We present a novel repository concept for the disposal of spent nuclear fuel and high-level nuclear waste. The concept consists of an array of deep horizontal drillholes bored into suitable sedimentary, igneous or metamorphic host rocks using off-the-shelf directional drilling technology. Waste is encapsulated in specialized corrosion-resistant canisters, which are placed end-to-end into the relatively small-diameter, cased and backfilled horizontal disposal sections of the drillholes. A schematic representation of a deep horizontal drillhole repository is shown in Figure 1
. The concept allows for consolidated waste disposal in a national or a few regional repositories or decentralized disposal in modular, smaller repositories located at or near the reactor sites or processing plants where the waste was produced.
High-level nuclear waste comes in many forms; we focus here on two types: (1) the small capsules of concentrated cesium and strontium (Cs/Sr) defense waste [1
] and (2) the spent nuclear fuel (SNF) from commercial nuclear reactors [2
]. Other waste forms, such as the granular solids of calcined high-level waste at the Idaho Nuclear Technology and Engineering Center [3
] or the borosilicate glass of vitrified high-level waste in the Glass Waste Storage Buildings at the Savannah River Site [4
], may also be considered for disposal in a horizontal drillhole repository but are not further discussed in this paper. The drillhole disposal concept takes advantage of the fact that nuclear waste is very compact. The following discussion refers to the current waste inventory in the United States.
The Cs/Sr defense waste is a by-product of chemical processing of spent fuel from nuclear reactors used for plutonium production and from reactors that were part of the U.S. Navy nuclear fleet. Because of the relatively short half-life of 137
Cs and 90
Sr (about 30 years), which results in high radioactivity and high heat generation, these isotopes were extracted from the spent fuel in the form of cesium chloride (137
CsCl) and strontium fluoride (90
), placed in about 2000 double- and triple-walled capsules [5
] and stored at the Hanford Waste Encapsulation and Storage Facility in Richland, Washington. (Note that the capsules also contain minor amounts of the much longer-lived 135
Cs isotope.) To dispose of all Cs/Sr capsules, only about a 2 km section of a small-diameter, horizontal drillhole would be needed.
The commercial spent nuclear fuel currently consists of about 80,000 tons of ceramic UO2
]. A total of about 280,000 SNF assemblies discharged from pressurized water reactors (PWR) and boiling water reactors (BWR) are presently held at about 75 sites in 33 states [6
]. About two-thirds of the waste is in cooling pools at the power plants and one-third has been transferred to dry casks, which are cooled by flow of gas rather than by immersion in water. Until a permanent disposal solution becomes available, the fuel stored in pools will gradually be moved to dry casks, either at the same site or at a consolidated interim storage facility [7
]. Placing SNF canisters (each containing a single PWR or BWR assembly) into drillholes end-to-end every 6 m (accounting for variable canister lengths and some spacing between them) amounts to a total disposal-section length of 1680 km (1050 miles), that is, on average about 22 km (14 miles) for each of the 75 current storage sites. Table 1
summarizes the number and dimensions of individual waste units, the diameter of a disposal canister, the required casing and drillhole diameters and the total length of horizontal disposal sections needed.
Horizontal drillhole disposal has become feasible and attractive because of the technological development of advanced directional drilling in the last two decades. A typical oil or gas well has an inner diameter from 5½ to 8¾ inches (14 to 22 cm), which is sufficiently wide to accommodate Cs/Sr waste capsules. As mentioned above, SNF disposal in unmodified assemblies requires drillholes with a diameter of up to 19 inches (48 cm). The technology for directional drilling of such large-diameter boreholes exists [10
]. In the U.S. alone, there are over 50,000 drillholes with a horizontal section of 3 km or longer and a depth of 1 to 5 km.
Disposal of radioactive waste in boreholes has been considered previously. An early evaluation was done by the U.S. National Academy of Sciences [12
]. The deep borehole disposal option discussed in that report considers the injection of liquids containing highly diluted waste into deep, permeable formations, that is, a concept very different from what is proposed here. In 1979, a study commissioned by the U.S. Atomic Energy Commission [13
] outlined the possibility of waste disposal in up to 6 km deep shafts and up to 15 km deep vertical boreholes. The report evaluates geotechnical, geophysical, environmental and safety issues, discusses data adequacy and identifies research and development needs. Technical feasibility was further examined in a report commissioned by the Office of Nuclear Waste Isolation (ONWI) [14
]. In 1989 a study concluded that waste disposal in very deep vertical boreholes (6 km) drilled into the crystalline basement rock in Sweden was considered both feasible and economical [15
]. Furthermore, a scheme was proposed [16
] that makes use of the decay heat generated by the high-level waste to partially melt the granitic host rock (which later recrystallizes) in the bottom disposal section of a 5 km deep vertical borehole. (Similar concepts involving rock melting and self-burial of the waste have been proposed as early as 1974 [18
].) In 2003, the interdisciplinary MIT study on the future of nuclear power [19
] recommended that deep vertical borehole disposal be investigated as an alternative to mined repositories. Starting in 2009, the vertical borehole disposal concept was further examined by Sandia National Laboratories [20
] and a reference design was developed [22
]. The project was expanded by the U.S. Department of Energy (DOE) in 2012, when an experimental program and demonstration project was initiated [23
]. (Note that the deep vertical borehole disposal program was canceled by the Department of Energy in 2017.) The Blue Ribbon Commission on America’s Nuclear Future considered deep vertical borehole disposal a potentially promising technology that should be pursued [24
]. In 2015, the Nuclear Waste Technical Review Board (NWTRB) dedicated one of its board meetings to technical presentations and discussions of the deep vertical borehole disposal concept [25
]. Finally, summary descriptions of European deep vertical borehole disposal programs can be found in References [8
These earlier concepts consider vertical boreholes or boreholes that are only slightly tilted to enable multiple disposal sections to be constructed from a single drill pad or as laterals from a main access hole [17
]. The horizontal drillhole concept discussed in this paper has been briefly mentioned elsewhere [8
] but was not examined in any detail. While waste disposal in vertical and horizontal boreholes appears very similar, these two concepts are quite different in their technical requirements and performance (see Section 4.2
The safety of high-level radioactive waste disposal in horizontal drillholes mainly stems from the depth of the repository and therefore, the effectiveness of the natural barrier system, which consists of thick, confining rock strata with very low hydraulic conductivities and low diffusivities, resulting in extremely long radionuclide travel times to the surface. In addition, there is a sequence of effective components of the engineered barrier system, which includes (1) the ceramic UO2 fuel pellets, (2) the zircaloy cladding of the assemblies, (3) any material that fills the spaces within the canisters, which (4) are made of corrosion-resistant alloy, (5) the buffer material between the canister and the casing, (6) the carbon steel casing and (7) the cement or other filling between the casing and the drillhole wall. The engineered barrier system also includes (8) plugs and (9) backfill materials that seal the vertical access hole. The compactness of the relatively small-diameter access and disposal sections of the drillhole repository strengthen these engineered components and reduce their vulnerability.
Finally, construction costs of drillhole repositories are expected to be considerably lower than those for conventional mined repositories, mainly because of the drastically reduced excavation volume, the use of off-the-shelf equipment for drilling and waste emplacement, the simplicity of the engineered barrier system, the potential avoidance of repackaging and transportation and the fact that no ancillary infrastructure is needed to support humans working underground. Moreover, the modular concept allows for staged, fast implementation and immediate closure, thus reducing the considerable costs for interim storage and operation of an open repository.
The main goal of this paper is to present the concept of using deep horizontal drillholes for decentralized, permanent, cost-effective disposal of high-level radioactive waste. The technical description of the concept (Section 2
) is followed by a general discussion of the expected repository performance (Section 3
). We compare deep waste isolation in horizontal drillholes with alternative, complementary concepts, such as mined repositories—specifically at Yucca Mountain, Nevada, the site designated for the disposal of spent nuclear fuel and other high-level radioactive wastes in the United States—and the deep vertical borehole disposal concept (Section 4
). The proposed concept’s main advantages and challenges will be discussed in Section 5
. Note that detailed assessments of the horizontal drillhole concept will be presented in separate technical papers.
2. Deep Horizontal Drillhole Disposal Concept
2.1. Radioisotope Inventory
We begin the description of the repository concept with some general remarks about the waste that needs to be disposed, focusing on SNF assemblies. The nuclear fuel pellets inserted into a reactor are made of UO2, with about 96% 238U and 4% 235U. Because of the long half-lives of 238U and 235U (4.5 and 0.7 billion years, respectively), the radioactivity of the fresh fuel rods is low. However, when uranium fissions in the reactor, it splits into two or more fragments, which are typically much more radioactive due to their significantly shorter half-lives. Most of the 235U generates decay products such as 3H, 90Sr, 137Cs, 129I and 99Tc. In addition, some of the 238U is converted by neutron absorption and decay into transuranics such as 239Pu, 240Pu, 241Am and 237Np. Other isotopes are created by the decay of their respective parent radionuclides and their concentrations may actually increase as time passes.
It is worth noting that the radioisotopes with very short half-lives decay quickly and thus cause little long-term concern. In contrast, isotopes with very long half-lives (such as 238U and 235U) decay so slowly that they have low radioactivity. Therefore, isotopes with intermediate half-lives (such as 137Cs and 90Sr) are the ones that are difficult to handle and that present disposal challenges. These fission fragments cause the high intensity of the SNF’s initial radioactivity; later, after the first few centuries, the transuranics contribute most to the long-lived radioactivity.
The initial isotope inventory of the nuclear waste to be disposed in a geologic repository is a key determinant of the overall risk. The abundance of each isotope, its half-life and that of its daughter products along the decay chain determine the radioactivity and toxicity of the waste. However, the key performance measure of a repository is not the waste inventory in the canisters but the dose received by a reasonably maximally exposed individual at the surface. This means that the release of isotopes from the waste containers and their transport through the engineered and natural barrier systems to the accessible environment have to be accounted for. The mobility of radioelements is influenced by many factors (such as the flow of liquids and gases, the radioelement’s solubility, adsorption behavior, diffusivity and volatility). These are properties of both the radioelements themselves as well as the man-made and geologic materials through which they migrate. Consequently, the importance of a given radionuclide for nuclear waste disposal is the product of its abundance and an index that accounts for all factors governing the fate of the radionuclide from the waste canister to the affected individual [28
]. These importance factors can only be determined by a comprehensive performance assessment analysis, which requires detailed design information and site-specific characterization data. Nevertheless, it is important to have a detailed waste inventory available as input to such an assessment; the inventory developed for the Yucca Mountain project [29
] is thus pertinent for a drillhole repository in the United States.
Although the spent fuel from a commercial nuclear reactor could be repackaged or reprocessed, we assume that it remains in the same assemblies that held it in the nuclear reactor, where they are handled as a unit. An assembly typically spends 3 to 5 years in the reactor, after which it is replaced with an assembly containing fresh fuel. During refueling, the reactor is shut down, the lid of the reactor is opened and the fuel assemblies are individually moved to a cooling pool. Despite the chain reaction having been stopped, the removed fuel assembly initially produces heat that is almost 7% of the heat production during reactor operation. This initially high thermal output declines rapidly as the short-lived radioisotopes decay. After ten minutes, it has been reduced to 2% of the operating power and after a day to about 0.5%. As more of the radionuclides decay, the heat production decreases further; after 1 year it is about 10 kW per ton of spent nuclear fuel; after 10 years, it is about 3 kW per ton. The impact of heat generation on repository performance is discussed in Section 3.5
2.2. Drillhole Repository Configuration
The overall configuration of a horizontal drillhole repository has been schematically presented in Figure 1
above. A vertical access hole is drilled and cased from the surface through confining geologic units to a kickoff point a few hundred meters above the target repository horizon. The purpose of the conductor and surface casings are, respectively, to guide the drilling and to protect freshwater aquifers, schematically shown in Figure 1
b. In the horizontal disposal section, the casing facilitates the emplacement (and potential retrieval) of the canisters and supports backfilling operations. A smaller-diameter hole is gradually curved, with a build angle of less than 0.25° per meter (8° per 100 feet), until it is horizontal. The radius of curvature is large enough to avoid any impedance during casing installation and waste emplacement. A third set of casing is installed and cemented in place and then yet a smaller diameter drillhole continues near-horizontally for a few hundred meters to several kilometers. This part of the drillhole is the waste disposal section of the repository, having a minimal diameter as shown in Table 1
to accommodate the canister size designated for a given drillhole. Drilling technology has advanced to the point where rotary steerable systems have high precision with absolute and relative accuracy of less than 1 m.
As the hole is being drilled, rock cores, fluid samples and formation evaluation well logs are collected to aid in site characterization and emplacement decisions. For larger-diameter canisters, the horizontal section may be drilled in two stages: a first small-diameter stage for evaluation and testing of the disposal section followed by a reaming operation to create a diameter large enough to house the canisters.
Once the horizontal section is drilled, another string of casing is cemented in place, potentially with monitoring systems embedded on or in the casing that communicate real-time data about the repository condition to the surface. The continuous casing in the curved and horizontal section of the drillhole facilitates the emplacement (and potential retrieval) of the canisters and supports backfilling operations. Moreover, the casing and the surrounding cement act as additional engineered barriers during the early periods after waste emplacement, providing mechanical protection and preventing fluid flow around the canisters during the initial equilibration phase. The casing also effectively redistributes heat, reducing temperature gradients and associated thermal stresses.
Several drillholes may be completed from the same surface pad, either using separate vertical access holes or possibly by drilling multiple laterals (potentially at different depths) from a single access hole. Modular drillhole repositories could be constructed at or near the sites where the waste is currently in interim storage, minimizing or even avoiding the need for waste transportation outside the boundaries of the nuclear facility. Alternatively, larger, regional repositories could be built if considered appropriate for technical reasons or if otherwise preferred.
The nominally horizontal repository section may have a slight tilt. Such a tilt of a few degrees may be beneficial in keeping fluids and dissolved radionuclides from migrating towards the vertical access hole by inducing density- or buoyancy-driven fluid gradients towards the dead-end of the drillhole.
2.3. Waste Emplacement
To prepare for disposal in a drillhole, the assemblies will be removed from cooling pools or temporary dry storage casks and each one placed in a canister made of a corrosion-resistant alloy. To simplify the transfer of SNF from dry casks to permanent disposal, a new cask storage design could be developed, in which fuel assemblies sitting in the cooling pools are directly loaded into disposal canisters, before being placed into a concrete cask for dry storage.
The canisters are sealed and transported to the drill rig. These operations require radiation shielding by using a transport cask or a modified dry cask system. The canisters are then lowered into the casing, then pushed by a wireline tractor, coiled tubing or drill pipe [30
] into the horizontal drillhole and released once the disposal location is reached. Canisters may be emplaced one-by-one or in strings of multiple canisters to reduce round-trip time.
In the event of a canister breaking free from its support cable and falling into the vertical access hole, there are several mitigating measures that can be devised to reduce or prevent damage to the canister or casing. The falling canister’s terminal velocity is limited by the presence of water, brine or other viscous fluids present in the casing. For a heavy canister, however, that velocity might be high enough to allow the canister to reach a speed which presents danger to the integrity of the canister, specifically if it hits another canister that had previously been put in place. The maximum velocity can be substantially reduced by self-deploying breaking devices, such as an expandable centralizer, a parachute mechanism or the use of foams. Damage from a collision can also be mitigated by the use of an impact absorber placed on the front end of the canister [32
]. Should a canister be damaged, a procedure has to be in place to de-contaminate the drillhole and associated equipment.
Another emplacement risk is that a canister may get jammed or otherwise stuck above the disposal section due to the casing’s tortuosity or other irregularities or obstructions. The large radius of curvature and the precision of current rotary steerable drilling technology make the risk of canister sticking or jamming during the emplacement process very low. However, to further mitigate this risk, a caliper or dummy canister can be lowered into the drillhole to ensure that the casing is free of obstacles and that the emplacement equipment functions as intended. Tools and procedures to retrieve a stuck canister from the drillhole have been routinely used for oil and gas operations and can be enhanced for nuclear disposal operations.
Many countries require that nuclear waste be retrievable after placement in a repository. (In current U.S. law, this is regulated in 10 CFR 60.111, requiring retrievability for “up to 50 years after the waste emplacement operations are initiated” or “until the completion of a performance confirmation program”.) Retrievability was mandated, in part, because of the possibility that a better disposal method or site might later be found or that the nuclear waste could have value in the future (e.g., as fuel for next-generation nuclear reactors).
To facilitate canister recovery, one option would be to leave the vertical access drillhole open for the first 50 years. Note, however, that sealing of the hole does not preclude recovery, as plugged boreholes can be reopened. In the drilling industry, recovery of objects in a deep drillhole is considered routine using “fishing” technology [33
]. Placement and retrieval of borehole equipment are highly developed in the oil industry and are commonly performed using an array of specialized tools and methods.
On 15 November 2018, we placed a dummy canister in a 690 m deep horizontal drillhole, released it, withdrew the placement cable and several hours later inserted a recovery cable, latched the canister and brought it back to the surface. This test was carried out under contract by a drilling company at a previously drilled horizontal borehole. The procedure was repeated on 16 January 2019 as a demonstration [34
The size and weight of the dummy canister used in the test was designed to represent a canister holding a Cs/Sr capsule. The canister was attached to a hanger, which had a releasable connector; the hanger was coupled to a tractor whose electrically-driven retractable wheels firmly pushed against the casing. The tractor was linked to the surface with a wireline cable. The canister was lowered in the vertical section to the kickoff depth at 242 m, where the hole gradually turned horizontal at a depth of 690 m. The tractor pushed the canister 122 m into the horizontal section. The canister and hanger were released and the wireline cable and tractor pulled to the surface. Subsequently, a special latching mechanism was attached to the tractor and the assembly lowered and pushed back into the horizontal section. The latching device engaged the canister-hanger combination and the canister was retrieved to the surface.
Although this initial demonstration was successful, retrievability of canisters from the disposal section needs to be further examined for the larger SNF canisters and for conditions prevalent in a backfilled, heated, potentially sealed disposal section. A comprehensive bibliography of reports discussing retrievability is available from the Nuclear Energy Agency (NEA) [35
2.5. Canister and Casing
The canister holding the spent nuclear fuel assemblies or other high-level radioactive waste is a key component of the engineered barrier system. The canister directly protects the waste from mechanical impact, exposure to the chemical environment and contact with fluids, which may dissolve the waste and act as the main transport vehicles for radionuclide migration. While radionuclides released from a breached canister (at early times) will very likely remain contained within the multi-barrier repository system, it is obviously desirable to use canisters with long-time durability. We propose to fabricate canisters made of highly corrosion-resistant nickel-chromium-molybdenum (Ni-Cr-Mo) alloys, such as Alloy 22 (UNS N0622) [36
] and Alloy 625 (UNS N06625) [37
], which are very stable in the expected hot, reducing chloride environments representative of the deep drillhole disposal sections. Elevated temperatures affect the corrosion behavior of the canister, because they impact both corrosion resistance of the metals and corrosivity of the environment. In the passive state, metals are protected by a thin, self-forming film on the surface, leading to very low general corrosion rates (on the order of 0.01 μm/y). A canister made with Alloy 625 with a wall thickness of 1 cm takes at least 50,000 years to degrade by general corrosion [38
Localized corrosion mechanisms, which are a particular threat to the durability of passive films, need to be accounted for when predicting the lifetime of a canister [39
]. The Ni-Cr-Mo alloys, with their durable, protective (passive) films that are self-forming even in harsh environments, have robust resistance to the localized corrosion processes of pitting, crevice corrosion and stress corrosion cracking [40
]. Passive films on Ni-Cr-Mo alloys are highly durable because of a combination of resistance to film breakdown even in highly aggressive environments and their ability to reform (repassivate) after being damaged. Microbially-induced corrosion (MIC) is another potential damage mode to canisters. Alloy 22 is not only one of the most resistant alloys for abiotic corrosion mechanisms but also among the most resistant to MIC [44
]. The MIC resistance of Alloy 22 and titanium alloys has been examined under a range of conditions with no evidence of surface damage [45
]. Galvanic corrosion is an additional damage mode to be considered and its effects will depend upon the other metals incorporated in the repository, the relative surface areas and the conductivity of filler materials and pore waters.
Mechanical breaching of the canister, for example by puncturing during handling and emplacement or shearing during seismic events or crushing by rock consolidation or volcanism, should also be considered. These risks are mitigated to some extent by the strength of the canister wall.
Calculations for carbon steel casings indicate that their lifetime in a reducing environment can reach several hundred years [38
]. This is long enough to provide a reliable and relatively smooth conduit for canister retrieval, should that become necessary or worthwhile. While casings are not designed for long-term waste protection, it is beneficial to have a barrier restricting fluid flow during the early pressure equilibration and thermal periods.
The corrosion of the canisters and casing forms metal oxides and generates hydrogen gas. Both processes lead to volume expansion and, consequently, pressure increases, which in turn induce fluid flow. The use of corrosion-resistant materials leads to a slow generation of corrosion products (hydrogen and oxides). Outside the containment, the slow gas generation allows most of the hydrogen to dissolve in the liquid and diffuse away, that is, reducing or avoiding the emergence of a free gas phase [46
]. Moreover, generating corrosion products from a linearly spread-out source expedites radial diffusion and dissipation of the pressure perturbation.
2.6. Backfilling and Sealing
Open space within the canister, between the canister and the casing, the casing and the drillhole wall, as well as along the vertical and curved access sections of the drillhole will be backfilled. The repository may be further sealed by plugs. One of the potential pathways along which radionuclides may migrate to the accessible environment is through advective transport along the drillhole and the excavation disturbed zone around it. Backfilling and sealing these preferential pathways are thus of primary importance.
Each backfill material will be designed for a particular purpose or specific barrier function. In general, low hydraulic conductivity, durability and self-healing abilities are among the main desirable characteristics of a backfill material. However, the choice of material is limited by the engineering challenges of installing the backfill in a small-diameter, deep drillhole. Backfill and sealing methods considered in the deep borehole disposal reference design are described in detail in References [22
As mentioned above, the annulus between the casing and the drillhole wall should be filled using cementitious materials with formulations currently used by the hydrocarbon and geothermal energy industries. Cementing is important to stabilize the casing during canister emplacement and to keep the casing centralized, which is necessary to reduce thermal stresses. Using fine-grained cements and other permeation grouts also helps seal the excavation disturbed zone.
Open space inside the canister can be filled with a suitable material that provides additional safety against mechanical damage from impact and vibration during emplacement or an earthquake and to increase thermal conductivity for more efficient heat dissipation. This filler could also include boron to absorb neutrons and lower the risk of reaching criticality (see Section 3.9
Once emplaced in the disposal section, backfilling the space between the canisters and the casing serves multiple purposes, including providing stability and mechanical protection, increased heat conduction, reduced fluid flow and, therefore, reduced advective transport of leaked radionuclides. It also helps to control the chemical environment, inhibiting corrosion and increasing the adsorption of radionuclides.
Compacted, expandable bentonite is the backfill material of choice in most mined repositories located in the saturated zone (for a review, see Selling and Leupin [48
]). However, the excellent barrier properties of bentonite can only be realized if installed under unsaturated conditions, allowing the clay to expand as it hydrates. In a deep drillhole, installation of dry granular bentonite or bentonite blocks is infeasible. Nevertheless, the use of a high-density bentonite suspension is a possibility to be considered. The requirement for pumpability limits the options to materials that set, gel, solidify or swell after emplacement, either driven by a chemical process, temperature changes or mechanical compaction, which induces creep flow or visco-plastic deformations. Such backfills include cementitious materials, oil-based materials (such as tar, bitumen or asphalt), pumpable colloidal silica and other grout mixtures, salts or fine pellets of lead-based alloys that melt and re-solidify to encapsulate the canisters [49
]. A detailed research and engineering program could help select or adapt these materials to the chemical, thermal and stress environment at the repository site. Moreover, verifiable methods will be needed to install the backfill material in a small-diameter drillhole and to examine its as-built performance.
When canister emplacement is complete and all initial performance confirmation tests yield satisfactory results, the disposal section is plugged, and the vertical access hole is backfilled and sealed. Effective plugs may be installed by reaming the drillhole over a relatively short section, which is preferably located adjacent to a clay, shale or salt rock layer. The reamed drillhole section will be filled with a suitable backfill material, which will be compacted by the overburden and plastic deformation of the surrounding rock [50
], effectively isolating the disposal section from the surface and at the same time sealing the excavation disturbed zone. Similar plugs may be installed throughout the disposal section to compartmentalize the disposal section, reducing the risk of axial radionuclide transport to transmissive zones that may intersect the drillhole. In the sections of the vertical hole that penetrate deformable formations, the casing or liner may be removed after waste emplacement to promote self-sealing. Rock welding with resistance heaters and ceramic plugs have also been proposed [52
]. The as-built effectiveness of a plug may be confirmed by performing an in-situ pressure test [53
]. However, the long-term stability is difficult to assess; multiple plugs may be installed along the drillhole for redundancy.
Backfilling of the disposal, curved and vertical sections of the drillhole provides an opportunity to add sorption-enhancing amendments that retard radionuclide transport; a similar concept has been proposed for the invert in the emplacement tunnels at Yucca Mountain [54
]. Backfilling and plugging might increase costs but should not compromise the ability to retrieve waste canisters. Finally, the sealing procedure has to account for the incorporation of sensors that may be installed for performance confirmation.
3. Expected Repository Performance
3.1. Basis of Performance Evaluation
Nuclear waste disposal in deep horizontal drillholes is a novel concept. While work on the detailed repository design and performance assessment is ongoing, an initial evaluation of the expected repository performance builds on the extensive research, analyses, developments and experiences of many organizations throughout the world that develop high-level nuclear waste repositories (for a review, see Faybishenko et al. [55
]). Of particular significance is the work that has been performed to assess the performance of mined repositories in a reducing, fully saturated environment—specifically those located in argillaceous host rocks—as well as deep vertical borehole disposal concepts. Complete performance assessments are available for mined repository concepts; a generic safety case for a deep borehole disposal scenario is presented in Reference [56
]. The following subsections discuss some performance aspects on a general level, with specifics being addressed in the cited references and upcoming publications.
3.2. Leakage Pathways
The performance of a nuclear waste repository is ultimately measured by the exposure risk to individuals. Evaluating this risk requires a deterministic or probabilistic assessment of the fate and transport of radionuclides from the waste form in the repository along some leakage pathway to the receptor. This pathway is part of several subdomains referred to as (1) the engineered barrier system (EBS), which includes the waste, canister, backfill and other engineered components, such as liners and seals, (2) the geosphere, also referred to as the far field or natural barrier system, which includes the host rock and aquifers and (3) the biosphere, which is the biological environment that determines the radiation dose to the receptor [57
]. Special consideration is given to the interface between the EBS and the geosphere, which consists of the damaged or disturbed zone around the excavation [58
It is essentially impossible to anticipate all potential failure modes and leakage pathways and to determine their probability, characterize the relevant properties and predict the consequences over the entire performance period. Inserting redundancies into a multi-barrier system and making realistically conservative assumptions in the performance assessment calculations are safeguards used against the inherent uncertainties and unidentifiable variabilities of a complex repository system. In this overview, we mention just a few leakage pathways that need to be considered when evaluating the performance of a deep horizontal drillhole repository.
In Section 2.5
, we briefly mentioned the role that the canisters play in protecting the waste during the initial, critical period after emplacement up to about 50,000 years [38
]. After a breach of the canister (either by corrosion or mechanical destruction, for example, during earthquake-induced fault reactivation), radionuclides dissolve into the resident brine and are released to the near field either by diffusion or advective and dispersive transport. This sequence of processes constitutes the so-called source term. The release rate from the canister may be limited by the solubility or the diffusivity of the radionuclide, along with the geometry of the canister perforation. The reducing conditions encountered in a deep, saturated formation tend to lower the solubility of most radionuclides, leading to smaller release rates and thus lower contaminant concentrations.
Once released from the canister, the radionuclides start migrating (either by advection or diffusion) within and across the engineered barrier system. Given the low permeability of the host rock, radionuclides may preferentially be transported in axial direction along the drillhole. Transport velocity is expected to be very low, mainly because of (1) the presence of low-permeable, absorbing backfill material, (2) plugs installed to prevent axial flow towards and within the vertical access hole and (3) the absence of significant driving forces. The last point deserves additional comments. Potential driving forces within the drillholes are either created by the repository itself or imposed by the natural system. Gradients induced by the repository include differences in chemical potential driving molecular diffusion, pressure perturbations caused by the decay heat (i.e., thermal expansion and contraction of fluids and the pore space during the heat pulse), pressure increases caused by corrosion (i.e., volume increase due to the generation of metal oxides and hydrogen gas) and general equilibration of drilling- and emplacement-induced pressure perturbations. All of these forces are expected to be minor, limited in time (from less than a year to a few centuries) and acting axially over the restricted cross-sectional area of the small-diameter drillhole. Consequently, radionuclide mass flow rates along the drillhole are expected to be very small.
Gradients in the natural system include (1) regional pressure differences caused by topography and recharge patterns, (2) thermal upwelling or convection cells, (3) pressure perturbations induced by stress redistribution (e.g., after an earthquake, volcanic eruption or due to erosion, uplift or glacial loading) and (4) the development of osmotic pressures across rocks with high clay content. Most of these gradients are expected to be small or they develop—and dissipate—slowly over geologic time. Notable exceptions are earthquakes and volcanism. Such disruptive events may generate new faults or magmatic intrusions or reactivate old faults. These discrete features may directly intersect the drillhole and impact the integrity of the engineered barrier system. At the same time, they provide potential pathways for fluid flow and radionuclide transport. Also note that seismic and igneous events may impact pressure gradients even if they occur at a considerable distance from the repository.
Finally, pressure gradients may be generated by human activity, specifically by nearby hydrocarbon production or by large-scale geologic carbon sequestration and energy storage projects.
The migration of volatile radionuclide-containing species such as 14
can occur in near-surface repositories located in the unsaturated zone [59
]. However, this is very unlikely in the case of a deep drillhole repository located within the saturated zone, where high fluid pressures inhibit the generation of a free gas phase and its migration through the rock.
A site- and disposal hole-specific hydrogeological characterization of the host rock as well as the overlying confining units will determine the properties of the heterogeneous formations, specifically their ability to transmit water in response to the local and regional pressure gradients discussed above. Particular attention needs to be paid to the geometric and hydrological characteristics of faults, deformation zones, contacts between hydrostratigraphic units and fracture networks. Nevertheless, the relatively great target depth (>1 km) of the horizontal drillhole repository makes it unlikely that long, continuous natural pathways exist or are created, or that they have sufficient permeability, cross-sectional area and sustained driving forces to induce significant upward flow rates over long time periods. Radioelement retention properties, such as adsorption onto mineral surfaces and diffusion into stagnant pore water in the surrounding rock matrix, would retard their transport to the accessible environment. Finally, the fact that nuclear waste emplacement is decentralized and stretched out linearly in multiple drillholes (rather than placed with high density at a centralized location) significantly decreases the amount of waste potentially affected by a disruptive event.
For disposal sections located in competent rock of low matrix permeability, molecular diffusion of dissolved radionuclides is the only remaining transport mechanism. Diffusion is driven by chemical-potential gradients, which decrease substantially at even short distances from the disposal drillhole. The small porosity and tortuosity of the pore space considerably reduce the effective diffusivity compared to that in free water, that is, radionuclide migration velocities are very small. Diffusing radionuclides reach very low concentrations after short distances as they are diluted and spread over a radially or even spherically increasing volume of pore water.
Initial calculations lead us to believe that the assessments comprising the safety case will likely demonstrate that these features and effects have either very low probabilities of occurrence or low consequences or that they do not compromise the safety of the repository even when making conservative assumptions and accounting for inherent and irreducible uncertainties. The horizontal drillhole disposal concept takes advantage of geological formations with sufficiently high barrier performance, integrated into a robust disposal system with multiple engineered barriers. Similar screening decisions as the ones presented here have been made previously based on a generic analysis of features, events and processes (FEPs) [61
3.3. Isolation of Brines from Aquifers
The disposal section of the horizontal drillhole repository is to be located in a deep formation whose pore space would most likely be saturated with brine. Demonstrating that such a brine has been isolated at depth for long time periods is a key factor in assessing the long-term security of the repository. Establishing the absolute age of brines is complicated by mixing processes and the lack of an ideal chronologic marker in the water. However, various isotopic systems can help place limits on age and mixing rates.
In addition to numerical modeling, a good indicator of isolation could be determined by measuring the presence of radioisotopes such as 81
Kr is a rare gas formed by cosmic ray interactions in the atmosphere and then incorporated into surface waters. Since it has minimal interaction with local rock and is not generated at depth in significant amounts, it can provide a direct measurement of the isolation of deep brines from surface waters. The presence of measurable 81
Kr (with a half-life of 229,000 years) in repository brines indicates that significant amounts of surface water are interacting with or replacing brines on a time scale less than one million years. Conversely, the absence of 81
Kr in brines suggests isolation of the brines over similar time scales. Concentrations of 81
Kr are extremely small (hundreds of atoms per liter); however, a laser-based resonance ionization technique measures this isotope with sufficient sensitivity [62
]. Note that low mixing of surface water with deep brines does not directly demonstrate that there is equally slow transport upward.
Upward flow limits can be estimated by measurements of other isotopes, specifically 36
Cl and 129
I, as a function of depth. These can be measured in deep brines from about one liter of sampled fluid using AMS (accelerator mass spectrometry) [64
]. These radioisotopes are produced naturally underground through the decay of uranium and thorium in the rock strata. In the absence of significant flow, each isotope maintains a natural background concentration in the rock that balances production and decay, a pseudo-steady state referred to as secular equilibrium. It takes approximately 4 to 5 half-lives for the system to reach this stable background concentration. These values can be predicted based on the measured rock composition and calculated or measured neutron fluxes from samples [65
]. Successive layers of rock strata that maintain distinct and different secular equilibrium concentrations of these radioisotopes indicate limited upward flow [67
]. We focus here primarily on 36
Cl, which has a half-life of 300,000 years and 129
I, which has a half-life of 16 million years. Maintenance of unique 36
Cl secular equilibrium concentrations can place limits on upward flow of Cl through the rock strata on time frames on the order of one million years. As a simple example, if an overlying formation has a lower value of 36
Cl or 129
I than does an underlying layer, that suggests absence of significant upward flow of these isotopes.
Helium gas analysis offers another independent means to assess upward flow. 4
He, the stable isotope of helium, is emitted in alpha decay. The balance of production and escape of 4
He from the rock strata determines the residence time of helium in the rock. 4
He gas is quite mobile; estimates of residence times for 4
He on the order of 100,000 years and greater suggest that other less mobile species in brines should not escape the rock strata over similar or even much longer time frames [68
In all of these isotopic studies, care must be taken to keep samples pristine. Small amounts of contamination from field sampling or handling in the laboratory can yield inaccurate and misleading results. Despite inherent difficulties, successful measurements of these isotopes in diverse settings and field conditions are now routinely undertaken.
In addition to isotopic methods, the confining properties of the overburden may be demonstrated by the presence of trapped gas in layers above repository strata. If it is determined that the natural gas was formed over millions of years earlier, then the existence of such gas provides strong evidence that the brine in deeper layers is not flowing upwards.
3.4. Reducing Environment
As mentioned above, the reducing environment encountered at repository depth is beneficial for the longevity of the waste canisters and casing. In addition, it affects the solubility and, therefore, the mobility of radionuclides in the subsurface.
In the event a canister is breached and the spent nuclear fuel is exposed to the saline fluids present in the host rock, the constituents of the waste can react with the brine and form new compounds of varying solubility. The subsequent advective and diffusive migration of dissolved radionuclides is dependent on their chemistry, that of the host rock and the dissolved constituents in the brine. Furthermore, most of these factors depend on temperature, alkalinity and redox potential.
The chemical factors that influence radionuclide migration can be divided into two groups, namely (1) those that immobilize the radionuclide by adsorption, ion exchange or mineral (co-)precipitation and (2) those that govern complexing of cationic radionuclides with anions dissolved in the brine, which may enhance their solubility and thus make them amenable to transport.
The valence state of several radionuclides depends on the ambient oxidation state, which in turn substantially affects their chemical properties and ability to migrate in pore fluids. The processes mentioned above are interlinked and in competition. Consequently, a prediction of the actual migration of a radionuclide requires integrating the critical thermodynamic and kinetic parameters affecting its solubility, which needs to be carefully examined by means of reactive transport modeling and, if necessary, supported with additional experiments.
3.5. Decay Heat
The temperature evolution in and around a drillhole containing heat-generating high-level radioactive waste is an important factor impacting repository performance. It affects corrosion rates, barrier properties of backfill materials, degradation of concrete, fluid density and viscosity, radionuclide solubilities, the evolution of the excavation disturbed zone and biogeochemical reactions; in summary, temperature is a key driver of many coupled processes.
The radiation from the spent nuclear fuel is converted to heat in the waste, the engineered barriers and the rock. The decay heat for Cs/Sr waste is initially up to 400 W per capsule [5
], decreasing with a half-life of about 30 years. The decay heat for spent nuclear fuel depends on the initial enrichment, burnup percentage and duration of post-reactor cooling. An initial value for spent nuclear fuel is on the order of 10 kW per metric ton of heavy metal (MTHM) [19
], decreasing with time as the initial radionuclide inventory and its daughter products decay, as shown in Figure 2
for SNF from a PWR reactor.
The temperature of the canister and the surrounding rock initially rises before declining as radioactivity decreases and as the rock conducts heat away. As an example, Figure 3
shows the temperature evolution around a drillhole holding Cs/Sr capsules that are emplaced with a 2-feet (0.6 m) spacing between them. The initial heat output is 100 W; the temperature change, ΔT
, shown in Figure 3
can be linearly scaled to different initial heat output values and can be converted to absolute values by adding the ambient temperature of the host rock. Although the maximum temperature (reached after about 3 years) is above the boiling point of water at atmospheric conditions, it is well below the boiling point for water at a depth of 1.0 km below the water table, which is about 310 °C. These simulations are described in detail by Finsterle et al. [69
]; similar studies for Cs/Sr capsules in water, air and deep vertical boreholes are presented by Arnold et al. [47
These analyses suggest that the linear arrangement of waste canisters with relatively low energy density combined with an approximately cylindrical heat dissipation regime decreases the risk of excessive temperature perturbations, both within the disposal section of the drillhole and the adjacent host rock. Given that the drillhole is filled with water during all stages of the repository’s lifetime (including the operation and post-closure phases), boiling is suppressed by the high hydrostatic pressure, drastically reducing the adverse effects of steam on repository components and lowering the complexity of predicting the overall system behavior.
The following two effects of earthquakes on the integrity of the drillhole repository are considered: (1) potential damage to the engineered barrier system and (2) the creation of new pathways for radionuclides to reach the accessible environment.
The potential damage to a canister from earthquake vibrations is small. The only examples of vibration-induced borehole damage occur in highly unconsolidated sediments, which are not suitable for a repository. Near the surface, accelerations of earthquakes rarely exceed 1 g, where g is the gravitational acceleration; below the surface, accelerations are considerably smaller [70
]. At early times, the casing and cement also substantially attenuate earthquake vibrations due to the elastic impedance contrasts between the rock, the cement and the casing. An added measure of protection from vibrational damage comes from the disposal assembly itself. The fuel rods are held within the canister by the structure of the fuel assemblies and the filling material, and the canisters are embedded in their respective backfill materials and a cemented-in casing.
There is the possibility that one or several canisters will be sheared by a new fault or by reactivation of an existing fault. The probability of a fault actually intersecting the repository is very low and can be further reduced through site and drillhole characterization methods and by orienting the drillholes sub-parallel to the direction of existing faults, so that the shear stress is unlikely to create a new fault that crosses the drillhole. A standoff distance of 100 m from an active fault is sufficient to mitigate the risk of damage to the drillhole. Moreover, for most types of rock, the ductility of the canister is greater than that of the rock and for small fault offsets, the canister would rotate or deform rather than rupture. For some rocks, such as granite, shearing may occur, albeit only affecting a very small number of canisters, even though secondary fracturing and faulting may extend to a distance on the order of 100 m perpendicular to the main fault plane.
Finally, as mentioned in Section 3.2
, the creation or reactivation of a fault or associated fracture zone might also create new pathways for fluid flow and radionuclide transport, along with earthquake-induced pressure perturbations acting as the driving force. It should be noted, however, that many faults are not water conducting and the induced pressure gradients are transient.
While strong earthquakes lead to high accelerations and substantial rupturing at and near the land surface, these effects decline with depth, where rock strength increases and no surface wave exists, limiting the impact of earthquakes on the engineered barrier system in a deep drillhole. Moreover, a modular, decentralized waste disposal concept reduces the risk that a large number of canisters are breached in a single event.
The risk of criticality arises if spent fuel undergoes a sustained chain reaction, which could be initiated by the intrusion of water that acts as a neutron moderator. There is no danger of criticality for the Cs/Sr waste, which is very low in 235
U or 239
Pu content. Estimating the probability of SNF reaching criticality will require the use of computer simulations [72
]. It can be noted, however, that the spent fuel rods are low in 235
U or 239
Pu—the very reason why they were removed from the reactor. They could still deliver energy in an environment in which most neutrons are created by fissions in other fuel rods. However, in a drillhole repository, they are placed in a linear configuration, not side-by-side as in the reactor. The empty space in canisters holding the fuel assemblies will be filled with a sand-like material, reducing the volume of water that may be present. If deemed necessary, neutron absorbers (such as boron) could be added to the filler. Nevertheless, for certain kinds of defense waste, particularly those containing high levels of plutonium, the criticality issue will have to be carefully evaluated.
3.8. Inadvertent Intrusion
Inadvertent intrusion refers to the accidental drilling into a repository during the exploration for water or mineral resources. The deep horizontal drillhole concept offers inherent protection against inadvertent intrusion, mainly because of its location well below potable water resources. Furthermore, the flexibility in siting modular horizontal drillhole repositories allows their placement in regions with no commercially viable natural resources. The probability may be higher in regions that have oil or gas bearing shale at the same geographic location. For the short term, this issue is addressed by the U.S. Nuclear Regulatory Commission (NRC) regulations that require mineral rights be secured at any location where nuclear waste is disposed. Finally, U.S. regulations mandate that a marker be placed above the site to alert future generations of the presence of the repository [74
]; the effectiveness of such markers, however, is difficult to demonstrate and the consequences of inadvertent intrusion needs to be evaluated regardless of the efficacy of the markers.
3.9. Terrorist Intrusion
Unlike interim storage in pools or dry casks on or near the land surface, waste disposal in deep repositories provides a geologic barrier that safeguards against a terrorist attempt to obtain spent nuclear fuel. Although the fuel is retrievable, removing even one canister after sealing of the drillhole requires installing a tall rig, removing several hundred meters of drillhole sealant and plugs and operating a specialized system to retrieve each canister. Although this procedure is feasible for a professional service company, it is not inexpensive or quick. Moreover, the retrieved canisters will be radioactive when they come to the surface, requiring terrorists to rig shielding and safety equipment. Removal of radioactive waste from a deep drillhole does not lend itself to a surreptitious terrorist attack.