Earthquake Induced Rock Shear Through a Deposition Hole. Effect on the Canister and Buffer
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(DUWKTXDNH,QGXFHG5RFN6KHDU7KURXJKD'HSRVLWLRQ+ROH (IIHFWRQWKH&DQLVWHUDQG%XIIHU L. Börgesson, L.-E. Johannesson and J. Hernelind1 Clay Technology AB, Ideon Research Centre, SE-223 70 Lund, Sweden 1 FEM-Tech AB, Pilgatan 8, SE-721 30 Västerås, Sweden $%675$&7 Existing fractures crossing a deposition hole may be activated and sheared by an earthquake. The effect of such a rock shear has been investigated in a project that includes both laboratory tests and finite element calculations. A number of laboratory tests have been performed with shearing of water-saturated bentonite samples at different densities and shear rates. From those tests a material model of the buffer that takes into account the shear rate has been formulated. Shear rates up to 6 m/s have been tested. The rock shear has been modelled with finite element calculations with the code ABAQUS. A 3D finite element mesh of the buffer and the canister has been created and a number of calculations with simulation of a rock shear have been performed. The rock shear has been assumed to take place perpendicular to the canister axis in either the centre of the deposition hole or at the ¼ point. The shear calculations have been driven to a total shear of 20 cm. Buffer densities at water saturation between 1950 and 2100 kg/m3 and shear rates between 0.0001 and 1000 mm/s have been modelled. The influence of buffer density, shear plane location, the shear rate and the magnitude of the shear displacement are analysed and discussed. The results show that the influence of especially the density of the buffer and the location of the shear plane are very strong but also that the shear rate and the magnitude of the shear displacement have a significant effect. ,1752'8&7,21 One important function of the buffer material in a deposition hole in a repository for nuclear waste disposal is to reduce the damage of rock movements on the canister. An earthquake of magnitude 6 yields 0.1 m displacement at the rate 0.1 m/s in a 200 m fracture placed 200 m from the epicenter [9]. The consequences of such a rock shear have been investigated earlier, both by laboratory tests, laboratory simulations in the scale 1:10 and finite element modelling [1-4]. A new investigation, which includes a new geometry of the copper/iron canister [5], a shear displacement up to 20 cm, a bentonite model updated for very fast shearing, a shear rate of 1 m/s and both centric and eccentric shear has been made. A number of laboratory tests with very fast shearing have also been performed and used as basis for the bentonite model. %$6,&%(1721,7(6+($53523(57,(6 Eqns 1 to 3 are used for defining the shear strength and the influence of density, pressure and rate of shear [6]. H S = S 0 H0
1 β
(1)
1
S T I = T I 0 S0
E
(2) Q
Y (3) T IV = T IV 0 V Y V0 where H= void ratio, H= reference void ratio (=1.1), S = swelling pressure (at H), S = reference swelling pressure (at H) (=1000 kPa),TI = deviator stress at failure at the swelling pressure S, TI
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