atw - International Journal for Nuclear Power | 05.2021

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atw Vol. 66 (2021) | Issue 5 ı September DECOMMISSIONING AND WASTE MANAGEMENT 44 Material Bulk modulus (GPa) Shear modulus (GPa) Density (kg·m -3 ) Elastic modulus (GPa) 1200 meters below surface with different anisotropic stress ratios (minimum to maximum principal stresses) up to 1:3. SSiC canister and foundation are modelled as elastic material. The falling rock blocks are simulated either as elastic material or as assembly of distinct blocks with calibrated elasto-plastic parameters, which allows to consider the rock disintegration during the impact. Damping was not applied for the dynamic simulations because data are not available, but viscous boundary conditions were applied to avoid unrealistic reflections at the lower bottom of the ground. This makes the simulations once again conservative. The interface stiffnesses at the contact between the colliding parts (e.g. between canister and rock block or ground) are adjusted in such a way, that impact induced stresses reach maximum values (corresponding lab data are not available). So far not explicitly otherwise mentioned the model parameters given in Table 3 are applied. µ jkn (GPa·m -1 ) jks (GPa·m -1 ) Rock & Foundation 40 29 2500 70 0.21 100 100 Buffer 1 0.5 2000 1.35 0.27 100 100 SSiC 200 180 3100 415 0.15 100 100 Clay-stone 40 18 2500 47 0.30 100 100 Clay-stone jkn TPa·m -1 75 | Tab. 3. Matrix and interface contact parameters. jks TPa·m -1 25 jcoh MPa 40 jtens MPa 10 jfric ° 0 res_jcoh MPa 0 jkn = jks = 440 TPa·m -1 for dynamic loading to avoid unrealistic penetration | Fig. 3. Left: Cross section of cylindrical canisters (see also Table 4); middle: Model set-up for canister completely imbedded inside the host rock incl. buffer; right: Impact constellations of canisters after free fall. Canister a/mm b/mm c/mm d/mm e/mm f/mm HTR (5 pebbles) 62 305 92 335 15 15 CANDU 102 510 142 550 20 20 PWR/BWR 400 4930 470 5000 35 35 Vitrified waste 450 1350 500 1400 25 25 | Tab. 4. Dimensions of canisters (see also Figure 3). | Fig. 4. Maximum tensile stress inside unprotected canister for different inclination angles of VW (up) and HTR (middle) canister (see Table 5). res_jtens MPa 0 res_jfric ° 27 Four different canister types are considered as given by Figure 3 (left) and Table 4. The considered waste canisters are hollow cylinders sealed at one, respectively the two ends using the technique of Rapid Sinter Bonding (RSB) as proposed by Knorr and Kerber [7]. Static loading scenarios This loading case considers anisotropic earth pressure on completely in a rock mass embedded VW and HTR canisters (Figure 3 (middle)). Table 5 lists all the considered earth pressure constellations in terms of principal stresses (X and Y (both horizontal) and Z (vertical): 1:1:1, 2:1:1, 3:1:1, 2:2:1, 3:2:1, 3:3:1. These constellations cover all typical stress states existing in potential host rocks. The considered maximum principal stress ratio is 3:1. The angle between canister axis and Z-axis is set to 0 °, 30 °, 60 ° and 90 °, respectively. Figure 4 documents the maximum induced tensile stresses inside the SSiC canisters. The considered depth is 1200 m, and the vertical earth pressure will be about 30 MPa. Overall, in any case the maximum tensile stress in the unprotected canister does not exceed 50 MPa which is significantly below the tensile strength of SSiC, which is 150 MPa. Maximum tensile stress is mainly distributed around the two lids of the canister as shown in Figure 5. The average stress in the VW canister is bigger than that in the HTR canister. The thickness of the canister is a controlling factor. For the VW canister, the thickness/ height ratio is 1/56, while the thickness/height ratio for the HTR canister is 1/22. Exemplary, for the HTR canister some selected simulations with a buffer sealing (200 mm thick) were performed. As Figure 6 documents, such a clay (bentonite) buffer can significantly reduce maximum tensile stresses. Decommissioning and Waste Management SSiC Nuclear Waste Canisters: Stability Considerations During Static and Dynamic Impact ı Yanan Zhao, Heinz Konietzky, Juergen Knorr and Albert Kerber
atw Vol. 66 (2021) | Issue 5 ı September | Fig. 5. Maximum tensile stress [kPa] distribution for unprotected canister, left: VW canister, rock stresses X: 30 MPa, Y: 10 MPa, Z: 10 MPa, inclination angle 0 °; right: HTR canister, rock stresses X: 30 MP, Y: 30 MPa, Z:10 MPa, inclination angle 30 °. Loading case X/MPa Y/MPa Z/MPa (X/Z) Case 1 10 10 10 1 Case 2 20 10 10 2 Case 3 30 10 10 3 Case 4 20 20 10 2 Case 5 30 20 10 3 Case 6 30 30 10 3 | Tab. 5. Primary stresses (see Figures 5 and 6). | Fig. 6. Maximum tensile stress of unprotected HTR canister and buffer-sealed canister for canister inclination angle of 0 ° and 30 ° (buffer thickness 200 mm). DECOMMISSIONING AND WASTE MANAGEMENT 45 | Fig. 8. Top: maximum tensile stress at impact [kPa]; Bottom: canister with deformed cover during the collision. | Fig. 7. Maximum tensile stress of unprotected canisters versus drop height. Dynamic loading scenarios Free fall of canister For impact, drop height and canister positions are the controlling factors. The considered drop heights (distance from the lowest point of the canister to the foundation) are 0.5 m, 1.0 m, 1.5 m, and 2.0 m, respectively. The canister positions during impact (0°, 30°, 60°, 90°) are illustrated in Figure 3 (right). Figure 7 shows the maximum induced tensile stresses in the different canisters for different drop height and documents, that peak tensile stresses can very locally reach values considerably higher than the material strength. Therefore, an additional simulation was performed assuming a protective cover (layer) around the SSiC canister. Simulation of a VW canister (drop height 1 m, inclination angle 0°) with soft cover (50 mm thick) confirms significant reduction of maximum tensile stress | Fig. 9. Top: Considered constellations of dropping rock pieces at the time of impact; Bottom: Induced maximum tensile stresses in VW canister for different simulation cases. Decommissioning and Waste Management SSiC Nuclear Waste Canisters: Stability Considerations During Static and Dynamic Impact ı Yanan Zhao, Heinz Konietzky, Juergen Knorr and Albert Kerber
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