Drop and fire testing of spent fuel and HLW transport casks at ‘BAM Test Site Technical Safety’

Abstract

BAM, as a competent German government institute for the mechanical and thermal testing of radioactive material transport and storage containers, operates unique drop and fire test facilities for experimental investigations on the open air BAM Test Site Technical Safety. To be able to perform even drop tests with full scale spent fuel or HAW casks (i.e. the German CASTOR cask designs), BAM constructed in 2004 a large drop test facility capable to handle 200 ton test objects, and to drop them onto a steel plate covered unyielding target with a mass of nearly 2600 ton. Drop test campaigns of the 181 ton GNS CONSTOR V/TC, the 129 ton MHI MSF-69BG and a 1∶2 scale model of the GNS CASTOR HAW28M (CASTOR HAW/TB2) have been performed since then. The experimental BAM drop testing activities can be supported also by drop testing of smaller packages (up to 2 ton) in an in-house test facility and by dynamic, guided impact testing of package components and material specimen inside a new drop test machine. In May 2008, a new modern fire test facility was put into operation. The facility provides two test stands fired with liquid propane. Testing in every case has to be completed by computational investigations, where BAM operates appropriate finite element modelling on appropriate computer codes, e.g. ABAQUS, LS-DYNA, ANSYS and other analytical tools.     

Spent fuel transportation risk assessment: cask impact analyses

Abstract

The Nuclear Regulatory Commission (NRC) has recently completed an updated Spent Fuel Transportation Risk Assessment, NUREG-2125. This assessment considered the response of three certified casks to a range of impact accidents in order to determine whether or not they would lose their ability to contain the spent fuel or maintain effective shielding. The casks consisted of a lead shielded rail cask that can be transported either with or without an inner welded canister, an all-steel rail cask that is transported with an inner welded canister, and a DU shielded truck cask that is transported with directly loaded fuel. Finite element analyses were performed for impacts at speeds of 48, 97, 145 and 193 kilometres per hour into a rigid target. Impacts in end-on, side-on, and CG-over-corner orientations were analysed for each cask and impact speed. Calculations were performed to equate these impacts onto rigid targets with higher speed impacts onto the yielding targets that exist in the real world. These analyses indicated that a cask with an inner welded canister or a truck cask would not release radioactive material in any impact accident and that only very high-speed impacts onto hard rock targets could result in either release of material or significant degradation of shielding for rail casks without an inner canister. Impacts other than those onto flat unyielding targets were also considered. Analyses show that an impact that bypasses the impact limiters on the ends of the casks does not result in seal failure and neither does an impact by a locomotive also between the impact limiters.     

Drop test program with half scale model CASTOR® HAW/TB2

Abstract

Federal Institute for Materials Research and Testing (BAM) is the competent authority for mechanical and thermal safety assessment of transport packages for spent fuel and high level waste in Germany. In context with package design approval of the new German high level waste cask CASTOR® HAW28M, BAM performed several drop tests with a half scale model of the CASTOR® HAW/TB2. The cask is manufactured by Gesellschaft für Nuklear Service mbH and was tested under accident transport conditions on the 200 tons BAM drop test facility at the BAM Test Site Technical Safety. For this comprehensive test program, the test specimen CASTOR® HAW/TB2 was instrumented at 21 measurement planes with altogether 23 piezo resistive accelerometers, five temperature sensors and 131 triaxial strain gauges in the container interior and exterior respectively. The strains of four representative lid bolts were recorded by four uniaxial strain gauges per each bolt. Helium leakage rate measurements were performed before and after each test in the above noted testing sequence. The paper presents some experimental results of the half scale CASTOR® HAW/TB2 prototype (14?500 kg) and measurement data logging. It illustrates the extensive instrumentation and analyses that are used by BAM for evaluating the cask performance to the mechanical tests required by regulations. Although some of the quantitative deceleration, velocity and strain values cannot be shown because of confidentially issues, they are provided qualitatively to illustrate the types of measurements and methodologies used at BAM.     

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.     

Internal cask content collisions during drop test of transport casks for radioactive materials

Abstract

In transport casks for radioactive materials, significantly large axial and radial gaps between cask and internal content are often present because of certain specific geometrical dimensions of the content (e.g. spent fuel elements) or thermal reasons. The possibility of inner relative movement between content and cask will increase if the content is not fixed. During drop testing, these movements can lead to internal cask content collisions, causing significantly high loads on the cask components and the content itself. Especially in vertical drop test orientations onto a lid side of the cask, an internal collision induced by a delayed impact of the content onto the inner side of the lid can cause high stress peaks in the lid and the lid bolts with the risk of component failure as well as impairment of the leak tightness of the closure system. This paper reflects causes and effects of the phenomenon of internal impact on the basis of experimental results obtained from instrumented drop tests with transport casks and on the basis of analytical approaches. Furthermore, the paper concludes the importance of consideration of possible cask content collisions in the safety analysis of transport casks for radioactive materials under accident conditions of transport.     

Dynamic impact characteristics of KN-18 SNF transport cask – Part 1: An advanced numerical simulation and validation technique

Domestic and international regulations for the transportation of radioactive materials strictly prescribe the design requirements for spent nuclear fuel (SNF) transport casks. According to the applicable codes, a transport cask must withstand a free-drop impact of 9 m onto an unyielding surface and a free-drop impact of 1 m onto a mild steel bar. However, the structural performance of a transport cask is not easy to evaluate precisely because the dynamic impact characteristics of the cask, which includes impact limiters to absorb the impact energy, are so complex.     

Results of full scale CONSTOR® V/TC prototype 9 m horizontal drop test

Abstract

In the context of the research on the mechanical safety of packages for radioactive material, full scale drop tests with spent fuel and high activity waste transport and storage casks have been performed by the Federal Institute for Materials Research and Testing (BAM). The research reflects national and international interest in acquiring comparative knowledge of full and reduced scale model drop tests as well as in finite element calculations. This paper presents the experimental, analytical and first numerical results of the full scale drop test with the full scale CONSTOR^® V/TC prototype, manufactured by GNS, Gesellschaft für Nuklear-Service mbH, Germany. The prototype was tested by BAM in a 9 m horizontal drop test onto the unyielding target of the BAM drop test facility in Horstwalde, Germany.     

Extended Drop Tests Of DCI Casks with Artificial Flaws Demonstrating the Existing Safety Margins

Abstract

The use of spent fuel shipping and storage casks made of ductile cast iron (DCI) has been common practice for about 15 years when the development of such casks started in Germany where qualified foundries are able to produce these heavy section castings at the high quality level needed for this kind of application. To promote the discussion on safety against brittle failure a lot of research had been carried out in different countries. The two test programmes in Germany on casks with big artificial flaws under severe impact conditions is summarised in this paper. The first test object was a thick walled DCI ‘pipe’ (150 mm wall thickness) with dimensions equivalent to a 1:2.5 scale cask model. It was dropped with a 40 mm deep laser sharpended flaw from heights of up to 9 m onto rails. As a second test object a full scale CASTOR VHLW cask was used. This specimen had a flaw with a depth of 120 mm in a 260 mm thick wall. With increasing drop heights (up to 14 m) and stress intensity factors (up to material fracture toughness) this object was also dropped onto rails. For both cases the measured data (decelerations, crack opening displacement, strains, material properties) are presented. No brittle failure occurred, although in the 14 m drop of the CASTOR VHLW Cask the impact was 6.5 times higher than the impact measured in the mechanical test of the type B package design. The results demonstrate that DCI casks have significantly high safety margins even in the hypothetical case of an impact beyond type B package design requirements.     

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.     

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.     

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.     

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.     

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.     

Finite element mesh design of a cylindrical cask under puncture drop test conditions

Abstract

Transport casks for radioactive materials have to withstand the 9 m drop test, 1 m puncture drop test and dynamic crush test with regard to the mechanical requirements according to the IAEA regulations. The safety assessment of the package can be carried out on the basis of experimental investigations with prototypes or models of appropriate scale, calculations, by reference to previous satisfactory safety demonstrations of a sufficiently similar nature or a combination of these methods. Computational methods are increasingly used for the assessment of mechanical test scenarios. However, it must be guaranteed that the calculation methods provide reliable results. Important quality assurance measures at the Federal Institute for Materials Research and Testing are given concerning the preparation, run and evaluation of a numerical analysis with reference to the appropriate guidelines. Hence, a successful application of the finite element (FE) method requires a suitable mesh. An analysis of the 1 m puncture drop test using successively refined FE meshes was performed to find an acceptable mesh size and to study the mesh convergence using explicit dynamic FE codes. The FE model of the cask structure and the puncture bar is described. At the beginning a coarse mesh was created. Then this mesh was refined in two steps. In each step the size of the elements was bisected. The deformation of the mesh and the stresses were evaluated dependent on the mesh size. Finally, the results were extrapolated to an infinite fine mesh or the continuous body, respectively. The uncertainty of the numerical solution due to the discretisation of the continuous problem is given. A safety factor is discussed to account for the uncertainty.     

Numerical Assessment of Spent Fuel Casks Impacting on Real Targets

Abstract

A reference container of high capacity was analysed for loads beyond those it has to withstand during a 9 m IAEA drop test onto an unyielding target. In doing this a lid-end drop with shock absorber onto a real target was simulated. This is a possible accident for the rail transport of such casks. In this case the most critical components of the containment system are the primary lid bolts. The behaviour of the lid system and its sealing function were investigated with finite element (FE) analysis. To correlate the findings with a corresponding impact velocity onto real targets an analytical method was used. Despite the conservative assumptions made in this study a two-fold safety factor compared to the 9 m drop tests onto the unyielding target could be shown. The quantification of the additional safety the cask might provide requires further basic investigations on the behaviour of the real targets considered as well as the reduction of the conservatism included in the assumptions made up to now.     

Plastic impact testing of shipping cask fin specimens

An impact test program was conducted to determine the energy absorption characteristics of typical heat transfer or structural fins on shipping casks as these fins undergo dynamic, gross plastic deformation. It was assumed that the energy absorption characteristics of fins welded parallel to the longitudinal axis of a cylindrical cask are linear with respect to their length, and the test program was designed to determine the energy absorption capability per inch of fin length along the cask body. Specimens of typical fin geometries were impacted with free-falling weight dropped from known heights, and the force-time relationships occurring during each impact were recorded. A computer program was used to convert the test data into deformation-time and energy-time relationships. These relationships were correlated in a set of curves that enable the cask designer or analyst to compute the energy absorption capacity of a longitudinal fin on a cask as a function of its geometry and the percentage of deformation it is expected to experience as well as the peak force the fin would experience during the impact resulting from a 30-ft free drop into an essentially unyielding horizontal surface.     

Free drop impact analysis of shipping cask

The WHAMS-2D and WHAMS-3D codes were used to analyze the dynamic response of the RAS/TREAT shielded shipping cask subjected to transient leadings for the purpose of assessing potential damage to the various components that comprise the cask. The paper describes how these codes can be used to provide an intermediate level of detail between full three dimensional finite element calculations and hand calculations which are cost effective for design purposes.Three free drops were addressed: (1) a thirty foot axialdrop on either end; (2) a thirty foot oblique angle drop with cask having several different orientations from the vertical with impact on the cask corner; and (3) a thirty foot side drop with simultaneous impact on the lifting trunnion and the bottom end. Results are presented for two models of the side and oblique angle drops; one model includes only the mass of the lapped sleeves of depleted uranium (DU) while the other includes the mass and stiffness of the DU. The results of the end drop aalyses are given for models with and without imperfections in the cask. Comparison of the analyses to hand calculations and simplified analyses are given.     

Design Assessment for Transport and Storage Casks

Abstract

The design assessment concerning the mechanical behaviour of transport and storage casks for radioactive material to fulfil nuclear safety criteria has to be based on two essential considerations: (1) Effective analysis of the stress–strain state of the cask components under both normal operational and test conditions including hypothetical accident scenarios with suitable accepted methods. (2) Economic estimation of the required properties and the structural state of the cask components with sufficient exactness. In an overview of the codes which are available at GNS/GNB for cask impact strength analyses (ANSYS, ADINA, VDI Codes), procedures and aspects of benchmarking and validation of calculation codes are described. The results of experimental full size cask drop test programs (CASTOR, POLLUX) and corresponding pre-test calculational analyses show the suitability of the codes used. The influence of dynamic effects on the mechanical properties of material (ductile cast iron, wood) has been investigated experimentally. By consideration of these dynamic values in strength analyses of casks at impact a good agreement between experimental and calculational results has been achieved.