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- Radioactive material (5)
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- Drop tests (2)
- Radioaktiv (2)
- Safety assessment (2)
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Organisationseinheit der BAM
Leakage testing during lateral displacement of metallic seals under mechanical accident impact
(2013)
In context with approval design tests, the IAEA regulations for the safe transport of radioactive material specify tests for demonstrating ability to withstand normal and accident conditions of transport. For 9-m drop test onto an unyielding target the drop orientation of the package must be chosen in a manner so that maximum damage occurs with regard to the safety criteria.
With respect to leak tightness after drop test impacts a horizontal or slap-down orientated test model can cause a lateral displacement of the packaging’s lids, affecting the leakage rate of the closure lid system strongly.
In Germany dual purpose casks with double lid sealing system composed of primary and secondary lid with bolts, metallic seals as well as cask body sealing surfaces are mainly used. Besides free drop tests with full-scale and half-scale models within licensing approval procedure BAM develops experimental testing methods to simulate defined stresses and deformations by means of components testing.
In order to investigate the relation between lateral lid displacement and leakage rate, BAM performs impact tests with small scaled flange-lid systems of transport packages in a drop test machine for guided drop tests.
The test setup is realized in a way that the flange part is fixed while a drop weight is impacting onto the lid’s edge. The dynamic displacement of the lid is measured simultaneously. Because of capturing the drop weight after first impact during a short rebound phase by pneumatic arrest mechanism a secondary impact on lid’s edge is excluded. The guided drop tests were accompanied by various measurements such as leakage rate recording during and after impact, identification of lateral shifting by electro-optical displacement transducer as well as decelerations of the lid and the impacting weight.
This paper describes the test setup, the test and measurement techniques as well as first results concerning the relation between lateral lid displacement and leaktightness.
Acceptable limits for activity release from transport casks for high-level radioactive material specified in the IAEA regulations must be kept by the integrity of cask body and the cask sealing system. BAM as the German competent authority for mechanical, thermal and containment assessment of packages liable for approval verifies the activity release compliance with the regulatory limits. One of the fundamental aspects in assessment is the specification of conservative package design leakage rates.
To ensure the required package tightness for both, storage, and transport of the cask before and after storage usually metal seals of the Helicoflex® Type are used. Due to the long-term use the seal behavior is influenced by temperature and time. The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport specified in the regulations can have a significant effect on the leak tightness of the sealing system. Whereas the safety for application of new, non- aged Helicoflex® seals is verified sufficiently, there are still technical data gaps concerning the efficiency of aged Helicoflex® seals.
BAM performed experiments to learn more about the sealing efficiency of aged Helicoflex® seals with Aluminum and Silver outer jackets. The seals were compressed in test-flanges and for artificial ageing the complete flange systems were stored in an oven for several month at a high temperature. During the compression and decompression tests after the aging, load-deformation characteristics of the seals, and leakage rates were measured. With these tests a load situation was simulated, which can occur in the regulatory drop test of the cask: Under high impact loads the bolted lid can lift a little for a short moment, allowing a little movement of the seal, so that the contact area can change before compressing again.
The poster presentation will show details about test conditions and first results.
Acceptable limits for activity release from transport casks for high-level radioactive material specified in the IAEA regulations must be kept by the integrity of cask body and the cask sealing system. BAM as the German competent authority for mechanical, thermal and containment assessment of packages liable for approval verifies the activity release compliance with the regulatory limits. The specification of conservative package design leakage rates is one of the most important aspects in assessment.
Metal seals of the Helicoflex® Type are usually used to ensure the required package tightness for both, storage, and transport of the cask before and after storage. Due to the long-term use the seal behavior is influenced by temperature and time. The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport specified in the regulations can have a significant effect on the leak tightness of the sealing system. Whereas the safety for application of new, non- aged Helicoflex® seals is verified sufficiently, there are still technical data gaps concerning the efficiency of aged Helicoflex® seals.
BAM performed experiments to learn more about the sealing efficiency of aged Helicoflex® seals with Aluminum and Silver outer jackets. The seals were compressed in test-flanges and for artificial ageing the complete flange systems were stored in an oven for several month at a high temperature. After this ageing procedure the flanges were opened completely, the seals were moved a little to vary the contact area, and the flanges were compressed again. During the compression and decompression tests after the aging, load-deformation characteristics of the seals, and leakage rates were measured.
With these tests a load situation was simulated, which can occur in the regulatory drop test of the cask: Under high impact loads the bolted lid can lift a little for a short moment, allowing a little movement of the seal, so that the contact area can change before compressing again.
Details about test conditions and first results will be presented in the poster.
The containment systems of transport casks for spent fuel and high radioactive waste usually include bolted lids with metallic or elastomeric seals.
The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport can have a significant effect on the leak tightness of such containment system.
Scaled cask models are often used for providing the required mechanical and thermal tests series. Leak tests have been conducted on those models.
It is also common practice to use scaled component tests to investigate the influence of deformations or displacements of the lids and the seals on the standard leakage rate as well as to study the temperature and time depending alteration of the seals.
In this paper questions of the transferability of scaled test results to the full size design of the containment system will be discussed.
The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport can have a significant effect on the leak tightness of the containment system of transport casks for spent fuel and high radioactive waste. The containment systems of such transport casks usually include bolted lids with metallic or elastomeric seals.
Scaled cask models are often used for providing the required mechanical and thermal tests series. Leak tests have been conducted on those models.
It is common practice to use scaled component tests to investigate the influence of deformations or displacements of the lids and the seals on the standard leakage rate as well as to study the temperature and time depending alteration of the seals.
In this paper questions and open points of the transferability of scaled test results to the fullscale design of the containment system will be discussed.
Damaged spent nuclear fuel (DSNF) can be loaded in Germany in dual-purpose casks (DPC) for transport and interim storage. Therefore, special encapsulations are needed to guarantee a safe handling and a reliable tight closure. These encapsulations must be durable and leak-tight for a possible long interim storage term and the transport thereafter. Because components inside the package containment like these special encapsulations are not accessible within periodical inspections of the DPC.
Due to the general design of DPCs for standard fuel assemblies, specific requirements needed to be considered for the design of the special encapsulations for DSNF to ensure the loading in existing package designs. Especially the content of the encapsulations with damaged spent nuclear fuel must be avoided to enter in the inner space of the DPC, due to the possible higher amount of corrosive and radioactive material as well as greater quantities of water, which can be trapped in defect fuel rods. The main difference of the encapsulations for damaged spent nuclear fuel is that they have no replaceable sealing as tightness barrier. They have a specific welded lid. Therefor the tightness needs to be shown in the design approval process for all transport conditions especially for accident conditions, where high internal impact loads can occur.
As German competent authority, BAM is responsible for the safety assessment of the mechanical and thermal design, the release of radioactive material and the quality assurance aspects of manufacturing and operation. Concerning the mechanical package design BAM carried out a comprehensive safety assessment for the specific encapsulations during the package design approval process of the commonly used DPC. For the proof of tightness of the welded lid no representative standards are available. For minor loads the German KTA standards can verify the tightness of a weld seam but only for low stresses. For higher impact loads physical tests are required. These tests were performed with representative drop tests and highly sensitive leak tests.
The present paper gives an overview of the applied assessments by BAM and points out the main findings of the mechanical design for the welded sealing. Both verification procedures are described, on one hand with German standards for lower loads and on the other hand with physical tests for higher impact loads. The drop tests based on complex numerical pre-calculations by the applicant which result in optimized dimensions and properties of the test specimens and representative drop heights for the drop tests. The horizontal drop orientation with a plane-parallel impact on the unyielding basement was most challenging and led to specific adaptations during preparation and conduction of the drop tests performed by BAM. With leak tests and the corresponding test evaluation, it could be finally approved that the specific welded sealing of an encapsulations for DSNF is tight for all transport conditions.
The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport can have a significant effect on the leak tightness of the sealing system of transport casks for spent fuel and high radioactive waste.
Applicants are requested by BAM to provide test programmes for verification of design leakage rates of the sealing system under the possible loads.
Two test series initiated to clarify the dependency of the standard leakage rate on dynamic lid displacement as well as the dependency of the useful elastic recovery ru of a metallic seal on temperature and time, are outlined in this paper as examples of present investigations:
Sliding tests simulating a lid displacement possible by a horizontal drop of the cask are carried out at the controlled drop test facility of BAM with test flange pairs equipped with metallic Helicoflex seals.
For specification of covering values for the useful elastic recovery ru GNS has started a very comprehensive test programme with overall 70 metallic seals installed in test flanges which are stored for a period of one year at three different temperatures up to 150°C.
This paper gives an overview about the current approach of BAM in the assessment of cask tightness and informs about the status of the running test series.
The design of a special form radioactive material has to resist a severe transport accident without undue loss or dispersal of radioactive material.
Safety assessment by authorities competent for design approval has to include besides the required test program (impact, percussion, bending and heat test) also the evaluation of the quality management system for design, manufacture, testing, documentation, use, maintenance and inspection.
These quality assurance measures have to assure that every specimen of the approved design is produced in the same verified quality and every specimen must be able to survive the severe mechanical and thermal tests without undue loss or dispersal of radioactive material at any time of its working life.
All important aspects in the design approval procedure by BAM as the competent authority for approvals of special form radioactive material in Germany are summarized in a guideline published in 2014. This paper will give additional explanations to some aspects in safety assessment, e.g.:
the applicability of leak test methods and the need to consider ageing aspects.
Three drop test campaigns have been performed with DN 30 Protective Structural Packaging (PSP) developed by DAHER NUCLEAR TECHNOLOGIES GmbH for the transport of natural, enriched and reprocessed uranium hexafluoride (up to 5 wt%) in 30B cylinders.
The mechanical prototype testing is intended to demonstrate that the package DN30 complies with regulatory requirements under normal and hypothetical accident conditions of transport (NCT, ACT) relevant to IF, AF and B(U)F packages, respectively.
The paper includes the results of the latest test campaign carried out in 2015-2016 at the drop test facility of BAM, Germany with new full scale prototypes of the DN30 PSP and 30B cylinders. Repetition of drop test sequences became necessary after changing to Polyisocyanurate foam as shock absorbing material with variable foam densities. Furthermore, the mechanical behavior of the UF6 content of the 30B cylinder is now simulated by a mixture of cement and steel grid as modified surrogate material; instead of small steel balls. The behavior of this new content simulation is assumed to be more realistic with respect to the properties of real UF6.
Drop testing of the NCS 45
(2007)