Geological repositories: scientific priorities and potential high-technology transfer from the space and physics sectors

Abstract The use of underground geological repositories, such as in radioactive waste disposal (RWD) and in carbon capture (widely known as Carbon Capture and Storage; CCS), constitutes a key environmental priority for the 21st century. Based on the identification of key scientific questions relating to the geophysics, geochemistry and geobiology of geodisposal of wastes, this paper describes the possibility of technology transfer from high-technology areas of the space exploration sector, including astrobiology, planetary sciences, astronomy, and also particle and nuclear physics, into geodisposal. Synergies exist between high technology used in the space sector and in the characterization of underground environments such as repositories, because of common objectives with respect to instrument miniaturization, low power requirements, durability under extreme conditions (in temperature and mechanical loads) and operation in remote or otherwise difficult to access environments.


Introduction
THE disposal of radioactive waste from nuclear applications (e.g. from the nuclear power industry, nuclear weapons, medical applications and research programs) is currently an environmental concern worldwide. According to the International Atomic Energy Agency's (IAEA) 2007 report, the worldwide amount of spent fuel mass (of heavy metal) is ∼18 × 10 7 kg and 1 × 10 7 kg are produced per year (International Atomic Energy Agency, 2007). The leading option to address this problem is the construction of long-term (i.e. over tens of thousands of years) subsurface geological repositories for the management of these wastes (Long and Ewing, 2004). According to the glossary of the U.S. Nuclear Regulatory Commission (NRC), the definition of a geological repository is: "An excavated, underground facility that is designed, constructed, and operated for safe and secure permanent disposal of high-level radioactive waste. A geological repository uses an engineered barrier system and a portion of the site's natural geology, hydrology, and geochemical systems to isolate the radioactivity of the waste" (NUREG-1350(NUREG- , 2013. For radioactive waste disposal (RWD), a multiple barrier approach is preferable given the difficulty in predicting the performance of a geological repository over very long time frames (Toth, 2011), and it is critical that the engineered barriers work together with the natural environment for isolation and containment (Vines and Beard, 2012).
Another class of geological repositories are those which are intended to safely and effectively store carbon dioxide (CO 2 ) gas (Carbon Capture and Storage; CCS). CCS repositories can be located in depleted gas fields (Jenkins et al., 2012), depleted hydrocarbon fields, deep saline aquifers and coal seams (Rütters and CGS Europe partners, 2013). However, repositories can also be located away from potential resources and therefore they are less well characterized. Depleted oil and gas reservoirs have an estimated storage capacity of ∼675-900 GtCO 2 (gigatonnes of CO 2 ), deep saline formations ∼1000 GtCO 2 or an order of magnitude higher and uneconomical coal formations ∼3-200 GtCO 2 (Benson and Cook, 2005). These are globally significant amounts of CO 2 , and highlight the potentially important role of geological repositories in addressing one of the major environmental challenges of the 21 st Century: climate change.
Burying waste far from populated areas (although this is often not possible), the biosphere, the atmosphere and the hydrosphere in the deep underground, confined by geological substrates/ strata/formations is thus currently the favoured approach (Long and Ewing, 2004;Toth, 2011). There are a number of challenges to be addressed that relate to the physical, geochemical and biological processes that might occur in repository sites worldwide. These challenges apply to either or both RWD and CCS, and any other use of the subsurface for geodisposal. In order to evaluate plans for geological disposal of different types of waste, and to gather sufficient information to make the case compelling, it is necessary that the geophysical, geochemical and geobiological processes that occur within the subsurface, and that might influence the long-term fate of the waste are understood thoroughly.
One of the important challenges is to identify technology transfer areas that could facilitate geological repository site selection, construction and monitoring. Many of the problems faced in building and maintaining such facilities have similarities to technology requirements in studying the physics, chemistry and biology of any extreme environment, and in particular environments encountered during space exploration.
Repository monitoring is one need, which could potentially benefit significantly from technology transfer. Monitoring could provide relevant information during all stages of repository development and maintenance, including during: (1) surface exploration of the repository site; (2) access just prior to construction or exploratory drilling and underground exploration; (3) construction or expansion of the repository; (4) emplacement (for RWD) or injection (for CCS) of the waste; (5) disposal tunnel backfilling (for RWD) or injection well plugging (for CCS); (6) backfilling and the sealing of the repository; and possibly (7) the long post-closure period. In the case of RWD, monitoring is required to ensure the stability of nuclear safeguards. The European Commission has funded a collaborative project (MoDeRn -Monitoring Developments for safe Repository operation and staged closure; Solente et al., 2013), for developing and possibly implementing monitoring activities during relevant phases of the RWD process (site characterization, construction, operation and staged closure, and post-closure control phase). Therefore, it is timely to review the potentially promising areas of technology transfer from space exploration and particle and nuclear physics sectors.

Science priorities and technology transfer opportunities
The establishment of geological repositories is a huge area in which a vast concentration of research and technology development has occurred (e.g. Benson and Cook, 2005;Birkholzer et al., 2012;Scott et al., 2013). In order to evaluate how technologies developed for the space sector can advance the scientific understanding needed for geological repository siting and operation, it is first necessary to identify some of the key science questions that need to be answered. This paper does not attempt to provide an exhaustive list of science questions, but instead identifies some of the high priority areas in geological disposal that could benefit from tools and approaches developed in high-technology areas such as the space sector. Science questions under the themes of geophysics, geochemistry and geobiology are reviewed, and potential technology transfer options are considered, noting that many of them could promote scientific advances under all three themes. These science areas and their potential for technology transfer were identified by GeoRepNet (Geological Repositories Network), a three year UK Science and Technology Facilities Council (STFC) network set up to investigate potential high-technology transfer into geological repositories.
The focus of this paper is on technologies that can be transferred from the particle and nuclear physics sector and, in particular, from the space sector to tackle the prioritized science questions (Table 1). Space instrument engineers are dedicated to a number of specific aims in instrument development including miniaturization, low energy requirements, long-term operation and operation under extreme conditions and in difficult to access locations (e.g. on the surfaces of other planetary bodies). Similar conditions and constraints are encountered in geological repository environments and thus it seems likely that some of these technologies and instrument strategies could be adapted with appropriate modifications, for use in the monitoring of geological repositories during construction and post-closure.
There are also a range of facilities, such as synchrotron sources and deep underground laboratories that could potentially play a role in testing technology transfer in all three areas discussed. The access to deep subsurface environments, such as underground laboratories, offers the potential to test technology appropriate for both planetary exploration/high-technology physics and geological repositories in environments where synergies between both applications can be developed.

Geophysics
Robust knowledge of the geophysical and/or of the geomechanical context of a repository site is essential to understanding mechanical, hydraulic, thermal and other physical processes (e.g. formation and propagation of fractures, temperature-induced effects in relation to heat-generating waste, swelling/sealing of clays) and other related facets like geochemistry and geobiology. Indeed, the geochemistry of a repository site and the way in which the chemistry interacts with the subsurface biota are constrained by the physical characteristics of the environment. Key questions specific to geological repository siting and monitoring include: (1) How can we accurately understand and monitor stress fields, before, during and after repository construction? This work is important for understanding how the geological repository environment will evolve over time and potentially identifying, and even forecasting, points of weakness. Examples of techniques currently used in carbon capture include the measurement of subsurface pressure, time-lapse 3D seismic imaging, vertical seismic profiling and crosswell seismic imaging, passive seismic monitoring, land surface deformation using interferometry and GPS, visible and infrared imaging from satellite or planes (Benson and Cook, 2005). Most of these techniques require periodic mobilization of resources for repeat surveys. Such episodic surveys conducted over time scales of tens of years are expensive (especially onshore seismic surveys) and their results are affected by environmental conditions not necessarily linked to the monitored parameters.
(2) Have we identified and understood the uncertainties and do we have confidence in the results? Mapping subsurface geology and its geophysical/geomechanical properties has inherent uncertainties. Characterizing these uncertainties is essential to predicting the behaviour of a repository, particularly in the long-term post-closure and yet carrying out this characterization of uncertainties remains in its infancy. The level of uncertainty varies between different locations, classes and type of repository (RWD or CCS), for example there is only limited experience in identifying and locating sites suitable for CCS beyond the regional scale (e.g. for saline aquifers) (Wilkinson et al., 2013). (3) What are the transport characteristics, fluxes and speciation of gases in environments with less than 1 km depth of relevance to geological repositories? This question relates to the natural environment in and around geological repositories, but also the influence of perturbation during construction, operation and closure. In particular, the way the subsurface structural characteristics either facilitate or mitigate against the transport of gases within the repository is relevant.
(4) Is the aim detection or quantification? For example, in CCS, technologies that are sensitive to detecting the presence of CO 2 are not necessarily the best technologies for quantifying spatially variable saturation. In practice, early detection of unintended migration of CO 2 may enable mitigating activities to be undertaken, whereas quantification is important for calculating a mass balance and imposing financial penalties that are dependent upon the amount of CO 2 that escapes. While for RWD in almost any geology/ geological environment, no leakage is achievable with appropriate, often costly, engineering.

Geophysics: technology transfer opportunities
A fundamental requirement for improving model capabilities, confidence in the results and in particular improving the modelling of gas flow through the subsurface is high-resolution data to categorize rock mass heterogeneities. Miniaturized, portable, low-power sensor technologies developed for space applications, which include X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) offer particular promise for on-site characterization of mineralogical heterogeneity of representative samples of the stratigraphy comprising and surrounding the repository, which might provide data and understanding to underpin models. Examples of possible technology transfer from space technology are given in Table 1. Non-intrusive techniques can also be developed to allow characterization of the underlying geology. A promising technique belonging to the particle and nuclear physics sector (muon tomography), is based on the observation of cosmic-ray muons, sub-atomic particles produced in collisions of highenergy primary cosmic radiation with nuclei in the atmosphere, in the deep subsurface (Kudryavtsev et al., 2012). This method allows for the mapping of geological structures. Muon detectors can be emplaced in the subsurface at different sites to monitor geological repositories. Although they may not be able to fully replace more conventional techniques, they have a clear advantage of providing a continuous monitoring capability and possibly a significant saving on resources over a long time period.
Another non-intrusive technique used in the space exploration sector but with potential for use in geological repositories is Ground Penetrating Radar (GPR). Depth of penetration is strongly influenced by the subsurface substrate and presence of ice/water and salinity levels, but typically can be tens of metres. An example of a GPR system applied in space exploration is the WISDOM (Water Ice and Subsurface Deposit Observation On Mars) instrument on the planned ExoMars rover (ESA and Roscosmos) (Ciarletti et al., 2011), which can penetrate up to 3 metres with cm-scale resolution. Such high-resolution subsurface analysis can be applied to monitoring the near-surface environment above a repository, particularly within terrains that would benefit from a miniaturized, low-power GPR. The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on ESA's Mars Express orbiter (Mouginot et al., 2012) (Table 1) also employs GPR technology and was originally designed to probe the ice-rich polar layered deposits up to four kilometres and up to several hundred metres in other lithic environments of Mars. Therefore this technology can also be applied in the identification of deep repositories.

Geochemistry
During the establishment of geological repositories, the local geochemical environment is perturbed. Some changes may be transient, but there may be long-term effects of the intervention of humans within the geological environment. For example, permanent changes to subsurface hydrology/groundwater flow regimes associated with repository construction (e.g. installation of low permeability walls) may lead to changes in redox states of subsurface fluids and minerals, most likely on a local scale. Questions related to geochemistry that may be particularly pertinent to geological repositories include: (1) How can we successfully obtain data that can be used to support the development of improved models such as the modelling of pore space chemical and physical processes and translate them to large-scale fate and transport models? To understand the likely fate of radionuclides or supercritical CO 2 in the subsurface, we require simulation approaches supported by in situ geochemical monitoring and mapping.
(2) What are the effects of impurities in the injected CO 2 streams (e.g. SOx, NOx, Hg in flue gas) on the nature and rates of chemical reactions during transport and injection, which would lead to mineralization and geologic sequestration of carbon? We require better parameterization of interactions of impurities with other molecules, which is important to fully understand the groundwater chemistry and flow in deep systems (at repository depths).
(3) In terms of storage security, what chemical reactions mobilize and immobilize CO 2 (including dissolution, residual saturation and mineralization as well as microbial effects)? Once waste is contained it is necessary to monitor, long-term, the environment to determine whether waste is escaping and what geochemical reactions determine the rate of escape. This requires a more in depth investigation of geochemical reactions occurring in natural geological repository environments.
(4) Can we identify potential geochemical signatures that could be used to indirectly indicate CO 2 leakage (i.e. other than measuring CO 2 itself ) or the leakage of radionuclides into the environment? During leakage of radioactive waste or CO 2 , these substrates are likely to geochemically modify the surrounding environment. Both smallscale and large-scale monitoring methods can be used to detect potential geochemical alterations.

Geochemistry: technology transfer opportunities
Technology transfer areas include methods for detecting the migration of radionuclides, CO 2 and other gases that could be indicative of subsurface repository change or leakage, although time scale may be an issue. Of particular interest are spectroscopy techniques (e.g. infrared spectroscopy, gas chromatography coupled with mass spectrometry (GC-MS), X-ray diffraction (XRD)) developed for planetary exploration and astronomy. Both of these communities seek to develop small, low-energy, robust instruments with very high sensitivity, particularly when this instrumentation is being used to search for trace gases such as methane at the ppt/ppb/ppm level (e.g. Formisano et al., 2004;Korablev et al., 2013). An example of such technology is the Tunable Laser Spectrometer (TLS) of the Sample Analysis at Mars (SAM) suite of Curiosity rover used for the recent in situ methane detection on Mars (Webster et al., 2015). Other examples include technologies to measure low concentrations of elements and impurities in the groundwater and in materials around a geological repository during construction and potentially post-closure, such as X-ray fluorescence (XRF) (see Table 1 for examples of possible technology transfer).

Geobiology
Microorganisms are pervasive in the environment, including the deep subsurface (e.g. Wouters et al., 2013). Near geological repositories, a variety of microbial metabolisms may be involved in changing the chemical speciation of elements and their mobility through the subsurface (Lloyd and Renshaw, 2005;Krawczyk-Barsch et al., 2012), or in affecting the performance engineered barriers, or in enhancing canister corrosion, concrete deterioration and the structure and performance of bentonite buffer materials used in nuclear waste disposal scenarios (Masurat et al., 2010;Pedersen, 2010), thereby affecting the performance of the engineered barriers and their functions (isolation and containment). Microorganisms and their activity can also change the transport properties of the host rocks of interest e.g. formation of biofilms can effect porosity (Coombs et al., 2010). Despite the growing and impressive understanding of microbiology in the deep subsurface and microbial interactions with radionuclides, several key questions for biological science related to geological repositories can be identified: (1) What are the main processes by which the microbial communities associated with different lithologies can influence RWD and CCS? We need to know more about subsurface microbial communities of geological materials at depth, and their dominant and potential metabolic pathways and how these respond to the changes imposed in and around a geological disposal facility. One way is to link the study of geological repository environments more directly with international continental and oceanic deep drilling programmes and especially permanent deep subsurface laboratories where longterm analysis of deep microbial communities can be accomplished.
(2) How do activities during excavation, construction and operation inhibit or promote specific microbial processes? During different phases of the development of a repository, new microbial inocula, interfaces and chemical species are likely to be introduced, which are expected to affect the resident microbial communities and their metabolic pathways. For instance, the introduction of oxygen during drilling or excavation is expected to affect subsurface microbial consortia that were previously based on anaerobic metabolisms such as anaerobic respiration (iron and sulfate), fermentation or methanogenesis. There is a need to better understand the impact of the disturbance associated with reservoir or repository construction and active operation on the long term ecology of deep repository environments and associated implications for safe waste isolation.
Related to point (2) is a priority to better understand how communities change after the closure of the facility. Ideally one would monitor the evolving microbial community over time, linking back to changes in geochemical and geophysical properties. Although permanent deep subsurface laboratories make such monitoring possible, such laboratory facilities are limited to specific sites for logistics and cost reasons. Therefore, one obvious priority would be to advance our capacity to quantify and predict microbial dynamics across temporal and spatial scales, which is itself directly linked to improved capacities for geochemical modelling.
(3) What are the effects of microbial presence and activity on local geochemistry, integrity of the repository and radionuclide transport? What are the exact roles of local microbial consortia in corrosion, as well as on microbial interactions with groundwater chemistry or radionuclides? For example, there is knowledge on the capacity of deep microbial communities to produce or metabolize gases (including those that are relevant for RWD safety assessments e.g. methane and hydrogen), but not much is known on intermediate and final gas mass balances in a repository system and on the subsequent effects on the local chemical processes. Also the biodegradation of naturally occurring or introduced organic material and the organic matter derived from dead microorganisms (Bassil et al., 2014) is expected to influence the local geochemistry.
(4) What is the role of microbial biofilms in disposal systems? In RWD, microbial biofilms on engineered interfaces such as concrete and metal can harbour strong chemical gradients which increase material deterioration rates, and can interact with radionuclides, changing geochemistry in a microenvironment. In CCS scenarios, extensive biofilms may interact with supercritical fluids. It is also not clear how microbial biofilm development is triggered and maintained in undisturbed and perturbed deep subsurface environments. (5) How can our current experiment-based understanding of microbial processes and their effects be abstracted and up-scaled, both in time and in space? Microbial influences can be studied in the laboratory or, in cases where the subsurface can be directly accessed, either by sample collection during initial drilling, in deep subsurface laboratories, or during the construction of the geological repository itself. However, a crucial objective must be to understand possible future interactions using improved modelling supported by empirical microbiological data. For example, there is a need to establish input values such as rates of metabolism for different redox couples for a given environment, and the rate of dispersal or transport of metabolic products and organic matter throughout the subsurface medium. (6) Can analogue environments be found that are representative of a geological repository environment? The use of modelling, laboratory studies and in situ investigations will advance our understanding of microbial interactions in geological repositories. However, one means to achieve improved modelling and real-world data is to find easily accessible and representative analogue environments. One such environment is the Boulby International Subsurface Astrobiology Laboratory (BISAL) dedicated to astrobiology analogue research in the deep subsurface and for the testing of space technology and technology transfer into the mining environment at 1.1 km depth in the Boulby Mine (Yorkshire, UK) (Cockell et al., 2013).

Geobiology: technology transfer opportunities
These identified challenges can be consolidated under technological innovation objectives that can be accomplished with technology transfer from a variety of areas, including molecular biology approaches, such as methodologies for the production of metagenomic libraries and their archiving and analysis. One promising area of research from the space sector that may find application to geological repositories is the 'lab on a chip' technology such as the Life Marker Chip (see Table 1), an instrument capable of recognizing small biomarker molecules by the use of an antibody-based detection system and developed for use on spacecraft (Sephton et al., 2013). This approach is a good example of a portable/low-power microfluidics technology that is transferrable to the deep subsurface. In general, this technology allows rapid identification of individual microbes and screening for specific metabolisms and metabolites that would allow for in situ analysis of microbial dynamics.
Other technologies, such as synchrotron spectroscopies offer the chance to better understand the interactions of microorganisms with radionuclides and the secondary mineral products formed following these interactions. The possibility of examining these effects at small spatial scales, e.g. using new techniques such as STXM (scanning transmission X-ray microscope), allows for improved mechanistic understandings of these interactions that could be applied at the larger scale. Computational resources ( possibly including all aspects of modelling and data analysis, processors, etc) from space physics and other areas could be mobilized to improve the ability to model microbial-groundwater interactions over time, and predict the effects of geological repository construction on geochemical and geobiological processes.

Concluding remarks
This paper concludes with the observation that there are strong synergies between high-technology areas such as space sciences and particle physics and the development of geological repositories. These synergies primarily arise because: (1) all these groups seek to build miniature, reliable, low energy, rugged instruments and (2) the scientific questions they address have similar technological solutions. Some concrete examples of these links are summarized in Table 1. We recommend that a stronger effort be made to link science and technology requirements in the development of geological repositories to these high-technology communities and ultimately to pool expertise and resources in the development of technology.