Olkiluoto Site Research Articles

Analysis and determination of the stress field at the Olkiluoto site

Hydraulic, overcoring and LVDT stress measurements have been conducted for characterizing the in situ stress state at the spent nuclear fuel repository at Olkiluoto site, located in Western Finland. The effect of foliation on the stress measurement data was investigated. The analysis of the hydraulic data conducted in foliated rock types shows that the hydraulically-induced fracture propagates along a foliation plane rather than developing along the true maximum horizontal stress direction as dictated by the theory of hydraulic fracturing. An integration of the stress measurement data considered to be reliable was conducted for establishing the most likely stress field and confidence intervals for the large (site) and small scales. At the depth of the planned repository, the stress magnitudes obtained for the two scales were found to be similar, thus concluding that the confidence in the derived stresses is high. The mean magnitudes of the major, intermediate and minor principal stresses are 22, 18, and 11 MPa, respectively. The estimated orientation of the major principal stress (WNW-ESE) agrees with the regional stress state.

Through diffusion experiments to study the diffusion and sorption of HTO, 36Cl, 133Ba and 134Cs in crystalline rock

The spent nuclear fuel in Finland will be deposited in crystalline granitic rock in Olkiluoto, Finland. As a part of the safety assessment of the repository, series of extensive in-situ sorption and diffusion experiments and supplementary laboratory work has been done in the Olkiluoto site. Through Diffusion Experiment in a laboratory (TDElab) aims to provide applicable data for the ongoing in-situ experiment in Olkiluoto. This laboratory scale experiment resembles the in-situ experiment and aims to gain information on possible effects in values of distribution coefficients, effective diffusion coefficient and porosity that are caused by differences in laboratory and in-situ conditions. The through diffusion and sorption of tracer solution with known activities of HTO, 36Cl, 133Ba and 134Cs were studied in a decimeter scale sample of veined gneiss, which is one of the main rock types in Olkiluoto. The measured breakthrough curves were modeled taking into account the porosity of the rock and diffusion and sorption of the radionuclides using Time-Domain Random Walk (TDRW) simulations. The porosities of 0.7–0.8% were determined for the rock and effective diffusion coefficients of (3.5 ± 1.0) × 10−13 m2/s and (3.0 ± 1.0) × 10−13 m2/s were determined for HTO and 36Cl, respectively. The porosity and effective diffusion coefficients were found to be in agreement with previous results for veined gneiss. Furthermore, distribution coefficients of (1.0 ± 0.3) × 10−4 m3/kg and (2.0 ± 0.5) × 10−3 m3/kg were determined for 133Ba and 134Cs, respectively, using information about the effective diffusion coefficient determined for HTO. The distribution coefficients were found to be significantly smaller than the ones determined for crushed rock in previous studies and slightly smaller than the ones from previous in-diffusion experiments.

The scaling effect on rock behaviour by comparing compressional wave velocities measured in the lab and field

Abstract. A careful comparison of rock behaviour at different scales is essential to determine if laboratory experiments can be useful for understanding the field scale. This research aims to investigate the scaling effect on rock behaviour by comparing P-wave velocities passing through rock at different scales in field and laboratory experiments. Field P-wave velocity was measured at specific ray paths in a fairly homogenous pegmatitic zone located at the third experimental hole ONK-EH3 of the POSE tunnel at OLKILUOTO site, Finland. Three true-triaxial experiments were carried out on pegmatite samples taken from the POSE niche to duplicate the stress path that was believed to exist at the most critical region around the ONK-EH3. Ultrasonic surveys were carried out during these experiments. Initially, reproducibility of the true-triaxial experiments was investigated and a good match was observed among the three samples. The evolution of the stress tensor around the ONK-EH3 was calculated and used in an ellipsoidal model to estimate P-wave velocities for all the points located on the ray paths in the field. Calculated field P-wave velocities along the ray paths were in reasonable agreement with those measured in the field. A linear trend was observed between absolute error per unit length of a ray path and it's average distance to the highest stress zone. This study corroborates the stress path dependency of rock behavior at different scales and the usefulness of the true-triaxial experiments in this regards.

Helium in Porewater and Rocks of Crystalline Bedrock from the Fennoscandian Shield, Olkiluoto (Finland)

Dissolved noble gases in matrix porewaters of low-permeable crystalline rocks are useful tracers for investigating porewater migration and interactions between porewater and groundwater. Samples from the 1160m deep OL-KR56 borehole, drilled at the Olkiluoto site (southwestern Finland), have been analysed for concentrations and isotope signatures of helium in porewater, groundwater and in the rock matrix. These are the first 3He/4He ratios combined with 4He concentrations in porewaters and rocks from the Fennoscandian Shield. High 4He porewater concentrations suggest long porewater residence times, while concentration differences of one order of magnitude between porewater and groundwater indicate a transient state between these reservoirs. Concentrations of 4He in porewater show small variations with depth, which are mimicked in the concentrations of conservative chemical tracers as well as in 4He concentrations of groundwaters from various regional boreholes. 3He/4He ratios in porewaters are higher than in the rock matrix, requiring either fractionation during redistribution of He between these reservoirs or the addition of an isotopically light component.

Porewater chemistry in compacted bentonite: Application to the engineered buffer barrier at the Olkiluoto site

Compacted bentonite is used as sealing and buffer material in engineered barrier systems (EBS) of high-level radioactive waste repositories. The chemical characteristics of this clay and its porewater affect the migration of radionuclides eventually released from the waste. They also determine the integrity and long-term performance of the clay barriers. Key features are the structural negative charge and the large proportion of structural (interlayer) water of the main mineral montmorillonite, which leads to exclusion of anions and a surplus of cations in a large part of the porosity space. The objective of this contribution was to assess the impact of different porosity model concepts on porewater chemistry in compacted bentonite in the context of the planned Finnish spent nuclear fuel repository at Olkiluoto. First, a structural model based on well-established crystallographic and electrostatic considerations was set up to estimate the fractions of the different porosity types. In view of the uncertainty related to the chemical properties of the interlayer water, two very different model concepts (anion-free interlayer, Donnan space), together with a well-established thermodynamic model for bentonite, were applied to derive the porewater composition of the bentonite buffer at Olkiluoto. The simulations indicate very similar results in the “free” water composition for the two models and thus support the validity of the reference porewater concept commonly used in performance assessment of waste repositories. Differences between the models are evident in the composition of the water affected by the surface charge (i.e. diffuse double layer and interlayer). These reflect the conceptual uncertainty in current multi-porosity diffusion models.

Evaluation of two conceptual approaches for groundwater flow simulation for a rock domain at the block-scale for the Olkiluoto site, Finland

To evaluate conceptual approaches for simulating a rock domain, which is a rock unit outside a fracture zone, the block-scale groundwater flow system of the radioactive waste disposal site in Olkiluoto, Finland, was simulated using two approaches: the equivalent porous medium (EPM) approach and the hybrid approach. For the EPM approach, the rock domain was considered to be a homogeneous continuum with an equivalent hydraulic conductivity, while in the hybrid approach, it was conceptualized as a heterogeneous continuum from the stochastically generated background fracture network. In both approaches, several large fracture zones were included in the simulation models, which were calibrated by comparing the simulated hydraulic heads to the observed ones. The results showed that the simulated heads were in good agreement with the observed ones in both approaches, although the root mean square errors of the EPM approach were smaller than those of the hybrid approach. The estimated groundwater flow pathways in the hybrid approach generally had more complex geometries and longer travel times than those from the EPM approach. The simulations were evaluated by comparing the simulated flow rates at the boreholes in the site to the observed ones from the flow loggings, and the results show that the hybrid approach reproduced the observations more similarly than did the EPM approach.

Complementary considerations in the safety case for the deep repository at Olkiluoto, Finland: support from natural analogues

A report entitled “complementary considerations” has recently been published by Posiva, the organization implementing the spent fuel disposal programme in Finland. It is part of the documentation (called TURVA-2012 safety case) submitted in 2012 for the construction license for a deep geological repository at the Olkiluoto site in Finland. The complementary considerations report addresses diverse and less quantifiable types of evidence and arguments for long-term safety which enhance confidence in the outcome of the safety assessment, especially for times greater than a few thousand years. Natural analogues form the core of the “complementary considerations” with the focus very much on geological occurrences of materials and/or processes which mirror those in the repository. For example, native copper is found in fractures in the Fennoscandian Shield and an understanding of its persistence would be of use when assessing the likely lifetime of the copper canister which surrounds the spent fuel. Based on the outcome of this Olkiluoto site specific report, research topics have been identified in relation to processes affecting the performance and future behaviour of the engineered barrier system materials, especially in relation to corrosion of copper and iron (used in waste container outer shell and insert, respectively), alteration and deformation of the bentonite buffer (clay used in the voids between the waste container and bedrock) and interaction with cementitious materials (tunnel supports etc.) “complementary considerations” is not the sole user of natural analogue information within the safety case, but its main aim is to allow thorough discussion and provide background that can be referred to in other safety case documentation, such as process descriptions, assessment of repository performance etc.

Reactive transport modelling of iron–bentonite interaction within the KBS-3H disposal concept: the Olkiluoto site as a case study

Abstract The interaction of steel with bentonite used as buffer material in high-level waste repositories may result in changes to the properties of the buffer. One of the repository designs (KBS-3H) developed by Posiva and SKB foresees the horizontal emplacement of so-called supercontainers, consisting of copper canisters surrounded by compacted bentonite and an outer perforated steel shell. The corrosion of the steel shell and the interaction of iron with the clay may impair the long-term safety functions of the buffer. The corrosion and iron–clay interaction processes within the KBS-3H concept were assessed with a kinetically based reactive transport model and a comprehensive thermodynamic database. The large uncertainty related to precipitation rates of corrosion products and iron silicates was considered by defining a series of test cases. The results generally indicate a limited effect on the stability of montmorillonite, thus affecting only a few centimetres next to the iron source. Upon complete corrosion only insignificant changes are predicted. These results are explained by (i) the diffusional constraint of mass transfer, (ii) low solubility of corrosion products and (iii) slow transformation kinetics of montmorillonite. Model results further suggest that the largest impact arises from ‘indirect’ processes, such as microbial sulphate reduction, which may lead to a strong increase in pH.

Dissolution rate of alpha-doped UO2 in natural groundwater

The objective of this work is to determine whether the presence of trace elements in natural groundwaters affects the dissolution rate of uranium dioxide in the presence of alpha radiation that causes radiolysis of water. The study is a part of the project Reducing Uncertainty in Performance Prediction (REDUPP) under the Seventh Framework Programme of the European Atomic Energy Community (EURATOM). The project aims to reduce uncertainties related to the extrapolation of the results of laboratory experiments to the conditions expected under geologic disposal. Thus far, synthetic groundwater has been normally used in the experiments. The synthetic groundwaters used do not contain all of the chemical elements that occur in natural groundwaters. Three natural groundwaters were chosen for the dissolution experiments with 0%, 5%, and 10% 233U-doped UO2 samples. These include a brackish groundwater, a saline groundwater and a low ionic strength groundwater. At the time of writing this paper, the dissolution experiments have been finished in the first groundwater, which was a moderately saline, brackish groundwater. The groundwater samples for the experiments were taken from a borehole in the Olkiluoto site in Finland. The measurements for dissolution rates were conducted under reducing conditions established using metallic iron in solution and an argon atmosphere in the glove box. The isotope dilution method was used to decrease uncertainties due to precipitation and sorption effects. The resulting dissolution rates in OL-KR6 natural groundwater were generally somewhat higher than the rates measured previously in synthetic groundwaters under similar redox conditions. No clear effect of alpha radiolysis could be seen for tests with lower SA/V, while those for higher SA/V indicated that the dissolution rate was higher for the 10% 233U-doped UO2, suggesting the effect of alpha radiolysis under these conditions.

Risk‐Informed Regulation and Safety Management of Nuclear Power Plants—On the Prevention of Severe Accidents

There are four operating nuclear power plant (NPP) units in Finland. The Teollisuuden Voima (TVO) power company has two 840 MWe BWR units supplied by Asea-Atom at the Olkiluoto site. The Fortum corporation (formerly IVO) has two 500 MWe VVER 440/213 units at the Loviisa site. In addition, a 1600 MWe European Pressurized Water Reactor supplied by AREVA NP (formerly the Framatome ANP--Siemens AG Consortium) is under construction at the Olkiluoto site. Recently, the Finnish Parliament ratified the government Decision in Principle that the utilities' applications to build two new NPP units are in line with the total good of the society. The Finnish utilities, Fenno power company, and TVO company are in progress of qualifying the type of the new nuclear builds. In Finland, risk-informed applications are formally integrated in the regulatory process of NPPs that are already in the early design phase and these are to run through the construction and operation phases all through the entire plant service time. A plant-specific full-scope probabilistic risk assessment (PRA) is required for each NPP. PRAs shall cover internal events, area events (fires, floods), and external events such as harsh weather conditions and seismic events in all operating modes. Special attention is devoted to the use of various risk-informed PRA applications in the licensing of Olkiluoto 3 NPP.

Faults in crystalline rock and the estimation of their mechanical properties at the Olkiluoto site, western Finland

Brittle deformation zones at the Olkiluoto nuclear repository site in western Finland play critical roles in the strength and hydrology of the host rock mass. We present a procedure implemented there for incorporating information on deformation zones obtained through boreholes into quantitative engineering design. First, ductile and brittle deformation zones are classified based on their characteristics in drillhole cores as brittle joint clusters, brittle fault zones, or semi-brittle fault zones, with an awareness of the geologic processes that caused the zones to develop as they did. Next, it is shown that the mechanical properties of the brittle deformation zones can be calculated by one of several methods, each of which has advantages and disadvantages. The site geology must be kept in mind at all stages to arrive at meaningful estimates of the mechanical properties of the deformation zones.

The socio-economic and communication challenges of spent nuclear fuel management in Finland: The post site selection phase of the repository project in Eurajoki

One of the key factors behind the expansion of nuclear energy policy in Finland is nuclear waste management. As in many countries, nuclear waste management agencies are still struggling with the siting of intermediate and low level nuclear waste management facilities, in Finland Posiva, the nuclear waste company owned by the two nuclear power utilities Teollisuuden Voima (TVO) and Fortum Power and Heat (FPH), has been excavating a underground characterization facility since 2004 in the Olkiluoto site in the municipality of Eurajoki. The facility under construction is meant to be for a part of the final repository for disposal of spent nuclear fuel (SNF). The site selection process of the repository was launched in Finland in the early 1980s. The site selection process included different measures to inform and involve local citizens. For example, an Environmental Impact Assessment process was implemented by the nuclear waste company in the late 1990s. The siting process, which proceeded step by step, was successful as in January 2000 the local council of the municipality of Eurajoki approved the siting of the repository by 20 votes to 7. The Council of State made a favourable decision-in-principle (DiP) in December 2000 and Parliament ratified the decision in May 2001. Posiva has also applied for a decision-in-principle to extend the facility. The aim of the paper is to introduce the first observations of the survey focused on local inhabitants. The paper addresses such questions as: What are the main impacts of the repository according to local residents? How can they obtain information about the repository? Which actors are seen as reliable sources of information regarding the final disposal of spent nuclear fuel? The survey was carried out in June 2008. The respondents ( N = 3000) were selected from the residents of the municipality of Eurajoki and the neighbouring municipalities using stratified random sampling. The information provided by the survey is important as the municipality of Eurajoki is the very first municipality in the world where the views of local residents on the construction of a repository for the final disposal of spent nuclear fuel could be elicited after the decision of the site selection. The post site selection phase the municipality of Eurajoki is currently experiencing can provide information useful for other countries likely to face this phase in the future. The paper is a part of the co-operation research project of the University of Jyväskylä and the University of Tampere. The research project “Follow-up research regarding socio-economic effects and communication of final disposal facility of spent nuclear fuel in Eurajoki and its neighbouring municipalities” is funded by the Finnish Research Programme on Nuclear Waste Management (KYT2010).

Concentration ratios for chemical analogues of key nuclides for different vegetation types at the Olkiluoto site

Olkiluoto Island on the western coast of Finland has been selected as a repository site for spent nuclear fuel in Finland. This study aimed at identifying differences in concentration ratios (CR), and their distributions, for the elements analysed on soil and vegetation samples taken on the island (Al, B, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Ni, P, S, Zn). Many of the elements can be considered to be chemically analogous to radionuclides that, potentially, can be released from the repository. Differences between the soil and vegetation in different tree age, tree species and site fertility classes typical of the forest ecosystems in Olkiluoto were investigated. Lognormal distributions were fitted to the different groupings of the CR data calculated on the basis of the results from 94 sampling plots. In most cases no significant differences were found between the different groupings for a specific element when the 95% confidence intervals were applied. According to the results based on real site data for CRs in forest ecosystems on Olkiluoto, it appears that the current CR-based approach to radionuclide modelling in forest ecosystems is problematic due to the large variation in parameter values and in their practical definition.

Geoscience and high-level nuclear waste disposal: the Nordic scene

In Norden, two countries, Sweden and Finland, are actively engaged in site investigations for the location of deep repositories for spent nuclear fuel from powerproducing nuclear reactors. These investigations are being carried out in crystalline rocks of the Fennoscandian Shield. In Sweden, a long history of site selection has led to the identification and investigation of two sites, Forsmark and Laxemar/Simpevarp, based on a strategy of combining favourable bedrock with consent by the local population. Surface-based geoscientific investigations of the two candidates, with extensive deep drilling, are now drawing to a close. A proposal as to which of the sites would be most suitable for the development of a deep repository will be submitted to the governmental regulatory authorities in 2009. In Finland, the site selection process was shorter and less politically controversial, and led to a Decision in Principle by the Finnish parliament, in May 2001, to develop a deep repository at the Olkiluoto site. The access tunnel to an underground rock characterisation facility at 400-500 m depth is at present under construction, accompanied by extensive geoscientific investigations in the subsurface. An application for a construction licence for a deep repository will be submitted in 2012. Although all sites are located in Precambrian crystalline rocks, the Swedish sites both lie in relatively homogeneous granitic rocks, whilst the Finnish site is located in an heterogeneous migmatite complex. The Nordic approach to high-level nuclear waste disposal in crystalline rock will be the theme of a Topical Symposium at the 33rd International Geological Congress at Oslo, in August 2008, and the three sites mentioned above will be the focus of Congress Excursion no. 14.

Winning Citizen Trust: The Siting of a Nuclear Waste Facility in Eurajoki, Finland

ment’s decision-in-principle (DiP) on the geological disposal of spent nuclear fuel in Finland. The government based its decision on the application of Posiva Oy, the nuclear waste management company owned by Finland’s two nuclear power plant companies. It meant that the repository could be sited in the Olkiluoto area near the Olkiluoto nuclear power plant in the municipality of Eurajoki, and the disposal could be based on a technical approach originally developed by SKB, the Swedish nuclear waste management company. This was the first time in the world that a site was selected for a high-level nuclear waste repository and was accepted by the majority of local people. Only a few years earlier this outcome looked far from likely. Opinion surveys showed a consistent lack of trust in the long-term safety of geological disposal, and the Eurajoki municipality had an official policy opposing any highlevel nuclear waste repository. To the extent that the topic was discussed in the public media, most opinions were sceptical if not completely negative. The DiP meant that Posiva could begin its underground research to characterize, or thoroughly examine, the bedrock at the Olkiluoto site. A few years earlier an official inquiry in the United Kingdom had stopped similar plans at the Sellafield site in Cumbria. In northern Sweden, referenda had stopped the SKB’s plans to investigate sites. In this context, the Finnish siting decision seemed to come against all odds. Its very existence suggests that something new Juhani Vira

Estimating the transversely isotropic elastic intact rock properties for in situ stress measurement data reduction: A case study of the Olkiluoto mica gneiss, Finland

An anisotropy in the elastic moduli values of intact rock with a ratio of more than 1.3–1.5 has been reported to have an effect on the calculated magnitudes and orientations of the in situ principal stress components as measured by the overcoring method. Work related to the on-going site investigation for a deep radioactive waste repository at the Olkiluoto site in Western Finland has shown that the migmatic mica gneiss is anisotropic which could therefore affect the interpretation of overcoring stress measurement results. This paper includes a summary of the theory of anisotropy concerning the intact rock moduli via the strain compliance matrix, a description of the core sample testing methods, and interpretation of results for the migmatic mica gneiss from two site investigation boreholes. In this case study, 19 specimens were tested and the results showed a modulus anisotropy of about 1.4. Because such anisotropy is high enough to produce significant errors in the estimation of the in situ principal stresses, it is recommended to take this into account in the interpretation of the stress measurement results, both in the context of the current work in Finland and in other projects where similar anisotropy is encountered.

Effect of excavation damaged zone on gas migration in a KBS-3H type repository at Olkiluoto

The quantities of steel in the KBS-3H disposal concept with long horizontal deposition holes are moderate but the gas production rates are significant. Based on current understanding of the geological situation at the Olkiluoto site in Finland, a simplified model of gas pressure evolution and migration of corrosion gases in the disposal system was developed and the effect of EDZ on the gas transport was evaluated. The results of the analysis suggest that gas is predominantly transported by two-phase flow through the EDZ and the fracture network in the host rock. For the major part of the deposition hole sections, there is sufficient transport capacity for gas to escape through the host rock and EDZ and the calculated gas pressure increases are found to be moderate. In tight deposition hole sections, transport through the EDZ dominates and the gas pressure development is sensitive to EDZ permeability. If the EDZ permeability in tight sections is of the order of 10% of the measured intrinsic permeability, the pressure there may rise to near the minimum principal rock stress. However, if the permeability is of the order of the measured intrinsic EDZ permeability, the pressure will rise only slightly above hydrostatic pressure. The results of the analysis show that the EDZ is capable of transporting the gas gradually out from tight hole sections without significantly disturbing the isolation characteristics of the repository system.

Further Steps Towards Licensing: Underground Characterisation Started for the Spent Fuel Repository in Finland

ABSTRACTPursuant to the Decision-in-Principle of 2001 the Finnish programme for geologic disposal of spent fuel has now moved to the phase of underground characterisation of the repository site. The main objective of this programme phase is to confirm – or refute – the suitability of the Olkiluoto site by investigations conducted underground at the actual depth of the repository. The construction work of the access tunnel to the rock characterisation facility (ONKALO) started in the late summer of 2004. The site research and investigations work aims at the maturity needed for submission of the application for construction license of the actual repository in 2012. This requires, however, that also the technology has reached the maturity needed. The design and technical plans form the necessary platform for the development of the safety case for spent fuel disposal. A plan, “road map”, has been produced for the portfolio of reports that should demonstrate the safety of disposal as required by the criteria set by the government and further detailed by the safety authority, STUK.

Advances in Classifications for Structural Modelling of the Olkiluoto Site

ABSTRACTThe site characterization for the construction of the repository of spent nuclear fuel is carried out at Olkiluoto in Finland. The results of geological, geophysical, and hydrogeological investigations are integrated in a bedrock model to describe rock types, fractured zones and potential hydraulic connections. In modelling it is essential to determine structures important for construction of underground facilities. This question encompasses objectives and scale of the modelling. Important parameters in structural modelling are fracture frequency and hydraulic conductivity. The definitions need to be evaluated site-specific, because the geological circumstances control the properties of structures. An important issue is the possible existence of a boundary zone close to structures.

Modelling of the bedrock response to glacial loading at the Olkiluoto site, Finland

The Olkiluoto Island is situated in Baltic Sea, near the southwestern coast of Finland. The most abundant rock type is migmatic mica gneiss, intruded by tonalites, granodiorites and granites. Response of bedrock at Olkiluoto site was modelled with 3DEC program considering four future ice age scenarios. Each scenario produces shear displacements of fractures with different time of occurrence and varying recovery rate. Generally, the larger the maximum ice load, the larger the permanent shear displacements will be. For basic case, the maximum shear displacements were few centimeters at suggested nuclear waste repository level. The sensitivity study performed showed that shear stiffness and joint friction angle of bedrock structures are the most critical parameters for bedrock displacements.