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The safety of transport packages may be demonstrated by numerical calculation of load scenarios defined in the IAEA regulations. Possible handling accidents of casks at interim storage sites or in a final repository are typically analyzed by dynamic finite element computations. In each case the investigated load scenario must be transferred into a mathematical model. Secondly the mathematical model must be transferred into a numerical model. Reliable finite element models should be developed by assembling verified sub-models of components. The finite element mesh, material modeling, initial and boundary conditions, contact definitions, and time integration as well as the benefit of pre- and post-calculations are discussed. The paper presents lessons learnt from modeling dynamic test scenarios for finite element analyses over the years.
Within the last years BAM has carried out numerous drop tests with prototype casks made of ductile cast iron onto targets according to the requirements for final disposal of non-heat generating waste in the German KONRAD repository. The results have shown that the target specifications in the acceptance criteria have to be defined more accurately to get reproducible test results with high precision. Hence, a suitable test stand foundation was developed with much effort. The integrity of the upper concrete layer of this target must be preserved during a test.
Recently the geometrical properties of a tested cubic cast iron container led to a concentration of the impact forces beneath the container walls. The target was damaged strongly with the consequence of inadmissible reduction of cask stresses. For that reason the target construction was modified. However, the basic design was not changed. A prefabricated concrete slab was still joined by a mortar layer to the IAEA target of the BAM drop test facility. In the course of the optimization of the test stand foundation the concrete slab dimensions and the reinforcement were enlarged. During the drop test repetition the target kept intact. Additionally, the mechanical behavior of the cast iron container and the target was analyzed by finite element calculations.
This improved target construction is suggested as a reference target for drop tests with casks whose mass and base area are covered by the container types VI or VII respectively according to the KONRAD repository acceptance criteria.
The measurements during the drop tests with cast iron casks have provided the strains on the cask surface at selected positions. This allows the verification of finite element simulations of drop tests which show the stress distribution also inside the component. In September 2008 a drop test was carried out with a cylindrical cast iron cask containing an artificial material defect which was designed under consideration of critical stress states in the cask body. This drop test could demonstrate the safety against failure by fracture of a cask made of a special cast iron with reduced fracture toughness.
The safety evaluation of cask components made of ductile cast iron includes investigations to prevent brittle fracture. Generally, ductile cast iron is endangered by brittle fracture especially at low temperatures (down to -40°C) and in combination with existing crack-like material defects. An applicable method is the assessment of fracture resistance using fracture mechanics according to the IAEA guidelines. The approach is based on the prevention of fracture initiation. For application of these principles for drop loads, account must be taken both of dynamic stresses within the component and dynamic material behavior. Basically, the dynamic stress intensity factor of postulated pre-existing crack-like defects is compared with the dynamic fracture toughness of the material. Applicable numerical and experimental methods for the safety assessment of cask components are demonstrated for the case of an artificially pre-cracked cylindrical cast iron cask which undergoes dynamic loading conditions as result of the hard impact between the cask and a concrete target. The proposed evaluation procedure is a combination of numerical and experimental steps. Exemplarily, the calculated stress intensity factor is compared with measured fracture toughness values from single edge notched bending specimens.
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.
Practical methods for the safety assessment of postulated crack-like material defects of cubic containers made of ductile cast iron are presented. A formula for the stress intensity factor of a crack in a fillet with a radius from 20 to 200 mm under static load conditions is given. From that, an assessment diagram is derived for the critical depth of a crack in a fillet. The formula may be used to estimate the stress intensity factor of a dynamically loaded crack for special cases. As an application the results of the estimation procedure are compared with the results of a dynamic finite element calculation.
Der erhöhte Zusatz metallischer Reststoffe aus der Stilliegung und dem Rückbau
kerntechnischer Einrichtungen bei der Herstellung von Behältern aus Gusseisen mit
Kugelgraphit führt einerseits durch das Recycling zu einer Reduzierung der Menge
des endzulagernden radioaktiven Metallschrottes, aber andererseits auch zu einer
nachteiligen Beeinflussung sicherheitsrelevanter Werkstoffeigenschaften. Unter den
hochdynamischen Beanspruchungsbedingungen der nicht mit Stoßdämpfern ausgerüsteten Behälter liegt die Bruchzähigkeit um mehr als die Hälfte unter dem Mindestwert für die bisher genehmigten Werkstoffqualitäten, was eine Anpassung der sicherheitstechnischen Bewertungsmethoden für Behälter aus Gusseisen mit Kugelgraphit an den neuen Werkstoff erfordert. Im ersten Teil des Projektes EBER wurden dazu die Grundlagen erarbeitet. In diesem zweiten Teil wird die Beanspruchungsanalyse der Behälter weiter vervollkommnet. Die Werkstoffdämpfung wird mittels einer Analyse der Ausbreitung von Spannungswellen im Gusseisen berechnet. Die Behälterstruktur wird genauer als bei den bisherigen Berechnungen in den Modellen abgebildet. Im Mittelpunkt steht die Weiterentwicklung des sicherheitstechnischen Bewertungskonzeptes. Zur Bewertung von rissartigen Werkstofffehlern in geometrisch
komplexen Behälterpartien wurden statische und dynamische bruchmechanische nu-
merische Berechnungen eines Risses in einer Hohlkehle und in der Mitte einer Behälterwand durchgeführt. Die Berechnungsergebnisse wurden für spezielle Anwendungsfälle verifiziert, für die bereits Lösungen bekannt sind. Die gewonnenen Erkenntnisse
wurden in einfach zu handhabende Bewertungsdiagramme umgesetzt. Das sicherheitstechnische Bewertungskonzept wurde schließlich bei einem Fallversuch mit einem
rissbehafteten Prototypbehälter definierter Werkstoffqualität überprüft. Eine Minderung
der Beanspruchung infolge konstruktiver Verbesserungen (größere Hohlkehlen, Bodenleiste) durch den Behälterkonstrukteur konnte dabei bestätigt werden.