Posiva Oy Research Articles

Thermal hydraulic and mechanical modelling of the full-scale in situ test (FISST)

In 2018, Posiva Oy started the Full-Scale in Situ System Test (FISST) within the ONKALO® underground facility. FISST is a full-scale application of the KBS-3 V reference disposal concept that involves the back section (approximately 50 m) of Demonstration Tunnel 2 (DT2) and its two experimental deposition holes. Two key objectives were identified in designing the test. The first one involved the demonstration that the KBS-3 V concept can be implemented safely and according to plan, whereas the second aimed at the provision of a full-scale simulation of the emplacement of high-heat-generating waste. This work focuses on the simulation of hydraulic, thermal and mechanical (THM) evolution of the FISST experiment. It describes the development of a holistic numerical methodology that integrates the hydrogeological description of the geosphere achieved through discrete fracture network (DFN) modelling, into a continuum THM modelling framework. Further, it provides an evaluation of the predictive capability of the models against available sensor data that capture the early phase of the test.

Effects of Excavation Damage on the Physical Properties of Rock Matrix

Posiva Oy has conducted investigations into excavation damage, including comprehensive laboratory testing of physical properties of rock specimens from excavation damaged rock mass. Laboratory testing was conducted on drill core specimens extracted from the excavated surface of a tunnel located at approximately 345 m depth in Olkiluoto, Finland. A total of 141 drill core specimens of three main rock types, a structurally isotropic coarse-grained pegmatoid (PGR) and structurally anisotropic veined gneiss (VGN) and diatexitic gneiss (DGN), were subjected to petrophysical testing, rock mechanics testing and petrographic analyses. Results from the various tests were subjected to rigorous statistical analysis in order to reveal the effects excavation damage has on the physical properties of the rock mass. Results of the study revealed changes that are credited to excavation damage in resistivity, S-wave velocity and various elastic properties of the rock specimens. Effects of excavation damage and the depth of the excavation damaged zone seem to be different to gneiss compared to pegmatoid. On microscopic level, the extent of excavation damaged zone appears to be 0.2 – 0.4 m depending on the measured property. This means that the deeper excavation damaged layer observed by geophysical surveys may be caused by larger scale fractures.

Comparison of microstructural features of three compacted and water-saturated swelling clays: MX-80 bentonite and Na- and Ca-purified bentonite

AbstractThe planned final disposal repositories of spent nuclear fuel in several countries, including Finland, pose significant scientific challenges due to their extremely long lifespan. One of the key materials proposed for use in Posiva Oy's repository in Finland is MX-80 bentonite in a compacted, water-saturated state. Border cases of calcium and sodium forms of purified bentonite were included in this study. The MX-80 in the repository is expected to undergo cation exchange due to the composition of the groundwater. The clays were studied at different dry densities between 0.7 and 1.6 g cm–3. The microstructure of the water-saturated, compacted clays was investigated using small-angle X-ray scattering, nuclear magnetic resonance and transmission electron microscopy. Additionally, atomic force microscopy was used to characterize the shape and size of the fine-fraction clay platelets. As expected, the average shape of the fine fractions was smaller than the bulk material, but a more elongated shape was present in the purified material. Mainly due to sample density, the pore structure was noticeably different for the Na form of purified bentonite at 0.7 g cm–3 density, but at higher degrees of compaction, no significant differences were noted between the samples. The laboratory results obtained in this study could be useful for safety and performance analysis of this high-level waste repository where sodium bentonite is used and is expected to change its charge-compensating cation composition during the repository's lifetime. Microstructural data may be used in modelling of diffusion and sorption by surface complexation modelling, for example, or as a basis for mechanical and water transport models.

State-of-the-art and proof-of-concept installations for repository concepts based in crystalline rock

AbstractOver the last five decades private and national energy programmes worldwide have been producing a variety of radioactive wastes. One of the safest ways of disposing of this waste is to bury it deep underground in purpose-built geological disposal facilities. Currently, there is no operating geological repository in Europe for high-level waste but the goal of the IGD-TP is that the first repository shall be fully operational before the year 2025. Several studies and experiments are ongoing at various potential repository sites in Europe with the goal to establish general approaches that can be adapted for any country in need of a geological repository.The Swedish Nuclear Fuel and Waste Management Co (SKB) in Sweden and Posiva Oy in Finland are developing a method for geological disposal of high-level long-lived nuclear waste in crystalline rock, the KBS-3 method. KBS-3V (vertical) is both organizations reference design, but KBS-3H (horizontal) emplacement is also being researched as a potential alternative. Of high importance in the development is demonstrating the technical feasibilityin situof safe and reliable construction, manufacturing, disposal and sealing of such geological disposal facilities. Parts of these demonstrations are carried out under the framework of EurAtom/FP7 and one of these projects is the LUCOEX project where SKB is demonstrating horizontal emplacement, the Multi Purpose Test (MPT), and Posiva is demonstrating vertical buffer installation processes.The MPT includes the key components of the horizontal design and comprises all essential steps; manufacturing of the full-scale components, their assembly, installation in the drift and monitoring of the early buffer evolution. The MPT installation was successfully performed in late 2013. By combining the components, an initial verification of the design implementation has been achieved. At the same time, integrating the components has meant the recognition of some design weaknesses and the design will be updated accordingly.Posiva's KBS-3V buffer installation equipment that places buffer blocks with high precision in vertical deposition holes is currently being developed and will be tested during 2014 and 2015 in real underground conditions. The machine uses vacuum lifting tools for moving the buffer blocks and laser scanning technology to position both the machine and blocks. Functionality of the concept and equipment selected will be confirmed by the tests and the installation tests will provide important information about the suitability of the selected buffer dimensions and tolerances.

Enhanced Sealing Project (ESP): evolution of a full-sized bentonite and concrete shaft seal

Abstract A full-scale shaft seal was designed and installed in the 5 m-diameter access shaft at Atomic Energy of Canada Limited's (AECL's) Underground Research Laboratory at the point where the shaft intersects an ancient water-bearing, low-angle thrust fault at a depth of c. 275 m in granitic rock. The seal consists of a 6 m-thick bentonite-based component sandwiched between 3 m-thick, keyed upper and lower concrete components. This design was adopted in order to limit the mixing of saline groundwater from the deeper regime with the fresher, near-surface groundwater regime. Construction of the shaft seal was done as part of Canada's Nuclear Legacy Liabilities Program. A jointly funded monitoring project, called the Enhanced Sealing Project (ESP), was developed by AECL (Canada) and jointly funded by NWMO (Canada), SKB (Sweden), Posiva Oy (Finland), and ANDRA (France), and since mid 2009 the thermal, hydraulic and mechanical evolution of the seal has been constantly monitored. The evolution of the type of seal being monitored in the ESP is of relevance to repository closure planning by demonstrating the functionality of shaft seals. Although constructed in a crystalline rock medium, the results of the ESP are relevant to the performance of seals in a variety of host rock types.

Characterization of Geotomographic Studies with the EMRE System

Posiva Oy carries out research and development on spent nuclear fuel disposal in Finland. The repository will be constructed deep in the crystalline bedrock of Olkiluoto island in Eurajoki. Posiva Oy and ANDRA (France) have cooperated actively in examining methods for revealing properties of granitic bedrock. One of the considered methods was an electromagnetic cross-borehole survey, and RIM measurements were conducted in 2009. Olkiluoto migmatitic bedrock has undergone polyphasic ductile-brittle deformation and resistivity in the bedrock varies strongly in range of tens to tens of thousands of Ωm. Field work was successfully performed in one borehole pair. The results are presented in this paper. The tomographic reconstruction of the borehole section is based on the far-field approximation of the electric field. The results prove that the method can be used between boreholes to delineate and follow sulphide-bearing horizons. The detected low and high resistivity zones and their apparent shapes and orientations are in fair agreement with geological and other geophysical results. The obtained information can be used, for example, in assessing the integrity of the rock mass, as the increased electrical conductivity is often associated with rock mass deformation (clay and water bearing fractures, sulphide and graphite bearing zones).

Heuristic Methods for Assigning Spent Nuclear Fuel Assemblies to Canisters for Final Disposal

The disposal of spent fuel assemblies (SFAs) by companies currently producing nuclear power in Finland is the responsibility of a company named Posiva Oy. Posiva Oy has decided to use the KBS-3 (Swedish abbreviation for nuclear fuel safety; version 3) concept. In KBS-3, SFAs are placed in metal canisters, which are themselves deposited deep into crystalline rock. The disposal process in Finland will last many decades. To efficiently assign SFAs to canisters, in this paper we study the minimax canister formation problem. In this problem, we assume we are given two sets of data: (a) a schedule specifying the number of disposal canisters per year and (b) the decay heat of each SFA for every disposal year. The goal in the problem is to assign SFAs to canisters so that the largest canister heat load is minimized. The minimax canister formation problem is a variant of a well-known optimization problem: makespan minimization on unrelated parallel machines. We developed heuristic methods for solving the minimax canister formation problem. Using our methods and predicted SFA amounts and properties for Finland, we obtained high-quality solutions in numerous test cases. We also investigated how the uncertainty in SFA burnups affects the canister heat loads.

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

Quality control of overcoring stress measurement data

The in situ state of stress is one of the key rock mechanics factors related to the safety and stability of underground excavations for civil and mining engineering purposes. However, measurement and interpretation of stress have their difficulties. In particular, practical and objective tools have not been developed to judge transient strain behaviour during overcoring. The work described in this paper was set up by the nuclear waste management companies Posiva Oy (Finland) and Svensk Kärnbränslehantering AB (Sweden) to improve the quality of interpretation of overcoring stress measurement results. Thus, the primary product of the project is a quality control capability for overcoring stress measurement data. For this purpose, a computer program was developed which can simulate the transient strains and stresses during the overcoring process for any in situ stress and coring load conditions. The solution is based on superposition of elastic stresses and the basic idea can be applied for different overcoring probes with minor modifications and recalculation of stress tensors. The measured strains can be compared to the calculated ones to check whether the measured transient behaviour accords with the interpreted in situ state of stress. If not, the in situ state of stress can be calculated based on any transient or final strain values. The transient stresses can also be compared to a strength envelope of intact rock and thereby the core damage potential can be estimated.