Karsten Müller

Comparison of experimental results from drop testing of spent fuel package design using full scale prototype model and reduced scale model

Abstract This paper presents a comparison between full scale prototype and reduced scale model drop test data in regard to similarity mechanics. Together with a current BAM research project, the paper contributes to the further development of mechanical evaluation methods for safety assessment of RAM transport and storage packages including the transferability of package impact response from reduced scale models to full scale packages.

Measurement techniques and preliminary results of drop tests with full scale spent fuel transport and storage casks

Abstract The experimental testing and quantitative stress analysis of new cask designs becomes more and more important with increasing size of casks and their impact limiters, in order to assess the safety margins as well as for calculation validation reasons. Full scale drop testing of spent fuel transport and storage casks is performed by the Federal Institute for Materials Research and Testing within the type B package approval procedure and for safety demonstration reasons respectively. In drop tests for demonstrating ability of a package to withstand normal and accident conditions of transport, the instrumentation of a specimen is an important tool to evaluate its mechanical behaviour during impact. Generally, the instrumentation incorporates the measurement of strains and accelerations at the package. Test results as deceleration–time and strain–time functions constitute a main basis for the validation of assumptions in the safety analysis, for the evaluation of calculations based on finite element methods and extrapolation of scale model testing on full sized package within approval design tests. Besides, all of the above mentioned instrumentation, the flatness of the sealing area and the circularity of the cask lid flange are determined by using three-dimensional coordinate measurements to evaluate the mechanical impact behaviour of the cask flange region after the drop test sequence. Indispensable are measurements concerning leak tightness of the closure lid system. Additionally, specific photogrammetric measurements are used to characterise the conditions of the closure lid system after drop test in more detail. The preparation and interpretation of the results is in progress and indicates possible correlations between the leak tightness rate and measures of the relative movement of the lid.

Drop test program with half scale model CASTOR® HAW/TB2

Abstract Federal Institute for Materials Research and Testing (BAM) is the competent authority for mechanical and thermal safety assessment of transport packages for spent fuel and high level waste in Germany. In context with package design approval of the new German high level waste cask CASTOR® HAW28M, BAM performed several drop tests with a half scale model of the CASTOR® HAW/TB2. The cask is manufactured by Gesellschaft für Nuklear Service mbH and was tested under accident transport conditions on the 200 tons BAM drop test facility at the BAM Test Site Technical Safety. For this comprehensive test program, the test specimen CASTOR® HAW/TB2 was instrumented at 21 measurement planes with altogether 23 piezo resistive accelerometers, five temperature sensors and 131 triaxial strain gauges in the container interior and exterior respectively. The strains of four representative lid bolts were recorded by four uniaxial strain gauges per each bolt. Helium leakage rate measurements were performed before and after each test in the above noted testing sequence. The paper presents some experimental results of the half scale CASTOR® HAW/TB2 prototype (14 500 kg) and measurement data logging. It illustrates the extensive instrumentation and analyses that are used by BAM for evaluating the cask performance to the mechanical tests required by regulations. Although some of the quantitative deceleration, velocity and strain values cannot be shown because of confidentially issues, they are provided qualitatively to illustrate the types of measurements and methodologies used at BAM.

Characterisation of shock absorber deformation by optical surface digitisation

Abstract The performance of shock absorbers has been tested in specific regulatory drop tests onto an unyielding target at the BAM drop test facility. Optical surface digitisation methods have been used to measure, analyse and evaluate permanent deformations of shock absorbers more systematically. The measurement principle of the used fringe projection technique and its technical application to shock absorber investigations is explained in detail. Furthermore, examples of shock absorber testing results as well as final data visualisation are given in the present paper.

Impact target characterisation of BAM drop test facility

Abstract BAM safety related research of containers for radioactive material focuses on advanced mechanical safety assessment methods for verification of the structural integrity and leak tightness under normal conditions of transport and hypothetical accident conditions during transport and storage. An essentially unyielding target with a rigid surface is required for impact tests performed for package approval according to IAEA regulations. In addition to specification of a target, e.g. with a combined mass more than 10 times that of the specimen for drop tests, unyielding target characteristics have been investigated with various package designs and different impact tests. The unyielding target of the BAM drop test facility, a reinforced concrete block together with an embedded and anchored mild steel plate, provides relatively large mass and stiffness with respect to the packages being tested. For monitoring reasons accelerometers and strain gauges are embedded in the concrete block of the foundation at several positions. Additionally, dynamic impact responses like vibrations and rigid body motion can be measured by seismic accelerometers. The mechanical characterisation of the targets rigidity is based on experimental results from various drop tests. Test containers with weights of 181 000 kg, 127 000 kg and 8010 kg hit the target with velocities up to 13˙5 m s–1 in the horizontal and vertical drop positions. The rigidity of the impact target can be demonstrated with experimental results confirmed by analytical approaches. Some conclusions can be drawn about experimental testing as well as analytical calculations in order to compare impact effects.

Mechanical design assessment approaches of actual spent fuel and HLW transport package designs

Abstract In recent years, BAM Federal Institute for Materials Research and Testing finalised the competent authority assessment of the mechanical and thermal package design in several German approval procedures of new spent fuel and high level waste package designs. The combination of computational methods and experimental investigations in conjunction with materials and cask components testing is the most common approach to mechanical safety assessment. The methodology in the field of safety analysis, including associated assessment criteria and procedures, has evolved rapidly over the last years. The design safety analysis must be based on a clear and comprehensive safety evaluation concept, including defined assessment criteria and constructional safety goals. In general, for new package designs, the implementation of experimental package drop tests in the approval process should be obligatory. Additionally, pre- and post-test calculations as well as components or material testing could be important. The extent to which drop tests are necessary depends on the individual package construction, the materials used and identified safety margins in the design.

Internal cask content collisions during drop test of transport casks for radioactive materials

Abstract In transport casks for radioactive materials, significantly large axial and radial gaps between cask and internal content are often present because of certain specific geometrical dimensions of the content (e.g. spent fuel elements) or thermal reasons. The possibility of inner relative movement between content and cask will increase if the content is not fixed. During drop testing, these movements can lead to internal cask content collisions, causing significantly high loads on the cask components and the content itself. Especially in vertical drop test orientations onto a lid side of the cask, an internal collision induced by a delayed impact of the content onto the inner side of the lid can cause high stress peaks in the lid and the lid bolts with the risk of component failure as well as impairment of the leak tightness of the closure system. This paper reflects causes and effects of the phenomenon of internal impact on the basis of experimental results obtained from instrumented drop tests with transport casks and on the basis of analytical approaches. Furthermore, the paper concludes the importance of consideration of possible cask content collisions in the safety analysis of transport casks for radioactive materials under accident conditions of transport.

Use of Charpy impact testing to evaluate crack arrest fracture toughness

Abstract Indirect measurements are commonly used for characterizing the crack arrest behaviour of ferritic structural steels instead of measuring the standard crack arrest fracture toughness. In the present study, correlations are proposed for estimating the nil-ductility temperature (NDT) and the crack arrest fracture toughness (KIa) from transition temperatures, based on instrumented Charpy-V crack arrest load information. Indirect experimental measures of the investigated pressure vessel steel show, that the transition temperature, T(Fa=0), at the arrest of the brittle crack is equivalent to the drop-weight nil-ductility transition temperature, TNDT. Furthermore, the transition criteria used are the 4kN crack arrest force from Charpy impact tests and the mean crack arrest fracture toughness of 100 MPa√m according to the master curve approach. Correlations between transition temperatures, T(Fa=4kN), T(KIa), and TNDT, which were proposed for various structural steels, work very well for the 18X2MFA material. The estimates of the correlation of the temperature for Fa=4kN with the temperature at 100 MPa√ level for a mean KIa “master” curve show that approximations of crack arrest fracture toughness values are possible.

Results of full scale CONSTOR® V/TC prototype 9 m horizontal drop test

Abstract In the context of the research on the mechanical safety of packages for radioactive material, full scale drop tests with spent fuel and high activity waste transport and storage casks have been performed by the Federal Institute for Materials Research and Testing (BAM). The research reflects national and international interest in acquiring comparative knowledge of full and reduced scale model drop tests as well as in finite element calculations. This paper presents the experimental, analytical and first numerical results of the full scale drop test with the full scale CONSTOR® V/TC prototype, manufactured by GNS, Gesellschaft für Nuklear-Service mbH, Germany. The prototype was tested by BAM in a 9 m horizontal drop test onto the unyielding target of the BAM drop test facility in Horstwalde, Germany.

Comparison of experimental results and numerical simulations of penetration tests with damping concrete

The shock absorbing material damping concrete is for the foundation in dry interim storage facilities for radioactive waste in Germany. In case of a potential cask drop damping concrete minimizes the mechanical loads to the cask. In course of safety analyzes this accident scenario is considered by numerical simulations using the finite element method. To get reliable results of numerical simulations a suitable material model is needed to take the characteristics of damping concrete into account. Due to the lack of sufficient material knowledge a research project was started to characterize the material’s behavior under different load conditions. This paper presents the test program to analyze the material behavior of damping concrete which is characterized by large volume change and strain rate hardening dependence. The determined Parameters were used to adapt an existing material model of the FE-code ABAQUS®. This model has to handle the mechanical damage behavior of damping concrete which occurs under compression and shear loads during a potential cask drop. To verify the material model numerical simulations are compared with dynamic penetration tests, which were conducted with specimens assembled similar to the real application of the damping concrete footings. The transferability of the material model to a real accident scenario was verified by a drop test with a full-scale cask on a damping concrete footing.