Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes
2. Deep Horizontal Drillhole Disposal Concept
2.1. Radioisotope Inventory
2.2. Drillhole Repository Configuration
2.3. Waste Emplacement
2.5. Canister and Casing
2.6. Backfilling and Sealing
3. Expected Repository Performance
3.1. Basis of Performance Evaluation
3.2. Leakage Pathways
3.3. Isolation of Brines from Aquifers
3.4. Reducing Environment
3.5. Decay Heat
3.8. Inadvertent Intrusion
3.9. Terrorist Intrusion
4. Comparison with Other Repository Concepts
4.1. Mined Repositories
- Defense in Depth—The currently proposed mined repositories are relatively shallow (typically 500 m) compared to the intended depth of the horizontal disposal section of a drillhole repository (>1000 m). While suitable host formations can be found at shallow depths (note that these sites would also qualify for horizontal drillhole repositories), the option to explore deeper formations has considerable advantages. It expands the rock volume and number of locations available for potential repository siting. More importantly, it increases isolation of the waste, because (1) the thick overburden with multiple confining layers considerably augments the natural barrier system, (2) the migration distance to the accessible environment (especially drinking-water aquifers) is increased substantially, (3) the rock at depth is more competent and hydrogeological conditions are more stable and less affected by the dynamics of near-surface processes and (4) the likelihood of inadvertent intrusion is considerably smaller. Since long-term safety is one of the main goals of any disposal solution, the ability to rely on the natural barrier system is paramount. Going deeper makes best use of the natural barrier system, which is the essence of geological disposal of nuclear waste.
- Size and Geometry—The volume of rock that needs to be excavated for a mined repository is substantially larger than that for a drillhole repository. Large-diameter access ramps that are long (due to their slope being limited to a few degrees) or shafts are needed for a mined repository, whereas a small-diameter, vertical or curved access hole is sufficient to reach the waste emplacement horizon. Reducing the diameter of the disposal section increases the stability of the opening. More importantly, it reduces the overall hydrological, chemical and mechanical perturbation of the host rock, specifically the extent of the excavation damaged zone, which is considered a potential pathway for radionuclide leakage. The cross-sectional area available for axial radionuclide transport along the backfilled opening and associated damage zone is also much smaller. Sealing deposition holes, constructing plugs and backfilling large access tunnels is more challenging and less reliable than sealing small-diameter drillholes. Finally, the linear end-to-end emplacement of small-diameter canisters holding individual SNF assemblies leads to a smaller waste density, with beneficial consequences for heat dissipation, corrosion-gas accumulation and diffusion and impacts from canister breaches, earthquakes and volcanism. Lower waste density and higher heat dissipation efficiency allows for waste disposal after short post-reactor cooling times at the surface and immediate site closure (i.e., no need for extended ventilation periods); it also reduces post-closure criticality issues.
- No Humans Underground—Much of the excavation volume and infrastructure of a mined repository is needed for enabling humans to work underground. This includes drainage and ventilation of the tunnels but also the installation of underground transportation and emergency equipment. Large machinery for excavation and waste emplacement require additional infrastructure. Humans will need to work underground during the construction of the repository, its operation and closure. This requires continued drainage and ventilation of the tunnel system until repository closure, that is, typically for a few decades. In many cases, ground support systems or liners must be installed. In addition to costs, changing the underground conditions to those needed for human life considerably affects the near field of the repository. Drainage over decades leads to fluid flow in the host rock and depressurization with associated deformations caused by effective stress changes. Ventilation not only leads to dry-out and potential desiccation cracking (in a clay-rich host rock) but introduces oxygen that remains underground until consumed by corrosion and other chemical reactions. While most of these perturbations are limited in time, some may be irreversible and need to be accounted for, complicating performance assessment calculations. While waste emplacement in a mined repository may be accomplished using remote-controlled machinery, the process of doing so is complex and costly. By comparison, constructing a drillhole repository and waste emplacement in the disposal section has minimal impact on the hydrology, geochemistry and stress conditions of the host rock. The perturbations that do occur are of considerably smaller extent and shorter duration. Construction, waste emplacement and site closure activities are simpler and require fewer workers and no underground life support system. These factors contribute to the lower costs of a drillhole repository.
- Site Characterization—Information and data from site characterization activities are key for site selection and performance assessment calculations and eventual licensing of a repository. While much of the initial characterization is done from the surface (e.g., using geophysical methods) and exploration boreholes drilled proximate to the disposal borehole, humans working in a mined repository have direct access to the host rock for detailed characterization and testing. For a drillhole repository, such direct inspection of the host rock by humans is limited to cores pulled from the drillhole. Nevertheless, testing, mapping and characterizing rock properties from boreholes are off-the-shelf technologies in the hydrocarbon and geothermal industries. Extensive data sets can be obtained from cores, fluid samples, geophysical well logs, borehole videos, caliper logs, hydraulic and compliance testing, in-hole tracer testing, flowing-fluid electric conductivity logging and high-resolution, continuous monitoring of temperatures, deformations and acoustic signals using fiber-optic cables . Sophisticated methods and computational toolsets for analyzing and interpreting borehole data are available. It should also be noted that direct underground observations available in mined repositories are limited to the tunnel surface, which is a very small fraction of the three-dimensional host formation. Moreover, the properties inferred from these observations reflect the excavation disturbed zone (which is much smaller and thus less relevant in a drillhole repository) and they may be biased due to the artificial environment created in the tunnel. For example, ventilation effects bias or render impossible the direct observation of water inflows that would occur under natural conditions. The need to account for such unwanted effects greatly complicates the analysis and interpretation of “direct” observations taken in a mined repository [75,76].
- Modularity—Mined repositories are typically conceptualized as large, centralized facilities, intended to accommodate all or a considerable fraction of a country’s high-level nuclear waste inventory. For such large projects, site selection, licensing, construction, waste emplacement and site closure are all technically challenging, expensive and time-consuming processes with a considerable risk of failure—for both technical and non-technical reasons. Deep horizontal drillhole repositories are suitable for flexible, modular, decentralized and fast implementation. Finding disposal solutions for a relatively small fraction of the waste (e.g., that produced by a nearby power plant or other nuclear facility) is more tractable. Long-distance transportation of nuclear waste can be reduced or eliminated altogether. Surface facilities are relatively small and the duration of an individual project comparatively short. Customized solutions for special conditions (e.g., small inventories) or special types of waste (e.g., the Cs/Sr capsules) can be found and implemented in a locally optimized, timely manner. We also believe that stakeholders are more amenable to accept a local solution that substantively reduces the risks associated with local interim storage at the surface and long-distance transportation.
- Unsaturated Zone—The pores of the fractured, volcanic rocks at the Yucca Mountain repository level are only partly filled with water. (Note that aridity was one of the main reasons why Yucca Mountain was selected as a potential disposal site ). Even given that the liquid saturation of the tuff matrix is typically above 80% , air flows primarily within the fracture network [79,80]. One consequence of ubiquitous airflow is that volatile radionuclides may be transported to the atmosphere [59,60]. However, the main consequence of the presence of air convection is that the repository is in an oxidizing environment, substantially affecting water chemistry and its impact on canister corrosion and radionuclide solubility. The considerable benefits of a repository located in a reducing environment have been discussed previously in Section 3.4. Furthermore, a principle of vadose zone hydrology is that water percolates essentially vertically through the fracture system but may be diverted laterally at hydrostratigraphic contacts, perched water bodies and notably around the emplacement drifts, reducing the amount of water that drips into underground openings . Moreover, a shadow zone develops beneath the emplacement drift, suppressing diffusive and advective releases of radionuclides into the near field . These beneficial effects are a direct result of capillary forces, which only occur in unsaturated rocks, that is, they are not active in a saturated zone repository. However, predicting the capillary barrier effect is complex, because flow diversion depends on two-phase parameters that are difficult to determine for fractured porous media [83,84]. Moreover, the amount and distribution of dripping depends on the difficult-to-characterize small-scale roughness of the drift ceiling, which can drastically change after partial drift collapse. Condensation further complicates the prediction of the in-drift environment, specifically during the thermal period. As a consequence, titanium drip shields had to be added to the engineered barrier system to prevent water from dripping directly onto waste packages, and—as a side benefit—to prevent mechanical damage due to rock fall. In summary, the unsaturated zone may be a viable location for a nuclear-waste repository, offering certain benefits. However, it has considerable drawbacks, such as the oxidizing environment and the fact that the vertical downward flow of contaminated water will eventually encounter a near-surface aquifer. Most importantly, unsaturated conditions make the system exceedingly complicated to characterize, analyze, understand, predict and, therefore, defend in a license application; no credit could be taken for many of the benefits mentioned above.
- Thermal Regime—In the unsaturated zone of Yucca Mountain is near atmospheric pressure conditions, where water boils at a temperature of approximately 96 °C (the boiling temperature may be higher in small rock pores or if a deliquescent brine forms). Evaporation and boiling changes the geochemistry of the near field, with complex impacts on mineralization, adsorption properties and corrosiveness of the waters entering the waste emplacement drifts. It also generates a pressure buildup that induces vapor flow and condensation, which are effective moisture and heat transfer mechanisms. Such coupled thermal-hydrological-geochemical processes have been studied for repository conditions during the thermal period, both experimentally and numerically [85,86,87]. Such analyses are very complicated, specifically because of the two-phase conditions induced by boiling, which make the resulting predictions of repository performance uncertain. By contrast, the hydrostatic pressure in a deep drillhole repository prevents boiling, even under highly elevated temperatures, reducing the impact of spatial variability as well as conceptual and parametric uncertainties on model predictions.
- Repository Site—It is far beyond the scope of this paper to comprehensively describe and evaluate the site-specific aspects of the Yucca Mountain repository. A summary of the natural barrier system and a brief history of the extensive program needed to characterize the site can be found in Rechard et al. . Much of the effort needed is a result of the complexity of: (1) its location in the unsaturated zone near the land surface and above the water table; (2) the geological environment indicating recent seismicity and volcanism; (3) the intricate stratigraphy, consisting of sequences of porous and highly fractured tuffs, some containing lithophysal cavities, intersected by faults, associated with a saturated alluvial formation extending to the compliance boundary; and (4) the engineered barriers and repository concept itself, which had to be adapted to reduce the impact of uncertainties in the expected system behavior. Building a repository in such an environment led to a large number of features, events and processes that needed to be considered . Moreover, addressing the complexity of the coupled processes induced by the heat-generating waste required an extensive research program.
4.2. Deep Vertical Borehole Disposal
- Stratigraphy—The subsurface is predominantly stratified horizontally. (Note that such horizontal stratification is less pronounced in the more monolithic crystalline basement, which is the target host rock of DOE’s deep borehole proposal). Similarly, most relevant conditions (specifically total stress, lithostatic and hydrostatic pressures, temperature and fluid density) are also stratified horizontally. This means that a vertical borehole is likely to traverse many discrete hydrostratigraphic interfaces and encounters constantly varying conditions. By contrast, the horizontal drillhole can be aligned in the principal dimension of the formation and encounter similar thermodynamic conditions and state variables along its length. A notable exception is the probability of intersecting steeply dipping faults, which is higher for a horizontal than a vertical borehole. While deep basement rocks can be thick, the vertical length of the disposal section is restricted by depth limitations, whereas relatively long horizontal disposal sections can be drilled.
- Vertical Gradients—The fact that pressure, temperature and salinity are relatively uniform in a horizontal drillhole implies that gradients inducing fluid flow and radionuclide transport are relatively small. In a vertical drillhole, decay heat from the high-level waste (superimposed on the natural geothermal gradient) induces thermal convection cells that are aligned with the orientation of the borehole , which may be regarded as a preferential flow path that passes through the horizontal, confining layers of the natural barrier system. By contrast, the density differences created by thermal expansion do not induce significant buoyancy-driven flow in a horizontal drillhole. Note that the vertical salinity increase and associated density stratification is beneficial in both the deep vertical disposal concept and the vertical access section of the horizontal drillhole repository. In the former, however, the stratification is disturbed during the thermal period and needs to be reestablished afterwards—a process that also leads to fluid flow. While minor, such gradients and associated flows should be mitigated, as is the case in the horizontal drillhole concept. Finally, the natural geothermal gradient leads to non-uniform and considerably higher ambient temperatures in the disposal section of a vertical borehole repository, with several implications on overall performance.
- Mechanical Load—The stacking of heavy waste packages in the vertical disposal borehole leads to a considerable accumulated load on the canisters. Detailed stress analyses  indicate that safety factors are relatively small, specifically for deep canisters that experience high loads and reduced axial and radial crush resistance because of reduced yield strength at high temperatures. As a precaution, multiple bridge plugs need to be installed to bear the weight of additional waste packages emplaced above. Plugs, however, are considered by some as the weakest link in the vertical borehole approach . Similar issues need to be addressed for the casing and compaction of backfill materials. None of these technical complications apply to canisters arranged horizontally, leading to a simplified performance assessment calculation with fewer scenarios. Moreover, a lighter design can be chosen for the canisters and casing, with added benefits regarding handling, emplacement, retrievability, effects from corrosion products and cost. Finally, the length of the disposal section is not limited by the accumulation of stacking loads.
- Drilling and Waste Emplacement—The drilling technology developed by the oil, gas and geothermal industries is sufficiently advanced to allow for deep vertical or directional drilling in different geological settings. Drilling costs depend mostly on depth, diameter, casing design and local geologic conditions . Vertical borehole disposal may consider the free-fall method for waste emplacement [23,31], an option not available for a horizontal drillhole repository, where a wireline tractor, coiled tubing or drill pipe is needed to push the canisters into the disposal section. Emplacement risks are discussed in detail in Reference .
5. Summary and Concluding Remarks
- Radioactive waste is effectively isolated from the accessible environment by the great depth of the repository and the thick, protective natural barrier system above it—this is the essence of geological waste disposal.
- The concept is comparatively simple. Complexity is reduced in all aspects related to building and operating the repository, including siting, construction, waste handling, transportation, emplacement, sealing and closure. Most importantly, the inherent simplicity of the concept leads to a more robust and more defensible safety assessment, with markedly fewer, less intricate features, events and processes that need to be examined.
- The repository has a compact geometry but a locally lower waste density. The small diameter of the disposal drillhole reduces the size of engineered barrier components, simplifying their construction and, consequently, increasing their robustness. The cross-sectional area available for fluid flow and radionuclide transport along the drillhole and excavation disturbed zone is substantially smaller than in a mined repository. The relatively low waste density in the linear drillhole arrangement distributes heat production, decreases the criticality risk and the consequences associated with disruptive events.
- The concept offers great flexibility, both globally and locally. Drillhole repositories of different sizes can be built in a modular fashion, tailored to the specifics of the waste inventory as well as geographical and geological conditions. In particular, relatively small repositories can be built on the sites where the waste is produced, limiting or avoiding transportation. Locally, the design of the repository can be adapted to the geologic conditions (and other constraints) by changing the number, length, orientation and inclination of the disposal sections. This global and local flexibility allows for a staged approach, with short implementation times and immediate site closure after waste emplacement, reducing overall risks and costs.
- The simplicity, compactness and flexibility of the drillhole disposal concept also reduce logistical complexity. Specifically, no humans need to be underground, transportation is reduced or avoided and established drilling technology can be used. This reduces the need to build and license additional, complex infrastructure. Logistical simplicity increases operational safety and reduces costs.
- In the United States, the absence of a legal framework and uncertainties about the regulatory environment complicate the planning and preparation of a license application for a deep horizontal drillhole repository. In the U.S., the Nuclear Waste Policy Act currently does not allow a second repository for commercial spent nuclear fuel until the Yucca Mountain facility is completed; note that there is no similar restriction for the disposal of defense waste (such as the Cs/Sr capsules). No licensing protocol exists yet for a modular, deep horizontal drillhole repository. While multiple, individual license applications need to be prepared, it is conceivable that these applications would be simpler than that for a large mined repository (due to the relative simplicity of the drillhole concept). Moreover, the parts of the application (e.g., related to drilling and emplacement technologies), which are not related to site-specific conditions, are likely to be transferable between sites. A regulatory perspective for deep borehole disposal is provided by Winterle et al. .
- Characterization by direct inspection of the host rock by humans working underground is not possible in a drillhole concept. However, as discussed in Section 4.1, we believe that the suite of advanced borehole-based coring, logging, testing and characterization methods currently available provide sufficient information about the safety-relevant properties of the host formation. Moreover, the data collected in drained, ventilated, large underground openings are difficult to interpret and may not accurately reflect post-closure repository conditions. Also note that a large component of site characterization (also for mined repositories) consists of surface-based geophysical surveys and geological mapping. Characterization needs may be different for drillhole repositories than mined repositories due to their relative simplicity and reduced perturbation of the host rock by excavation and operation of large underground openings. And finally, drillholes can be visually inspected with remote cameras. (Videos of such inspections are readily available online on YouTube.) To take advantage of such a capability, the hole needs to be filled with a clear fluid for an inspection stage that would take place prior to casing the drillhole and the placement of the canister. Such borehole inspection is common practice in the drilling industry.
- Scientists, engineers, regulators, stakeholders and the public need to gain confidence in the expected performance and safety of the repository. This is only possible after comprehensive performance assessment studies and a related safety case have been presented. Developing a defensible license application depends—in part—on the legal and regulatory framework, which may need to be adapted to accommodate the deep horizontal drillhole concept. Nevertheless, many aspects of the proposed concept can be based on decades of research, testing, engineering analyses and experiences made worldwide by independent institutions, nuclear waste management organizations, as well as the oil, gas and geothermal industries. Additional technical arguments specifically related to a drillhole repository were presented in Section 3 above.
Conflicts of Interest
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|Waste Type 1||Number 2||Capsule or Assembly||Disposal Canister ID 5||Casing ID 5||Drillhole Diameter||Total Disposal Section|
|Diameter 3 (cm)||Length 4 (cm)||(cm)||(cm)||(cm)||Length 6 (km)|
|Feature||Mined Repository||Horizontal Drillhole Repository|
|depth||0.3–1.0 km||>1 km|
|access hole diameter||3–8 m||<0.5 m|
|deposition hole diameter||1.5–5.0 m||<0.5 m|
|ground support||shotcrete/rockbolts/wire mesh||casing/liner|
|drainage and depressurization||yes||no|
|Ventilation||pre- and post-emplacement||no|
|local waste and heat density||high||low|
|temperature limit||~100 °C (bentonite)||in situ boiling temperature|
|repository closure||>50 years||immediately|
|Feature||Yucca Mountain||Horizontal Drillhole Repository|
|depth||0.3 km||>1 km|
|aquifer||below disposal section||above disposal section|
|transportation||inter-state||none or local|
|length of tunnels/drillholes to dispose 63,000 tons of SNF||50 km||1320 km|
|diameter of disposal hole||6 m||0.45 m|
|excavated volume||1,400,000 m3||200,000 m3|
|boiling point of water at depth||96 °C||>300 °C|
|Feature||Vertical Borehole Disposal 1||Horizontal Drillhole Repository|
|depth||3–5 km||>1 km|
|geologic setting||crystalline basement rock||igneous/metamorphic/sedimentary|
|pressure, temperature, stress||higher||lower|
|conditions and properties||depth-dependent||constant|
|buoyancy forces||aligned with borehole||absent|
|waste emplacement||easier||more difficult|
|disposal-section length||limited by depth and load||potentially long|
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Muller, R.A.; Finsterle, S.; Grimsich, J.; Baltzer, R.; Muller, E.A.; Rector, J.W.; Payer, J.; Apps, J. Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes. Energies 2019, 12, 2052. https://doi.org/10.3390/en12112052
Muller RA, Finsterle S, Grimsich J, Baltzer R, Muller EA, Rector JW, Payer J, Apps J. Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes. Energies. 2019; 12(11):2052. https://doi.org/10.3390/en12112052Chicago/Turabian Style
Muller, Richard A., Stefan Finsterle, John Grimsich, Rod Baltzer, Elizabeth A. Muller, James W. Rector, Joe Payer, and John Apps. 2019. "Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes" Energies 12, no. 11: 2052. https://doi.org/10.3390/en12112052