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- 2011 (8) (entfernen)
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- Cask (2)
- Interim storage (2)
- Safety (2)
- Aging mechanisms (1)
- Compression set (1)
- Degradation mechanisms (1)
- Design testing (1)
- Dual purpose casks (1)
- Dynamic mechanical analysis (1)
- Elastomer (1)
In Germany the concept of dry interim storage of spent fuel and vitrified high active waste in dual purpose metal casks is implemented (currently for periods of up to 40 years). The casks being used have an approved package design in accordance with the international transport regulations. The license for dry storage is granted on the German Atomic Energy Act with respect to the guidelines of the German Reactor Safety Commission for dry interim storage of spent fuel. Currently part of the assessment process of the cask design for transport on public routes is to evaluate the suitability of the used materials with respect to their properties and their compatibility for possible transport periods. For transport on public routes during or after long term interim storage periods, it has to be ensured that the transport and storage casks fulfill the specifications of the transport approval or other sufficient properties which satisfy the proofs for the compliance of the safety objectives at that time. This paper describes the state-of-the-art technology in Germany and points out arising prospective challenges and which related questions have to be answered in future. Present research and knowledge concerning the long term behavior of transport and storage cask components (such as gaskets and shielding components) have to be consulted as well as experiences from interim cask storage operations. Challenges in the safety assessment are therefore i.e. the behavior of aged metal and elastomeric gaskets under IAEA test conditions to ensure that the results of drop tests can be transferred to the compliance of the safety objectives at the time of transport after the interim storage period (aged package). Assessment methods for the material compatibility, the behavior of fuel assemblies and the aging behavior of shielding parts (e. g. influence of radiation) are further issues as well.
Dry cask storage of spent fuel and high active waste front reprocessing is licensed and established in Germany for up to 40years so far. About 10years ago, the strategy has been shifted from centralized storage using facilities in Ahaus and Gorleben to cit-site storage facilities in Order to avoid immediate cask shipping from nuclear power plants. At present, several hundred casks are stored safeiy and without any major problem. BAM is highly involved in cask specific safety evaluations within the scope of licensing procedures as well as of quality assurance procedures during cask fabrication, loading and storage.
A major prerequisite for any cask storage license is a valid transport Type B license. Subsequently, cask transportability has to be ensured at any point during the entire storage period. So far, this requirement is met by continuous maintenance of the Type B license. Anyhow, IAEA Type B regulations do not consider permanently loaded storage casks that have the sole purpose of being finally shipped öfter decades of storage inducing ageing effects. No transport safety demonstration has to be performed on aged cask components.
Metal seals are in common use as an essential component for the leak tight closure of transport and storage casks for spent fuel and vitrified high active waste from reprocessing. They are placed between the massive monolithic cask body made of ductile cast iron or forged steel and the primary and secondary lids, which are fixed with screws around the lid circumference, in order to ensure long term safe enclosure of the radioactive cask inventory. Hereby, long term reliable pressure forces in combination with a tight surface contact are as important as well maintained seal and surface qualities and dry and clean conditions during seal assembly and operation. A few years ago BAM started systematic investigation on the long term seal behavior with re-spect to longer storage periods and seal behavior in accidental cask scenarios with dynamic de-formations of the seal groove geometry or short term drop in pressure force up to lid lifting. BAM has developed test flanges for seals with full scale cross section diameter but much small-er outer diameter to gain representative data for metal-sealed lid systems. They are placed in appropriate testing machines for relevant mechanical loading under static and dynamic condi-tions with the ability to measure the standard helium leakage rate continuously during each test phase. This paper focuses on BAM long term tests of different seal types with aluminum and silver jackets at three different temperatures of +20°C, +100°C and +150°C under static conditions over longer periods of time. Due to creeping effects, a reduction of the pressure force does ap-pear during loading and unloading depending on prior holding times. So far, test results of up to 2 years indicate a clear correlation between seal pressure forces and holding time, temperature and seal type allowing for extrapolating to much longer periods of time. The paper presents basic correlations, test and evaluation procedures as well as important provisional results and gives an outlook on further investigations.
The Konrad repository for not heat generating radioactive wastes was licensed first in 2002. But due to legal actions this license has not been finally confirmed before 2007. Subsequently, the Federal Office for Radiation Protection (BfS) began scheduling backfitting of the former iron ore mine into a repository. The licensed repository volume is 303,000 m3 based on estimations of expected waste volumes to be disposed off although the mine itself would offer a much larger volume. Once the repository is ready for Operation, waste packages can be disposed, but this is not expected before the end of this decade. Nevertheless, there is already today a great interest in qualified and certified waste conditioning and packaging for disposal requiring Containers, tested, evaluated and certified by BAM and BfS.
Key points of the Konrad license concern comprehensive requirements for waste products and Containers. Details are described in BfS reports, which constitute the basic documents for the license. Following the final license confirmation a revision of these documents has been requested in order to include considerations of water poisoning substances. This work is currently done by BfS in Cooperation with the NLWKN of Lower Saxony and the Institute for Safety Technology (ISTec). Concerning the specification for Containers themselves no or only minor changes are expected.
Proper predictions of the behaviour of shock absorber materials are of utmost importance in safety assessments for licensing casks for transport and storage of highly active waste. After having identified significant discrepancies between numerical results and the actual response of polyurethane foam limiters subjected to accidental scenarios, a new research project ENREA was established by BAM. A major objective is to enhance and to develop advanced material models intended to simulate limiters under impact loading. They should account for all major factors influencing the load–deformation relationship like temperature, strain rate and specific stress state. The corresponding test program, applicable plasticity models, the overall parameter identification strategy based on local and global optimization techniques as well as experimental and numerical results are presented here in particular for closed cell foams.
Elastomers are widely used as the main sealing materials for containers for low and intermediate level radioactive waste and as an additional component to metal seals in spent fuel and high active waste containers. The safe encapsulation of the radioactive container inventory has to be guaranteed according to regulation and appropriate guidelines for long term storage periods as well as for temperatures as low as -40°C during transport. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance. It is known that the material properties of elastomers are strongly temperature dependent. At low temperatures, this is caused by the rubber-glass transition (abbreviated: glass transition). During continuous cooling, the material changes from a rubber-like entropy elastic to a stiff energy elastic behaviour, which allows nearly no strain or retraction. Hence, rubbers are normally used above their glass transition, but the minimum working temperature limit is not defined precisely; this can cause problems during the above noted applications. Therefore, the lower operation temperature limit of elastomer seals must be determined in dependence of the material properties. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are combined with standardised measurements to determine the compression set according to ISO 815. To reduce the test time of the standard tests, a faster technique than normally used was developed. Additionally, the breakdown temperature of the sealing function of complete O ring seals was measured in a component test set-up to compare it with the results of the other tests. The experimental set-up is capable of measuring the leakage rate at low temperatures by the pressure rise method. A fluorocarbon rubber (FKM) was selected for this investigation as it is often used for radioactive waste containers. The materials (seals and test sheets) were purchased from a commercial seal producer.
The aging management system for the mechanical components of nuclear power plants (NPPs) must be established and used by the licensee in such a way that the quality of safety relevant components is guaranteed for the completely designed lifetime of the NPP. This demands an extensive plant life management with special emphases on the knowledge of the degradation in material properties. The basic safety concept in Germany observes this circumstance. Lifetime extension of the German NPPs is an aim of the current valid coalition agreement of the German government. Operational extension of interim storage facilities requires, in comparison to the aging management system for NPP, an aging management system adapted to the Special circumstances of spent fuel storage casks. Extension of interim storage periods for spent fuel casks beyond the designed lifetime requires, in comparison to the components of an NPP, an increasing knowledge of material degradation with potential impact on cask integrity, e.g. leak tightness. Dry interim storage in Germany has been approved for 40 years. After that time, according to the present strategy, a final repository should be available. However, until now, such a final facility still does not exist, and the German exploration and licensing process is heavily delayed. Currently, discussions are continuing regarding further exploration of the Gorleben salt mine. There is willingness to overcome this situation that is clearly described in the available coalition agreement of the federal government. Anyway, however, the prediction is viewed; a repository for heat generating radioactive waste in Germany will not be available in the near future and may not be available when first storage facilities and casks reach their 40 years of approved lifetime, which will occur in ~25 years starting from now. Therefore, the question must be asked:
what has to be done with the existing storage casks in the interim facilities? May these casks be fit for continued use, with an extension of the storage period? One option is to have an aging management system, which creates enough information about the technical condition of safety relevant cask properties. This is the basis for safety evaluation for extended storage periods. In the present paper, possible aging mechanisms for high level waste storage casks are discussed, as well as the influence of the time dependent changes of the component properties.
Within the framework of the G8 partnership, the German Ministry of Economics and Technology has been realizing a 300 EUR million project of the storage of decommissioned Russian nuclear submarines based on a German-Russian agreement. The main parts of the project are the construction of the long-term storage facility as well as a radioactive waste storage facility at Sayda Bay. Long-term interim storage facility for this particular case has a period of about 70 years. The Energiewerke Nord GmbH (EWN) under the technical controlling of BAM carried out the project management. Foremost in this presentation is the finalization of the long-term storage facility for nuclear submarine compartments and the planning and realization of the radioactive waste storage building with special emphases of the parts carried out by BAM. Because BAM staff cannot stay permanently on site, a random inspection of different topics was the bases for the activities of BAM. The key aspects of activities are the long-term storage of the nuclear compartments, maintenance building, the decommissioning of the compartments and the transportation of the very heavy weight compartments (1600 metric tons). Details relating of the progress of the construction work as well as information about technical recommendations will also be briefly illustrated. Within this scope, problems and difficulties during the construction of the storage facility as well as logistical challenges including transportation of the reactor compartments will also be discussed.