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- Dual purpose cask (3) (entfernen)
Existing spent nuclear fuel (SF) and high active waste (HAW) management policies and practices worldwide are the result of past presumptions that sufficient reprocessing and/or disposal capacity would be available in the near term. Consequently, in the past many countries have developed specific solutions for different periods of time due to their individual national nuclear policies.
In Germany the concept of dry interim storage in dual purpose metal casks before disposal is being pursued for SF and HAW management and transport and storage licenses have been issued accordingly. The current operation licenses for existing storage facilities have been granted for a storage period of up to 40 years. This concept has demonstrated its suitability for over 20 years so far. Relevant safety requirements haven been assessed for the short-term as well as for the long-term for site-specific operational and accidental storage conditions. But in the meantime significant delays in the national repository siting procedure occured which will make extended storage periods necessary in the future.
This paper describes the current situation in Germany with regard to dry cask storage and focuses on current perspectives considering regulatory, technical, and scientific aspects for storage license renewal. Since there is one case of a storage license limitation to only 20 years due to administrative reasons, first experience is currently gathered in case of an expiring storage license. Subsequent license options have been pursued intensively including the extension of the initial storage license as well as shipping all casks to another storage facility considering an extended storage period at that storage facility as well. All safety relevant aspects have to be reviewed on basis of the current state-of-the-art which might be different from the initial safety demonstrations. That includes new safety assessment standards as well as improved knowledge base. Major issues are e.g. improved accident scenario analyses, assessment methods, and consideration of aging effects from previous operation periods. Inspection programs with respect, e.g. to bolts and seals to verify leak-tightness and lid screw pre-stress have been initiated to demonstrate proper cask conditions for extended storage as well as transportation to another storage facility.
It is important to evaluate the effect of thermal ageing on the sealing performance of metal gaskets under high temperature for long-term usage. Therefore, in order to gain representative data for this kind of metal-sealed lid System, BAM is currently performing laboratory tests of different gasket types with aluminum and silver jackets at three different temperatures under static conditions up to four years so far, using test flanges for gaskets with full scale cross section diameter but much smaller outer diameter. On the other hand, in order to investigate the applicability of the numerical methodology to evaluate the long-term behavior of the metal gaskets, such as a correlation between seal pressure force and holding time, CRIEPI is developing a modeling method including material tests (tensile and creep tests) at high temperature. In this paper, the applicability of the finite element method (ABAQUS) to predict the recovery displacement and residual seal pressure force of the gasket complex was verified by comparing the calculated values with BAM’s laboratory test results under the joint research agreement between BAM and CRIEPI.
In Germany dual purpose casks for spent nuclear fuel (SF) or high level radioactive waste (HLW) are used for safe transportation and interim storage. Key safety issues in both fields are the safe enclosure of the radioactive material, the safe removal of decay heat, securing nuclear criticality safety, limitation of radiation exposure to acceptable levels and keeping it as low as reasonable achievable (ALARA principle).
Whereas these safety requirements during transportation are ensured by the Type-B(U)-design testing and approval procedure on basis of internationally agreed IAEA (International Atomic Energy Agency) requirements storage licenses are issued on national and site specific safety analyses.
This paper presents experiences from recent interim storage safety evaluation and licensing procedures in Germany on basis of a Type-B(U) certified cask designs concerning specific differences from Operation and accident conditions inside the storage facility. The focus is laid on the interaction of cask and building structures with regard to shielding, heat removal and accident analyses including aircraft crash. Basic safety is assured by thick-walled metal casks with monitored double lid Systems. They also reduce radiation to levels where workers can safely operate and maintain the casks inside the storage facility. The storage building e. g. provides additional shielding and protection against extemal hazards depending on the building construction. In addition, the storage building helps to reduce radiation levels at the boundary of the storage site well below regulatory limits given by the German Radiation Protection Ordinance (StrSchV).