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Packages for the transport of radioactive material are often equipped with impact limiters consisting of wood, encapsulated by steel sheets. These impact limiters shall ensure that transport casks meet the IAEA safety requirements. After damage caused by the mechanical tests the package has to withstand a severe fire scenario. It is required that the mechanical tests have to produce maximum damage, taking into account the thermal test. Furthermore, any damage, which would give rise to increased radiation or loss of containment or affect the confinement system after the thermal test, should be considered. Concerning the thermal test, the IAEA safety requirements state that during and following the fire test, the specimen shall not be artificially cooled and any combustion of materials of the package shall be permitted to proceed naturally. Different works from the French Institute for Radiological Protection and Nuclear Safety (IRSN) and BAM show that additional energy supply from a pre-damaged impact limiter to the cask could occur caused by smoldering of the wood. This effect should be considered within the safety assessment of the package. A heat wave from the fire could overlap with the additional energy from the impact limiter in the sealing system. In 2015 BAM conducted small scale
fire tests with wood filled metal drums showing continuing combustion processes during the cooling down phase. As not much is known about smoldering processes in wood filled impact limiters, it is highly complex to define pre-damage of impact limiters, which are conservative, regarding the most damaging energy flow from the impact limiter to the containment system in dependence of time. More research has to be done to develop models to examine the effects of smoldering impact limiters on the containment of packages for the assessment. The process of smoldering is described with regard to the requirements in the thermal safety assessment. Parameters influencing the smoldering process are identified. BAM operates test facilities to examine the issue of mechanical damage, combustion and heat transfer of packages for transport of radioactive material. A thermal test will take place with a wood filled test specimen with a diameter of about 2.3 meters. The aim is to understand the phenomena of smoldering under the consideration of relevant regulatory boundary conditions.
German packages for the transport of spent nuclear fuel are assessed with respect to specific transport conditions which are defined in the safety regulations of the International Atomic Energy Agency. In general, gastight fuel rods constitute the first barrier of the containment system. The physical state of the spent fuel and the fuel rod cladding as well as the geometric configuration of the fuel assemblies are important inputs for the evaluation of the package safety under transport conditions. The objective of this paper is to discuss the methodologies accepted by German authority BAM for the evaluation of spent fuel behavior within the package design approval procedure. Specific test conditions will be analyzed with regard to assumptions to be used in the activity release and criticality safety analysis. In particular the different failure modes of the fuel rods, which can cause release of gas, volatiles, fuel particles or fragments, have to be properly considered in these assumptions. The package as a mechanical system is characterized by a complex set of interactions, e.g. between the fuel rods within the assembly as well as between the fuel assemblies, the basket, and the cask containment. This complexity together with the limited knowledge about the material properties and the variation of the fuel assemblies regarding cladding material, burn-up and the operation history makes an exact mechanical analysis of the fuel rods nearly impossible. The application of sophisticated numerical models requires extensive experimental data for model verification, which are in general not available. The gaps in information concerning the material properties of cladding and pellets, especially for the high burn-up fuel, make the analysis more complicated additionally, and require a conservative approach. In this context some practical approaches based on experiences by BAM within safety assessment of packages for transport of spent fuel will be discussed. Ongoing research activities to investigate SNF mechanical behavior in view of gas and fissile material release under transport loads are presented.
Accident safe packages for the transport of spent nuclear fuel and high-level waste shall fulfil international IAEA safety requirements. Compliance is shown by consecutive mechanical and thermal testing. Additional numerical analysis are usually part of the safety evaluation. For damage protection some package designs are equipped with wood filled impact limiters encapsulated by steel sheets. The safety of these packages is established in compliance with IAEA regulations. Cumulative mechanical and fire tests are conducted to achieve safety standards and to prevent loss of containment. Mechanical reliability is proven by drop tests. Drop testing might cause significant damage of the impact limiter steel sheets and might enable sufficient oxygen supply to the impact limiter during the fire test to ignite the wood filling. The boundary conditions of the fire test are precisely described in the IAEA regulatory. During the test the impact limiter will be subjected to a 30 minute enduring fire phase. Subsequent to the fire phase any burning of the specimen has to extinguish naturally and no artificial cooling is allowed. At BAM a large-scale fire test with a real size impact limiter and a wood volume of about 3m3 was conducted to investigate the burning behaviour of wood filled impact limiters in steel sheet encapsulation. The impact limiter was equipped with extensive temperature monitoring equipment. Until today burning of such impact limiters is not sufficiently considered in transport package design and more investigation is necessary to explore the consequences of the impacting fire. The objective of the large scale test was to find out whether a self-sustaining smouldering or even a flaming fire inside the impact limiter was initiated and what impact on the cask is resulting. The amount of energy, transferred from the impact limiter into the cask is of particular importance for the safety of heavy weight packages. With the intention of heat flux quantification a new approach was made and a test bench was designed.
Packages for the transport of radioactive material are often equipped with impact limiters consisting of wood, encapsulated by steel sheets. These impact limiters shall ensure that the transport casks meet the mechanical and thermal IAEA regulatory test requirements. According to the accident conditions of transport it is mandatory to expose the specimens to a cumulative effect by mechanical and thermal impacts. The mechanical tests consist of a free drop from 9 m onto a flat unyielding target and a 1 m drop onto a puncture bar. After damage caused by mechanical test sequences the package has to withstand a severe fire scenario. Corresponding to the IAEA advisory material it is required that the impact attitudes for the 9 m drop test and for the puncture test have to be such as to produce maximum damage, taking into account the thermal test. Moreover, any damage, which would give rise to increased radiation or loss of containment or affect the confinement system after the thermal test, should be considered. During and following the thermal test, the specimen shall not be artificially cooled and any combustion of materials of the package shall be permitted to proceed naturally. Different works from the French Institute for Radiological Protection and Nuclear Safety (IRSN) and BAM show that additional energy supply from a pre-damaged impact limiter to the cask could occur. This effect should be considered within the safety assessment of the containment. Thermal effects at the closure system of the cask, which might result in an elevated activity release, have to be excluded. BAM conducted small scale tests with wood filled metal buckets showing continuing combustion processes during the cooling down phase. These test results are presented. As not much is known about smouldering processes in wood filled impact limiters, it is highly complex to define pre-damage of impact limiters, which are conservative, regarding the maximum damaging energy flow from the impact limiter to the containment system. More research has to be done to develop models to examine the effects of smouldering impact limiters on the containment of packages for the transport of radioactive material. Aspects of assessment and its difficulties are shown. BAM as a competent authority for the approval of transport casks for radioactive material in Germany operates the test facilities to examine the issue of mechanical damage, combustion and heat transfer for such kind of package systems. For this purpose the knowledge from real drop tests with casks of a mass partly over 100 tons was transferred to a test application. A thermal test will take place with a wood filled test specimen with a diameter of about 2.3 meters. The aim is to understand the phenomena of smouldering under the consideration of relevant regulatory boundary conditions. The process of smouldering is described with regard to the requirements in the thermal assessment of safety of packages for the transport of radioactive material. Requirements concerning the pre-damage of packages for the maximum damage of impact limiters are discussed. Parameters influencing the smouldering process are identified.
In the approval procedure of transport packages for radioactive materials, the competent authority mechanical and thermal safety assessment is carried out in Germany by BAM Federal Institute for Materials Research and Testing.
The combination of experimental investigations and numerical calculations in conjunction with materials and components testing is the basis of the safety assessment concept of the BAM.
Among other mechanical test scenarios a 1 meter drop test onto a steel bar has to be considered for hypothetical accident conditions of Type B packages according to IAEA regulations.
Within the approval procedure for the new German package design of the HLW cask CASTOR® HAW 28M, designed by GNS Gesellschaft für Nuklear-Service Germany, a puncture drop test was performed with a half-scale model of the cask at -40°C.
For independent assessment and to control the safety analysis presented by applicant, BAM developed a complex finite element model for a dynamical ABAQUS/ExplicitTM analysis. This paper describes in detail the use of the finite element (FE) method for modeling the puncture drop test within an actual assessment strategy.
At first investigations of the behaviour of the steel bar are carried out. Different friction coefficients and the material law of the bar are analysed by using a 'rigid-body' approximation for the cask body.
In the next step a more detailed FE model with a more realistic material definition for the cask body is developed. Strain verification is possible by results of the strain gauges located at the relevant points of the cask model. The influence of the finite element meshing is described.
Finally, the verified FE half-scale model is expanded to full-scale dimension. Scaling effects are analysed. The model is used for safety assessment of the package to be approved.
Die IAEA Transportempfehlungen stellen die Basis der sicherheitstechnischen Anforderungen von Behältern zum Transport radioaktiver Stoffe dar. Sie gelten als international harmonsiert und werden regelmäßig an den Stand von Wissenschaft und Technik angepasst. Der laufende Revisionsprozess berücksichtigt mehr als 150 Vorschläge zur Weiterentwicklung des Regelwerks. Wesentliche Themen, die für die hoheitlichen Prüfungen im Zuständigkeitsbereich der BAM eine Rolle spielen, werden im Vortrag vorgestellt. Das Implementieren der besonderen sicherheitstechnischen Anforderungen an Zwischenlagerbehälter werden im Detail behandelt. Ebenfalls von Interesse ist die Weiterenticklung der Anforderungen, die den Einfluss des radioaktiven Inventars auf das Verschlusssystem bei Fallprüfungen diskutiert.
The Federal Institute for Materials Research and Testing (BAM) is assessing the mechanical and thermal safety performance of packages for the transport of radioactive materials. Drop testing and numerical calculations are usually part of the safety case concepts, where BAM is performing the regulatory tests at their own test facility site.
Among other mechanical tests the 1 meter drop onto a steel puncture bar shall be considered for accident safe packages. According to the IAEA regulations “the bar shall be of solid mild steel of circular section, 15.0 ± 0.5 cm in diameter and 20 cm long, unless a longer bar would cause greater damage…”.
Particularly with regard to the German transport- and storage cask designs, often made from ductile cast iron, an accurate determination of the puncture bar length to guarantee a load impact covering the worst case scenario can be imperative. If the fracture mechanical proof for the cask material shall be provided by a test, small deviations in the concentrated load applied can be decisive for the question if the cask fails or not.
The most damaging puncture bar length can be estimated by iterative procedure in numerical simulations. On the one hand, a sufficient puncture bar length shall guarantee that shock absorbers or other attachments do not prevent or reduce the local load application to the package, on the other hand, a longer and thus less stiff bar causes a smaller maximum contact force. The contrary influence of increasing puncture bar length and increasing effective drop height shall be taken into account if a shock absorber is directly placed in the target area. The paper presents a numerical approach to identify the bar length that causes maximum damage to the package. Using the example of two typical package masses the sensitivity of contact forces and puncture bar deformations to the initial length are calculated and assessed with regard to the international IAEA package safety requirements.
Investigation of the internal impact during a 9 m drop test of an accident-safe waste package
(2021)
The safety assessment of packages for the transport of radioactive material follows the IAEA regulations and guidance. The specified regulatory tests cover severe accidents and demonstrate the package containment system integrity.
Special attention must be drawn to the behaviour of the content which could move inside the package due to unpreventable gaps caused by the loading procedure and the structure of the content. A possible internal impact of the content which occurs during the drop tests onto the lid system is investigated. The IAEA regulations SSR-6 and the Guidance SSG-26, revised recently, consider input from Germany and France related to the significance of internal gaps.
In the context of a waste package design assessment, a model was equipped with a representative content to conduct a drop test with an internal impact. The weight and kinetic impact of this content covered all possible real contents. The objective of the test was to maximize the load onto the lid system and to prove the mechanical integrity by complying with the required leak tightness. The test was conducted
conservatively at a package temperature lower than -40 °C at the BAM Test Site Technical Safety.
This paper gives an overview of efforts to address internal gaps and their consequences, and the BAM efforts with the implementation of this topic into IAEA regulations and guidance material. The paper then focuses on the conduction of a drop test and investigation of internal component impact.
CASTOR-Behälter sollen den Auswirkungen schwerster Unfälle standhalten und müssen dabei ihre Sicherheitsfunktionen beibehalten. Dazu zählen der sichere Einschluss des radioaktiven Inventars mit nachgewiesener Behälterintegrität und -dichtheit, die ausreichende Abschirmung der radioaktiven Strahlung, die Ableitung der von den Brennelementen ausgehenden Wärme und der Ausschluss des Entstehens einer nuklearen Kettenreaktion, d.h. die Kritikalitätssicherheit sind zu gewährleisten.
Zum Nachweis dieser Fähigkeiten werden die Behälter gegen Stoß, Aufprall, Durchstoßen, Feuer sowie beim Eintauchen in Wasser geprüft. Durch diese Tests werden Beanspruchungen aus potentiellen schweren Unfällen abdeckend simuliert. Die hierfür erforderlichen experimentellen Tests, aber auch die Prüfung und Entwicklung komplexer rechnerischer Simulationen des Behälterverhaltens unter Anwendung der Finite-Elemente-Methode erfolgen bei der Bundesanstalt für Materialforschung und -prüfung (BAM).
Die Basis bildet das Regelwerk der Internationalen Atom- und Energieorganisation (IAEO) zum sicheren Transport von radioaktiven Stoffen, welches international harmonisierte Schutzziele und Prüfanforderungen definiert. Die seit 60 Jahren praktizierte stetige Weiterentwicklung der Sicherheitsanforderungen trägt aktuellen Erkenntnissen, Erfahrungen und Risikoanalysen Rechnung. Die Brennelementbehälter werden mechanischen Prüfungen sowie Brand- und Wasserdruckprüfungen unterzogen. Hierdurch wird nachgewiesen, dass sie den Auswirkungen eines schweren Unfalls standhalten können.
Nur Behälter die zweifelsfrei die kumulativen mechanischen und thermischen Tests und auch die Wasserdruckprüfung mit Erfüllen aller Schutzziele bestehen, bekommen eine Zulassung, die es erlaubt die Behälter zu transportieren.
Transportierbarkeit von Transport- und Lagerbehältern für radioaktive Stoffe, welche nach eine längerfristigen trockenen Zwischenlagerung
transportiert werden müssen. Überblick über die aktuellen Regelwerke und Richtlinien sowie die Zeitpläne für das Finden eines Endlagers für hochradioaktive Stoffe und die damit verbundene verlängerte Zwischenlagerung. Des Weiteren werden das für die verlängerte Zwischenlagerung notwendige Alterungsmanagement und die noch erforderlichen Forschungstätigkeiten vorgestellt.