European Geologist Journal 59

Importance and Role of Geological Investigations in the Expected Expansion of Nuclear Energy to Combat Climate Change

 

by Esa Pohjolainen 1, Markku Paananen 1, Ismo Aaltonen 1 and Heini Reijonen 1

1 Geological Survey of Finland

Contact: esa.pohjolainen@gtk.fi

Abstract

Nuclear energy is a key tool in reducing greenhouse gas emissions, and achieving climate goals will require an increase in nuclear capacity. Correspondingly, uranium demand is expected to rise in the coming decades. Likewise, the need for site investigations for both nuclear power plants and nuclear waste disposal facilities is growing. Geological investigations play a significant role both in the front-end and back-end of the nuclear fuel cycle activities. The discovery of a uranium deposit, the definition of uranium resources, and the development of uranium mines require long-term commitment to geological investigations. Similarly, a wide range of geological data must be collected and assessed during site investigations for radioactive waste disposal facilities, as well as in evaluating the long-term safety of high-level waste.

Cite as: Pohjolainen, E., Paananen, M., Aaltonen, I., & Reijonen, H. (2025). Importance and Role of Geological Investigations in the Expected Expansion of Nuclear Energy to Combat Climate Change. European Geologist, 59. https://doi.org/10.5281/zenodo.16442990

Creative Commons License This work is licensed under a Creative Commons Attribution 4.0 International License.

1. Introduction

Climate change is one of the greatest threats to current and future generations. A mean global warming of 1.5°C above pre-industrial (1850–1900) levels is considered the threshold beyond which the adverse impacts of climate change will become increasingly difficult for ecosystems and human lives [1]. Human-induced global warming had already reached about 1°C above pre-industrial levels around 2018, and it is likely to exceed 1.5°C during the 21st century even under very low greenhouse gas emission scenarios [2]. Therefore, significant reductions in global greenhouse gas emissions are needed to achieve a sufficient slowdown in global warming within the coming decades. The United Nations Sustainable Development Goal 13 (Climate Action, Figure 1) also calls for urgent action to combat climate change and its impacts [3].

Figure 1: Sustainable Development Goals. Figure by the United Nations [3].

Nuclear energy plays a vital role in climate change mitigation due to its ability to produce baseload electricity in a reliable and cost-effective manner without greenhouse gas emissions. A total of 440 commercial nuclear reactors are currently in operation worldwide, with a net electricity generating capacity of 399 gigawatts electric (GWe), requiring approximately 67,500 tonnes of uranium (tU) annually [4]. Achieving climate goals requires a considerable increase in nuclear generating capacity, as nuclear energy is one of the most efficient tools to reduce greenhouse gas emissions. Limiting global warming to 1.5°C will require a tripling of installed nuclear capacity, from 399 gigawatts in 2025 to 1,160 gigawatts by 2050 [5].

Nuclear energy’s key role in meeting climate change targets is currently widely recognised, which is demonstrated by the inclusion of nuclear energy in the outcomes of the first Global Stocktake under the Paris Agreement on climate change and by the declaration issued at the 2023 United Nations Climate Change Conference (COP28) by 25 countries pledging to triple nuclear capacity by 2050 [6]. Consequently, the need to reduce greenhouse gas emissions will be the primary driver for global projected nuclear growth, particularly to replace fossil fuels in baseload electricity generation in the coming decades. Moreover, one of the greatest advantages of nuclear energy is that nuclear power plants can operate continuously, providing a stable source of electricity that is not affected by weather conditions, while the share of wind and solar photovoltaic in total electricity generation is increasing [7].

A total of 66 commercial nuclear reactors are currently under construction around the world [4]. The addition of nuclear power plants to the global electricity generation mix will help meet rising electricity demand without increasing greenhouse gas emissions, but nuclear energy also has significant potential to contribute to emission reductions in other, non-electric applications, including district heating, water desalination, industrial process heat, and hydrogen production that currently largely rely on fossil fuels [6].

Due to global projected nuclear growth, uranium demand is expected to increase significantly over the coming decades. For the same reason, the need for activities related to site investigations for nuclear power plants and nuclear waste disposal facilities is increasing. It is noteworthy that geological investigations play a significant role both in the front-end and back-end of the nuclear fuel cycle activities (Figure 2). The exploration and discovery of an economic uranium deposit, the definition of uranium resources, and subsequent uranium mine development require a comprehensive understanding of the regional and local geology, as well as long-term commitment to geological investigations. Similarly, a wide range of geological information needs to be collected, evaluated, and understood in site investigations for nuclear power plants and radioactive waste disposal facilities, particularly to assess the long-term safety of high-level waste.

This article presents the role of a variety of geological investigations in the expected growth of nuclear energy as an integral part of global efforts to mitigate climate change.

Figure 2: Steps of the nuclear fuel cycle. The front-end of the nuclear fuel cycle is highlighted in green, and the back-end in orange. Geological investigations play a significant role, particularly in uranium exploration and mining activities, and in site investigations for the final disposal of spent nuclear fuel. Photos by Esa Pohjolainen (exploration and mining), IAEA (milling), Cameco’s copyright ownership and grant of permission (conversion and fuel fabrication), © Orano copyright (enrichment and reprocessing), TVO (nuclear power plant, electricity generation, and spent fuel storage), and Posiva Oy (final disposal).

2. Role of Geological Investigations in Uranium Exploration and Production

All commercial nuclear reactors are currently fuelled by uranium, and this raw material cannot be sourced without geological investigations to first explore, locate, discover, and characterise uranium deposits, then to define uranium resources and ore reserves, and finally to mine and extract uranium from these ore deposits. As an integral part of uranium exploration, geological investigations form a vital basis for the entire nuclear fuel cycle.

A global increase in projected installed nuclear capacity drives significant growth in uranium demand over the next decades, creating a variety of challenges for the uranium supply sector. Global identified uranium resources are sufficient to meet projected future high-demand reactor-related uranium requirements through the year 2050, but the main challenge is to turn these resources into economically viable uranium production in a timely manner due to the long mine development time [8]. Meanwhile, uranium supply from the existing mines will decrease significantly due to depletion of resources. In the long run, intense development of new uranium mines will be needed to fill in the supply-demand gap. However, this also requires that new uranium deposits be discovered, and new uranium resources be defined through comprehensive geological knowledge and significant investments in uranium exploration projects. The exploration and discovery of an economic uranium deposit and subsequent uranium mine development require long time frames, significant geological expertise, and long-term commitment. Exploration may take 15-20 years before discovering an economic uranium deposit, and the time required to bring a uranium mine into production from the time of discovery ranges from 10 to over 30 years [9]. Due to these factors, the uranium supply sector will have significant challenges to supporting a tripling nuclear capacity by 2050. However, timely investments in geological investigations to generate new geological knowledge in the exploration areas may expedite the discovery of economic uranium deposits.

2.1. Genetic Uranium Deposit Type Models in Uranium Exploration

Economic uranium deposits can be discovered only through long-term commitment to geological investigations and by understanding ore-forming processes that can lead to economic uranium concentrations within the context of a genetic uranium deposit model [9]. It is crucial to integrate an evaluation of favourable ore-forming processes with the regional and local geology to understand where economic uranium deposits could be discovered, in order to identify prospective exploration areas to focus on. Therefore, the availability of skilled and experienced geologists is essential for exploration success. Uranium deposits are classified by the International Atomic Energy Agency (IAEA) into 15 types [10], which are listed in Table 1.

Table 1: Classification of uranium deposit types showing reasonably assured (Measured and Indicated) recoverable uranium resources in the <USD 130/kgU (equivalent to USD 50/lb U3O8) cost category, by deposit type, with the main examples of each deposit type. The uranium resource figures presented in this table are a snapshot of the situation as of 1 January 2023, as reported in a joint report (Uranium 2024: Resources, Production and Demand, also known as the “Red Book”) by the Nuclear Energy Agency and the International Atomic Energy Agency [8]. It should be noted that the uranium resource figures for the black shale type do not include very low-grade uranium resources of the Talvivaara black schist-hosted Ni-Zn-Cu-Co deposit (at 18 ppm U) which are considered unconventional resources from which uranium is only recoverable as a minor by-product. Resources of the Talvivaara deposit are about 19,200 tU in Measured and Indicated Resources [8].

It is noteworthy that uranium deposit types are genetically related to highly variable geological environments, and therefore each deposit type model requires different knowledge of the genesis of uranium deposits. Similarly, each deposit type demands different exploration approaches and methods. Geological investigations, such as geological mapping, are essential for the discovery of any type of uranium deposit, as they increase the understanding of the regional and local geology together with the ore-forming factors that control uranium mineralisation. In particular, one of the most important tasks for field geologists is to search for the characteristics of the target uranium deposit type, which is selected during the exploration planning and area selection phase [9].

As illustrated in Table 1, uranium deposits and ore-forming processes are closely associated with specific rock types, host rock sequences, geological settings, structural framework, and phases of geologic time. Therefore, the potential for uranium deposits is commonly evaluated using genetic uranium deposit type models, based on analogies with economic uranium deposits in similar geological settings worldwide [9]. For example, quartz-pebble conglomerate uranium deposits were formed in a highly anoxic fluvial environment between 3.5 Ga and 2.2 Ga in Earth’s history [11]. In contrast, the formation of unconformity-type uranium deposits has taken place in the geologic record after the Great Oxidation Event (~2.2 Ga), under oxidising conditions that enable the mobilisation and transportation of uranium in oxidising ore-forming solutions.

Most current global uranium production comes from basin-related deposits, especially from sandstone-hosted deposits in Kazakhstan and Proterozoic unconformity-related deposits in Canada. Therefore, the identification of prospective sedimentary basins in the vicinity of potential uranium-bearing source rocks is a common approach in uranium exploration planning and the area selection phase [9]. Along with comprehensive geological knowledge and an understanding of genetic uranium deposit type models, exploration also requires adequate knowledge and experience to plan, manage, implement, and supervise exploration programs, and to interpret and report the exploration results. Widely used international reporting standards also require that a supervising geoscientist must have relevant experience in the style of mineralisation and the deposit type in the activity that the person is undertaking and reporting [12].

2.2. Understanding the Geological Context within the Exploration Area and Identification of Uranium Anomalies

The main objective of early-stage, regional-scale uranium exploration (also known as the reconnaissance exploration phase) is to identify viable uranium anomalies in the selected exploration area. For this purpose, the exploration team needs to first understand the geological context within the exploration area, and this geological knowledge is typically gained through the comprehensive compilation and review of existing geoscientific data, and subsequent regional-scale geological mapping in the field. One of the most important tasks of the fieldwork is to test and confirm that the selected uranium deposit type model(s) are applicable in the exploration area. This first phase of exploration is aimed at acquiring sufficient information for the assessment of the investment value for potential follow-up exploration within the selected exploration area [9].

Regional airborne geophysical surveying (especially airborne radiometric surveying) and ground follow-up of aeroradiometric anomalies are among the most important methods in uranium exploration, in particular during the reconnaissance exploration phase. A national geological survey organisation commonly has an important role during the area selection and early-stage exploration phases. In some countries, geological survey organisations have conducted high-resolution airborne geophysical surveys which cover the whole country. For example, the Geological Survey of Finland (GTK) holds publicly available national geodatabases which include airborne geophysical datasets (radiometric, magnetic and electromagnetic) that provide full national  coverage [13].

One of the first activities of an exploration organisation is to review existing airborne radiometric data for the region of interest, particularly if the uranium deposit type model favours the possibility of exposed or near-surface uranium mineralisation, such as in targeting for intrusive, metamorphite, metasomatite, sandstone and calcrete (surficial) types of uranium deposits. Airborne radiometric surveying usually results in the identification of a large number of aeroradiometric uranium anomalies. The purpose of ground follow-up of aeroradiometric anomalies is to characterise and assess these anomalies in the field using handheld scintillation count rate metres or portable gamma ray spectrometers. The number of promising uranium anomalies typically decreases significantly during field checking of aeroradiometric anomalies when the main goal is to locate viable anomalies for follow-up exploration. New uranium anomalies are also commonly located during ground radiometric surveying of aeroradiometric anomalies because significant anomalies can exist between airborne survey coverage, especially if older airborne geophysical surveys have limitations, such as wide flight line spacing, high flight altitude, poor location accuracy, or old sensor technology such as the small crystal size. Ground radiometric surveying through ground follow-up of aeroradiometric anomalies and geological mapping are typically carried out together so that the level of radioactivity of the bedrock outcrops is measured at the same time as these outcrops are geologically mapped by a field team [9].

Ground follow-up of aeroradiometric anomalies and geological mapping can be accompanied by a variety of other supporting exploration methods, such as radiometric boulder tracing, satellite-based remote sensing work, geochemical bedrock sampling, geochemical stream sediment sampling, and soil sampling. It should be noted that relatively few anomalies commonly result in positive outcomes and require subsequent more detailed follow-up surveys. Therefore, the number of promising anomalies and potential uranium targets typically decreases significantly during the reconnaissance exploration phase.

2.3. Developing Anomalies to Exploration Targets and Locating High-grade Uranium Mineralisation

If viable uranium anomalies are located during the first phase of exploration, follow-up exploration may be warranted to determine the extent of the most promising anomalies and to identify positive indicators of uranium mineralising systems. This phase aims to develop anomalies into exploration targets which are tested by early-stage drilling with the main objective of locating and intercepting high-grade uranium mineralised intervals for more detailed exploration [9]. During this phase, exploration typically evolves from less expensive techniques (such as geological mapping) to more expensive techniques (such as drilling).

The identification of positive indicators of uranium mineralising systems requires sufficient geological knowledge of ore-forming processes, such as potential uranium source, transport from source to deposition site, structural and physiochemical traps to promote uranium precipitation, and conditions preserving mineralised volumes and grades of uranium to form an economically viable uranium deposit. It is essential that uranium occurs predominantly in the tetravalent (4+) oxidation state that has low solubility. However, near-surface oxidation of U4+ to the hexavalent valence state U6+ results in uranyl (U6+O2)2+ complexes that are highly soluble [11]. The ability of uranium to form highly soluble uranyl complexes is important for most of the uranium mineralising processes and deposit type models, such as unconformity-related and sandstone uranium deposits. These geological processes include leaching of uranium from uraniferous source rocks (such as granites or rhyolites) by oxidising fluids, transportation of uranium in oxidising uranyl-bearing solutions, and precipitation and deposition of uranium from mineralising fluids as a result of the reduction of U6+ to U4+ at redox boundaries to form mineralised volumes and grades of uranium [9]. It should be noted that uranium mineralising systems vary between different uranium deposit type models, requiring different exploration approaches. For example, the mechanisms associated with the formation of intrusive uranium deposits include a variety of considerations, such as partial melting of uraniferous source rocks, magmatic chemistry, aluminium saturation index, magma polymerisation, key aqueous complexes, and the degree of fractional crystallisation in magmatic systems.

The follow-up exploration phase commonly involves geological mapping, ground and airborne geophysical surveys, radon surveys, trenching, widely spaced drilling, geochemical assaying, geological modelling and mineralogical characterisation [9]. In particular, ground radiometric measurements play an important role if the deposit type model favours the possibility of exposed or near-surface uranium mineralisation. In this case, trenching is typically carried out on the most promising anomalies. The main objective of this stage is to identify and locate high-grade uranium targets, and therefore widely spaced drilling is typically carried out at the end of this exploration phase to investigate possible continuity and trends of anomalies, and to intercept high-grade uranium mineralised intervals for subsequent detailed exploration. Drill hole planning requires that the overall geological context and extent of mineralisation at the surface is well-defined. A series of inclined drill holes are typically drilled perpendicular to the orientation of the target, if the strike and dip of the target can be determined from previous targeting work. For example, geological mapping, ground radiometric surveying, other geophysical surveying, or radon surveys are often conducted prior to drill hole planning in order to select appropriate drill hole locations, and to determine the potential orientation of the radioactive zones hosting mineralisation. At this stage, clusters of shallow holes are commonly most informative, and therefore it is recommended to drill several short holes very close to the target to test the true dimension, continuity, and geometry of geological features and uranium mineralisation before drilling longer holes.

Unexposed, subsurface or deeply buried anomalies without direct surface expression require very different exploration approaches, such as in exploration for unconformity-related or sandstone uranium deposits. For example, one of the primary methods suited to the exploration for roll-front type sandstone uranium deposits is drilling to identify and locate potential strata and conditions for the development of redox fronts to trap uranium-bearing fluids. In exploration for unconformity-related uranium deposits, airborne electromagnetic surveying and diamond drilling of geophysical anomalies, such as drilling of basement conductors, serving as proxies for graphite-bearing faults, play a significant role [14].

In addition to sampling for chemical analysis, mineralogical samples from bedrock outcrops and drill cores are taken to investigate the mineralogy of the exploration target. This mineralogical characterisation commonly includes petrographic observations, identification of uranium-bearing minerals in their textural settings, examination of mineral paragenesis using optical microscopy on polished thin sections, and X-ray diffraction to provide valuable information about the mineralogy of uranium and other associated metals of potential economic importance [9].

2.4. Discovery of a Deposit, Geological Modelling and the Definition of Uranium Resources

If high-grade uranium mineralised intervals are intercepted by drilling during previous exploration, detailed exploration and intensive drilling with a dense drill hole spacing may be warranted with the main objective of identifying and delineating significant uranium mineralised zones. This stage typically represents the discovery of a uranium deposit if drilling results are successful in delineating mineralised volumes and grades of uranium. This phase requires a systematic approach using closely spaced drilling to carefully and systematically trace mineralised zones. If geological evidence is sufficient to support geological and grade continuity of the uranium mineralisation, it may be possible to define Mineral Resources (Figure 3) usually at the confidence level of Inferred Resources [12]. It should be noted that different deposit types require different exploration techniques. For example, trenching and test pitting are typically the primary methods suited for the surficial uranium deposits (such as calcrete-hosted deposits) to assess their near-surface geological and grade continuity.

The definition of uranium resources, which is one of the main goals in deposit-scale studies at this stage, requires a comprehensive geological understanding of the uranium deposit. Modelling of Mineral Resources can be carried out only if the drilling and sampling provide sufficient evidence of geological and grade continuity. The main geological inputs to Mineral Resource estimations are geological characteristics, uranium grade, volume, and bulk density. Significant drill core sampling for geochemical laboratory analyses is carried out during this exploration phase to establish the grade of uranium in different geological domains within a uranium deposit. Alongside geochemistry, downhole radiometric probing can be utilised as an indirect measurement of uranium grade to obtain high spatial radiometric reading coverage and immediate results on site in the field. Radiometric downhole probing data is particularly important for fracture zones and highly mineralised intervals where drill core recovery can be poor due to high degrees of alteration or structurally damaged rocks associated with faulted uranium-mineralised zones.

The verification of the geology and modelling of geological continuity, grade continuity and grade distribution within a uranium deposit is the key factor in resource estimation. Other important parameters in preparing a Mineral Resource estimate include the verification of geological and analytical datasets, data adequacy, and quality assurance. For quality assurance purposes, quality control samples are inserted in the sample streams that are processed through the analytical laboratory to monitor and control the quality of sampling, sample preparation and sample assaying to establish acceptable levels of accuracy and precision of the chemical assay data, and to detect any potential cross-contamination which can take place at the laboratory during sample preparation due to improper cleaning of the crusher and pulverising equipment between the samples.

The supervising geoscientist needs to carefully consider the style of mineralisation and cut-off grade when assessing geological and grade continuity for the purposes of Mineral Resource classification [12]. The cut-off grade is the lowest grade of mineralisation that can be mined economically. The integration of geological, economic, and engineering parameters is needed for the determination of the economic cut-off grade. Other key parameters in a Mineral Resource estimate include mineralisation controls and geological model, surfaces, volumes, and other features used to constrain the Mineral Resource estimate, methodology for modelling of geological domains, Mineral Resource estimate data analysis (such as sample support, treatment of outliers, continuity analysis, rock density), and Mineral Resource classification including block model parameters and interpolated variables, interpolation methodology and resource model validation [9].

Figure 3: Relationship between Exploration Results, Mineral Resources and Ore Reserves. After CRIRSCO [12].

It should be noted that a Mineral Resource is a concentration of solid material of economic interest in such form, grade, and quantity that there are reasonable prospects for eventual economic extraction [12]. Therefore, the economic viability of the quantities of mineralisation is always an essential consideration when reporting and disclosing any uranium resources in accordance with the reporting template of Committee for Mineral Reserves International Reporting Standards (CRIRSCO), with the objective of promoting reliability, transparency, and high standards of reporting of Exploration Results, Mineral Resources and Ore Reserves. The determination of reasonable prospects for eventual economic extraction needs to consider a variety of aspects, including the technical and economic support for the cut-off assumptions, potential mining methods, metallurgical considerations, cost assumptions, uranium prices, and constraints applied to the Mineral Resource estimate.

Uranium grade, dimensions and extension of the mineralised zones (geological and grade continuity), and mineralogical characteristics are essential parameters for the evaluation of a uranium deposit. Therefore, chemical assaying, mineralogical characterisation and metallurgical testing are commonly closely linked to each other. It is particularly important to study whether the deposit contains any uranium-bearing minerals from which uranium may be difficult and expensive to extract. These mineral phases are called “refractory minerals”, which are common hosts to uranium in polymetallic uranium deposits where uranium can be incorporated in complex titanium, niobium, tantalum and rare earth element (REE) minerals, such as brannerite, pyrochlore, steenstrupine, davidite, betafite and fergusonite. In contrast, uraninite (or pitchblende), coffinite and hexavalent uranium mineral phases are considered to be non-refractory minerals from which uranium can be extracted much more easily and at lower costs. Therefore, uranium exploration efforts are commonly focused on deposit types that typically have favourable mineralogy (predominantly uraninite), such as unconformity-related and sandstone uranium deposits.

Drill core sampling is also done for detailed mineralogical characterisation, such as optical microscopy, X-ray diffraction and electron probe microanalyses. Mineralogical characterisation is crucial to define dominant hosts to uranium and the distribution of uranium between various minerals if several uranium-bearing mineral phases exist in a deposit. This mineralogical characterisation is typically followed by laboratory-scale metallurgical test work and preliminary process flowsheet development to obtain information on the ability to recover uranium from its hosting minerals. Alongside Mineral Resources, these mineral processing assumptions are essential parameters for the Preliminary Economic Assessment (also known as the Scoping Study) which is commonly carried out at this stage to understand the economic viability of the uranium deposit at very high levels [9].

The reliable definition of the bulk density is also an important component for an accurate tonnage estimate in uranium resource estimation. A sufficient number of bulk density samples needs to be taken and measured to obtain an adequate representation for all geological material types across a uranium deposit [9]. Special attention should be given to the physical characteristics of the sample material, especially if samples are friable, soft, or porous to adequately account for porosity and vugs in bulk density measurements.

The definition of initial Mineral Resources does not by itself justify to progressing to the final phase of exploration because the evaluation of the potential economic viability of initial uranium resources is also needed. Therefore, the completion of the Preliminary Economic Assessment is highly advisable at this stage because the outcome of the Preliminary Economic Assessment typically determines whether the subsequent, highly expensive and advanced exploration activities are reasonably justified or not. The Preliminary Economic Assessment provides appropriate assessments of realistically assumed Modifying Factors (Figure 3) together with relevant operational factors relating to mining, processing, metallurgy, infrastructure, economics, marketing, legislation, environment, social responsibility and governmental aspects [12]. If the exploration company does not carry out the Preliminary Economic Assessment at this stage, there is a risk that the company may unnecessarily progress to the final phase of exploration with marginal or even uneconomic projects with costs that may not be warranted [9].

2.5. Increasing Geological Knowledge and Confidence Levels of Uranium Resources, and Verification of Economic Viability of the Uranium Deposit

If initial uranium resources, usually at the confidence level of Inferred Resources, are defined during previous exploration, and the outcome of the Preliminary Economic Assessment indicates that the uranium deposit has good potential to be economically viable, the final phase of exploration may be warranted. The main objective of this advanced exploration phase is to confirm the economic viability of the uranium deposit, with the ultimate goal of bringing the deposit into uranium production. This phase includes very detailed and intensive geological, geochemical and geophysical investigations to better define the uranium deposit. The level of deposit-scale geological knowledge is typically increased through additional, intensive drilling with a very dense drill hole spacing to increase the density of sampling, and to systematically define the extent, dimensions, and grades of mineralised volumes. High-density sampling is needed to upgrade the confidence level of the existing Mineral Resources to the Indicated and Measured Resources (Figure 3). If geological evidence is sufficient to assume geological and grade continuity between sampling points based on adequately detailed and reliable exploration, it may be possible to define Mineral Resources at the confidence level of Indicated Resources. For the definition of Measured Resources, geological and grade continuity needs to be confirmed between sampling points based on geological evidence that is derived from detailed and reliable exploration [12].

Metallurgical test work is one of the most important activities in the advanced exploration phase to confirm the ability to recover uranium from its hosting minerals and to develop processing flowsheet. Detailed mineralogical characterisation is done as a prelude to metallurgical testing. For these purposes, drill core and/or bulk sampling is carried out to represent all major mineralised types that have been identified within a defined uranium deposit. Comprehensive metallurgical test work is conducted on a bulk sample and/or composited drill core samples in order to develop definitive metallurgical flowsheet. First, laboratory-scale metallurgical test work is carried out to define the process parameters and processing conditions, and to determine process design options. The results of the laboratory-scale tests are then used to define a pilot plant test work program. The flowsheet at conventional uranium mills typically includes radiometric ore sorting, crushing, grinding, leaching (either acid or alkaline leaching), liquid-solid separation, solvent extraction (loading, scrubbing, and stripping), precipitation, drying, calcining and packing. Depending on the mineralogy and chemistry of the uranium deposit, either an acid (usually sulphuric acid) or alkaline (usually sodium carbonate and sodium bicarbonate) leaching agent is used. If the uranium ore contains abundant carbonate minerals, alkaline leaching is used to minimise acid consumption. Along with the leaching agent, an oxidant (such as sodium chlorate, manganese dioxide, or hydrogen peroxide) is added to the leach solution to oxidise U4+ to U6+ in order to maximise the rate of uranium dissolution from the ore to the leach solution.

Pilot plant testing is carried out to define and validate the key process parameters, such as particle size, leaching agent, acid consumption, properties and quantities of oxidant, the effects of reagents, process temperature, acidity, redox potential, contaminants, plant throughput (tonnes/day), mass balance, the rate of uranium recovery (%), and the grade of uranium in concentrate. These results will be used for the process definition, design, and engineering activities.

An Ore Reserve is the economically mineable part of a Measured and/or Indicated Mineral Resource [12]. Therefore, one of the main objectives of the final exploration phase is to develop Ore Reserves from Mineral Resources. This is why an intensive drilling program is typically aimed at upgrading the confidence level of the Inferred Resources to at least the level of Indicated Resources, and preferably to Measured Resources. An Inferred Resource has a lower level of confidence and cannot be converted to an Ore Reserve. An Indicated Mineral Resource has a lower level of confidence than that applying to a Measured Mineral Resource and may only be converted to a Probable Ore Reserve (Figure 3). A Measured Mineral Resource has a higher level of confidence than that applying to either an Indicated Mineral Resource or an Inferred Mineral Resource. A Measured Mineral Resource may be converted to a Proved Ore Reserve or under certain circumstances to a Probable Ore Reserve. Modifying Factors (Figure 3) are used to convert Mineral Resources to Ore Reserves. These Modifying Factors are considerations that include mining, processing, metallurgical, infrastructure, economic, marketing, legal, environmental, social and governmental factors [12]. In other words, Probable and Proven Ore Reserves are identified through the assessment of the foregoing Modifying Factors to demonstrate that the uranium deposit can be mined in an economically viable, environmentally sustainable, and socially acceptable manner. For example, the estimation of dilution and ore loss is an essential mining factor in the definition of an Ore Reserve.

Ore Reserves are defined by the Pre-Feasibility and Feasibility Studies that include the application of Modifying Factors [12]. For example, pilot plant testing provides critical input parameters for the Pre-Feasibility Study and the Feasibility Study. Uranium deposits are typically advanced through these two economic feasibility studies to the uranium mining stage. The purpose of these studies is to assess the economic viability of the uranium deposit. The Pre-Feasibility Study is carried out to demonstrate the economic viability of the uranium deposit based on a variety of economic and engineering studies that support mining and metallurgical considerations. If the outcome of the Pre-Feasibility Study is positive in terms of economic viability, it is typically followed by subsequent studies such as infrastructure investigations, environmental impact assessment, environmental baseline study, mine planning, geotechnical tests, tailings management plan, evaluation of the capital and operating costs, economic analysis, project infrastructure design and reclamation plan, which provide key input parameters for the Feasibility Study that is carried out to confirm the economic viability of the uranium deposit, and to describe how the uranium mine will be built [9]. It should be noted that confidence in the estimate of Inferred Mineral Resources is not sufficient to allow the results of the application of technical and economic parameters to be used for detailed planning in Pre-Feasibility or Feasibility Studies.

Sandstone uranium deposits may be amenable to in situ leach (ISL) mining, depending on water table depth, the appropriate permeability of the host rock, and the confining conditions required for ISL mining operations. In ISL (also known as situ recovery, ISR), uranium is first extracted from sandstone using a leach solution and is then recovered at the surface at the processing facility. ISL is conducted by injecting a uranium-dissolving leach solution (acid or alkaline) along with an oxidant through injection wells down into the uranium deposit below the groundwater table, thereby oxidising, complexing and mobilising the uranium, and then pumping the uranium-bearing solution up through production wells to the surface for further processing and uranium recovery. Once the leach solution is returned to the surface, uranium is recovered using hydrometallurgical techniques. Hydrogeological and leachability studies play an important role when assessing the economic viability of sandstone uranium deposits that have the potential for ISL [9]. Instead of conventional pilot plant testing, pilot well-field testing is conducted for ISL operations to increase technical confidence before ISL mining development.

2.6. Geological Investigations During Uranium Mining and After Decommissioning of a Uranium Mine

Uranium mine development follows an investment decision based on the positive outcome of the Feasibility Study where the economic viability of the uranium deposit has been confirmed. Geological investigations continue during mine development and throughout the life of a uranium mine. For example, resource estimation is an ongoing geological activity throughout the mining period to support long-term uranium production from the mine and processing plant, aimed at growing the Mineral Resource and Ore Reserve base because previously defined Mineral Resources and Ore Reserves are inevitably depleting due to ongoing mining. Therefore, new geological knowledge is typically generated through geological investigations throughout the uranium mining process. Near-mine exploration (also known as brownfield exploration) is a common strategy by many companies as it offers the low-cost opportunity to add uranium resources.

A uranium mine and processing plant are typically located at the same site, or close to each other. Management of radioactive mining waste generated at a uranium mine and processing facility requires a high level of technical and regulatory competence, such as geotechnical, geochemical, environmental, and hydrogeological knowledge [15]. Typical types of radioactive waste generated from uranium mines and processing facilities are waste rock, tailings and contaminated waters. Tailings management facilities require well-designed, long-term geotechnical and geochemical stability, and monitoring throughout the life of a uranium mine and processing plant. Environmental monitoring is an ongoing activity that continues after decommissioning to ensure that uranium mine closure techniques are adequate and that they are functioning as planned [15].

3. Role of Geological Investigations in Siting Activities for Nuclear Power Plants and Nuclear Waste Disposal Facilities

Geological studies play an important role in siting activities for both nuclear power plants and nuclear waste disposal facilities. A significant amount of existing geoscientific data needs to be reviewed and evaluated, and a wide range of new data will also be acquired. These datasets include geographical, topographical, geological, geophysical, geotechnical, hydrogeological, meteorological, hydrological, oceanographical, and environmental data that are typically stored in a geodatabase and analysed in a geographic information system (GIS).

3.1. Deep Geological Repository Concept

It is currently widely recognised that the final disposal in a deep geological repository is the best way to safely manage long-lived intermediate and high-level radioactive waste (including spent nuclear fuel) in the long term [16]. In particular, selection of a final repository site for spent nuclear fuel requires careful consideration of various geological aspects of the candidate site. The final disposal of spent nuclear fuel in a deep geological repository needs to meet the requirements for long-term safety without reliance on active monitoring and management [16]. Therefore, the selection of a suitable host geology is one of the most important factors in deep geological repository programs. Information collected from geological investigations includes structural geology, the degree of fracturing, hydrogeology, geochemistry, lithology and mineralogy.

Geological disposal takes place in a facility that is constructed in tunnels, vaults, or silos in a geological formation at least a few hundred metres below ground level [17]. Because a geological disposal facility is typically designed to receive high-level radioactive waste, including spent nuclear fuel, long-term geological stability and hydrogeological properties need to be well-characterised. However, a geological disposal facility can receive all types of radioactive waste depending on design [17].

The deep geological repository concept (Figure 4) involves both natural and engineered release barriers to isolate and confine the radioactive waste in stable geological formations, allowing the decay of the radionuclides within the waste. Most deep geological repository programs rely upon final disposal at depths of between 250 and 1,000 m to provide a substantial natural barrier [16]. The depth of the disposal facility and the properties of the host rock, such as well-characterised geological stability and low permeability to water, should provide sufficient isolation and confinement of the radionuclides within the disposal facility for a long period of time, typically hundreds of thousands of years. Engineered barriers, such as the disposal container, buffer material, and backfill, are used to prevent the dispersion of radionuclides from the disposal facility into the biosphere. For example, buffer material (such as compressed bentonite) is placed between the host rock and the disposal canister to protect the canister from minor bedrock movements and to prevent the dispersion of radionuclides if a canister gets broken for any reason.

The combination of multiple release barriers enhances the safety of final disposal and ensures that long-term safety does not solely rely on any one component of the disposal system [16]. The periodic safety assessments of the deep geological repository and its environment are used to evaluate the long-term safety of the disposal facility.

Figure 4: The deep geological repository concept. Figure by Posiva Oy.

3.2. Siting Criteria

The selection and evaluation of a suitable site for a nuclear power plant or a nuclear waste disposal facility require the application of both exclusionary and discretionary criteria. In addition to various geological and geographical criteria, also additional criteria involving different communities should be considered. Exclusionary criteria are used to discard sites, and these include factors that preclude the construction of a nuclear power plant or a nuclear waste disposal facility at a location, such as volcanic activity, geological and geotechnical hazards, population density, archaeological, historical heritage and cultural sites, environmentally protected areas, and national parks. For example, insufficient availability of cooling water is a typical exclusionary attribute for potential nuclear power plant sites.

Discretionary criteria are used to compare and rank the candidate sites. Attributes for which there is no adequate or useful data, or that are generally similar across the entire area of interest, are not used for the siting process as they do not show any differences between sites. However, in further site characterisation, additional specific information on all relevant attributes will be acquired, such as hydrogeochemical conditions that potentially have a significant effect on the long-term safety of the deep repository. Furthermore, it is commonly considered that the criteria for siting spent fuel storage facilities (Figure 2) are similar to those for a nuclear power plant.

3.3. Siting Process

The appropriate workflow of the siting process is based on IAEA guidance [17–23], but it should also consider national legislation. The siting process (Figure 5) typically starts with the conceptual planning stage, followed by the area survey stage, including regional-scale screening from numerous sites to one or several sites [19]. The area survey stage commonly consists of desktop studies involving a review of existing information on suitable large or country-wide areas for hosting a repository [18]. The information acquired during the area survey stage should allow a comparison between the areas, screening out areas based on exclusionary and discretionary criteria, and the identification of smaller areas and specific sites that would be suitable for more detailed characterisation. The next steps include the site investigation stage and detailed site characterisation stage (Figure 5).

Figure 5: The main stages in the siting process. The area survey stage covers a country-wide or regional geological assessment, the site investigation stage includes areas based on the survey stage, and the detailed site characterisation stage covers the actual possible sites based on the previous steps. After IAEA [18].

It is noteworthy that the siting of a nuclear waste disposal facility or other nuclear installation is not only a geological or a technical consideration, but it must also include different socio-political aspects and the needs and concerns of people and communities. This consent-based siting approach [24] aims to interact with various communities and prioritise and involve them through the different stages of the siting program. Paying attention to the viewpoints of all interested and affected parties is crucial for the acceptance of the program, enabling also its smooth progress. The consent-based approach should be considered at different siting stages as follows [25]:

  • Stage 1 – Planning and Capacity Building: building relationships, mutual learning, development of a common understanding of waste management;
  • Stage 2 – Site Screening and Assessment: initial criteria for suitable sites, national call for volunteers, community-based additional criteria, and preliminary and detailed assessment;
  • Stage 3 – Negotiation and Implementation: negotiation of agreements with willing and informed host communities with licensing, construction and operation.

3.4. Area Survey and Site Screening Stage

The approach in area survey and site screening is primarily based on avoiding existing bedrock weakness zones (fracture zones and deformation zones) to ensure the seismic and mechanical stability around the facility, as well as to ensure that the majority of the groundwater flow is confined to the major deformation zones, located far from the repository. The initial idea is that the bedrock has a block mosaic structure, and large bedrock blocks are surrounded by major fracture zones. Within these major blocks, smaller blocks can be delineated. Accordingly, the optimal setting would be a block-within-block concept (Figure 6), where a well-defined, large regional block (target area >100 km2) surrounds a smaller block (potential investigation area), embodying the actual site for the facility [22].

Figure 6: Siting strategy based on the block-within-block concept.

The area survey is a regional GIS-based desktop work, focusing on a large area like the whole country, a voluntary municipality area, or some other region of interest, defined by specific preferences. The goal of the area survey is to identify regions and target areas including numerous potential sites. Subsequently, based on these regional results a more detailed site screening is needed to delineate one or several potential sites within the target areas for further evaluation as the outcome [18]. The area survey and site screening stages typically include the following geological and geographical data:

  • Elevation models (LiDAR and other elevation data);
  • Airborne geophysical data (magnetic, electromagnetic, and radiometric);
  • Geological maps;
  • Databases of mineral potential;
  • Databases of natural conservation, groundwater areas, and land use;
  • Base maps;
  • Maps of Quaternary formations;
  • Aerial photos and satellite images;
  • Gravity data;
  • Other data if adequate and relevant.

In practice, by utilising elevation models, geophysical maps, base maps, aerial photos, and satellite images, a lineament interpretation is the initial outcome (if not existing yet), representing the best understanding of the locations of the deformation zones (lower left image in Figure 6). Based on this interpretation, a number of regional target areas and investigation areas within them are delineated.

The next stage considers direct elimination of unfavourable target areas according to mineral potential, geophysical heterogeneity (indicating geological heterogeneity), and a set non-safety-related geological factors including vicinity of population centres, nature preservation and groundwater areas. After this exclusion process, the remaining target areas are classified and ranked according to the size of the block, unambiguity of the block structure, lithological and structural features, topography, exposure rate and geophysical homogeneity.

Subsequently, from each regional block (target area), one or more investigation areas are defined, each being large enough (5-10 km2) to host a repository [22]. An investigation area is bordered by internal faults, which are smaller than the ones defining the target area. The definition of the investigation areas requires more detailed lineament interpretation within the target areas. Typical discretionary criteria for scoring and ranking the investigation areas are the size of the block, topography and relief of the block (flat relief is favourable), faulting and fracturing (intact blocks are preferred; intensive faulting and fracturing are unfavourable), exposure rate (well-exposed is favourable, indicating good investigability), lithological and geophysical homogeneity (homogeneous is favourable), and distance to known historical seismic events (long distance is favourable). Accordingly, the final outcome of the site screening phase is a set of scored and ranked potential investigation sites, with considerations of consent-based aspects.

3.5. Site Investigation Stage

Based on the outcome of site screening, the most favourable areas are selected for site investigations. At this stage, preliminary investigations at the sites are started, including geological mapping, various geophysical ground surveys and drilling. Furthermore, a wide range of drill hole investigations is carried out, including geological and geophysical logging, and hydrogeological, hydrogeochemical, and rock mechanical studies. The objective at this point is to reduce the number of prospective sites to one or a maximum of three sites, each of which would be characterised in more detail during the next and final stage to demonstrate site suitability to host a repository or a nuclear installation [18].

It is notable that in the case of a disposal facility, the nature of the waste, preferences regarding the engineered system, and the characteristics of the geological and surface environment collectively define the disposal concept. Therefore, the whole disposal system will be delineated in more detail during site investigations, providing input for repository design and safety assessment studies [18].

The goal of the site investigation phase and needed geoscientific studies in site investigations are described at a general level in the international guidance [17,18]. The site for a disposal facility shall be characterised at an adequate level of detail to support a general understanding of the site. This shall include the present condition, probable natural evolution, and possible natural events, as well as human plans, and actions in the vicinity that may affect the safety of the facility over the period of interest. It shall also include a specific understanding of the impact on safety of features, events, and processes associated with the site and the facility. Site investigations typically include the following geological considerations [17]:

  • The regional geological setting of a site to provide context and understanding for more local (site-scale) studies;
  • The topographic form of the site and its surrounding region to provide the framework within which the investigations, and the disposal facility are located;
  • The geomorphology of the site as an aid to interpreting the history of the development of the site, the nature of potential hazards, and the surficial distribution of the geological materials present at the site;
  • The nature, distribution, and properties of the soils and any superficial sediments at the site (such as alluvium and regolith);
  • The nature, distribution, and properties of any cover rocks overlying the host formation at and around the site;
  • The nature, distribution, and properties of the host rock formation within which it is proposed to construct the disposal facility;
  • The nature, statistics, and characteristics of the structural geological features present at the site, such as folds, faults, bedding planes, and joints;
  • The nature, extent, distribution, and history of exploitation of mineral deposits and other natural resources within the area (as well as any indicators of potential future exploitation);
  • An assessment and description of the spatial heterogeneity of the geological units present at the site;
  • The geological evolution of the area including the genesis, relative age and nature of fracture filling materials and studies of displacements and movements along discontinuity surfaces;
  • The nature of volcanic activity at the site, from a knowledge of Quaternary and earlier volcanism;
  • The nature of tectonic activity at the site from knowledge of geologically recent fault movements and the aid of seismological monitoring;
  • The nature of geodynamic processes at a site, such as erosion, uplift, and subsidence.

3.6. Site Selection

The site selection process involves evaluating potential sites and ranking them to determine the final selection. The primary objective of this stage is to identify a site that meets all necessary requirements for the planned facility [23]. However, experience from advanced programs has shown that multiple sites may satisfy these requirements, allowing for the consideration of non-technical siting factors as part of the evaluation process [22].

Typically, this stage assesses the findings and conclusions from the site investigation phase regarding the properties and conditions of the candidate sites (see Section 3.5), as well as the feasibility of safe disposal implementation [20]. The suitability of potential sites is evaluated based on factors such as long-term safety, constructability, adaptability, social impact, land use, and environmental considerations, infrastructure, and cost [22].

Candidate sites are compared using ranking criteria, which may include discretionary considerations (see Section 3.2) in addition to non-safety-related factors. Once a site is selected, its suitability must be confirmed through comprehensive site characterisation. This includes completing the derivation of the design basis to support further development [20].

3.7. Site Characterisation Stage

The objective of the site characterisation stage is to confirm the acceptability of the selected site through a comprehensive characterisation process [20]. During this stage, confirmatory site studies are conducted to demonstrate the site’s suitability for final disposal. These studies focus on gathering the information required for safety assessment, engineering design, licensing, and site preparation. Investigation planning specifically addresses the need for additional data, potential data gaps, and any issues that could raise concerns about site suitability [17,22].

During the site characterisation stage, it is also essential to implement a site monitoring program. This program is designed to collect information and long-term data series on the site’s characteristics and processes. The primary objectives of the monitoring program are to establish a baseline understanding of the site, track effects of natural variations such as seasonal changes and fluctuations in rainfall, and assess disturbances related to human activities, including changes in land use or construction activities. Based on the monitoring program, it can be demonstrated that the site conditions remain favourable for long-term safety despite repository construction and operation [21].

Site characterisation methods can be categorised as either intrusive or non-intrusive techniques. Non-intrusive methods include remote sensing, ground geophysical surveying and geological mapping, while intrusive methods involve drilling along with related downhole geophysical and hydrological surveys, measurements, and sampling [21].

When planning site investigations and selecting appropriate methods, several factors are considered, including the geological and ecological characteristics of the site environment. For instance, investigation techniques suitable for a sedimentary rock environment may differ from those used in crystalline bedrock. Additionally, the selection process considers the availability of methods and expertise, as well as any other relevant requirements and constraints. The site characterisation process must be sufficiently detailed to ensure a comprehensive understanding of the site’s characteristics and its long-term evolution [17]. The characterisation studies and the models developed from them play a critical role in assessing long-term safety (see Chapter 4).

4. Role of Geological Investigations in the Assessment of Long-term Safety of Disposal Facilities

The role of geological studies further expands from the site investigations (see Chapter 3) when the overall long-term safety of a disposal facility is concerned. These can be grouped into the following three wider entities:

  1. External processes affecting the site evolution in the future;
  2. Geological studies supporting the understanding of the performance of the disposal facility components;
  3. Geological studies supporting the understanding of the potential radionuclide release and transport from the disposal facility in the case of leaks.

These studies are often referred to as natural analogues [26–28] and they need to be discussed in relation to the individual properties of the disposal system considered, as the engineered barrier system components and site properties vary [29,30]. The need for natural analogue studies arises from the extremely long service lifetime required for the disposal facilities; depending on the waste to be disposed, safety must be assessed beyond 1 million years, and this requires an understanding of geological time scales.

In addition to regional geodynamic processes studied during the site investigations (see Section 3.5), dedicated studies regarding external processes may be required to better understand what will happen to the repository in the future. During the past few decades, driven by the disposal facility developments in countries affected by Quaternary glaciations, analogue studies related to climate forcing and ice-age scenarios [29,30] have been one of the main focus areas, ranging from permafrost studies [31], glaciodynamics, hydrogeology and hydrogeochemistry [32], glacially induced seismicity [33] to potential disturbances caused by meteoric recharge effects for deep groundwater systems [34,35].

Disposal facilities may have both natural and man-made materials as part of the engineered barrier systems. Waste packaging is usually based on metallic containers (copper and/or steel or cast iron) sometimes accompanied by cementitious binders. Underground openings are planned to be sealed with bentonite, other clays, concrete structures, bitumen, and rock materials with variable compositions. Natural materials such as metallic copper, bentonite clay, and rock aggregates have multiple analogous occurrences that can be studied to better understand the stability of these materials in the disposal facilities [29].

Man-made engineered barrier system materials, such as concrete and steel, have multiple industrial and archaeological analogues, but even for them, natural occurrences have been identified [29]. For example, in Jordan, the stability of 2-million-year-old natural cements shows support for endurance over geological time frames [29,36]. Even exotic materials, only considered in some repository designs, such as ceramics and bitumen, occur in the geological formations.

High-level radioactive waste is in multiple forms depending on the type of nuclear power plant and the processing of the spent nuclear fuel, for example as spent nuclear fuel (mainly uranium with fission products), or as vitrified high-level waste (mainly borosilicate glass containing radionuclides). Geological occurrences of natural glasses and uranium deposits have been used to better understand the long-term stability of these materials [37]. In addition to stability, potential leaks from the disposal facilities are considered in safety assessments. This is a topic beyond the scope of this article, but it deserves a mention, as geological investigations have also helped to understand the overall behaviour of radionuclides in the geosphere [37].

As exploration and mining aim to produce the uranium to be used in nuclear power plants, the disposal of spent nuclear fuel aims to do the opposite, i.e., keep the waste stored and away from the biosphere. In some cases, the geological studies originally conducted for discovering uranium deposits have been later developed and utilised to better understand the processes in geological disposal of radioactive waste. One example is Canada’s Cigar Lake, which is one of the world’s largest high-grade uranium deposits. Cigar Lake has been used to communicate overall repository performance, as the uranium deposit is located at repository-relevant depth and it has an even clay halo around it, mimicking the bentonite seals planned to be used around the waste in repositories. In addition, Cigar Lake has served to assess overall stability and dissolution behaviour of spent nuclear fuel [38, 29].

From the vast amount of literature, only a few examples have been mentioned here to showcase the versatility of geological investigations that are needed for long-term safety assessments of disposal facilities. The work is ongoing, and the use of the existing natural analogue literature needs careful screening when used in different programmes [28]. However, there are still plenty of opportunities for further work, both for reducing potential uncertainties in the current long-term safety assessments [37], as well as for use as a tool for further optimisation of them [37,39].

5. Conclusions

Climate change is one of the greatest threats to sustainable development, as many of the impacts of global warming fall disproportionately on poor, vulnerable, and disadvantaged populations [1]. Nuclear energy plays an important role in climate change mitigation efforts, as it is one of the most efficient tools to reduce global greenhouse gas emissions. In the coming decades, significant nuclear growth is required to replace fossil fuels in baseload electricity generation in order to achieve climate goals.

Geological investigations form the basis for the entire nuclear fuel cycle as they are an integral part of uranium exploration for discovering uranium deposits and defining uranium resources to secure the supply of uranium raw material for nuclear fuel. Consequently, nuclear growth requires an increase in the uranium resource base through sustained geological expertise and long-term commitment to uranium exploration and mine development, to ensure an adequate multi-decade supply of uranium to fuel a growing global nuclear fleet.

It is noteworthy that geological investigations play a significant role throughout the nuclear fuel cycle, from the exploration of uranium to the final disposal of spent nuclear fuel. Economic uranium deposits cannot be discovered without a wide variety of geological investigations, a comprehensive understanding of genetic uranium deposit models, and long-term investments in uranium exploration and mine development. Moreover, geological investigations are critical in the final disposal of spent nuclear fuel, as they provide essential parameters for the site selection of a final geological repository and for the evaluation of the long-term safety of the radioactive waste disposal facilities.

Most importantly, geological knowledge and geological investigations are vital to understanding and identifying where economic uranium deposits can be found, and where spent nuclear fuel can be disposed of in a safe, environmentally sustainable, and socially acceptable manner. Consequently, the importance of geological investigations – within the context of the entire nuclear fuel cycle – will increase in the coming decades, as nuclear energy is a key tool to reducing global greenhouse gas emissions and helps countries meet their sustainable development and climate goals.

Funding: This research received no external funding.

Conflicts of Interest: The authors declare no conflict of interest.

References

  1. Intergovernmental Panel on Climate Change (IPCC). Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, 2018, 616 pp. https://doi.org/10.1017/9781009157940.
  2. Intergovernmental Panel on Climate Change (IPCC). Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, Geneva, Switzerland, 2023, 115 pp. https://www.doi.org/10.59327/IPCC/AR6-9789291691647
  3. United Nations Department of Economic and Social Affairs. The Sustainable Development Goals Report: Special Edition, United Nations, New York, United States of America, 2023, 76 pp.
  4. World Nuclear Association (WNA). World Nuclear Power Reactors & Uranium Requirements. Available online: https://world-nuclear.org/information-library/facts-and-figures/world-nuclear-power-reactors-and-uranium-requireme (accessed on 8 April 2025).
  5. OECD Nuclear Energy Agency. Meeting Climate Change Targets: The Role of Nuclear Energy, OECD Publishing, Paris, France, 2022, 49 pp.
  6. International Atomic Energy Agency (IAEA). Climate change and nuclear power 2024: financing nuclear energy in low carbon transitions, Vienna, Austria, 2024, 99 pp.
  7. International Atomic Energy Agency (IAEA). Nuclear energy in climate resilient power systems, Vienna, Austria, 2023, 24 pp.
  8. OECD Nuclear Energy Agency. Uranium 2024: Resources, Production and Demand. A Joint Report Prepared by the OECD Nuclear Energy Agency and the International Atomic Energy Agency, 2025, 603 pp.
  9. International Atomic Energy Agency (IAEA). Uranium Exploration Planning, Management and Practice, Vienna, Austria, 2024, 167 pp.
  10. International Atomic Energy Agency (IAEA). Geological Classification of Uranium Deposits and Description of Selected Examples, IAEA-TECDOC-1842, IAEA, Vienna, Austria, 2018, 417 pp.
  11. Hazen, R.M., Ewing, R.C. and Sverjensky, D.A. Evolution of uranium and thorium minerals, American Mineralogist, 2009, 1293–1311, https://doi.org/10.2138/am.2009.3208
  12. CRIRSCO, International Reporting Template for the public reporting of Exploration Targets, Exploration Results, Mineral Resources and Mineral Reserves, 2019, 78 pp.
  13. Airo, M. Aerogeophysics in Finland 1972-2004: Methods, System Characteristics and Applications, Geological Survey of Finland, Special Paper 39, 2005, 197 pp.
  14. International Atomic Energy Agency (IAEA). Unconformity-related uranium deposits, IAEA-TECDOC-1857, IAEA, Vienna, Austria, 2018, 295 pp.
  15. International Atomic Energy Agency (IAEA). Milestones in the Development of National Infrastructure for the Uranium Production Cycle, Nuclear Energy Series No. NF-G-1.1, IAEA, Vienna, Austria, 2023, 164 pp.
  16. OECD Nuclear Energy Agency. Deep Geological Repositories and Nuclear Liability, NEA No. 7596, 2023, 65 pp.
  17. International Atomic Energy Agency (IAEA). Disposal of Radioactive Waste: Specific Safety Requirements, IAEA Safety Standards Series, SSR-5, IAEA, Vienna, Austria, 2011, 62 p.
  18. International Atomic Energy Agency (IAEA). The Management of Site Investigations for Radioactive Waste Disposal Facilities: IAEA Nuclear Energy Series No. NW-T-1.40. IAEA, Vienna, Austria, 2024, 235 pp.
  19. International Atomic Energy Agency (IAEA). Geological Disposal Facilities for Radioactive Waste: Safety Standards Series No. SSG-14, IAEA, Vienna, Austria, 2011, 104 p.
  20. International Atomic Energy Agency (IAEA). Site Survey and Site Selection for Nuclear Installations: Specific Safety Guide No. SSG-35, IAEA, Vienna, Austria, 2015, 61 pp.
  21. International Atomic Energy Agency (IAEA). Managing siting activities for nuclear power plants, Nuclear Energy Series No. NG-T-3.7, IAEA, Vienna, Austria, 2022, 78 pp.
  22. McEwen, T. and Äikäs, T. The site selection process for a spent fuel repository in Finland – Summary report. Posiva Report 2000-15. Helsinki, Finland: Posiva Oy, 2000, 224 pp.
  23. Saanio, T., Hammershaug, S. and Wanne, T. Development process for the Norwegian National Facility. NND Technical Memorandum, 2021, 31 pp.
  24. Lyons, P.B. Lessons learned from the U.S national strategy – a personal perspective. In: Management of Spent Fuel from Nuclear Power Reactors – Learning from the Past, Enabling the Future. Proceedings of an International Conference Vienna, Austria, 24 – 28 June 2019, 2020, pp. 73 – 81.
  25. S Department of Energy. Consent-based Siting Process for Federal Consolidated Interim Storage of Spent Nuclear Fuel – Exploring the background, fundamentals, roles, and more associated with DOE’s consent-based siting process, 2023, 37 pp.
  26. Miller, W. M., Alexander, W. R., Chapman, N. A., McKinley, I. G. & Smellie, J. A. T. Natural Analogue Studies in the Geological Disposal of Radioactive Wastes. Studies in Environmental Science 57. Amsterdam: Elsevier, 1994, 395 pp.
  27. Miller, W.M., Alexander, W.R., Chapman, N.A., McKinley, I.G. & Smellie, J.A.T. The geological disposal of radioactive wastes and natural analogues: Lessons from nature and archaeology. Waste management series Vol. 2, Oxford, UK: Elsevier Science Ltd. Pergamon, 2000, 332 pp.
  28. Reijonen, H., Alexander, W.R. & Norris, S. Resilience in knowledge management – the case of natural analogues in radioactive waste management. Process Safety and Environmental Protection, 180, 2023, 205-222. https://doi.org/10.1016/j.psep.2023.10.008
  29. Safety Case for the Operating Licence Application – Complementary Considerations (CC). Posiva 2021-02. Posiva Oy, Eurajoki, Finland, 2021.
  30. Alexander, W.R., Reijonen, H.M., Norris, S. Assessing the performance of a radioactive waste repository over geological timescales: past experience and the way forward. Geoenergy, https://doi.org/10.1144/geoenergy2023-046
  31. Stotler, R.L., Frape, S.K., Ruskeeniemi, T., Ahonen, L., Onstott, T. C. & Hobbs, M. Y. Hydrogeochemistry of groundwaters in and below the base of thick permafrost at Lupin, Nunavut, Canada. Journal of Hydrology 373 (1-2), 2009, 80-95.
  32. Claesson Liljedahl, L., Kontula, A., Harper, J., Näslund, J.-O., Selroos, J.-O., Pitkänen, P., Puigdomenech, I., Hobbs, M., Follin, S., Hirschorn, S., Jansson, P., Kennell, L., Marcos, N., Ruskeeniemi, T., Tullborg, E.L., Vidstrand, P. The Greenland Analogue Project: Final report (Technical Report No. TR-14-13). Swedish Nuclear Fuel and Waste Management Co., Stockholm, 2016.
  33. Ojala, A.E.K., Mattila, J., Ruskeeniemi, T., Markovaara-Koivisto, M., Palmu, J-P., Nordbäck, N., Lindberg, A., Sutinen, R., Aaltonen, I., Savunen, J.. Postglacial Faults in Finland – A Review of PGSdyn Project Results. Posiva Report 2019-1, 2019.
  34. Kuusisto, J., Lamminmäki, T., Pitkänen, P., Joutsen, A., Kuusisto, M., Tapiola I. The Saimaa Analogue Project: Final Report. Working Report 2021-04. Posiva Oy, Eurajoki, Finland, 2023, 140 p.
  35. Reijonen, H.M., Al-ani, T., Elminen, T., Kuva, J. Issukka, K., Lamminmäki, R. Fracture smectite in bedrock at Kivetty and Romuvaara sites, Finland as natural analogues for nuclear waste repository buffer. Geoenergy, https://doi.org/10.1144/geoenergy2023-053.
  36. Alexander, W.R. (ed.). A natural analogue study of the Maqarin hyperalkaline groundwaters. I Source term description & thermodynamic testing. Technical Report 91-10. Wettingen, Switzerland: Nagra, 1992.
  37. Alexander, W.R. & Reijonen, H. (Eds). A Catalogue of Analogues for Radioactive Waste Management – an update. GTK research report for RWM,
  38. Cramer, J.J. & Smellie, J.A.T. Final report of the AECL/SKB Cigar Lake Analogue Study. AECL-10851, Pinawa, Canada: Atomic Energy of Canada Ltd (AECL), 1994.
  39. Reijonen, H.M. New insights on natural analogues for bentonites used in geological disposal of radioactive waste – unveiling physical and chemical signatures of bentonites and fracture filling clays. Dissertationes Universitatis Helsingiensis 56/2025. ISBN 978-952-84-0597-9 (print); ISBN 978-952-84-0596-2 (PDF); ISSN 2954-2898 (print); ISSN 2954-2952 (PDF). Unigrafia, Helsinki, 2025, 140 p. http://hdl.handle.net/10138/590234.

This article has been published in European Geologist Journal 59 – UN Sustainable Development Goals – where the geology lies