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Eingeladener Vortrag
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An extensive series of large deformation crushing tests with spruce wood specimens was conducted. Material orientation, lateral constraint and loading rate were varied. Regarding material orientation, a reduction in the softening effect and the general force level was observed with a higher fiber-load angle. A comparison with characteristics gained by application of Hankinson's formula showed discrepancies in compression strength and the beginning of the hardening effect. Lateral constraint of the specimens caused a multiaxial stress state in the specimens, which was quantified with the applied measuring method. Further, a higher force level compared to specimens without lateral constraint and significant hardening effect at large deformations resulted. Thus, the influence of a multiaxial stress state on the force level could be determined. An increase in the loading rate led to higher force levels at any displacement value and material orientation.
TN®24 E, a new package design, was developed and applied for by Areva TN for German transport approval certificate. The certificate was issued by BfS, Federal Office for Radiation Protection, on 24th of July 2013. The package is designed as a dual purpose cask for transport and storage of up to 21 PWR spent fuel assemblies from German NPPs. BAM is the German competent authority responsible for the design assessment of RAM packages regarding mechanical and thermal safety cases, activity release analysis and all issues of quality assurance during manufacturing and operation of packages. Certain assessment experiences as well as new developments resulting from the BAM TN®24E approval procedure are presented. The mechanical safety case of the TN®24E is based mainly on finite element calculations, which were verified by the TN®81 1/3 scale drop test program performed at BAM. Thermal analyses rely upon calculations, while the activity release criterion is based upon leakage rate results of TN®81 drop tests. The BAM-GGR 012 guideline for the analysis of bolted lid and trunnion systems has been fully implemented. Due to requirements by BAM, AREVA TN developed a new assessment strategy for fracture mechanical evaluation of welding seams. The material qualification and documentation is also an important aspect of BAM assessment; the qualification of borated aluminum basket material, the determination of strength values for thermal aged hardened aluminum alloys for the basket or consideration of high burn-up fuel assemblies are remarkable issues in this context. In addition, the consideration of the material compatibility, especially taking into account a transport after 40 years of dry interim storage in German facilities, has gained significant importance in the licensing process of the TN®24E. Next to obvious mechanical issues such as the assessment of shell ovalization under 9 m drop test scenario and its impact on basket load, thermo-mechanical interactions had to be addressed in the safety case. Due to BAM requirements, AREVA TN performed a full thermo-mechanical analysis of the cask behavior under fire test conditions.
Federal Institute for Materials Research and Testing (BAM) is the responsible authority for the
assessment of mechanical and thermal safety as well as quality issues within the licensing procedure
for transport casks for radioactive materials. The assessment includes a brittle fracture analysis of all
relevant cask components.
For cask bodies where a steel bottom is welded to a steel shell, specimens for testing of sufficient
fracture resistance can normally not be extracted directly from the welding seam area of the cask.
Therefore, a methodology has to be developed to ensure compliance with the safety analysis
considerations and the approved design specification. This paper describes such a methodology
including brittle fracture analysis and de-termination of required material properties as well as aspects
of quality assurance during manufacturing.
The moisture content of wood is known to have a significant influence on the woods mechanical properties. Using wood as an energy absorber in impact limiter of packages for the transport of radioactive material, it is of particu-lar 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 ana-lysis of the accuracy of hand-held moisture meters using the electrical resistance method are discussed. Conclu-sions are drawn regarding the measurement of moisture content of wood upon delivery as well as of complete im-pact limiter assemblies. Requirements for quality surveillance during manufacturing of wood filled impact limiter 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.
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.
Diffusion of Cr, Fe, and Ti ions from Ni-base alloy inconel-718 into a transition alumina coating
(2012)
Heat treating metals at high temperatures trigger diffusion processes which may lead to the formation of oxide layers. In this work the diffusion of Cr, Fe and Ti into an alumina coating applied to Inconel-718 is being investigated. Mass gain measurements, UV–vis spectroscopy and transmission electron microscopy were applied in order to study the evolution of the diffusion process. It was found that mainly Cr as well as minor amounts of Fe and Ti are being incorporated into the alumina coating upon prolonged heat treatment at 700 °C. It could be shown that alumina coatings being void of Cr have the same oxidation related mass gain as uncoated samples. However, incorporation of Cr into the alumina coating decreased their mass gain below that of uncoated substrates forming a Cr oxide scale only.
BAM Federal Institute for Materials Research and Testing is the competent authority for mechanical safety assessment of transport packages for radioactive material in Germany. The further development of state-of-the-art technology concerning assessment methods is essential for a qualified work of involved designers and authority experts. The paper gives an example of current development done to improve understanding and modeling capabilities of wood filled impact limiter. In order to reduce the loads applied to the package containment, which result from regulatory drop tests, most packages are protected by energy dissipating impact limiter. Wood, encapsulated by steel sheets, is one of the materials typically used for energy dissipation in these impact limiter. Very often, mechanical safety cases regarding the 9 m drop test are performed computationally, where it is essential to use reliable and verified computational methods and models. In this context, the paper presents an approach for a finite element material model for wood. Thereby, the mechanical behavior of wood under compression loading is the focus of the development work. Additionally, material orientation as well as strain rate, temperature and lateral constraint may vary. A large number of experiments, particularly compression tests, was designed and performed to establish an adequate experimental database for modeling verification. The experimental results enabled the derivation of necessary requirements: The material model has to take into account strain rate and temperature dependencies as well as the anisotropic characteristics of the material, a proper yield criterion, flow rule and hardening law. Such a material model is currently not available in established commercial dynamic finite element codes. Thus it is necessary to create a user-defined material model considering the mentioned requirements. A first step was done by determining a yield surface as well as detecting flow and hardening mechanisms from experimental force-deflection curves. In a next step the LS-DYNA material model MAT_75 was altered according to conclusions of former BAM development work, regarding the modeling of post-peak softening as a function of lateral constraint. Future research will contain the further development, implementation and verification of a material model for wood.
Porous ceramics prepared by the foam replication technique have a high porosity and low mechanical stability. It has been reported that coating such porous ceramics with gelatin allows for an improved compressive strength. Little details regarding the influence of important gelatin parameters such as concentration, temperature and drying conditions as well as bloom number which is a measure of the gel rigidity, on this toughening effect are available. This paper investigates the influence of these parameters on compressive strength of gelatin coated porous hydroxyapatite ceramics. It was found that concentration in the gelatin sols has a marked impact whereas sol temperature, bloom number and differing conditions during subsequent drying have only little influence on the compressive strength of the coated ceramics.
This paper demonstrates exemplarily how numerical and experimental approaches can be combined reasonably in mechanical assessment of package integrity according to the IAEA regulations. The paper also concentrates on the question about how static mechanical approaches can be applied, and what their problems are in relation to dynamic calculation approaches. Under defined impact tests, which represent accident transport conditions, the package has to withstand impact loading, e.g. resulting from a 9 m free drop onto an unyielding target in sequence with a 1 m puncture drop test. Owing to the local character of the interaction between the puncture bar and the cask body, it is possible to develop a dynamic numerical model for the 1 m puncture drop which allows an appropriate simulation of the interaction area. Results from existing experimental drop tests with prototype or small scale cask models can be used for verification and validation of applied analysis codes and models. The link between analysis and experimental drop testing is described exemplarily by considering a regulatory 1 m puncture bar drop test onto the cask body of a recently approved German high level waste transport package. For the 9 m drop test of the package, it is difficult to develop a dynamic numerical model of the package due to the complexity of the interaction between cask body, impact limiters and unyielding target. Dynamic calculations require an extensive verification with experimental results. The simulation of a 9 m drop of a package with impact limiters is thereby often more complex than the simulation of a 1 m puncture drop onto the cask body. A different approximation method can be applied for the consideration of dynamic effects on the impact loading of the package. In a first step, maximum impact force and rigid body deceleration of the cask body during the impact process can be calculated with simplified numerical tools. This rigid body deceleration can subsequently be applied on a verified static numerical model. Dynamic effects, which cannot be covered by the static numerical analysis, have therefore to be considered by using an additional dynamic factor. The paper describes this approach exemplarily for a 9 m horizontal drop of a typical spent fuel cask design.
Impact limiting components of packages for the transport of radioactivematerials are often designed as wood filled steel constructions. Wood absorbs major part of the impact energy in order to minimise the impact load acting upon the containment. Dynamic impact experiments with wood filled impact limiters showed different crushing mechanisms for axially loaded wood depending on their lateral constraint. Tests on spruce wood samples (Picea abies) were performed in order to clarify the influence of strain rate from static to 30 s-1 on a) compression strength, b) stress at a global strain level of 50%, and c) energy absorption capacity at 50% deformation, including statistical evaluation of the results. Results were as follows: strain rate increase led to significantly higher compression strength, stress and strain energy at a strain level of 50%. Lateral strain restriction had no effect on compression strength; it had a significant effect on stress and strain energy at strain level of 50%. Therefore, the definition of a general yield curve for wood under large deformations is not possible, the yield curve has to be chosen taking into account lateral constraints.
Impact limiters of packages for transport of radioactive materials are in Germany typically steel-wood-sandwich-constructions, combining a relatively stiff steel structure bolted to the cask body, outer steel plates and different types of wood. By crushing the wood-steel-sandwich-structure between unyielding target and cask, kinetic energy of 9-m-free-fall is absorbed. Main energy absorber is wood under a high level of deformation. Wood under large deformations exhibits destruction of the fibre matrix. By analysing compression of impact limiter wood after drop tests with prototype casks for radioactive material, underlying mechanisms of wood crushing and corresponding energy absorption under large deformations are identified. Softening occurring at compression of wood is a function of lateral strain restriction of wood. Against the background of continuum mechanics an analogous model for compression of fibre bundle is presented. The model takes lateral strain restriction as triaxiality of stress state into account. Further modelling possibilities for wood with a continuum approach are described. Different material laws in the explicit Finite Element code LS-DYNA are analysed for possible application on the analogous model for the fibre bundle. Small scale compression tests with wooden specimens were modelled in order to evaluate the ability of different modelling techniques to simulate softening. Although modelling of compression of wood under large deformations is possible, softening could not be simulated purposefully. A drop test of a cask with impact limiting devices similar to existing impact limiters is simulated with different material laws for wood. Behaviour of impact limiting devices could not be simulated universally including influence of lateral strain restriction; nevertheless loading of the cask by crushing of impact limiter could be simulated purposefully. Verification with experimental results is compulsory.
Alumosols in combination with well dispersed corundum were successfully used to form up to 2.5 ìm thick coatings on the Ni-base alloy SC16. These coatings withstood heat treatments at 700 °C in a water vapour containing atmosphere. The heat treatment caused formation of delta-alumina in the coating and diffusion of chromium and titanium into the coating.
Dynamic and quasi-static FE calculation of impact from 9 metre drop of spent fuel transport cask
(2009)
The drop from 9 m height onto a rigid target is one of the required proofs of safety for packages of radioactive materials. Direct dynamic finite element method (FEM) calculations and combined two step analytical quasi-static finite element (FE) calculations are both applied for the simulation of a 9 m drop test. This paper gives a comparative example of both approaches for the 9 m side drop simulation of the spent fuel transport cask with wood filled impact limiter. The model for dynamic FEM consists of cask body and impact limiter. Detailed material properties and geometry descriptions from each component of the impact limiter are required. The results (stress fields in the cask body) are obtained directly from the calculation. The combined method provides as intermediate results the force-deformation characteristic of impact limiter. The maximum impact limiter force determined by the law of energy conservation during the drop is then - in a second step - applied on the cask body in a quasi-static FE model in order to calculate the stresses. In this paper, the rigid body deceleration and the maximum stress in the middle of the cask body are used for the comparison between the dynamic FEM and the combined method. Similar maximum rigid body deceleration-time curves were obtained by both methods for the horizontal 9 m free fall. Concerning the stress in the cask body the dynamic FEM results oscillate about values calculated by the combined quasi-static approach. If the combined quasi-static approach is used in the safety assessment of a cask, a suitable factor has to be applied on its results to take into account the additional dynamic effects.
Modified Yoldas sols can be used to prepare crack-free well adherent layers on different substrate materials such as sodalime glass, corundum and metal (IN-718). These layers having thicknesses of 0.6 µm are strain tolerant and withstand thermal cycling between 810 °C and room temperature. Introducing alpha-Al2O3 filler particles into modified Yoldas sols allows the preparation of thicker layers of 1.6 µm, which as well can be thermally cycled without noticeable delaminations. Chemical bonding seems to be the predominant layer bonding mechanism.