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.     

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.     

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.     

'Design to manufacture': journey through criticality considerations for new design fissile material transport packages

Abstract

The development of new fissile material transport packages is an increasingly complex and costly business. The IAEA regulations stipulate performance requirements for the behaviour of the package under both normal and accident conditions. The principal performance drivers are: impact/structural, thermal, shielding and criticality safety. The needs of these disciplines can often compete, with refinements becoming necessary to optimise the design of the package. Maximising the payload while achieving the required levels of safety can be time consuming and expensive. Early communication with the criticality safety analysts can frequently speed up the design process. Numerous variables are significant to criticality safety, (e.g. mass, geometry and spacing of fissile material, presence of moderators, presence of fixed poisons, potential damage to package, etc.). Small changes in the design can result in big changes in the reactivity and this may lead to a very large number of scoping calculations. Increased computing power and improved code abilities can make criticality scoping calculations much more efficient. The development of software to enable a single parameterised input to analyse multiple variations of a basic model provides significant benefits in the production of the criticality analysis and to the overall design process. Criticality analysis of a specific design and of variations in that design can now be performed more quickly and efficiently. Through frequent interaction between members of the design team, a 'fluid' design can be created, and an iterative process can then optimise the package configuration. Retaining the criticality input throughout the design, development, prototype manufacture and testing ensures that the key safety principles are progressed to the manufactured package. In turn, the time and cost of the overall project in producing an acceptable package may be much reduced. The paper presents experience from a criticality viewpoint of the development of new package designs for transport of fissile materials, emphasising the benefits of the utilisation of software/computing developments in this process, with statistics illustrating the efficiencies now achievable.     

New approach to evaluate lattice expansion of light water reactor fuel elements on criticality safety of transport packages under impact accidents

Abstract

In order to safely transport packages containing light water reactor fuel assemblies, it is essential to maintain the fuel assemblies in a subcritical state in accidents during transport. To evaluate nuclear criticality safety, an estimator is required to determine an absolutely safe level based not only on hypothetical accidents but also on practical accident levels which, to some extent, are based on actual accidents. The purpose of the present study is to suggest the arrangement of the deformation range of the fuel assembly after an actual accident, and to obtain the maximum value of the neutron effective multiplication factor based on the criticality safety assessment for the transport cask. In the present study, two kinds of criticality calculations for the package were considered: large scale pin pitch shift and small scale pin pitch shift. For the large scale pin pitch shift, a parameter which determines the location of each fuel pin which constitutes the fuel assembly was introduced so that the criticality calculation for the fuel assembly with non-uniform lattice pitch can be performed parametrically. The result of the criticality calculation using the parameter made it clear that the fuel pin pitch is sensitive to the neutron reactivity because each of the fuel pin pitches is related to a ratio of the fissile to the moderator, and that the relationship of the ratio to the neutron reactivity depends on the type of the fuel assembly involved, i.e. the type of a nuclear reactor in which a fuel assembly is used. For the small scale pin pitch shift, the study focused on the small displacement of each fuel pin. The small displacement of each fuel pin pitch can be described probabilistically using the stochastic geometry routine in MCNP code. Using the scheme in combination with the scheme for the large scale pin pitch shift, the maximum value of the neutron effective multiplication factor of the package after an accident can be obtained. This scheme is useful to determine the maximum neutron effective multiplication factor for the criticality safety evaluation.     

Mechanical characteristics of fuel rod claddings in transport conditions

Abstract

A synthesis on the mechanical characteristics of unirradiated and irradiated fuel rod claddings was performed by the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in order to have reference data for the assessment of the safety demonstrations in normal and accident conditions of transport required by the procedure of package licensing. Indeed, the transport conditions correspond to a range of cladding temperatures (200–550°C) which is only partly covered by the data acquired within the framework of the safety demonstration relative to the reactor normal operating conditions, especially beyond 400°C. This work concerned Zircaloy-4 cladding material (Zry-4) and M5^TM. Data about mechanical properties (elastic and ductile properties, creep behaviour), oxidation (in reactor and under air during transport), hydrides and fracture toughness have been collected and synthesised. The laws presented in the document can be used to obtain orders of magnitude of oxide layer thickness, hydrogen content and creep deformation rate. The following phenomena which could influence the mechanical behaviour of the cladding were more particularly studied: oxidation which could become very important during transport in case of cladding temperatures of ～500°C; creep for which only a few data ～500°C are available and which depends in particular on the internal pressure of the rods, the cladding oxidation and the presence of the hydrides; and recrystallisation of Zry-4 at ～500°C, which could have consequences on the mechanical properties of the cladding after cooling during the storage. For other topics of interest for the study of the mechanical behaviour of the cladding, such as the fracture toughness for example, it was identified that the data available is scarce.     

Early accident simulating testing of radioactive material packages in road vehicles

Abstract

Beginning as early as the 1960s, concerns were voiced as to the adequacy of the package test standards imposed by the transport regulations promulgated by the International Atomic Energy Agency. One concern that was frequently raised and has continued to the present time is that the test standards do not necessarily simulate real accidents. The purpose of the crash tests described here that were done with typical packages carried in full scale vehicles was to assess the IAEA standards, their adequacy and to suggest changes to them that might be needed. It was also hoped that the tests, which were performed in the USA and in the UK, would show to regulators, to users of the regulations and to the public that current regulations already provide a very high level of safety for real world accidents. With time, much of the original information regarding these tests and their results has been lost. The few documents that remain have been surveyed and this paper presents summaries from this survey of the tests and their results.     

Assessment for radiological impact of irradiated nuclear fuel package sunk in marine environment

Abstract

To gain the public acceptance for the transport of irradiated nuclear fuel, environmental impact assessments have been made by assuming that a transport package may be sunk into the sea. General circulation models for the Japan Sea and for the world ocean were employed to simulate radionuclide concentration in the ocean near shore and in deep sea areas respectively. Radionuclide concentrations were simulated for the hypothetical release from a submerged transport package of fresh mixed oxide fuel in the Japan Sea and the global ocean by general circulation models. Simulated concentrations were quite small compared to the background concentration by the fallout. The effective doses of radiation exposure to the public for both cases were much less than the effective dose limit (1 mSv year^–1) by the ICRP recommendation.     

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.     

Radiological impact of transport of radioactive material in UK by road and rail

Abstract

A survey has been carried out on the transport of radioactive materials by road and rail in the UK by the Health Protection Agency's Radiation Protection Division. This survey, carried out in 2004, is the latest in a series of periodic studies on the transport of radioactive materials by all modes of transport. Questionnaires were sent to hospital departments, radionuclide manufacturers, suppliers, and carriers, and to the nuclear and other industries, in order to obtain data on shipments of radioactive materials. Visits were made to hospitals, suppliers, carriers and some railway premises in order to observe working practices and radiological surveys were made while packages were handled. Assessments of individual doses to workers and members of the public were made, and occupational dosimetry data were also obtained. It was found that ～500 000 package movements take place by road annually in the UK, with ～4000 package movements annually by rail. Radiation doses to most transport workers are low, with an average dose of 0·6 mSv to general workers transporting medical and industrial sources. Doses to individual members of the public are very low, with average annual doses of less than 0·02 mSv.     

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.