Computer modelling of wood-filled impact limiters

The performance of impact limiters, composed of balsa wood encased in steel shells, has been studied by experimental tests and computer modelling. The aim of the work was to develop a validated capability for simulating the impact behaviour with the code DYNA3D. The code is thus available to support new package designs employing similar shock absorbing features—in principle, any crushable material encased in a metallic shell.

A brief review is given of the mechanical properties of balsa wood, including methods for modelling orthotropic crushable materials. With DYNA3D, the material model for metallic honeycombs was adapted for application to wood. Both static crush tests and impact tests have been simulated, with good results for deformation and energy absorption. Dynamic strength enhancement and friction effects are examined.     

Vulnerabilities of legacy fuel casks when evaluated using modern structural analysis tools

Abstract

Cylindrical fuel casks often have impact limiters surrounding the ends of the cask shaft in a typical 'dumbbell' arrangement. The primary purpose of these impact limiters is to absorb energy to reduce loads on the cask structure during impacts associated with a severe accident. Impact limiters are also credited in many packages with protecting closure seals and reducing peak temperatures during fire events. For this credit to be taken in safety analyses, the impact limiter attachment system must be shown to retain the impact limiter following normal conditions of transport (NCT) and hypothetical accident conditions (HAC) impacts. Large casks are often certified by analysis only because of the cost associated with testing. Therefore, some cask impact limiter attachment systems have not been tested in real impacts. A recent structural analysis of the T-3 spent fuel containment cask found problems with the design of the impact limiter attachment system. Assumptions in the original safety analysis for packaging (SARP) concerning the loading in the attachment bolts were found to be inaccurate in certain drop orientations. This paper documents the lessons learned and their applicability to impact limiter attachment system designs.     

Mechanical design assessment approaches of actual spent fuel and HLW transport package designs

Abstract

In recent years, BAM Federal Institute for Materials Research and Testing finalised the competent authority assessment of the mechanical and thermal package design in several German approval procedures of new spent fuel and high level waste package designs. The combination of computational methods and experimental investigations in conjunction with materials and cask components testing is the most common approach to mechanical safety assessment. The methodology in the field of safety analysis, including associated assessment criteria and procedures, has evolved rapidly over the last years. The design safety analysis must be based on a clear and comprehensive safety evaluation concept, including defined assessment criteria and constructional safety goals. In general, for new package designs, the implementation of experimental package drop tests in the approval process should be obligatory. Additionally, pre- and post-test calculations as well as components or material testing could be important. The extent to which drop tests are necessary depends on the individual package construction, the materials used and identified safety margins in the design.     

Impact target characterisation of BAM drop test facility

Abstract

BAM safety related research of containers for radioactive material focuses on advanced mechanical safety assessment methods for verification of the structural integrity and leak tightness under normal conditions of transport and hypothetical accident conditions during transport and storage. An essentially unyielding target with a rigid surface is required for impact tests performed for package approval according to IAEA regulations. In addition to specification of a target, e.g. with a combined mass more than 10 times that of the specimen for drop tests, unyielding target characteristics have been investigated with various package designs and different impact tests. The unyielding target of the BAM drop test facility, a reinforced concrete block together with an embedded and anchored mild steel plate, provides relatively large mass and stiffness with respect to the packages being tested. For monitoring reasons accelerometers and strain gauges are embedded in the concrete block of the foundation at several positions. Additionally, dynamic impact responses like vibrations and rigid body motion can be measured by seismic accelerometers. The mechanical characterisation of the target's rigidity is based on experimental results from various drop tests. Test containers with weights of 181 000 kg, 127 000 kg and 8010 kg hit the target with velocities up to 13˙5 m s^–1 in the horizontal and vertical drop positions. The rigidity of the impact target can be demonstrated with experimental results confirmed by analytical approaches. Some conclusions can be drawn about experimental testing as well as analytical calculations in order to compare impact effects.     

Topical BAM cask design evaluation using drop tests and numerical calculations: accidental cask drop without impact limiters onto storage building foundation

Abstract

Cask impacts without impact limiters onto unyielding targets result in totally different mechanical reactions from those of relatively smooth impacts using impact limiters. During the licensing procedure of the new GNS CASTOR HAW 28M design for vitrified high activity waste, BAM therefore decided to perform an additional drop test with a 1 : 2 scale test cask (CASTOR HAW/TB2). In spite of a small drop height of only 0˙3 m onto the unyielding target of the BAM drop test facility, which conservatively covers any storage building foundation, the impact caused considerable stresses to the cask structure with high stress and strain rates. This paper presents the evaluation strategy of BAM including the drop test results and the development and qualification of appropriate finite element modelling to achieve sufficient agreement between test and calculation results. Further steps include mechanical analyses of reduced and full scale cask designs to determine the most critically stressed areas of the structure, verify scaling factors and demonstrate safety with respect to cask integrity and tightness.     

Analyses to demonstrate structural performance of KN18 in hypothetical drop accident scenarios

Abstract

The KN18 is a new cask design by KONES for KHNP for the dry or wet transportation of up to 18 PWR spent nuclear fuel assemblies in South Korea. The containment vessel consists of a cylindrical thick-walled forged steel body, closed by a stainless steel lid with bolts. Spent fuel assemblies are located in a basket which consists of a tube disc system. Two pairs of trunnions are attached for lifting, manoeuvring and tie-down. A pair of impact limiters manufactured from wood and encased in steel cladding provide impact energy absorption during the hypothetical accident conditions. The package complies with the requirements of 10 CFR Part 71 for Type B(U)F packages. It received its transport license from the Korean Competent Authority KINS in early 2010 and is expect to enter service in 2011. Structural performance of the package in the normal and accident conditions were demonstrated against the requirements of 10 CFR Part 71 by analysis including extensive calculations by state-of-the-art finite element methods, and confirmed by tests carried out on a one-third scale test model which were also used to verify the numerical tool and methods used in the analyses. For the analyses of the hypothetical accident drop conditions, the models consisted of the complete package, including the impact limiters, the containment structure and the basket, which was modelled explicitly in detail and in three dimensions, to take into account the complex interaction between the components and the non-linearities in the geometry, the material behaviour and overall behaviour. The analyses were carried out using the explicit transient finite element method so that the transient behaviour could be robustly simulated. This paper presents two of the analyses from the suite of analyses for demonstrating the performance of the package in the hypothetical accident drop scenarios, discussing the analyses methodology, modelling technique and evaluation methodology, as well as analyses results and package response. The one-third scale model drop testing and benchmarking of the model to the scale model tests are the subject of a separate paper.     

Mechanical Properties of Aluminium Honeycomb Impact Limiters

Abstract

Aluminium honeycombs have been extensively used as impact limiters in nuclear waste transport casks. The mechanical behaviour of these shock absorbing materials was studied to develop an extensive experimental database. A series of tests were performed along various loading paths. Different densities of aluminium honeycombs were tested in different orientations. Static tests included uniaxial tension, uniaxial compression and torsion. Dynamic tests were conducted at different strain rates of up to 100 S^?1, to generate experimental data relevant to accident situations. Dynamic studies included the effects of specimen size and confinement. The purpose of using different loading paths was to generate an extensive experimental database which may also be used to develop constitutive models for these materials. Design charts were constructed which can be accessed by various cask designers to optimise and economise on cask development.     

Thermal load on the TFR Tokamak limiter during additional heating pulses and major plasma disruptions

Inconel and graphite have been tested as limiter materials in the TFR Tokamak. Their behaviour during MHD activity and plasma current disruptions in high density low impurity content plasmas has been studied. The discharge energy balance with auxiliary heating has been established by using infrared measurements of the limiter temperature increase and bolometric techniques. Measurements of the temperature distribution on different limiters show that the maximum limiter temperature can be reduced by a large factor by an appropriate choice of the limiter shape and of its total area. The characteristics of the scrape-off layer in the limiter shadow has also been investigated in different limiter configurations. The experimental results and observations indicate a low physical and chemical sputtering of the graphite limiters during plasma discharges and their good properties for thermal shocks during major disruptions.     

A simple sizing optimization technique for an impact limiter based on dynamic material properties

According to IAEA regulations, a transportation package for radioactive material should perform its intended function of containing the radioactive contents after a drop test, which is one of the hypothetical accident conditions. Impact limiters attached to a transport cask absorb most of the impact energy. So, it is important to determine the shape, size and material of impact limiters properly. The material data needed in this determination is a dynamic one. In this study, several materials considered as those of impact limiters were tested by drop weight equipment to acquire the dynamic material characteristics data. The impact absorbing volume of the impact limiter was derived mathematically for each drop condition. A size optimization of the impact limiter was conducted. The derived impact absorbing volumes were applied as constraints. These volumes should be less than the critical volumes generated based on the dynamic material characteristics. The derived procedure to decide the shape of the impact limiter can be useful at the preliminary design stage when the transportation package's outline is roughly determined and applied as an input value.     

Moisture content of wood: influence on mechanical behaviour of wood filled impact limiters and importance for quality surveillance during manufacturing

Abstract

The moisture content of wood is known to have a significant influence on the wood’s mechanical properties. Using wood as an energy absorber in impact limiters of packages for the transport of radioactive material, it is of particular importance to ensure the moisture content and thus relevant mechanical properties to be in specified limits. The paper surveys the influence of wood moisture content on the mechanical properties of wood. Different measuring methods are discussed with respect to in situ applicability, accuracy and effort. The results of an experimental analysis of the accuracy of hand held moisture metres using the electrical resistance method are discussed. Conclusions are drawn regarding the measurement of moisture content of wood upon delivery as well as of complete impact limiter assemblies. Requirements for quality surveillance during manufacturing of wood filled impact limiters are derived and it is exemplified how to meet them. Construction, manufacturing and inspection of impact limiter encapsulation with regard to leak tightness are addressed.     

Safety Analysis of Casks Under Extreme Impact Conditions

Abstract

The determination of the inherent safety of casks under extreme impact conditions has been of increasing interest since the terrorist attacks of 11 September 2001. For nearly three decades BAM has been investigating cask safety under severe accident conditionslike drop tests from more than 9 m onto different targets and without impact limiters as well as artificially damaged prototype casks. One of the most critical scenarios for a cask is the centric impact of a dynamic load onto the lid-seal system. This can be caused, for example, by a direct aircraft crash (or just its engine) as well as by an impact due to thecollapse of a building, e.g. a nuclear facility storage hall. In this context BAM is developing methods to calculate the deformation of cask components and — with respect to leak-tightness — relative displacements between the metallic seals and their counterparts. This paper presents reflections on modelling of cask structures for finite-element analyses and discusses calculated results of stresses and deformations. Another important aspect is the behaviour of a cask under a lateral impact by aircraft or fragments of a building. Examples of the kinetic reaction (cask acceleration due to the fragments, subsequent contact with neighbouring structures like the ground, buildings or casks) are shown and discussed in correlation to cask stresses which are to be expected.     

Fatigue crack propagation testing using subsized rotating bending specimens

In many industrial applications, mechanical properties characterization is needed yet sufficient amount of material for standardsized specimens is not available. Miniaturized specimen testing technique has to be employed. Currently there are a lot of effort in developing subsized specimen technique for impact, fracture toughness and tensile properties. Work on fatigue properties testing is limited and largely confined to stress/strain life tests. In the current work, evaluation of fatigue crack propagation behaviour using surface crack growth in a rotating bending rod has been attempted. Nine different rod geometries have been tested. The resulting fatigue crack propagation data is more sensitive to rod diameter than to rod length. Difference in crack growth behaviour can largely be understood when crack closure is taken into account. All in all, the crack propagation data obtained from these small sized rods agree well with those obtained from standard testing employing compact tension specimens. A more precise picture about the crack growth behaviour can be obtained if crack closure is considered. If crack closure is not monitored, rods with longer length and smaller diameter are more likely to give the conservative upper bound fatigue crack propagation behaviour.     

Investigation of the impact behaviour of wooden impact limiters

Wooden impact limiters are used to reduce the forces acting on packages for irradiated fuel assemblies in hypothetical accident situations. For spruce specimens designed corresponding to the filling of real impact limiters, the dynamic force-compression characteristic has been evaluated in experiments according to the IAEA 9 m drop. Force-compression curves have been obtained, which are compared with those of equivalent static experiments. An analytical and a finite element approach have been used in combination with the dynamic compression curves to calculate the maximum cask deceleration and the maximum impact limiter deformation for a 9 m drop in an end-on, a side and an edge impact. The results are compared with those of 9 m drop experiments performed on original casks. An agreement between calculation and experiment has been found.     

Advanced instrumented impact testing facility for characterization of dynamic fracture behavior

The convenient and cheap instrumented Charpy impact test used for the evaluation of dynamic fracture toughness suffers many disadvantages and limitations. As an effort to enhance the reliability and accuracy of the instrumented impact testing, a new pendulum type of test apparatus has been designed and constructed at the Technical Research Centre of Finland (VTT).The new tester contains several novel features which help to avoid the drawbacks of the normal designs and which make the impact testing more reliable and appropriate from the fracture mechanics point of view. For that purpose the tester is equipped with a fully computerized data acquisition system and an optical COD measuring device. The latter has been developed and constructed through the co-operation between Materialprüfungsanstalt (MPA) and VTT. In the new testing facility the determination of fracture toughness is based on the application of the optical COD measuring device. In this paper testing equipment and procedures are described and discussed.     

From experiment to appropriate finite element model-safety assessment for ductile cast iron casks demonstrated by means of IAEA puncture drop test

Abstract

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 metre drop test onto a steel bar has to be considered for the application of the hypothetical accident conditions to 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 the applicant, BAM developed a complex finite element (FE) model for a dynamical ABAQUS/Explicit^TM analysis. This paper describes in detail the use of the FE method for modelling the puncture drop test within an actual assessment strategy. At first, investigations of the behaviour of the steel bar were carried out. Different friction coefficients and the material law of the bar were 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 was developed. The validation of calculated strains was carried out by comparison with the results of the strain gauges located at the relevant points of the cask model. The influence of the FE meshing is described. Finally, the validated FE half-scale model was expanded to full-scale dimension. Scaling effects were analysed. The model was used for safety assessment of the package to be approved.     

Experimental and analytical studies of shear behaviour of slabs subjected to multiaxial stress conditions

Prestressed concrete pressure vessels include structural members which rely for their safety on the knowledge of the capacity of concrete to resist shear stresses. The shear forces developed on some of the structural components under operating conditions may be several times higher than those occurring in conventional structures. This paper reviews the experimental and analytical studies which have been initiated over the years by various organisations and research workers to examine the problem of high shear stresses and the ultimate load behaviour of end slabs for PCPV. The experimental programmes were carried out to assist in the development of various designs of PCPV and to introduce a more analytical approach for designs of deep members against shear failures.     

Dynamic Fracture Mechanics Assessments for Cubic Ductile Cast Iron Containers

Abstract

An improved BAM safety assessment concept for the cask material ductile cast iron (DCI) to cover higher stresses in the cask body, highly dynamic load scenarios, and a broader range of material qualities will require more extensive fracture mechanics analyses based on a combination of material testing, calculation of applied stresses, and inspection standards. As an example, the brittle fracture mechanics assessment of a surface crack in a plate due to the dynamic load from the 5 m drop of a cubic container (not equipped with impact limiters) onto a reinforced concrete target is investigated. The numerically calculated time-dependent stress intensity factor is compared with a previous static solution with the same loading history inserted. For the scenario studied the differences between the curves are negligible because a dynamic load of the cask within a time scale of millisec9nds can be considered as a quasi static load for the crack.     

Use of impact analysis in licensing industrial container for transporting new fuel

Abstract

The finite element (FE) method is a powerful tool for the simulation of mechanical and thermal behaviour of structures. In recent years, the explicit FE method has increasingly been used in the development of transport packages and as part of approval applications to demonstrate the performance of packages. Testing and analysis are the two methods specified in the IAEA Regulations for the Safe Transport of Radioactive Material for demonstrating the structural and thermal performance of a transport package against the requirements of the Transport Regulations. The roles of testing and analysis, and the relative prominence of the two, may vary between Competent Authorities in different countries. This can range from analysis being regarded as the primary mode of demonstration with testing as confirmatory, to testing being the primary mode of demonstration supplemented by analysis. This paper describes the use of the non-linear FE code LS-DYNA in the licensing of a new container for the transport of new nuclear fuel. The package was classified as an Industrial Package (Fissile) in accordance with the IAEA Regulations, and hence it was necessary, among other things, to demonstrate that criticality criteria were satisfied under postulated impact conditions. Physical drop tests were carried out and the results are compared with LS-DYNA computer calculations using the same FE models developed to support the design of the new container. The analyses and tests clearly demonstrate the novel use of polyurethane foam as the container main energy absorber. The FE predictions are compared for accelerations, bolt loadings and global deformations of the container. In general good correlation was obtained between predictions and tests and the differences, which did occur, particularly for accelerations, are discussed and reconciled. The paper concludes that explicit analysis codes are now so reliable for container impact calculations that minimal test work should be pursued basically for key confirmatory impact scenarios.     

Analysis of thermal problems in development of spent fuel transport cask for research reactors

A packaging for the transport of irradiated fuel from research reactors was designed by a group of researchers to improve the capability in the management of spent fuel elements from the reactors operated in the region. Two half scale models for MTR fuel were constructed and tested so far and a third one for both MTR and TRIGA fuels will be constructed and tested next. Four test campaigns have been carried out, covering both normal and hypothetical accident conditions of transportation. The thermal test is part of the requirements for the qualification of transportation packages for nuclear reactors spent fuel elements. In this paper, both the numerical modelling and experimental thermal tests performed are presented and discussed. The cask is briefly described as well as the finite element model developed and the main adopted hypotheses for the thermal phenomena. The results of both numerical runs and experimental tests are discussed as a tool to validate the thermal modelling. The impact limiters, attached to the cask for protection, were not modelled.     

Japanese perspectives and research on packaging,transport and storage of spent fuel

Abstract

The present paper gives an overview of Japanese experimental studies of dual-purpose metal casks. The studies included: cask drop without impact limiters, drop of a heavy weight onto a cask due to building collapse, burial of a cask in debris from building collapse, tipping over of a cask during an earthquake, long-term containment of metal gaskets and transportability of casks after long-term storage. Most of the studies employed full-scale casks for the experiments.