A fracture mechanics safety concept to assess the impact behavior of ductile cast iron containers for shipping and storage of radioactive materials

Within the scope of the German licensing procedures for shipping and storage containers for radioactive materials made of ductile cast iron, BAM performs approval design tests including material tests to ensure the main safety goals of shielding, leaktightness and subcriticality under “Type B accident conditions”. So far the safety assessment concept of BAM is based essentially on the experimental proof of container strength by prototype testing under most damaging test conditions in connection with complete approval design tests, and has been developed especially for cylindrical casks like CASTOR- and TN-design. In connection with the development of new container constructions such as “cubic cast containers”, and the fast developments in the area of numerical calculation methods, there is a need for a more flexible safety concept especially considering fracture mechanics aspects.

This paper presents the state of work at BAM for such an extended safety concept for ductile cast iron containers, based on a detailed brittle fracture safe design proof. The requirements on stress analysis (experimental or numerical), material properties, material qualification, quality assurance provisions and fracture mechanics safety assessment, including well defined and justified factors of safety, are described.     

Development of Assessment Methods For Transport and Storage Containers with a Higher Content of Recycled Metal

Abstract

The mechanical behaviour of transport and storage containers made of ductile cast iron melted with a higher content of recycled metal from decommissioning and dismantling of nuclear installations is investigated. Using drop tests with cubic container-like models, the influence of different real targets on the stresses in the cask body and the fracture behaviour is examined. A foundation for a test stand is suggested, which is simple to manufacture and which greatly improves the reproducibility of the test results. Dynamic fracture mechanics analyses of artificial crack-like defects in the test objects were performed by means of finite-element calculations to uncover safety margins. Numerous test results have shown that containers for final disposal can be built from a ductile cast iron with a fracture toughness of more than 50 per cent less than the lower bound value for the current licensed material. The limits of application of the material are also determined by the opportunities for safety assessment.     

Abstracts on Packaging,Transport, Storage and Security of Radioactive Material

Abstract

The design testing of packages for radioactive materials considers normal operating conditions and accident conditions. A mechanical test, especially under accident conditions, must include the safety assessment of possibly undetected material defects. BAM has developed improved assessment methods, using fracture mechanics, for cracks in the most highly stressed regions of cubic containers made of ductile cast iron. Postulated surface cracks in the centre of the container walls and grooves are investigated numerically. In the static case relations between the crack tip parameters (stress intensity factor or the J integral, respectively), stress load, crack depth, container geometry and material behaviour are derived. In the dynamic case it can be shown by numerical simulations of the drop test of containers onto different targets, even without shock absorbers, that the dynamic crack tip parameter may be estimated by static formulae with the dynamic stress inserted in the intact component. This somewhat surprising result can be explained by the fact that the drop event happens over milliseconds. That is slow enough for the crack to behave quasistatically although the crack is loaded with a dynamic, i.e. time-dependent, stress. Based on these calculations, the critical crack depth is given as a function of the stress, the material quality (defined by the fracture toughness) and the wall thickness for surface cracks in the centre of walls as well as in grooves of a cubic container.     

Mechanical Properties of Ductile Cast Iron and Cast Steel for Intermediate Level Waste Transport Containers

Abstract

UK Nirex Ltd is developing Type B re-usable shielded transport containers (RSTCs) in a range of shielding thicknesses to transport intermediate level radioactive waste (ILW) to a deep repository. The designs are of an essentially monolithic construction and rely principally on the plastic flow of their material to absorb the energies involved in impact events. Nirex has investigated the feasibility of manufacturing the RSTCs from ductile cast iron (DCI) or cast steel instead of from forgings, since this would bring advantages of reduced manufacturing time and costs. However, cast materials are perceived to lack toughness and ductility and it is necessary to show that sufficient fracture toughness can be obtained to preclude brittle failure modes, particularly at low temperatures. The mechanical testing carried out as part of that programme is described. It shows how the measured properties have been used to demonstrate avoidance of brittle fracture and provide input to computer modelling of the drop tests.     

放射性物质运输货包力学试验评价技术

放射性物质运输容器是放射性物质安全运输的唯一物理屏障,运输容器需能抵抗可能的碰撞事故,GB 11806和IAEA的SSR-6针对碰撞事故情景规定了相应的力学试验项目。本文结合GB 11806和SSR-6规定的试验要求,介绍了中国辐射防护研究院自由下落冲击力学试验装置和应力、加速度、形变、影像测量系统。针对3m3六氟化铀运输容器、XAYT-Ⅰ型医用伽马刀治疗头及密封放射源运输容器、ZHQY-QG-001型退役辐照源运输容器,采用试验和有限元仿真计算相结合的方法,分别研究了容器关键部件的形变、应力、加速度数据在容器安全性能评价中的应用。结果表明,综合应用有限元仿真计算与试验技术,采集和分析影像、应力、加速度、形变等数据,可分析货包结构失效模式和评价货包安全性能。     

Packaging and security of radioactive material

Abstract

Elastomers are widely used as the main sealing materials for containers for low and intermediate level radioactive waste and as an additional component to metal seals in spent fuel and high active waste containers. The safe encapsulation of the radioactive container inventory has to be guaranteed according to regulation and appropriate guidelines for long term storage periods as well as for temperatures as low as ?40°C during transport. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance. It is known that the material properties of elastomers are strongly temperature dependent. At low temperatures, this is caused by the rubber–glass transition (abbreviated: glass transition). During continuous cooling, the material changes from a rubber-like entropy elastic to a stiff energy elastic behaviour, which allows nearly no strain or retraction. Hence, rubbers are normally used above their glass transition, but the minimum working temperature limit is not defined precisely; this can cause problems during the above noted applications. Therefore, the lower operation temperature limit of elastomer seals must be determined in dependence of the material properties. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are combined with standardised measurements to determine the compression set according to ISO 815. To reduce the test time of the standard tests, a faster technique than normally used was developed. Additionally, the breakdown temperature of the sealing function of complete O ring seals was measured in a component test set-up to compare it with the results of the other tests. The experimental set-up is capable of measuring the leakage rate at low temperatures by the pressure rise method. A fluorocarbon rubber (FKM) was selected for this investigation as it is often used for radioactive waste containers. The materials (seals and test sheets) were purchased from a commercial seal producer.     

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.     

Strain analysis on ductile cast iron containers at drop tests

Ductile cast iron (DCI) containers for transportation and deposition of radioactive waste have to be designed carefully in order to avoid unacceptable damages and leakages in case of an accident. Therefore various calculations and experimental methods are used during development and licensing of the containers. Besides others the container has to suffer severe impacts (e.g. falling from a height of several meters onto a concrete base). The level of strains must not exceed a value which would adversely affect the package in such a way that it would fail to meet the applicable requirements. In practice complex events such as drop tests are very difficult to calculate. Both the position of maximum stress and the time of its occurrence are not easily predicted with the method of FEM. The uncertainty of the material modelling for plastic deformation by dynamic loading rates is the limiting factor. Therefore holography as an integral measuring technique in combination with strain gauge techniques were used to fit the FEM. By using the FEM calculations in the case of licensing, the FE and the material model have to be verified. The verification of the FE model has to be done by comparison of the local maxima measured by strain gauges and by comparison of the vibration modes. These vibration modes we take from holographic measurements. In this paper we explain container vibrations after impact analysed with holographic measurements, FEM calculations and the comparison of the results. The comparison of the local maxima (strain gauges/FEM) is reported elsewhere (Schreiber, D. et al., 1993. Trans. 12th Int. Conf. J04, 5, pp. 101–108; Völzer, W. et al., 1997. Technical Semin. on Brittle Fracture. Krefeld 27–28.10.94, RAMTRANS (in press)).     

Sandia National Ladoratories Cask Drop Test Programme: A Demonstration of Fracture Mechanics Principles for the Prevention of Brittle Fracture

Abstract

Sandia National Laboratories recently completed a cask drop test programme. The aims of the programme were (1) to demonstrate the applicability of a fracture mechanics-based methodology for ensuring cask integrity, and (2) to assess the viability of using a ferritic material for cask containment. The programme consisted of four phases: (i) materials characterisation; (ii) non-destructive examination of the cask; (iii) finite element analyses of the drop events; and (iv) a series of drop tests of a ductile iron cask. The first three phases of the programme provided information for fracture mechanics analyses and predictions for the drop test phase. The drop tests were nominally based upon the lAEA 9 m drop height hypothetical accident scenario, although one drop test was from 18 m. All tests were performed in the side drop orientation at a temperature of ?29°C. A circumferential, mid-axis flaw was introduced into the cask body for each drop test. Flaw depths ranged from 19 to 76 mm. Steel saddles were welded to the side wall of the cask to enhance the stresses imposed upon the cask in the region of the introduced flaw. The programme demonstrated the applicability of a fracture mechanics methodology for predicting the conditions under which brittle fracture may occur and thereby the utility of fracture mechanics design for ensuring cask structural integrity by ensuring an appropriate margin of safety. Positive assessments of ductile iron for cask containment and the quality of the casting process for producing ductile iron casks were made. The results of this programme have provided data to support IAEA efforts to develop brittle fracture acceptance criteria for cask containment.     

The Application of Fracture Mechanics to The Safety Assessment of Transport Casks For Radioactive Materials

Abstract

BAM is the responsible authority in Germany for the assessment of the mechanical and thermal design safety of packages for the transport of radioactive materials. The assessment has to cover the proof of brittle fracture safety for package components made of potentially brittle materials. This paper gives a survey of the regulatory and technical requirements for such an assessment according to BAM's new 'Guidelines for the application of ductile cast iron for transport and storage casks for radioactive materials'. Based on these guidelines, higher stresses than before will be permissible, but it is necessary to put more effort into the safety assessment procedure. The fundamentals of such a proof using the methods of fracture mechanics are presented. The recommended procedure takes into account the guidelines of the IAEA's advisory material which are based on the prevention of crack initiation. Examples of BAM's research and safety assessment practices are given. Recommendations for further developments towards package designs with higher acceptable stress levels will conclude the paper.     

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.     

BAM Production Control Programme for Containers for Transport and Storage of Nuclear Materials

Abstract

In the process of testing spent fuel casks, BAM is gaining a lot of relevant data regarding the quality level of ductile cast iron (DCI). The basic parameters governing the material behaviour of ferritic and ferritic pearlitic DCI are dicussed and the development of container quality over recent years is summarised. The high quality level of German DCI containers is outlined. The effect of microstructure, sample size and loading rate on the fracture toughness of DCI is discussed in the second part of the paper.     

Probabilistic Evaluation of Brittle Failure Design Criterion of DCI Cask

Abstract

As a cask material, ductile cast iron may be susceptible to failure in a brittle manner under certain temperature and load conditions. A design criterion for ductile cast iron casks against brittle failure due to drop tests, has been proposed by Central Research Institute of Electric Power Industries. This design criterion includes a safety factor which presents the extent between the detectable flaw size and the critical flaw size and may be interpreted as ‘uncertainty factor’ as to the uncertainties regarding stress prediction, fracture toughness and so on. In this report, to verify the proposed design criterion, probabilistic evaluation was performed according to a series of drop tests using a full scale cask and material tests, and it is confirmed that the proposed design criterion is applicable and reliable. Furthermore, applicability of the safety design method described in the IAEA-TECDOC-717 published in August 1993 was investigated.     

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.     

Application of IAEA TECDOC 717 to the Assessment of Brittle Fracture in Transport Containers with Plastic Flow Shock Absorbers

Abstract

UK Nirex is developing re-usable shielded transport containers (RSTCs) in a range of shielding thicknesses (from 70 nun to 285 nun) to transport immobilised intermediate level radioactive waste (ILW) to a deep repository. The RSTCs are being designed to meet the requirements of the IAEA Transport Regulations for Type B packages, including the requirement to maintain shielding and containment following a drop of 9 m onto an unyielding surface. The RSTCs are essentially monolithic in construction and the heaviest version weighs up to 65 tonnes when loaded with contents. They rely principally on plastic flow of the material of construction to absorb the high energies involved in impact events. Specific features of the designs, such as the solid metal comer shock absorbers and side ribs have been optimised for this purpose. Nirex has investigated the feasibility of manufacturing the RSTCs from ductile cast iron (DCI) or cast steel instead of from forgings, since this would bring advantages of reduced manufacturing time and costs. In this paper the methodology set out in IAEA-TECDOC-717 is applied to the Nirex RSTC, including the application of elastic plastic fracture mechanics methods.     

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.     

Fresh Fuel Shipment: the FCC Concept

Abstract

For more than 25 years, Framatome, the foremost world supplier of PWR fuel, has been delivering fuel elements directly or through Fragema, from the factories of its subsidiary FBFC to the various French and foreign sites. Since 1995, the French Safety Authority has undertaken a process of reviewing the concepts of the radioactive material shipping containers, with the purpose of ensuring that their safety files are consistent with the regulations in force. For now the safety analyses on the former packages, known as the RCC type, have shown the limits of this design. Framatome has committed itself to this process by performing a global review of the RCC containers transporting uranium-bearing fresh fuel assemblies, which has led to upgrading of the design of these former containers to an up-to-date model: known as the FCC type. The design of the new FCC container is presented from the mechanical strength and criticality standpoints, together with the regulatory tests which validated the basic hypotheses of the FCC design.     

Independent Design Appraisal and Container Certification

Abstract

Under the IAEA Safety Series 6, waste and transport containers for radioactive materials must comply with specified standards of design, manufacture and test. In all cases, the consignor carries responsibility, but will generally neither design nor manufacture the container himself. Design appraisal, inspection during manufacture, and witnessing of tests by an independent third party can therefore be of benefit. In the case of ISO freight containers, such services can also incorporate Certification of containers as required by the International Container Safety Convention (CSC).     

Qualifying a 40 ft ISO Freight Container as a Type IP-2, IP-3 Package by Performing a Full Scale Drop Test

Abstract

The first successful worldwide free fall drop test with a 40 ft ISO freight container took place in Bremen (Germany) at the dry dock of the former Vulkan shipyard on 25 September 1998. This drop test had to be performed to qualify the ISO Boxcontainer as a Type IP-2, IP-3 package in accordance with the new IAEA Safety Standards Series No ST-1 (1996 Edition). Dynamic impact requirements will become mandatory for freight containers to be qualified as Type IP-2,3 packages in compliance with IAEA ST-1 paragraph §627 ‘Alternative Requirements for IP-2,3 Packages’ (comes into force in January 2001). STM has fulfilled the dynamic impact requirements in performing a full scale drop test. The 40 ft ISO freight container prototype (L × W × H = 12192mm × 2438 mm × 2491 mm) was fully loaded with 28 t of steel plates together with shock absorbing material to simulate the load and load securing system. The total drop test weight was 35.6 t. In accordance with the new IAEA Safety Standards Series No ST-1 requirements, the so-called LONGFORCE® container was dropped onto an unyielding foundation in a position which produced the maximum damage in respect of the package safety features. The package was dropped on its comer, door side down on the roof, with the centre of gravity over the impact area (slap-down drop). The container was lifted 12.6 m high (highest point) and 0.3 m (lowest point) under a drop angle of 70°. The combined mass of the concrete block and the steel plate was more than 100 times that of the container test specimen. The first impact resulted in an acceleration of about loog where the maximum was just before the impact. The second impact, however, turned out to be decisive showing maximum acceleration readings in the range of 250g. The container has been inspected after the drop test and deformations of the container rear comer castings (area of second impact) and a small weld crack in one of the comer casting welds was found. On the container floor one third of transverse profiles showed S-form distortion. The LONGFORCE container was leak tested prior to and after the drop test in compliance with the STM leak test procedure. The leak tests consisted of filling the container with pressurised air up to 5 kPa and recording a possible pressure drop over a determined test period. The container was considered leak tight prior to and after the drop test based on the permissible limits set in the leak lest procedure. The free fall drop test is considered a full success qualifying the 40 ft LONGFORCE container as Type IP-2, Ip-3 package in compliance with the new IAEA Safety Standards Series No ST-1 requirements.