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Eingeladener Vortrag
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Validation of numerical simulation models for transport and storage casks using drop test results
(2012)
The safety assessment of new designs for transport and storage casks for radioactive materials is a challenging task accomplished using different methods such as prototype tests, model tests, calculations and analogy reflections. At BAM (Federal Institute for Materials Research and Testing), the test procedures for the mechanical IAEA (International Atomic Energy Agency) test conditions often start with preliminary finite element (FE) calculations mostly with a small-scale 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 series of drop tests consisting of different test sequences is performed.
Following the drop tests, numerical post-analyses are carried out. These analyses offer the possibility of a detailed calculation and assessment of stresses and strains in the entire test cask construction. The calculation results have to be carefully compared with the measurement data over the impact history to find out all relevant parameters for a realistic simulation of the impact scenario. The desired ideal boundary test conditions often cannot be met exactly during the drop tests. Therefore, the numerical post-analyses are carried out by using the real boundary conditions of the drop tests. The objective is to find a validated model, where the results of the numerical simulations satisfactorily meet the experimental results.
Under test conditions according to the IAEA transport regulations, casks are usually equipped with impact limiters and dropped onto a so-called unyielding target. In general, it is difficult to verify a complex FE model by using results from only one drop test because of the complex impact process and the complex structure of such packages. After each drop test, numerical post-analyses should be carried out. Only if all drop tests were simulated successfully by using the same FE model under different test conditions, it is possible to obtain a validated numerical model for further investigations. In this case the results of the numerical simulations meet satisfactorily the experimental results. In this paper a study is presented, where the influence of different components on the cask loading is investigated systematically.
Validation of Numerical Simulation Models for Transport and Storage Casks using Drop Test Results
(2012)
Metal seals of O-ring form are often used in lid-systems of transport and storage casks for radioactive waste in Germany. To investigate their mechanical behaviour, three dimensional (3D) finite element (FE) models were created using solid elements for all of the seal components. The material behaviour of each component is described with a unified static elastic-plastic material model. The total strain is defined as the sum of linear elastic strain and plastic strain with power-law hardening. The model was carefully validated by comparison of Simulation results with experimental results. The influence of material fluctuation of each seal component due to varying properties and the sensitivity of different seal diameters on the seal force are analysed and discussed.
The results show that the material properties of helical spring have major influence on seal force and that the influence of seal diameters is negligible small in the studied range. This is very important to use the test results from seals with small diameter for the assessment of seal behaviour with larger diameter as used in transport and storage cask.
Three-dimensional finite element analysis of O-ring metal seals considering manufacture tolerances
(2019)
Metal seals are widely used in various industrial branches with severe working conditions (e.g. high pressure, high temperature, corrosion, or radioactive radiation). For example, O-ring metal seals are applied in the closure lid system of transport and storage casks for radioactive materials to guarantee an approved specified leak-tightness and the safe enclosure of the radioactive inventory. Within safety assessments of those casks under normal or accident conditions during transport and long-term interim storage for several decades, numerical simulations of the thermo-mechanical behavior of metal seals by using finite element (FE) analyses are suitable and effective.
In general, finite element codes provide the possibility to construct a complex three-dimensional (3D) modelling of metal seals with solid elements, cf., where all components of the metal seals are modeled in detail. This modelling is complex but allows the consideration of underlying physical effects such as elastic-plastic deformation, thermal expansion, creep/relaxation, friction and possible local damage. Therefore, this approach permits the investigation and understanding of the complex behavior of metal seals in detail which can hardly be measured for all seal components.
In a prior work, the complex 3D modelling approach was applied to investigate the influence of each seal component on the global seal force by considering varying material properties and different seal diameters. In the present contribution, the influence of manufacturing tolerances is discussed. At first, the results of prior work are summarized. After that, the manufacturing tolerances of the studied O-ring metal seal types are described and the corresponding FE model with all individual components is introduced. Finally, the influence of varying manufacturing tolerances of each seal component on the global seal force is analyzed and discussed.
Metal gaskets in the lid system of transport and storage casks for radioactive material have to guarantee leak-tightness and safe enclosure of the radioactive inventory under normal and accident conditions during transport, in case of accidents and for the longterm interim storage. For safety assessments by nurnerical simulation of the thermomechanical behaviour of a lid system with metal gaskets, the finite element method offers three options to use: i) special gasket elements, ii) complex three-dimensional modeHing with solid elements, and iii) a simplified axisymmetric approach. Gasket elements can be adjusted by many parameters, but they give only a global representation of the experimentally observed gasket behaviour. For calculations of the entire cask including the lid system with gaskets, nonetheless this approach is recommended. To investigate the hardly measurable impact behaviour in detail or to extrapolate the long-term behaviour, a local modeHing of all parts of a gasket and their interactions with effects like elastic-plastic deformation, creep, relaxation, and friction is necessary. The three-dimensional model can describe the change of contact area between outer jacket of the gasket and flange dependent on the load conditions, what overcomes an essential limitation of special gasket elements. To simplify the problern of investigating the underlying physical effects, an axisymmetric lid system can be modelled with axisymmetric finite elements. Usually an approximation is only necessary for the helical spring of a metal gasket. This paper explains basic ideas for an adequate finite element simulation of cask lid systems with metal gaskets and their thermo-mechanical behaviour under specific load scenarios.