Introduction

In early 2020s Finland will be the first nation in the world starting the geological disposal of the spent nuclear fuel. The road towards disposal has been systematic and straightforward since 1978 when the disposal studies started, just after the first nuclear power reactors were built. Currently the Loviisa power plant in Southern Finland has two pressurised water reactors (VVER) each producing 510 MW of electricity and first taken into use in 1977. The Olkiluoto plant in Western Finland consists of two Boiling Water Reactors (BWRs) each producing 880 MW of elec-tricity, taken into use in 1979 and 1982. The third reactor of the Olkiluoto plant, a 1600 MW European Pressurized Reactor (EPR) has been under construction since 2005 and will be soon start to operate. Besides these, sixth reactor (latest VVER type) producing 1200 MW of electricity when ready is being built in Northern Finland.

While the new power plants are being built, the geological disposal of the spent nuclear fuel is continuing forward. One of key success factors of disposal of spent nuclear fuel has been the political commitment and the two-phase-app-roach where a second generation underground research laboratory (URL) was built to the disposal site, before applying for the construction license for the final disposal facility. The confirming rock characterization conducted in the ONKALO in Olkiluoto Island, located in Western Finland (Fig. 1), has increased the confidence to the rock properties to that point that it was possible to grant the construction license for the spent nuclear disposal facility.

Fig. 1.

The location of the Olkiluoto site on the western Finland.

In the geological spent nuclear fuel (SNF) disposal concept, knowledge of in situ stresses, excavation damage zone (EDZ), rock mass strength, hydraulic conductivity and other rock mass properties are of high importance for long-term safety and also for the acceptance of the project by public and nuclear safety authorities. To accomplish this, the Finnish nuclear waste disposal projects, managed by Posiva Oy, have initiated research, method development and in situ experiments on these subjects for waste disposal in Finland since Posiva was established in 1995. Also close collaboration with the Swedish Nuclear Fuel and Waste Management Co. (SKB) has benefited the Finnish progamme, not only by knowledge exchange, but also by having access to Äspö Hard Rock Laboratory in Sweden. Also in Sweden the nuclear waste disposal project is heading for the implementation phase in the next decade or so.

Disposal concept

The Swedish and Finnish plan for disposal of spent fuel is based on the multi-barrier KBS-3 concept. In the KBS-3 approach, the spent fuel will be deposited in a cast iron insert surrounded by copper canister with copper lid that is friction stir welded in place (Posiva, 2014). The iron insert provides mechanical strength while the copper canister protects the spent fuel from corrosion. The copper canisters will be disposed at approximately 400–500 m depth in crystalline rock at a selected geological domain. The canisters will be holes surrounded by compacted bentonite clay, and tunnels will be sealed using concrete plugs and backfilled using bentonite and rock mixture.

The reference concept for the nuclear waste disposal is the vertical KBS-3V disposal concept as illustrated in Fig. 2 on the left. The horizontal KBS-3H disposal concept (Fig. 2 on the right) is being developed simultaneously with the vertical concept (Suikkanen et al., 2016). In both disposal concepts, the principle with multiple protective barriers is followed. The barriers include final disposal canister, bentonite buffer, tunnel backfill and bedrock (Saanio et al., 2012).

Fig. 2.

Vertical- and horizontal variants of the KBS-3 deposition concept.

The deposition holes are filled with compressed blocks made from bentonite clay (buffer), and the gaps between blocks and rock surface with bentonite pellets. The purpose of the bentonite buffer is to provide support pressure for the deposition hole (Juvankoski, 2013). The maximum temperature of the bentonite buffer is limited to +100 ºC (Ikonen and Raiko, 2012). The deposition tunnels will be filled with backfill material, which is required to have sufficient swelling pressure to contribute to the stability of the tunnels (Keto et al., 2013). The rock mass surrounding the deposition tunnel is affected by stress-induced excavation damage zone (EDZ_SI) and construction-induced excavation damage zone (EDZ_CI) (Siren et al., 2015b), which decreases the rock strength and increases the hydraulic conductivity of the rock mass.

Finnish progamme structure

The Finnish programme for the disposal for spent nuclear fuel is organized through a private company, Posiva Oy, which has been established by the two power companies, Teollisuuden Voima Oyj (TVO) and Fortum Power and Heat Oy, currently operating nuclear power plants in Finland to dispose owners spent nuclear fuel (see Fig. 3). The repository being build by Posiva will deposit 3,250 canisters that contain 6,500 tons of uranium.

Fig. 3.

Organization of finnish spent nuclear fuel disposal programme. (Courtesy of Posiva Oy).

Third company, Fennovoima Oy, is also currently building a nuclear power plant in Finland. Fennovoima is at the moment studying alternatives for final disposal to be started in 2090’s and Posiva Solutions is assisting them in the project.

First plans for the geological disposal of the spent nuclear fuel started already at 1978 by TVO operating a powerplant constructed by ASEA-Atom. However the power plants operated by Fortum (formely named Imatran Voima) were built by the former Soviet Union and Fortum was able to return the spent nuclear fuel to Russia. This changed in 1995 when Finland joined the European Union and signed the Euratom Treaty, leading to the amendment of the Finnish Nuclear Energy Act banning the export (and import) of spent fuel from 1997 on, and the only remaining option was disposal in the Finnish bedrock. This led the two companies join their forces in disposal and establish a joint venture, Posiva Oy, with mission to take care of the final disposal of spent nuclear fuel of the owners. However both companies still operate their own operational waste repositories for low and medium level nuclear waste.

Timetable

In 1983 the Finnish Government made a policy decision of the main goals and timetable of the management of nuclear waste. According to the decision several sites were to be evaluated by the end of the year 2000. Finally the detailed site investigations were started in 1993 (McEven and Äikäs, 2000).

In the site selection studies several suitable candidate sites were recognised to be suitable for disposal of the spent nuclear fuel. In the site selection studies following items were considered: long-term safety, the feasibility of repository development, the possibility of repository enlar-gement, repository operation, social acceptance, land usage and the environmental burden, infrastructure and costs. Finally there were four candidate sites that were studied in detail. In geological aspect, all of the sites were considered to be suitable for disposal of spent nuclear fuel, however especially the infrastructure, transport costs and social acceptance were better in sites with existing nuclear facilities – Olkiluoto and Loviisa. Based on these aspects, the Finnish Parlament made a decision-in-principle that the spent nuclear fuel shall be disposed in Olkiluoto. The decision was almost unianimous, 159 parlament members voted for the decision and only 3 members opposed. The result is politically encouraging since for example tradition-ally the Green League (with 11 parlament members in 2001) generally are against nuclear energy. The decision is a statement that no political party is willing to leave any possible waste problems for the future generations to handle, although the political parties strongly disagree on nuclear energy.

In 2004, Posiva Oy started to build ONKALO rock characterization facility in the Olkiluoto Island. After years of confirming research, at the end of 2012, Posiva Oy submitted an application for construction licence for a final disposal facility. Based on the application and their constant review of the project, the Finnish Radiation and Nuclear Safety Authority stated in February 2015 that final disposal facility can be built to be safe (STUK, 2015). Finally in November 12 2015 the Finnish Government granted a licence to Posiva for the construction of world’s first final disposal facility for spent nuclear fuel (TEM, 2015). The decision made Finland an international pioneer in nuclear waste management and provides commercial opportunities in developing nuclear waste management in other countries.

The main goal of the Government decision in 1983 is to start the disposal of spent fuel in 2020s. Generally this has been interpreted to mean the beginning of the decade and most probably the disposal will commence in 2024. After the disposal facilities are ready an operation license appli-cation will be applied. Cooling time depends on the burnup of the spent fuel and will be ca. 30 to 40 years. This means that the facility will be open while nuclear power is used and 30 to 40 years after the closure of the nuclear reactors. The current estimate is that the facility will operate for 120 years before it is closed. The general timeline is presented in Fig. 4.

Fig. 4.

Timeline for the disposal of the finnish nuclear waste (Courtesy of Posiva Oy).

Confirming site studies in ONKALO

The site selection phase suggested that an underground research laboratory (URL) should be build to study local the hydraulic heads, hydrochemistry and the properties of the bedrock. The special issues at Olkiluoto were the presence and significance of saline water at depth and groundwater flow. The underground research laboratory was designed to study the effects of repository construction and to provide confirmation of the understanding of the site (McEven and Äikäs, 2000). One example in designing the URL has been Äspö Hard Rock Laboratory (HRL) in Sweden, being build already at 1990s, and served as a place for testing in situ properties of rock and testing disposal concept. Although the Äspö HRL was not designed as to ever host spent fuel, the rock conditions are suitable for disposal.

Based on the example, in 2004, Posiva Oy started to build ONKALO rock characterization facility (Fig. 5). The ONKALO consists of an access tunnel and three shafts that have been excavated to anticipated repository level. The access tunnel has been excavated by using the drill-and-blast method to -457 m depth. Three shafts (one personnel shaft Ø4.5 m and two ventilation shafts Ø 3.5 m) have been raise bored to the depth of -450 m. At level -437 m Technical facilities have been excavated and at a depth of -420 m the demonstration tunnels, where the current main research focus is concentrated. In the Demonstration tunnels the disposal concept components and other large scale tests are carried out and the methodology for locating suitable rock volumes is demonstrated.

Fig. 5.

ONKALO underground facility from SW direction (Courtesy of Posiva Oy).

Geology

Geology in Olkiluoto is complex, characterised by crystalline bedrock dominated by migmatitic, foliated gneiss with some brittle deformation zones crossing the repository area. Massive, coarse-grained pegmatitic granites also occur as dykes in the area (see Fig. 6). The bedrock has gone through a multiphase ductile deformation which is evidenced in the structure and texture of the migmatites, which are strongly controlled by the metamorphic environ-ment, past tectonic stress regimes and by the rheological properties and composition of different bedrock units. A total of four separate deformation phases have been identified from the Olkiluoto and each phase is characterised by the unique deformation and migmatite structures and by the intensities of the deformation phases. The fault zones in Olkiluoto are mainly SE-dipping thrust faults or NE-SW striking strike-slip faults, the low-angle SE dipping zones also being the main hydraulically conductive zone. All in all, the bedrock in Olkiluoto is very heterogeneous and anisotropic both in the small- and medium-scale (Aaltonen et al., 2016).

Fig. 6.

Bedrock map of Olkiluoto (Posiva, 2013).

The detailed geotechnical and geological tunnel mapping has frequently stopped the tunnel excavation, making the excavation time long, however providing never seen set of tunnel mapping data. There are over 100,000 mapped fractures in the Posiva’s research data base, with full mapping data and accurately measured trace (Fig. 7). The average rock mass quality has in general been good or very good after first 130 m depth (Johansson et al., 2015).

Fig. 7.

Digitized tunnel mapping data from ONKALO (Courtesy of Posiva Oy).

Rock mechanics

The complicated geology reflects to the rock mechanics of Olkiluoto. The rock stress field in Olkiluoto is affected and rotated by large brittle deformation zones (Valli et al., 2016), and to overcome the problem of measuring stresses a new LVDT-cell stress measurement technique was taken in use in ONKALO. The results are very promising in that it can reliably measure the local rock stresses even in very complex and heterogeneous geology. (Hakala et al., 2012).

The in situ rock strength has been investigated in a series of Posiva’s Olkiluoto Spalling Experiments (POSE) reported in detail by Johansson et al. (2014), Valli et al. (2014) and Siren et al. (2015a). In the in situ experiment it was concluded that the rock strength is largely defined by the heterogeneous geology, because the weakest parts of the rock fail in structurally controlled (see Fig. 8) manner (Siren et al., 2015a). The mean rock UCS strength is 108 MPa, however it ranges from 40 MPa to 180 MPa. After excavation new fractures have been created subcritically at the repository level, however no brittle spalling type failures has been observed.

Fig. 8.

One observed structurally controlled shear failure in disposal hole perimeter under compressive stress field. The dotted line indicates crack growth in the ONK-EH1 experiment hole that occurred sometime after boring of the hole and before boring of the second experiment hole. The crack growth was observed on the 2nd of July 2010.

The observed discontinious failure pattern causes a discontinious flow path that reduces the hydraulic conduc-tivity from the disposal facility to the ground surface. Continuous spalling formations could create radionuclide transport routes in situations in which nuclear waste canisters are damaged.

Based on the experimental results, the vertical disposal concept (KBS-3V) suffers from initiation of new fractures sometime after excavation. However, according to the calculations, the horizontal disposal concept suffers from no fracture initiation when the tunnels are excavated; however, approximately 60 years after commencement of the disposal, both concepts will be prone to fracture initiation. Neither of the disposal concepts is expected to suffer from any major rock mass failure. This stems from the secondary stresses being well below the rock mass strength (Siren, 2015).

Based on the modelling, the vertical disposal concept is not particularly sensitive if the trend of the tunnels is within 30° of the major principal stress direction. In overall the rock mechanics research conducted during last 10 years reveal that, the rock damage in Olkiluoto is modest and rock is well-suited for nuclear waste disposal (Siren, 2015).

Hydrogeology

The groundwater flow is one of the most important research areas for the long-term safety of the repository. The groundwater flow is a significant potential pathway for transport of components which may be harmful for containment and for radionuclides that are released from the spent fuel. Groundwater also affects to the evolution of the groundwater composition, possibly being harmful for the engineering barrier (EBS) and canister. The hydraulic properties in large depend on connectivity of the fractures through the repository space. The transmissivity of fractures intersecting boreholes has also been measured by Posiva’s Flow Log (PFL) tool. While excavating the tunnels of ONKALO, the hydraulic head of different sections of surface boreholes have been monitored. Based on this it has been possible to characterize the transmissivity of the bedrock. Also the disturbance of tunnel excavation has been monitored during construction. Particularly interesting sub-vertical zones HZ20A and OL-BFZ020A are crossing the access tunnel of ONKALO and shafts at depth of approximately -300 m. The zones have significantly higher transmissivity (in scale of 10^-5 m²/s) compared to the adjanced bedrock (Fig. 9). The total groundwater inflow to whole ONKALO facility was only 32 l/min in 2016 (Posiva, 2017).

Fig. 9.

Transmissivity of brittle deformation zone at disposal site. Boreholes shown in the figure is from the surface down to ca. 1 km (Modified after Posiva, 2013).

Geochemistry

The prevailing groundwater composition affects to the performance of the engineered barrier system system and the solubility of the radionuclides. Therefore the ground-water composition has been studied in detail in ONKALO. The groundwater compositions at different time periods are based on the groundwater flow modelling and the general understanding of the geochemical evolution at the site. The groundwater at repository depth is brackish and saline, and dilution with time due to infiltration of meteoric water is modelled when the safety of repository is assessed for 100,000 years. An interpretation of history and current composition of Olkiluoto bedrock geochemistry is presented in Fig. 10 (Hellä et al., 2014).

Fig. 10.

Schematic representation of interpreted history and current composition of Olkiluoto bedrock geochemistry since the last glacial period. Dashed lines implies minor mixing of groundwaters (Posiva, 2013).

Conclusions

Posiva Oy has developed it’s expertise for decades aiming for their sole goal of safe disposal of spent nuclear fuel. The Finnish spent nuclear fuel programme is proceeding determinedly towards the start of disposal in 2020s. So far the programme has been following the milestones set in 1983. The construction of the on site rock characterization facility ONKALO has significantly increased the site specific knowledge. The ONKALO has enabled direct in situ measurements, experiments and testing of different investigation tools underground. In addition to the in situ tests, all the observations made during the ONKALO excavations and constructions have also provided valuable information for the rock mass property characterisation to build up more reliable site descriptive models. The ongoing and future tests will confirm the elements of the final disposal concept and will demonstrate the design, construction and host rock suitability assessment process (RSC) to be utilized during the final disposal. All that information is essential for the repository design and for the safety analyses. The ONKALO facility plays an important role in Posiva’s repository development programme.

The Radiation and Nuclear Safety Authority evaluated in 2015 that the facility for spent nuclear fuel can be built to be safe (STUK, 2015). Later in 2015 Finnish Parlament decided to grant a Construction license for the final disposal facility (TEM, 2015). The decision made Finland an inter-national pioneer in nuclear waste management and provides commercial opportunities in developing nuclear waste management and construction of responsible and safe repository in other countries.