About Project
The Onkalo Spent Nuclear Fuel Repository is a deep tunnel system for the final disposal of spent nuclear fuel. It is first of such repository in the world. It is currently under construction at the Olkiluoto Nuclear Power Plant in the municipality of Eurajoki, on the west coast of Finland, by the company Posiva. It is based on the KBS - 3 method of nuclear waste burial developed in Sweden by Swedish Nuclear Fuel and Waste Management Company (SKB).
Project Name
Onkalo Spent Nuclear Fuel Repository
Location
Eurajoki, Finland
Construction Period
2004 – under construction
Contractor
Posiva Oy
Designer
Rockplan Ltd
As the mines and nuclear waste disposal facilities expand deeper, the increasing stress magnitudes cause the probability of a failure to increase. The understanding and reliable prediction of the failure process are the key to optimal layout and adequate rock support measures which enable cost-effective and safe construction of a deep underground facility. As the stresses at the excavation boundary exceed the rock mass strength, a brittle failure occurs, that is often referred to as “spalling”. This phenomenon occurs when strong compressive pressure induces crack growth behind an excavated surface and eventually leads to buckling of thin rock slabs. In order to predict spalling, an approximate value of in-situ stress, stress direction and rock mass spalling strength are required. The shape and direction of the tunnel have a drastic effect to the tangential stresses that exist on the tunnel boundary. Therefore a tunnel oriented parallel to the direction of the principal stress, suffers minimum spalling damage compared to a tunnel that is oriented orthogonally to the direction of principal stress. The complex geometry of an excavation affects stress vector directions and boundary stresses which are hard to predict without three-dimensional numerical modelling.
ONKALO is an underground rock characterisation research facility for future nuclear waste disposal. ONKALO consists of a series of tunnels and shafts starting from the surface and reaching the depth of -460 m. The zone of interest in a large facility was divided into 6 parts. In order to obtain the actual value of stress with high level of accuracy, the order of the elements (around 200,000 per every model) was set to quadratic. An essential problem was to set up the direction and magnitude for the principal stresses. In order to cover the whole spectrum of variation, different scenarios with different in-situ stresses were created. The in situ stress is high in the Technical Room area (up to 35 MPa in this depth), which required calculation of induced stresses around the designed excavation. The most important parts to look at, were the connecting galleries between shafts and a tunnel, where spalling zones were expected. The calculation was carried out using both elastic models in order to predict the stresses and plastic models in order to predict the deformations. The calculation and interpretation implied that the most critical area will be located at the bottom of a technical shaft that starts from the top of the crown and that will be excavated after the tunnel. This area is a high-stress zone even without the shaft which can be seen as high compression in the arc of the tunnel. After the construction of the shaft, the stress increases up to 120 MPa, which is higher than the compressive strength of the intact rock.
