- Experimental Container Tests
- Measurement Value Logging and Data Analysis
- The Finite Element Method (FEM)
- Drop Tests with Prototype Shock Absorbers
- EBER Research Project
Experimental Container Tests
Within the framework of the approval process based on dangerous goods regulations and nuclear legislation, it is necessary to perform experimental type tests for containers to be used for the transport, intermediate storage and final storage of radioactive materials. These include drop tests, leak tightness tests and fire tests for containers, as well as tests for special form radioactive materials.
Particularly, the mechanical tests include the test sequence involving a drop from 9 metres onto an unyielding foundation and a drop from 1 metre on a rigid steel bar, respectively in that particular position in which the most serios damage of the package occurs. Due to the different container components or component areas, it is necessary to examine a number of different drop test positions for each test specimen.
Elaborate testing and measuring facilities are available at BAM for conduction corresponding test activities. Regular work performed in the BAM drop test facility includes the following
- 9 metre drop onto an unyielding foundation
- 1 metre drop on a steel bar (puncture test)
- 9 metre drop of a steel plate onto the test object (crush test)
- 5 metre drop onto the permanent storage foundation
Comprehensive experience in testing and research provide a substantial knowledge base for the up to 60 minute fire test that BAM performs with containers and container components on BAM Test Site Technical Safety, which operates two fire test stands.
Experimental testing work includes closure lid system examinations as well as helium leak-tightness tests, in which leak sensor equipment of the highest sensitivity is used.
BAM performs detailed analyses and assessments of mechanical and thermal stress behaviour within the scope of experimental container studies. Furthermore, BAM continually develops corresponding test methods and measurement technology. In performing their research and testing work, BAM researchers focus not only on experimental type tests, but also on analytical and numerical examinations geared to simulate mechanical and thermal accident effects, as well as studies dealing with the tight enclosure of dangerous substances. Simulations are performed using finite element software. The programs used include, for example, ABAQUS, ANSYS, DYNA3D, TASEF, TOPAZ3D, HEATING6.
Measurement Value Logging and Data Analysis
During the impact of the test object, the mechanical stresses, to which the cask body is subjected, and the involved impact kinematics are investigated.
For high-level mechanical impact measurments the cask body is extensively instrumented with various miniature piezoelectric or piezoresistive accelerometers (in the figure, using ENDEVCO and ENTRAN sensor types). Strain gauges are used to determine the time dependent magnitude of any deformation as well as associated stresses.
Appropriate electronic devices concerning range of analogue bandwidth, sample rate, etc. are utilised to acquire, record and store data.
Additionally, the impact process is visually analysed using a high-speed camera in connection with an intensive lighting unit.
For example, BAM investigated the mechanical stresses resulting from the impact of a 4000 kg transport cask for radioactive materials, which was dropped from a height of 9 metres onto an unyielding target; measurement values were recorded at numerous individual measurement points along the cask body.
In the shown example referred to, the interval between the initial impact onto the left side of the cask and the secondary impact was approx. 10 ms, whereby comparatively high stress maxima occurred during slap down.
The following leakage test methods and measurement techniques are used for testing of leakage rates of enclosed radioactive substances:
|Methods for leakage test|
(DIN 25426, T. 3)
|Vacuum method (Helium)||Mass spectrometer leak detector|
|Sniff test method (Helium)||Mass spectrometer leak test with sniff probe|
|Vacuum bubble test||Vacuum pump, vacuum chamber|
|Bubble test after pressurisation||Pressure chamber, vacuum pump|
The Finite Element Method (FEM)
The mathematical determination of container stresses, particularly in accident situations such as fires or container drops, can be performed either analytically or numerically.
Because analytical approaches in mechanics and thermo-mechanics are generally based on very idealized assumptions (e.g. constant wall thickness, rigid restraint), these are often no longer adequate for a sufficiently accurate calculation of complex container geometries, particularly when the actual stress limits of the containers are to be increasingly exploited.
Corresponding to this development, today there is the possibility to apply the finite element method (FEM) in which a real structure is mapped out as one more or less finely subdivided model structure that is meshed of the so-called finite elements. Material properties, initial conditions and boundary conditions can then be assigned to the model via these elements and their nodes. Based on conventional mechanical and/or thermal equilibrium relationships, and depending on the degree of refinement of the finite element model, this leads to very extensive equation systems that can only be solved using highly developed algorithms on high-performance modern computers. As the performance of computers is being continually developed, this means that it is increasingly possible to solve more and more complex problems.
For more in-depth information about the background, the principles and the opportunities offered by the method of finite elements, reference is made to the numerous sources of information in the relevant literature, e.g.:
- Bathe, K.-J.: Finite Element Procedures, Pearson Higher Education, 1997, ISBN 0-13-349697-X
- Zienkiewicz, O. C., Taylor, R. L.: The Finite Element Method, 3 Vols., 5th ed., Butterworth Heinemann, 2000 ISBN 0-7506-5160-1
Further information can also be found in numerous places on the Internet; two websites worth mentioning, for example, are: Universität Stuttgart
Since calculations using the finite element method always provide extensive and detailed mathematical results, particular importance must be attached to the careful examination and verification of these data so that no false conclusions are drawn. (See also nafems)
Such an examination can be performed under a wide range of aspects, and several independent methods should be used for this process. These methods include, for example, analytical approximation considerations aimed at confirming basic interrelationships, or parameter variations aimed at estimating the influences of different model parameters, e.g. the mesh refinement of the models. However, another essential aspect for the verification of mathematical results is always the focused experimental confirmation of various assumptions and results. However, this does not mean that drop tests with a complete container are necessary in every single case. Rather, experimental examinations can serve to determine the constitutive equations and parameters of the material or to confirm the behaviour of selected partial structures, e.g. the lid system of a container.
In general it can also be said that the increasing utilization of the safety reserves of a container design will require an increasingly more precise analysis of the load scenarios that occur, so that the effort of calculation and verification increases and may, under circumstances, require further experimental examinations using full-scale containers.
Drop Tests with Prototype Shock Absorbers
During transport, CASTOR casks and other casks for the transport of radioactive materials are protected by impact limiters. At BAM, prototypes of such shock absorbers filled with wood or with polyurethane material were mounted to a dummy cask and then subjected to drop tests.
The 1:4 scaled test specimens had a weight of approx. 1.000 kilograms. They were dropped with different orientations from a height of 9 metres onto an unyielding target.
The tests served to collect experimental data for the use in computer simulations. The specimens were instrumented with accelerometers. With the help of this data it is possible to design shock absorbers in original sizes and to do design calculations for future damper models.
The measurement results obtained show that, when using wood as a filling material, the deceleration values are higher and the impact period is shorter than using polyurethane. This might be particularly evident for edge drops and vertical drops, while for horizontal drops the differences are relatively small.
EBER Research Project
The three projects for the development of assessment methods for transport and storage containers with higher content of metallic recycling material ("Entwicklung von Beurteilungsmethoden für Transport- und Lagerbehälter mit erhöhten metallischen Reststoffanteilen") (acronym EBER) are used to examine inhowfar radioactively contaminated scrap metal from decommissioning and dismantling of nuclear facilities can be used as container material.
In Germany today, containers for the transport and storage of radioactive waste are often made of ductile cast iron with nodular graphite. Scrap metal is hardly added to the cast iron during its production. On the one hand, by increasing the proportion of recycled metal used for container manufacture, the amount of contaminated scrap metal for final disposal is decreased. On the other hand, this increase also adversely changes the properties of the cast iron because, in particular, the fracture toughness of the metal is reduced. At BAM we investigate the requirements that containers made of this new material „cast iron with nodular graphite / smelted with higher content of metallic recycling material “ still meet. These projects are used to develop suitable methods for assessing such containers with respect to safety engineering.
The first project, EBER from 1995 through 1998, examined whether cast iron with a high percentage of recycling material is generally suitable as a material for manufacturing containers. In this study, the requirements for final disposal of radioactive waste in the KONRAD repository were used as a basis. Reference containers were defined for different requirement profiles. The reference container for high requirements (similar to cast iron container Type VI) was subjected to numerically simulated drop tests from a height of 5 metres on to the substructure found in the final repository. Using the finite element method (FEM), it was possible to determine the stresses and strains that would occur in the container if real drop tests were made. Theses investigations showed that the applicability of ductile cast iron having an increased proportion of recycling material primarily depends on the fracture-mechanical properties of the material. The final report for this project has been published as BAM Research Report 240 ( Abstract in german).
The follow-up project, EBER II from 1998 through 2001, focused on the fracture-mechanical assessment of material defects in cast iron. The material faults were modelled as sharp cracks. Based on the postulated cracks, the fracture-mechanical behaviour of the cast iron components was analysed numerically under static and dynamic loads. The knowledge gained from these examinations was represented in readily usable assessment diagrams. The safety-engineering assessment methods established were then successfully examined in a drop test using a crack-containing prototype container having a defined material quality. The final report for this project is available from the TIB Hanover.
The BAM has been working on Project EBER III since 2001, in which the goal is to finally ensure the permissible material limits for the application of ductile cast iron having an increased proportion of recycling material. The aim of the project is to establish a more general formulation of the assessment methods that are independent of the actual final repository location. Stresses in the container during a fall depend considerably on the nature of the respective substructure on to which the fall takes place. In drop tests, this substructure is simulated with a corresponding crash foundation used in the test stand. That is why the results obtained in previous research results are to be transferred and expanded to any type of real targets. For this purpose, drop tests using cast iron components dropping on to concrete slabs are studied with systematically changing properties and different contact conditions to the substructure.
Videofilm: Drop test of a cast iron container (MPG).
The projects named here were or are being supported by the Federal Minister for Education and Research under the contracts 02 S 7584 (EBER), 02 S 7788 (EBER II) and 02 S 8021 (EBER III).