Jens Sterthaus

NUMERICAL APPROACH FOR CONTAINMENT ASSESSMENT OF TRANSPORT PACKAGES UNDER REGULATORY THERMAL TEST CONDITIONS

The requirements of the IAEA safety standards for Type B(U) packages include the thermal test as part of test sequences that represents accident conditions of transport. In comparison to mechanical tests, e.g., 9 m drop onto an unyielding target with short impact durations in a range of approximately 10 ms to 30 ms, the extended period of 30 min is defined in regulations for exposure of a package to a fire environment. Obviously, the required containment capability of the package has to be ensured not only after completing the test sequence but also over the course of the fire test scenario.Especially, deformations in the sealing area induced by the non-uniform thermal dilation of the package can affect the capability of the containment system. Consequently, thermo-mechanical analyses are required for the assessment.In this paper some aspects of finite element analysis (FEA) of transport packages with bolted closure systems under thermal loading are discussed. A generic FE model of a cask is applied to investigate the stress histories in the bolts, lid, and cask body as well as the deformations in the sealing area and the compression conditions of the gasket. Based on the parameter variations carried out, some recommendations in regard to modeling technique and results interpretation for such kind of analyses are finally given.Copyright

Methodological Aspects for Numerical Analysis of Lid Systems for SNF and HLW Transport Packages

The regulatory compliance of the containment system is of essential importance for the design assessment of transport packages for radioactive materials. The requirements of the IAEA transport regulations SSR-6 for accident conditions implies high load on the containment system of Type B(U) packages. The integrity of the containment system has to be ensured under the mechanical and thermal tests.The containment system of German transport packages for spent nuclear fuel (SNF) and high level waste (HLW) usually includes bolted lids with metal gaskets. BAM Federal Institute for Materials Research and Testing as the German competent authority for the mechanical and thermal design assessment of approved transport packages has developed the guideline BAM-GGR 012 for the analysis of bolted lid and trunnion systems. According to this guideline the finite element (FE) method is recommended for the calculations. FE analyses provide more accurate and detailed information about loading and deformation of such kind of structures. The results allow the strength assessment of the lid and bolts as well as the evaluation of relative displacements between the lid and the cask body in the area of the gasket groove.This paper discusses aspects concerning FE simulation of lid systems for SNF and HLW transport packages. The work is based on the experiences of BAM within safety assessment procedures. The issues considered are the assessment methods used in the BAM-GGR 012 for bolted lid systems along with the nominal stress concept which is applied for bolts according to that guideline. Additionally, modeling strategies, analysis techniques and the interpretation of the results are illustrated by the example of a generalized bolted lid systems under selected accident conditions of transport.Copyright

Methodological aspects for finite element modelling of lid systems for Type B(U) transport packages

Abstract The regulatory compliance of the containment system is of essential importance for the assessment process of Type B(U) transport packages. The requirements of the International Atomic Energy Agency safety standards for transport conditions imply high loading on the containment system. The integrity of the containment system has to be ensured in mechanical and thermal tests. The containment system of German spent nuclear fuel and high level waste transport packages usually includes bolted lids with metal gaskets. The finite element (FE) method is recommended for the analysis of lid systems according to the guideline BAM-GGR 012 for the assessment of bolted lid and trunnion systems. The FE analyses provide more accurate and detailed information about loading and deformation of such kind of structures. The results allow the strength assessment of the lid and bolts as well as the evaluation of relative displacements between the lid and the cask body in the area of the gasket groove. This paper discusses aspects concerning FE simulation of lid systems for type B(U) packages for the transport of spent nuclear fuel and high level waste. The work is based on the experiences of the BAM Federal Institute for Materials Research and Testing as the German competent authority for the mechanical design assessment of such kind of packages. The issues considered include modelling strategies, analysis techniques and interpretation of results. A particular focus of this paper is on the evaluation of the results with regard to FE accuracy, influence of the FE contact formulation and FE modelling techniques to take the metallic gasket into account.

Moisture content of wood: influence on mechanical behaviour of wood filled impact limiters and importance for quality surveillance during manufacturing

Abstract The moisture content of wood is known to have a significant influence on the wood’s mechanical properties. Using wood as an energy absorber in impact limiters of packages for the transport of radioactive material, it is of particular importance to ensure the moisture content and thus relevant mechanical properties to be in specified limits. The paper surveys the influence of wood moisture content on the mechanical properties of wood. Different measuring methods are discussed with respect to in situ applicability, accuracy and effort. The results of an experimental analysis of the accuracy of hand held moisture metres using the electrical resistance method are discussed. Conclusions are drawn regarding the measurement of moisture content of wood upon delivery as well as of complete impact limiter assemblies. Requirements for quality surveillance during manufacturing of wood filled impact limiters are derived and it is exemplified how to meet them. Construction, manufacturing and inspection of impact limiter encapsulation with regard to leak tightness are addressed.

Numerical Analysis of Bolted Trunnion Systems of Packages for Radioactive Materials

Packages for the transport of radioactive material are generally equipped with particular components for crane operations and supporting the package during transport. As an example of such components the bolted trunnion system of heavy packages are analysed in this paper.According to the German guideline BAM-GGR 012 created by the BAM Federal Institute for Materials Research and Testing for analysis and assessment of bolted lid and trunnion systems, the finite element (FE) method is to be used preferably in the analysis of such kind of structures to obtain more accurate and detailed information about their stressing. The FE model of a trunnion system should include the trunnion itself, the bolts and an appropriate part of the packaging wall with necessary contact conditions on all interfaces between these components. The use of solid finite elements, which is generally recommended in the BAM-GGR 012 guideline, leads to local stress and strain fields as a result of the calculation. The modelling strategy, e.g. the way of modelling interaction between the components of the trunnion system, has an essential impact on these local values.The effect of different assumptions in a FE model is demonstrated in this paper using the calculation results for a reference trunnion system. The approaches for preparation and interpretation of results in connection with local or nominal assessment criteria are discussed as well.Copyright

Material Characterization and Modeling Within Safety Analysis of Packages for Transport of Radioactive Material

In mechanical design safety assessment of packages for the transport of radioactive material the International Atomic Energy Agency (IAEA) regulations [1, 2] require the use of one of four methods of demonstration of compliance with regulations: tests with prototypes, reference to previous satisfactory demonstrations, tests with scaled models as well as calculation or reasoned argument. In all cases characterization of material behavior is necessary. The need for material characterization can be split into three groups: characterization for material modeling in the design and verification process, determination of assessment criteria and inspections during manufacturing.The paper deals primarily with material characterization for the material modeling in the design phase and authorities approval process. Main package components are discussed: cask body and lids, screws, trunnions, impact limiter and basket (structure holding the content in specified position). Depending on the material used for these components this paper shows exemplary a possible set of material investigations in order to allow modeling in design and assessment during the approval process.Copyright

Experimental Setup for the Determination of Mechanical Solder Materials Properties at Elevated Temperatures

The fabrication of microelectronic and micromechanical devices leads to the use of only very small amounts of matter, which can behave quite differently than the corresponding bulk. Clearly, the materials will age and it is important to gather information on the (changing) material characteristics. In particular, Young’s modulus, yield stress, and hardness are of great interest. Moreover, a complete stress-strain curve is desirable for a detailed material characterization and simulation of a component, e.g., by Finite Elements (FE). However, since the amount of matter is so small and it is the intention to describe its behavior as realistic as possible, miniature tests are used for measuring the mechanical properties. In this paper two miniature tests are presented for this purpose, a mini-uniaxial-tension-test and a nanoindenter experiment. In the tensile test the axial load is prescribed and the corresponding extension of the specimen length is recorded, both of which determines the stress-strain- curve directly. The stress-strain curves are analyzed by assuming a non-linear relationship between stress and strain of the Ramberg-Osgood type and by fitting the corresponding parameters to the experimental data (obtained for various microelectronic solders) by means of a non-linear optimization routine. For a detailed analysis of very local mechanical properties nanoindentation is used, resulting primarily in load vs. indentation-depth data. According to the procedure of Oliver and Pharr this data can be used to obtain hardness and Young’s modulus but not a complete stress-strain curve, at least not directly. In order to obtain such a stress-strain-curve, the nanoindentation experiment is combined with FE and the coefficients involved in the corresponding constitutive equations for stress and strain are obtained by means of the inverse method. The stress-strain curves from nanoindentation and tensile tests are compared for two mate-rials (aluminum and steel). Differences are explained in terms of the locality of the measurement. Finally, material properties at elevated temperature are of particular interest in order to characterize the materials even more completely. We describe the setup for hot stage nanoindentation tests in context with first results for selected materials.

Analysis of Bolted Flange Joints for Lids of Packages for Radioactive Materials

The design of bolted flange joints for the German double barrier closure system of spent fuel and high-level waste transport and storage casks is described. Additionally, the basic load assumptions are presented for service conditions and for hypothetical accident scenarios. The modelling details and the results of numerical analyses are discussed with respect to those of experiments. The treatment of the numerical results for the bolts is presented considering safety factors derived by nominal stress concepts. The lids of the double barrier system are designed for a specified helium leak tightness. In addition to interior pressure, the accelerations during usual service and in particular during accidental drop tests are considered. The sliding and the gap formation at the gasket surfaces is analysed with respect to drop test loads. The consequences for the containment analysis are discussed on basis of the tightness characteristics. Concerning the bolts, the transfer of local stresses provided by numerical analysis to nominal stress values is presented. This transfer enables the use of safety factors based on nominal stress concepts for the local stresses given by numerical analysis.Copyright