Characterising polyurethane foam as impact absorber in transport packages

Abstract

Transport and storage packages used for the safe transport of radioactive materials are required to satisfy IAEA regulations. One key design requirement for a radioactive material transport package is that under a 9 m regulatory drop test, containment functions are maintained. For certain payload types, such as fuel assemblies, impact loads on the payloads may need to be controlled in order to maintain spacing and confinement. To achieve all of this, detailed and accurate characterisation of the impact absorbing material is important in order to design an effective shock absorber. Polyurethane foam is an excellent energy absorbing material because it has a relatively high specific strength, a large compressive deformation, much of this at constant force, and a predictable compressive strength characteristic. Traditionally various types of wood have been used for this purpose, however foams are a more cost effective alternative, which are readily available, and can be formed and shaped easily. Some grades may have the added advantage of providing an almost isotropic crush response, combined with significant thermal protection. The general compressive strength properties of foams and their temperature dependencies are well documented by manufacturers; however, strain rate sensitivity and stiffness variation with orientation are not readily available. Hence impact compression tests for polyurethane foams for a range of densities from 56 to 320 kg m^–3 were specified by Rolls-Royce and performed by the Health and Safety Laboratory. These tests included dynamic conditions for a range of strain rates and temperatures and a selection of orientations of the foam. Following collation of the test results, property curves were derived for the range of temperatures at which the package was expected to operate in service between –10 and +75°C. The properties for a given specification of foam will vary within a defined tolerance range, mainly due to the variables inherent during manufacture. Hence nominal static curves were derived for each foam and a number of factors were taken into account to derive the full range of foam properties: density, compressive strength, temperature and manufacturer supplied tolerance. The net result of this work was a series of force displacement plots, depicting upper and lower bounds to account for the cumulative effects of many variables. Accounting for these upper and lower performance bounds is an essential approach in justification of any modern package design. This paper describes the characterisation and mathematical modelling of polyurethane foam for use as the main impact energy absorber in a new design of package for transporting fresh fuel. The non-linear finite element (FE) code LS-DYNA was used to carry out simulation of the tests. The HONEYCOMB material model available in LS-DYNA was used to accurately predict the test measurements of the foam material. The properties derived for the foam were then used as input to the full FE model used for the licensing of the new package design. Full scale drop testing of the package demonstrated good correlation of deformations between test and FE model analysis, providing good validation evidence of the foam characterisation in the transport package.     

Modelling of polyurethane foam thermal degradation within annular region subject to fire conditions

Abstract

Nuclear materials are placed in shielded, stainless steel packaging for storage or transport. These drum type packages often employ a layer of foam, honeycomb, wood or cement that is sandwiched between thin metal shells to provide impact and thermal protection during hypothetical accidents, as those prescribed in the Code of Federal Regulations (10 CFR 71·73). The present work discusses the modelling of the thermal degradation of polyurethane (PU) foam within an annular region during an 800°C fire. Measurements and analysis by Hobbs and Lemmon [M. L. Hobbs and G. H. Lemmon: ‘Polyurethane foam response to fire in practical geometries’, Polym. Degrad. Stab., 2004, 84, 183–197.] indicate that at elevated temperatures, PU foam exhibits a two-stage, endothermic degradation. The first stage produces a degraded solid and a combustible gas; the second stage reaction consumes the degraded solid and produces another combustible gas. As a result, during a prolonged fire, a gas filled void develops beside the outer metal shell and grows inward toward the inner shell and the containment vessel. As a result of the radial symmetry in the drum geometry, a one-dimensional finite difference model is constructed for the annular foam region. Heat flux is applied to the inner surface to model the decay heat of the containment vessel contents. Thermal radiation and convection boundary conditions with a specified environmental temperature are applied to the outer surface. The material and reaction rate properties determined by Hobbs and Lemmon are applied to the foam. The annular region temperature and composition are determined as functions of radius and time after the environmental conditions are changed from room temperature to those of an 800°C fire. The effects of surface to surface radiation between the package’s outer skin and the undegraded foam and the reaction rate reduction due to material damage during the reaction are evaluated for fires lasting 20 h. The peak package liner temperature caused by a 30 min fire is predicted to be 129°C, well below the short term limit for containment vessel seal (377°C).     

Criticality assessments using polyurethane foam

Abstract

Rolls-Royce has designed a package to transport and store fresh fuel assemblies and anticipates approval from the regulators for the new package design in the near future. The space between the inner and outer steel shells is filled with shaped blocks of rigid polyurethane foam, of two different densities. The criticality safety case for the fresh fuel package had to consider single packages and arrays of packages under routine, normal and accident conditions. IAEA regulatory requirements state that the criticality assessment must include investigations on the effect on the neutron multiplication factor k _eff due to impacts, flooding and fire. Sensitivity studies must also be carried out to determine the effects on the k _eff due to any uncertainties in the composition of the fuel and container materials. An important part of the criticality safety case is the treatment of the foam. The approach adopted to model the polyurethane foam is the subject of this paper. The following were investigated: (1) the effect on the k _eff of varying the elemental composition of the foam, including the removal of hydrogen; (2) the experimental analysis of burnt foam; (3) the effect of addition of water to the foam to simulate water absorption; (4) a simple representation of crushed foam to simulate knock-back in the package; (5) extreme representations of burnt foam, such as replacing foam with solid carbon or as randomly distributed spheres of carbon to represent soot. These investigations were most informative and should be considered in any criticality assessments of transport packages containing large amounts of foam in the future.     

Study on the two-way corrugated aluminum honeycomb as a filler material in impact limiters for spent fuel transport casks

Impact limiters installed on nuclear spent fuel transport casks are used to absorb energy and limit overload during transport and accidents. The two-way corrugated aluminum honeycomb, a new kind of filler material, is designed based on the action mechanism of the impact limiter. Quasi-static compression tests are used to compare the properties of the honeycomb and the traditional filler material, paulownia wood. Experimental results indicated that the compressive and energy absorption properties of paulownia wood with axial wood grain and radial wood grain were very different. Also, the moisture content of paulownia wood led to a significant decrease in its properties. The two-way corrugated aluminum honeycomb, as a new porous material, showed better compression and energy absorption properties than paulownia wood in the x, y, and z directions. The peak stress (σ_pe), platform stress (σ_pl), and energy absorption capacity (W_EA) were 2.10 times, 2.07 times, and 1.69 times higher than that of paulownia wood with axial wood grain. The two-way corrugated aluminum honeycomb is a filler material that has essential application value in impact limiters of spent fuel transport casks.     

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.     

Shock d eformation diagram of fireclay

A method is proposed for constructing the shock compression and load-relief diagrams of a foam material when a one-dimensional stress-strain is realized in it. The method is based on the mathematical analysis of experimental information. The shock-compression and load-relief diagrams of fireclay (strain rate ε=10² sec⁻¹) are constructed from the experimental data obtained in experiments on the compression of samples of this foam material. 5 figures. 10 references. NITs TIV OIVT RAN. Translated from Atomnaya énergiya, Vol. 88, No. 1, pp. 38–43, January, 2000     

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.     

An Absorbent Material for Use with a Type a Package for a Liquid Radioactive Isotope for Medical Use

Abstract

Radioactive isotopes must be safely transported from the production centre to the point of use. The shipping package to safely transport radioactive isotopes should be able to withstand the conditions prescribed by law. In the field of nuclear medicine, radioactive isotopes are used in liquid or capsule form. A Type A package, which is used to transport liquid radioactive materials, should have a containment system comprising a primary inner and a secondary outer containment or it should be provided with a sufficiently absorbent material to absorb twice the volume of the liquid contents. Hospitals prefer to use not only convenient but also reusable packages. In this work the free absorbency of absorbents for use in a Type A package for transporting liquid radioactive isotopes was estimated. Among the absorbent materials studied, it was found that in the case of a liquid with 0.4% NaOH, the free absorbency of the melamine foam was best at 91 g g^-1. In the case of a liquid with 0.9% Na, the free absorbency of the melamine foam was also the best at 88 g g^-1.     

Testing of packages with LSA materials in very severe mechanical impact conditions with measurement of airborne release

Abstract

To assess the risks associated with transport accidents involving solid LSA-II and LSA III materials a comprehensive experimental programme was conducted to quantify and characterise airborne release of radioactive particulate matter in transport and handling accidents with mechanical impact of varying severities and to determine the dependency from influencing parameters such as LSA material and packaging properties and size. The experimental approach combined well-controlled and very reproducible impact experiments with small scale specimens and drop tests of larger scale specimens from different heights up to 27m. In both cases the associated airborne release of particulate matter is determined by measuring the amount and aerodynamic particle size characteristics of released dust. The small scale tests revealed fundamental results on airborne release and size distribution which helped to design the test matrix of the large scale experiments, especially with brittle material. In the large scale tests, volumes of specimens were varied systematically up to 200L and the LSA material was contained either within packaging or without protective packaging in order to determine the influence of the packaging on the airborne release and to be able to extrapolate other configurations of package sizes and impact severities. The LSA surrogate materials were either concrete, used to immobilize radioactive wastes as representative brittle material, or appropriately chosen powders representing dispersible materials. Based on the experimental results it can be concluded that the requirements of the current IAEA Transport Regulations sufficiently limit potential radiological consequences from transport accidents with mechanical impact involving packages with LSA-II or LSA-III materials.     

PMBP-聚氨酯泡沫塑料对水溶液中钍的萃取研究

聚氨基甲酸酯泡沫塑料简称聚氨酯泡沫塑料,由聚醚(或聚酯)树脂与多异氰酸酯反应发泡制得,是常见的一种生活用品。软性泡沫塑料具有多孔网状结构、表面积大和弱硷性阴离子交换剂的性质,对多种无机或有机物有吸着作用,在气-固、气-液和液-固体系中可用作吸着剂或吸附剂;同时,泡沫塑料还能吸附一定量的各种有机萃取剂,而且水力学性质很     

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.     

Effect of the New Neutron Quality Factor (ICRP 60) on the Transport of Radioactive Material

Abstract

The ICRP 60 has introduced a new definition of the neutron quality factor. This paper aims to assess the impact of the new neutron quality factor on the transport of radioactive material in France. By using two examples, the transport of fresh mixed oxide (MOX) fuel assemblies and of spent UO₂ fuel assemblies, an overview is given of the technical solutions which could be implemented to meet the new regulatory requirements. The options investigated include the modification of packaging designs and resulting operational changes. The influence of each choice on the amount of transport needed is also addressed.     

快堆MOX元件运输容器的缓冲器研究

快堆MOX元件运输容器的缓冲器是决定其放射性包容边界安全的重要部件。某型号MOX元件运输容器的缓冲器材料首次选择泡沫铝,通过自由下落试验的标准姿态进行吸能原理分析,设计出了适用于缓冲器材料的型号、结构及关键参数。对选定的材料进行了拉伸、压缩、剪切3种准静态和动态力学性能试验,获得了用于数值模拟计算的材料本构关系参数,并对模型参数进行了测试,用弯曲试验进行了验证。有限元分析和试验结果对比显示：运输容器缓冲器材料的本构关系具有适用性,可用于快堆某型号MOX元件运输容器的自由下落分析计算。     

Scale model impact limiter in type assessment of radioactive material transport packages

Abstract

In Germany, the Federal Institute for Materials Research and Testing (BAM) is the competent authority for the mechanical and thermal design safety assessment of transport packages for radioactive material according to IAEA regulations. The combination of experimental and numerical safety proof forms the basis for a state of the art evaluation concept. Reduced scale models are often used in experimental investigation for design assessment of transport packages corresponding to IAEA regulations. This approach is limited by the fact that a reduced scale model cask can show different behaviours from a full scale cask. The paper focuses on the peculiarities of wood filled impact limiter of reduced scale models. General comments on drop testing with reduced scale models are given, and the relevant paragraphs of the IAEA regulations and Advisory Material are analysed. Possible factors likely to influence the energy absorbing capacity of wood filled impact limiting devices are identified on the basis of similarity mechanics. Among possible significant influence factors on the applicability of small scale models are strain rate and size effects, failure mechanisms, underground compliance, gravitational and friction effects. While it was possible to derive quantitative estimations for the influence of strain rate, size effects and target compliance, it was not possible to evaluate the influence of compression mechanisms and gravitation. In general, if reduced scale models are used in proof of safety, uncertainties increase in comparison with full scale models. Additional safety factors to exclusively cover the uncertainties of reduced scale model testing have to be demanded. The possible application of reduced scale models in regard to crucial aspects for proof of safety has to be analysed critically.     

INES Scale: French Application to Radioactive Material Transport

Abstract

After gaining control of radioactive material transport in June 1997, the French Nuclear Safety Authority (ASN) decided to apply the International Nuclear Event Scale (INES scale) to transport events. The Directorate General for Nuclear Safety and Radioprotection (DGSNR) requests that radioactive material package consignors declare any event occurring during transport, and has introduced the use of the INES scale adapted to classify transport events in order to inform the public and to have feedback. The INES scale is applicable to events arising in nuclear installations associated with the civil nuclear industry andevents occurring during the transport of radioactive materials to and from them. The INES scale consists of seven levels. It is based on the successive application of threetypes of criterion (off-site impact, on-site impact and degradation of defence in depth) and uses the maximum level to determine the rating of an accident. As the transport in questiontakes place on public thoroughfares, only the off-site impact criteria and degradation ofdefence in-depth criteria apply. This paper deals with DGSNR's feedback during the past 7 years concerning the French application of the INES scale. Significant events that occurred during transport are presented. The French experience was used by the International Atomic Energy Agency (IAEA) to develop a draft guide in 2002 and the IAEA asked countries to use a new draft for a trial period in July 2004.     

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.     

聚氨酯泡沫塑料的辐射效应研究

用DTG和万能材料试验机等分析测试手段研究了聚氨酯泡沫塑料在辐照前后的热稳定性和力学性能变化 ,并利用GC对辐解气态产物进行了定性定量分析。结果表明 ,聚醚型聚氨酯泡沫塑料在 8.0× 10 5Gy时综合力学性能仍然较好 ,但受辐射降解的作用已有相当的气态产物生成。经辐照以后 ,材料的热性能较为稳定     

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.     

The 1996 Edition of the IAEA's Regulations for the Safe Transport of Radioactive Material — What's New?

Abstract

The 1996 Edition of the International Atomic Energy Agency's Regulations for the Safe Transport of Radioactive Material has been published and is being implemented by Member States and international transport organisations. Several major changes were made in the requirements, particularly those applicable to uranium hexafluoride packaging, air transport of larger quantities of material, and the specification of material which is exempt from the regulations. The international transport organisations are working toward a uniform effective implementation date of 1 January 2001. Shippers and carriers should begin preparation for compliance with the revised requirements in order to ease implementation.     

Comparison of collective dose in India and USA owing to transport of radioactive material by road

Abstract

This paper estimates probable collective doses owing to transport of radioactive material by road mode in India and the USA for an identical hypothetical case under respective transport conditions for the two countries, using the INTERTRAN2 computer code. The differences observed in the transport conditions in the two countries are reflected in the DNORM input parameters for the incident free transport conditions of transport and the accident rates. Owing to differences in the parameter values, it is found that the estimated probable collective dose values owing to transport of radioactive material by road mode for India are much higher than those for USA. Sensitive parameters are identified which contribute maximum to the estimated probable collective dose.