dropped without impact limiters from a height of 0.3 m vertically onto a nearly ... Keywords: impact, simulation, cask, safety, drop, foundation, unyielding, target.
NUMERICAL SAFETY ASSESSMENT OF A TRANSPORT AND STORAGE CASK FOR RADIOACTIVE MATERIALS WITHOUT IMPACT LIMITERS BY THE 0.3 M DROP TEST ONTO AN UNYIELDING TARGET L. Qiao, U. Zencker, G. Wieser, H. Völzke BAM Federal Institute for Materials Research and Testing, Berlin, Germany
Abstract The safety of a transport and storage cask for radioactive materials in an accident scenario inside a storage facility is investigated. A half-scale model of the cask was dropped without impact limiters from a height of 0.3 m vertically onto a nearly unyielding target. A post-test finite element (FE) calculation is presented with a detailed model of the cask under the test conditions to develop a sufficiently accurate computer model and to provide detailed interpretation of the large amount of measurement data for achieving good correlation between experimental and numerical results. Finally, a full-scale FE model is used to examine the stresses and strains of the original cask design in combination with a yielding target, i.e. the storage facility foundation. Keywords: impact, simulation, cask, safety, drop, foundation, unyielding, target.
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Introduction
The safety assessment of new designs for transport and storage casks for radioactive materials is a challenging task by using different methods like prototype tests, model tests, calculations, and analogy reflections. At BAM (the German Federal Institute for Materials Research and Testing), the test procedures for the mechanical IAEA (International Atomic Energy Agency) test conditions [1] or the storage site specific accident conditions start often with preliminary finite element calculations mostly with a scaled cask model for verification of the proposed test cask instrumentation and test plan. On that basis the extensive test cask instrumentation is applied and checked. After that, a drop test series consisting of different test sequences is performed. Following the drop tests, numerical post-analyses are carried out. The analyses offer the possibility of a detailed calculation and assessment of the entire test cask construction. The calculation results are carefully compared with the measurement 1
data over impact history to find out all relevant parameters for a realistic simulation of the impact scenario. Because the desired ideal boundary test conditions often cannot be met exactly in practice, the post-analyses are carried out under the real conditions of the test. At the end of this step a validated model is found. Then, the validated finite element model with ideal, i.e. worst case, boundary conditions can be scaled up to full-scale dimensions to analyse the applied cask design and to check scaling laws. With that the maximum stressed cask areas and components can be identified and evaluated to get final safety assessments with respect to e.g. fracture mechanics, plastic deformations and leak-tightness of sealed lid systems. It has to be taken into account that especially nonlinear effects cannot be scaled with simple rules. Under these circumstances the investigations must be carried out directly at a full-scale model. BAM develops its own FE models independently of the applicant and calculates these models also with an alternative FE code. In some cases the material behaviour is described with other suitable material models when equivalent models are missing in the material library of the alternative code. Also, the concrete implementation of a complex material behaviour may differ. In this way possible errors of the numerical simulation are uncovered both at the FE model and in the used FE code. Under test conditions according to the IAEA transport regulations [1] casks are usually equipped with impact limiters and are dropped onto a so-called unyielding IAEA target. However, it is also necessary to evaluate different scenarios for storage site specific accident conditions where casks are not equipped with impact limiters. This means that the cask body hits a very hard foundation in direct contact which leads to a totally different mechanical reaction and stress state compared to the relatively smooth impact mainly affected by the impact limiter design. In this study the chosen test scenario covers one critical accident situation to be considered if the cask drops from the crane of the storage building during handling operation and hits the ground. For that, a half-scale model of the cask CASTOR® HAW 28M [2] of the German vendor GNS (Gesellschaft für Nuklear-Service mbH) is investigated.
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Storage site specific accident scenario
The CASTOR® HAW28M cask is designed for the transport and storage of 28 canisters of vitrified nuclear waste from reprocessing [2]. Within the safety assessment of this type of cask, a drop test series with its 1:2 scale model CASTOR® HAW/TB2 cask was carried out by BAM [3]. As the storage site specific accident scenario, this test cask without impact limiters was dropped from a height of 0.3 m vertically onto the IAEA target of the test site [4]. In this context the IAEA target appears as a real foundation representing conservatively the hard ground inside the German Gorleben interim storage facility. The test provided a large amount of test data. The cask was equipped with a large number of strain gauges and accelerometers. Recorded high speed videos help to understand important effects for a correct numerical simulation of the impact sequence. Figure 1 shows the drop test configuration. Compared with a 9 m drop test with an impact limiter, the hard impact of the 0.3 m drop test has a shorter impact time (in the range of some milliseconds). The test results are very sensitive to the 2
impact orientation of the cask. This is a well-known effect. Only a small change of the impact angle will cause a large change in the measured strains [5]. Since an ideal flat drop (impact angle = 0°) is almost impossible, a post-test analysis must show the exact test conditions.
Figure 1: Vertical drop test with the cask CASTOR® HAW/TB2 from a height of 0.3 m onto BAM’s 2600 Mg IAEA target. As the interpretation of videos and measurement data has shown, the cask hit the ground in nearly perfect orientation and only a very small angle < 0.1° was registered between target and cask bottom plane. After the primary impact and before the following rebound the cask inclined visible and came back with additional slight rotation. The cask inclination during the secondary impact led to a load concentration and a visible imprint at the target steel plate.
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Half-scale cask model
For the calculation of the investigated drop test, BAM has developed a detailed finite element model of the cask and its main components including the basket and the inner canisters as shown in Figure 2. Because slight impact angles in combination with selected strain measuring points have to be considered during the evaluation process, it was essential to have a complete cask model and not only a segment model using symmetry boundary conditions. The finite element model of the 1:2 scale test cask consists of the following main components: 3
•
the cask body made of ductile cast iron with drill holes in it for neutronshielding material, • the primary lid made of stainless steel, • the basket with 28 canisters made of steel, • the anti-twist block made of steel for the basket, • the 4 trunnions made of stainless steel, • 4 graphite pillar substitutes made of steel, • the jacket impact limiter made of aluminium (not of interest here), • 32 screws M36 made of stainless steel for the primary lid, • a neutron-shielding plate at the cask bottom and • a closure plate made of steel. The total weight of this cask model is about 14 Mg. Especially the cask bottom is modelled with a fine mesh to describe the high stress and strain gradients in this region. All free surfaces of the cask (between cask body and canisters, cask body and primary lid, etc.) are defined as contact without friction.
Figure 2: BAM finite element model of cask CASTOR® HAW/TB2.
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FE model of the test site foundation
In addition to the cask model it is also important to model the test site foundation precisely. Caused by the impact, stress waves are induced into the foundation and their transmission and reflection is mainly influenced by structural transitions between different materials or components. An over-simplified model, e.g. with a rigid body, would neglect the energy absorption of the target. For that reason it is necessary to build a detailed foundation model with all components and dimensions large enough to avoid unrealistic reflections of stress waves and their possible influence on the cask reaction during the calculation time (Figure 3). The test site foundation has a base of 14 m x 14 m, is 5 m deep and consists of reinforced concrete. This block is covered by three 0.22 m thick steel plates forming 4
a 10 m x 4.5 m impact area. The steel plates are fixed with the concrete block with 40 steel rods (2 m long with a diameter of 33 mm). Additionally, there is a 0.5 m thick concrete transmission layer between steel plate and reinforced concrete block. The soil around and below the reinforced concrete block is built up with so-called infinite elements which avoid unwanted stress wave reflections from boundaries.
Figure 3: FE model of BAM’s 2600 Mg IAEA target.
Figure 4: Definition of impact parameters of a vertical drop of a cylindrical cask.
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Simulation of the drop test
The modeling and the following dynamic calculations were performed with the ABAQUS/Explicit finite element code which has been used very successfully at BAM for several years. The main criterion for a sufficiently accurate simulation of the drop test scenario is a good correlation between experimental data and calculation results at representative measuring points spatially distributed over the cask structure during the whole significant impact history. One-dimensional truss elements with a very small cross section area were attached along the strain gauge direction at the measuring points on the model surface to get directly the local
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strains. A set of seven measuring points at the bottom area of the cask is of special interest. The considered time period of 20 ms is more than 4-times the duration of the primary impact. The orientation of the cask in a global coordinate system is given in Figure 4. The point P marks the position of first contact with the ground. The angle α defines the rotation of the cask and is measured positive counter-clockwise from the axis X to point P. The angle β describes the inclination of the cask bottom with respect to the X-Z-plane.
a) Strain at cask bottom centre.
b) Strain at inner side wall near cask bottom. Figure 5: Comparison of normalized strain histories from FE calculation and measurement data. Figure 5 shows two representative strain histories, one for the cask bottom centre and one at the inner side wall near the cask bottom. The considered very small
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impact angle β of 0.05° came out of the evaluation of the video recordings and a variation analysis with different angles of 0.0°, 0.05° and 0.1° showing a significant influence on the results. Another verification performed is the consideration of the canisters inside the cask. The differences between the dotted and the continuous line in Figure 5 show that the canisters are considered in a sufficient manner. The oscillations after 5 ms represent stress waves running through the cask body which are influenced by the interaction of the canisters among themselves and with the cask. Other verifications referred to finite element modeling of the cask (e.g. FE mesh refinements), the foundation and contact conditions. Hence, the impact conditions of the investigated drop test have been realized with the angles α of 270° and β of 0.05°. The current results represented by Figure 5 show a good correlation of test and calculation data. Therefore, the numerical model can be considered as a sufficient description of the physical reality. With that an appropriate basis for further strain and stress analyses of the whole cask structure under worst case conditions without an impact angle is given.
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Full-scale cask model
Following the demonstration of a sufficient correlation between experimental data and calculation results for the 1:2 scale cask, the underlying FE model was scaled up to a 1:1 scale model (Figure 6). Only the cask dimensions were enlarged without any other changes. It is postulated now, that the model derived in this way is a suitable representation of the original cask design.
a) 1:2 scale cask model.
b) 1:1 scale cask model.
Figure 6: FE model of the cask on top of the IAEA target. Figure 7 shows exemplarily the strain history at the cask bottom centre for the 1:1 scale model compared with the 1:2 scale model both calculated under worst case conditions, i.e. for an exactly flat impact (β = 0°). It can be seen that the impact time is approximately doubled while the maximum strains remain at the same level. The course of the strain is only expanded in time without a change of the characteristic behaviour as expected from theoretical considerations. Altogether the simulation 7
results for the drop of the original cask from a height of 0.3 m onto the test site foundation do not show any critical strains in the cask structure.
Figure 7: Comparison of strain histories at cask bottom centre for half-scale and full-scale cask drop simulation onto BAM’s IAEA target (impact angle=0°).
Figure 8: Comparison of stress histories at the cask bottom centre for the reference case, the flat impact onto the Gorleben site foundation from 6.0 m and the test result for the half-scale cask with non-zero impact angle.
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Target effects
The safety assessment of the original cask design is based on a postulated drop from a height of 0.3 m flat onto the BAM test site foundation representing an IAEA target (Figure 7). The strength of the BAM test site foundation is definitely higher than that of any real ground of existing storage facilities in Germany. Hence, the safety 8
evaluation for the test site foundation is sufficient. However this strategy is possible, only if the safety margins of the investigated cask design are high enough for the selected test scenario. This prerequisite is fulfilled for the given cask in the handling accident scenario with a very limited drop height of 0.3 m. Additionally the loadreducing effects of a more yielding real storage site foundation are investigated. For that purpose the foundation of the developed FE model is changed with the concrete foundation of the German Gorleben storage facility built with a 350 mm thick reinforced concrete layer, a 50 mm grout layer and 8 m deep subsoil. It must be emphasized that only simplified material models have been used because of the lack of verified constitutive laws describing the mechanical behaviour of the given storage site foundation including dynamic effects. The concrete layer is missing the reinforcement bars. Concrete and grout are described by von Mises plasticity. The subsoil is modelled as a linear-elastic material according to the common approach in soil engineering literature. In Figure 8 calculated stresses at the cask bottom centre for the reference case with BAM’s IAEA target (full-scale cask, 0.3 m drop height, impact angle 0.0°) are compared with a cask flat impact onto the real Gorleben site foundation from 6.0 m height as an example. The drop test result for the half-scale cask with the non-zero impact angle is also given for comparison purposes. In further calculations the maximum stresses in the cask structure were examined for drop heights from 0.3 m to 6.0 m for the special case of a full-scale cask drop onto the Gorleben site foundation. The maximum stresses are highly dependent on the foundation stiffness with values significantly below the stress level for the reference case. On the other hand the performed drop test with non-ideal impact angle still provided maximum stresses higher than at the flat impact onto the Gorleben foundation in the investigated drop height range. It must be taken into account however, that the calculated stresses in dependence of the drop height are encumbered with uncertainties because only simplified material models and possibly non-conservative material properties have been used. In particular the discussed relation between maximum stresses and drop height may not be generalized to other existing storage sites. Nevertheless, the parameter study shows clearly that the foundation stiffness is a dominant parameter and a drop height higher than 0.3 m may be acceptable in cases of yielding targets. An acceptable maximum handling height for an existing storage facility can be defined only on the basis of a verified FE model for its storage foundation. These findings show impressively that postanalyses are necessary to uncover the true test conditions and to demonstrate real safety margins.
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Conclusions
The accident scenario inside a storage facility has been investigated by a cask drop without impact limiters onto a nearly unyielding target. The test scenario and experimental results were shown. On one hand a comprehensive three-dimensional finite element model of the cask and the foundation was used to understand the experimental data, and on the other hand this model was verified by means of the measured data. After scaling of this model to original cask size it was used to calculate all stresses and strains in the whole cask as the basis for the safety 9
assessment. It could be shown that a post-analysis of a drop test is necessary to find the results for the worst case accident scenario. Additionally cask stresses are significantly smaller when the real and less stiff foundation of a storage facility is included in the FE calculations.
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