Techno-Economic Comparative Assessment of Geological Disposal of CO₂ and Nuclear Waste

Dalia Streimikiene
Complex Energy Research, Lithuanian Energy Institute, Lithuania

Introduction

Climate change mitigation options covers a broad portfolio of strategies to reduce carbon emissions via carbon capture and storage, enhanced efficiency of power generation and use, application of low-carbon fuels, use of renewable energy sources and nuclear energy. Recent discussions on pots-Kyoto GHG emission reduction strategies and energy options have raised the increasing role of nuclear power in enhancing energy security and mitigating climate change together with concerns over disposal of nuclear waste. Simultaneously, increasing interest in carbon capture and the geological storage of CO₂ has brought up a range of questions as well. Except for a few sporadic efforts dealing with selected topics, no systematic comparison has been prepared so far concerning the issues involved in the geological storage of CO₂ and nuclear waste. Carbon capture technologies are expected to mature and become commercially available over the next decade or so. Yet there are many open issues related to the geological disposal of the captured CO₂. On a life-cycle basis, nuclear power is a low-carbon technology but the industry needs to take care of the high-level waste generated and deposit them in stable geological formations. The issues involved in the geological storage of these two profoundly different materials involve, of course, many differences, but a significantly large number of similarities that warrant a comparative assessment. The specific objective of the presentation is to review the state-of-the-art in these two fields focusing on in-depth comparative assessment of the similarities and differences of the geological disposal of CO₂ and nuclear waste and to identify the already resolved issues and the remaining key challenges based on comparative case study for Lithuania. The main tasks to achieve this objective are:

Evaluation of CO₂ storage potential in Lithuania

The territory of Lithuania contains a number of different formations. However, the prospective media for CO₂ storage should meet certain requirements, among which the most important are the large volume of the reservoir, suitable depth and temperature, and presence of a reliable seal (including structural tightness). Deep saline aquifers are by far the most popular proposal for large-scale CO₂ storage. These are water-saturated porous layers in the subsurface of sandstone or limestone, at present not used for any other purpose. Only two large aquifers of Lithuania meet requirements for geological CO₂ storage, i.e. the Lower-Middle Devonian (Pärnu-Kemeri formations) and Middle Cambrian aquifers buried to depths exceeding 800 m in the central and western parts of the Baltic basin [1]. The thickness of Cambrian aquifer is in the range of 20-70 m [1, 2]. Due to considerable variations in depth and temperature, the porosity of sandstones changes drastically across the basin, from 20-30% in the northern and eastern shallow part of the basin to less than 5% in the central and western parts of the basin [3]. The Middle Cambrian aquifer is sealed by a thick (500-900 m) shaly package of Ordovician-Silurian age representing a reliable seal rock. The Pärnu-Kemeri aquifer is distributed in the central part of the basin. Its depth exceeds 800 m only in West Lithuania. The aquifer is composed of arkosic sandstones containing siltstone and shaly layers. [4]. Total thickness of the aquifer varies from 100 to 160 m in West Lithuania. There are 3 potential geological aquifer structures in South-West of Lithuania: Vaskai 8.7 Mt), Syderiai (21.5 Mt), D11 (11.3 Mt) which can store totally 41.5 Mt of CO₂ [18]. The average costs of CO₂ geological disposal in geological structures makes 8.2 EUR/t CO₂ [5]. The average costs of CO₂ storage in such formations in Lithuania would make more than 3 billion EUR (41.5 Mt*8.2 EUR/tCO₂=3.4 bilion EUR). The main characteristics of these geological structures are presented in Table 1 bellow.

Table 1. The main characteristics of geological structures in Lithuania [6]

The solubility trapping is not restricted to particular structures. The solubility of CO₂ in Cambrian formation water varies from 25-30 kg/m³ in West Lithuania to 40-50 kg/m³ and in East Lithuania. The Pärnu-Kemeri CO₂ solubility ranges from 36 kg/m³ in the deep part of the basin to 60 kg/m³ in the shallow periphery of the basin. The CO₂ storage potential changes westwards from 0.4 Mt/km2 to 0.05 Mt/km². The calculated total solubility trapping capacity is as high as 11 Gt of CO₂ within the area of the supercritical state of the carbon dioxide [7]. The Pärnu-Kemeri aquifer is characterised by better reservoir properties, but has a smaller area of extent than the Middle Cambrian reservoir. CO₂ solubility ranges from 28 kg/m³ in the deep part of the basin to 41 kg/m³ in the shallow periphery of the basin. In West Lithuania the storage capacity of the reservoir is about one Mt of CO₂ in one km² area. The total onshore potential of this formation is estimated as high as one Gt of CO₂. In Table 2 the main characteristics of Cambrian reservoir for CO₂ solubility is presented.

Table 2. The main characteristics of Cambrian reservoir for CO₂ solubility in Lithuania [7]

The calculated total solubility trapping capacity in Lithuania is 5.5 Gt. The Piarnu-Kemeri aquifer onshore storage potential is 3 Gt of CO₂. The Cambrian aquifer onshore storage potential is 2.5 Gt [6]. The Middle Cambrian reservoir comprises quartz sandstones that are practically not reactive to carbon dioxide. The Pärnu-Kemeri sandstones contain clay admixture (up to 10%) and feldspar grains (up to 15%). Therefore they have a potential for permanent immobilisation of carbon dioxide in mineral form. Assuming the rock capacity of 10 kg/m³, the sequestration potential for mineral trapping can reach 5.6 Gt CO₂ (onshore) [2]. Mineral trapping is also possible in ultramafic rock. Roughly, the assumed ratio of immobilized CO₂ to serpentinite is assumed 1:2. The estimated volume of serpentinites of the largest Varena Iron Ore Deposit is 1–2 Gt. Consequently, the sequestration potential in ultramafic rock is evaluated to be as high as 0.5–1 Gt. In Lithuania, ten oil fields are presently exploited. The size of oil fields ranges from 16,000 tonnes to 1,400,000 tonnes of the recoverable oil. The storage potential of the largest oil fields in West Lithuania reaches 2 Mt of CO₂. The total potential in Lithuania is estimated at 7.6 Mt of CO₂. Table 3 presents the capacities of CO₂ storage in the main oil fields in Lithuania.

Table 3. Capacities of CO₂ storage in the main oil fields in Lithuania [6]

No coal seams exist in the Baltic area, but thin lignite layers have been identified in Jurassic succession of Lithuania. Salt has accumulated in the Zechstein lagoon in the Kaliningrad district, while only one small salt pillow is found in Southwestern Lithuania [1]. The total CO₂ storage in Lithuania makes 12.21 Gt. however the current well developed CO₂ storage technologies such as structural trapping has very low potential in Lithuania – 41.5 Mt. The analysis of CO₂ storage potential in clayey formations by mineral trapping is necessary seeking to compare with possible nuclear waste storage facilities in Lithuania.

Analysis of spent nuclear fuel geological storage options in Lithuania

It has been universally acknowledged that in terms of environment protection the only cohesive and safe way of final disposal of spent nuclear fuel and other long-lived high-level radioactive waste is in deep geological repositories. The final disposal of this type of waste in deep geological repositories may be justified by the usage of stable geological environment. Radioactive waste is isolated by several passive barriers that reinforce and complement one another. Therefore the repositories for disposal of spent nuclear fuel and high-level waste generally rely on a multi-barrier system to isolate waste from the biosphere. The multi-barrier system usually comprises a natural geological barrier and engineered barrier system [8]. The host rock types currently under investigation are salts (in either salt domes or bedded formations), granite and similar crystalline rocks, argillaceous rocks, tuff and basalt. The most investigated host rocks to date are crystalline rocks.
The overview of the geological structure of Lithuania was carried out for feasibility studies in terms of suitability for a deep geological repository. Several major candidates of geological media (clayey formations, rock salt and anhydrite formations, crystalline basement rocks) were selected for future considerations (Table 4).

Table 4. Potential geological formations for spent nuclear fuel geological disposal in Lithuania [9]

As a result of the overview, four clayey formations perspective as a host rock were distinguished in the sedimentary cover of Lithuania. They are represented by the Lower Cambrian, Lower Silurian, Middle Devonian and the Lower Triassic sequences. The most perspective from clayey formations is Lower Triassic sequences. The Lower Cambrian Formation occurs only in the eastern part of Lithuania (proximity to the INPP region). Lower Triassic sequence occurs only in southwest Lithuania (quite far from the INPP) [10]. In Lithuania there are quite large blocs of crystalline basement not much affected by tectonic processes lying at a depth suitable for a deep geological repository to construct. Such blocs are a promising location for constructing a deep geological repository. In Lithuania the crystalline basement is covered by sediments the thickness of which varies from 200-300 metres in south-east Lithuania to 2000 metres in the Baltic seacoast. With the use of the state-of-the-art technologies, it is possible to construct shafts in rocks of crystalline basement and to bore 150-500 metres long tunnels at a depth of some 500 metres in which containers with spent fuel may be placed. All gaps between the copper containers with fuel and the rocks would be loaded by special impervious clay (bentonite). Clayey rocks are also a promising medium since clay is quite impervious, distinguished by its properties of sorption. Clay, however, is not so steady and less stable than rocks of a crystal base. Therefore, to construct a repository in the medium of clay would be a much harder task than to do so in the rocks of crystalline basement however clay is considerably less permeable than crystalline basement rocks, therefore it would contain radionuclides better. As it is less stable the construction of a repository in clay is much more difficult than in crystalline basement rock. According to preliminary estimates, the repository for disposal of spent nuclear fuel from Ignalina NPP in Lithuania would cost about 3 bill. EUR. The costs of construction of spent nuclear fuel repository for new NPP is still not clear but the possible cost rages from 67,000/m³ and £201,000/m³ of spent nuclear waste and amounts to 14 billion EUR. Therefore two geological media: clayey formations and crystalline basement rocks merit further studies [11; 12].

Methodological and conceptual similarity of CO₂ and nuclear waste geological disposal

Lithuania currently faces two major challenges in energy sector: final closure of Ignalina Nuclear Power plant (Ignalina NPP) in the end of 2009 and nuclear waste disposal and climate change mitigation issues having in mind replacement of nuclear capacities with fossil one and anticipated increase in GHG emissions. Lithuania has two options: to construct new nuclear power plant also taking into account nuclear waste disposal issue or to burn fossil fuel by applying carbon capture and storage (CCS). The construction of new nuclear PP in Lithuania would allow to reduce GHG emissions by 4-5 Mt/year [13]. The expected life-time of new NPP is about 50 years. These 2 options need to be investigated in Lithuania. The comparison of geological carbon and nuclear waste storage options in Lithuania would allow to make recommendations for GHG mitigation and energy options in post-Kyoto period. In Table 5 the comparison of CO₂ and nuclear waste geological disposal options in Lithuania is presented based on analysis of these back- end technologies conducted above.

Table 5. Comparison of CO₂ and nuclear waste geological disposal options in Lithuania

As one see from information provided in Table 5 the methodological and conceptual similarity of CO₂ and radioactiove waste geological disposal enables to use experience, knowledge and know-how acquired during several years of research in radioactive waste disposal in order to be used in the field of CO₂ disposal as this back-end technology is completely new and need to be elaborated further seeking to make decision in selecting GHG emission reduction options in Lithuania. Comparison of two back-end energy technologies based on several technical, safety, economic and environmental characteristics in Lithuania revealed that CO₂ storage in geological media by structural trapping which has many similarities with spent nuclear fuel geological disposal has very low GHG emission reduction potential comparing with new NPP construction in Lithuania therefore other CO₂ storage options, for example mineral trapping in clayey formation needs to be investigated further and compared with spent nuclear fuel storage alternatives for example in clayey formations etc. The costs presented in Table 5 are only indicative and do not include transportation and other handling costs.

Conclusion

1. Geological and hydrogeological investigations performed in Lithuania revealed that crystalline rocks and clayey geological formations are most promising for the geological disposal of spent nuclear fuel in Lithuania. These two geological formations can also be applied for CO₂ geological storage as CO₂ storage in deep saline aquifers through mineral trapping is possible.
2. Total CO₂ storage potential in Lithuania amounts to about 11.21 Gt however the current well developed CO₂ storage technologies such as structural trapping has very low potential in Lithuania – 41.5 Mt.
3. The geological disposal of CO₂ and spent nuclear fuel have methodological and conceptual similarities. Potential geological formations for both disposal formations in Lithuania are Middle Devonian and Cambrian. Crystaline host rocks are potential geological media for both disposal facilities. The required depth of disposal facilities, thickness of geological formations and time life is also similar. Radioactive waste disposal requires multibarier system and metal capsules for it’s disposal and CO₂ can be disposed directly.
4. The safety requirements for both storage facilities is related with tectonic and rocks stability however for nuclear waste disposal the main safety concern is related with radionuclides escape from deep geological repository and for CO₂ geological disposal the CO₂ leakage is the main issue.

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