even if the borehole is carefully sealed and tested as part of the repository closure activities, it is difficult to demonstrate that the seals will remain tight for the long time periods over which the safety of the repository must be assessed. Rather than studying the effectiveness of different sealing methods, we decided to examine the impacts of a poorly sealed borehole (or a seal that has degraded over time) on safety, to better understand how much we will have to rely on the long-term integrity of the seal. It is important to note that the design of a Deep Isolation repository includes proper sealing of the boreholes.

Q. The results seem to imply that a tight seal is not really necessary. Yet, Deep Isolation plans to install an impermeable sealing barrier. Given the results of your study, why spend the time and resources to do so?

A. There will always be irreducible uncertainties in predicting the long-term behavior of both the engineered and natural barrier systems. It simply makes good sense to install plugs at strategic locations and to backfill the access hole, specifically since such safety measures are relatively inexpensive. For example, installing a sealing plug at the beginning of the horizontal disposal section or another point along the access hole within the host rock would be very effective in retarding axial radionuclide transport. Special attention should also be given to the uppermost section of the vertical access hole. A suitable backfill would reduce the near-surface hydrological disturbances that propagate along the borehole into the repository, specifically pressure drawdowns caused by climate change effects or groundwater pumping. It would also directly protect the aquifer and inhibit inadvertent or malicious human intrusions into the repository. Hydraulic feed or thief zones identified during drilling and borehole logging can be plugged, and the drilling-disturbed zone around the borehole can be grouted at certain intervals. Sealing of boreholes and abandoned wells is required by regulation in other areas of engineering, specifically oil and gas production, energy storage, and geologic carbon sequestration systems. Similar requirements are expected for a borehole repository for nuclear waste. Nevertheless, it is certainly reassuring to know that a deep borehole repository does not need to rely on the long-term integrity of its backfill materials and sealing methods.

Q. What do you see as next steps or a follow-on study to this one?

A. I’m looking forward to engaging in discussions that further probe our assumptions about potential axial driving forces, the dissipation of pressure and dispersion of radionuclides into the overburden, and the overall arguments about the inherent, passive safety afforded by the geometry of the borehole repository. The topic of sealing will definitely need to be revisited for each potential repository site, as the site-specific geology and design adjustments will influence the effectiveness of the seals as well as alter seal degradation processes. In summary, extending the current, generic study to a site-specific performance analysis will be the next step in further examining the sealing of a deep borehole repository.  

Two sessions of the webinar “Safety in Depth Part 2: Sealing of a Deep Horizontal Borehole Repository for Nuclear Waste” will be offered. Click here to register for the 10:30 a.m. PST March 30 session. Click here to register for the 3:30 p.m. March 31 CET session. Both sessions will feature a live question-and-answer period following the presentation.