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- 2014 (16) (entfernen)
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- Drop test (3)
- Aging (2)
- Aging mechanisms (2)
- Corrosion (2)
- Impact limiter (2)
- Metal seals (2)
- Safety assessment (2)
- Spent fuel (2)
- Storage (2)
- Transport (2)
Eingeladener Vortrag
- nein (6)
Storage of spent nuclear fuel and high-level waste in dual purpose casks (DPC) is related with the challenge of maintaining safety for transportation over several decades of storage. Beside consideration of aging mechanisms by appropriate design, material selection and operational controls to assure technical reliability by aging management measures, an essential issue is the continuous control and update of the DPC safety case. Not only the technical objects are subject of aging but also the safety demonstration basis is subject of "aging" due to possible changes of regulations, standards and scientific/technical knowledge. The basic document, defining the transport safety conditions, is the package design safety report (PDSR) for the transport version of the DPC. To ensure a safe transport in future to a destination which is not known yet (because of not yet existing repository sites) periodical reviews of the PDSR, in connection with periodic renewals of package design approval certificates, have to be carried out. The main reviewing tool is a gap analysis. A gap analysis for a PDSR is the assessment of the state of technical knowledge, standards and regulations regarding safety functions of structures, systems and components.
In context with new cask designs and their approval procedure, the experimental testing of impact limiters under drop test conditions becomes more and more important in order to assess the damage mechanics behaviour and safety margins for validation reasons. In recent years, various designs of impact limiters have been tested by the Federal Institute for Materials Research and Testing within specific component testing and particularly with regard to type B package design approval procedures. The paper focuses on the experimental realisation of impact limiter tests and presents implemented measurement techniques to determine the amount of deformation and to explain the impact behaviour by means of photogrammetric metrology and three-dimensional fringe projection method, high speed motion analysis and adjusted deceleration measurements.
When storage of spent nuclear fuel or high level waste is carried out in dual purpose casks (DPC), the effects of aging on safety relevant DPC functions and properties have to be managed in a way that a safe transport after the storage period of several decades is capable and can be justified and certified permanently throughout that period. The effects of aging mechanisms (e.g. radiation, different corrosion mechanisms, stress relaxation, creep, structural changes and degradation) on the transport package design safety assessment features have to be evaluated. Consideration of these issues in the DPC transport safety case will be addressed. Special attention is given to all cask components that cannot be directly inspected or changed without opening the cask cavity, like the inner parts of the closure system and the cask internals, like baskets or spent fuel assemblies. The design criteria of that transport safety case have to consider the operational impacts during storage. Aging is not the subject of technical aspects only but also of intellectual aspects, like changing standards, scientific/technical knowledge development and personal as well as institutional alterations. Those aspects are to be considered in the management system of license holders and in appropriate design approval update processes. The paper addresses issues that are subject of an actual International Atomic Energy Agency TECDOC draft 'Preparation of a safety case for a dual purpose cask containing spent nuclear fuel'.
Dicht und sicher abgeschirmt
(2014)
Die Sicherheitsanforderungen, die an Behälter für den Transport radioaktiver Stoffe auf öffentlichen Verkehrswegen gestellt werden, orientieren sich an dem Gefährdungspotenzial des radioaktiven Inhalts. So müssen für den Transport abgebrannter Brennelemente oder hochradioaktiver Abfalle aus nuklearen Anlagen Behälter eingesetzt werden, die auch schweren Unfällen standhalten. Zu ihnen gehören z. B. die CASTOR®-Behälter (Cask for Storage and Transport of Radioactive Material).
Packages for the transport of SNF and HLW are usually equipped with impact limiters to reduce the loads that result from the regulatory 9 m drop test. A common impact limiter design in Germany is a welded steel sheet structure filled with wood. The material wood is the main energy absorber, while the steel sheet provides the integrity of the impact limiter. The IAEA allows mechanical safety cases of transport packages to be carried out computationally, as long as the models used are reliable. In this context, a Finite Element (FE) modeling approach for wood and its application to impact limiters in the calculation of a 9 m drop test is presented.
A user material model for wood was developed for the dynamic FE-Code LS-DYNA. Its features are based on a series of crush tests with spruce wood specimens. The model considers wood as a material with transversely isotropic properties, i.e. in the directions parallel and perpendicular to the fiber. The plastic material behavior depends on the state of stress. This has shown to be important to account for the lateral constraint of wood in impact limiters resulting from steel sheet encapsulation. Lateral constraint or respectively, a multiaxial stress state, increases the compression strength level of wood, limits the softening effect and increases the hardening effect. Lateral constraint also increases volumetric and reduces deviatoric deformation. The wood material model considers various hardening and softening characteristics via input flow curves. It considers effects of temperature and strain rate on strength as well. The development of a multi-surface yield criterion and a plastic potential that enables the user input of plastic Poisson's ratios were the challenges during the development of material model.
A dynamic FE calculation of a horizontal drop test with an 18,000 kg test package was performed. The wood material model was used to model the wooden impact limiter inlays. The impact limiter deformation and the package deceleration were compared to the experimental drop test results to rate the performance of the wood material model.