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.     

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.     

Evaluation of the Shock Absorption Properties of Rigid Polyurethane and Polystyrene Foam Used in a Small Transport Package

Abstract

The research described in this paper concerns the characterisation of polyurethane and polystyrene foam for use as shock absorbing filling materials in impact limiters for nuclear transport packages. For a comparatively small cylindrical transport package the mechanical properties of the foam material in compression tests, a preliminary determination of the best foam density and its co-relation with other material properties, appropriate selection of polyurethane and polystyrene from the relation between the reaction force due to the drop impact and their compression characteristic properties are discussed.     

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.     

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

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

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

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

Computer modelling of wood-filled impact limiters

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

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

Beneficial influence of the weldment rupture of cask impact limiter case on the impact absorbing behavior

In the cask impact limiter design, the functions of steel case should be evaluated for the protection of the filler materials and the impact energy absorption by the buckling deformation in both the fire and impact accidents. The objective of this paper is to evaluate the beneficial influence of the intermittent weldment of the cask impact limiter case on the cask impact behavior. This paper describes the test results for the joint strength evaluation of intermittent weldment and the cask impact analysis considering the weldment rupture of the impact limiter case. The weldment rupture of the impact limiter case causes to lose their constraining effect for the wood blocks, which are filled into the metal incasement between the case and the gussets. The weldment rupture of the impact limiter case reduces the impact force which acts on the impact target significantly in vertical and horizontal drop impacts. Therefore the beneficial effect of weldment rupture should be considered in the impact limiter design and the cask impact analysis.     

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.     

The design and application of the movable limiter in EAST

There are two movable limiters that have been installed on EAST. The movable limiter consists of three parts that are blade, support components and drive components. The blade which consists of heat sink and graphite tiles has to face plasma during plasma discharge. The support components consist of a long shaft and a movable plate. The drive components consists of a screw shaft, a step motor, four guide shafts and two support plates. There is an extra support which has fit in Tokamak and a frame on the outside of the flange. The specific shape of the blade has been optimized so that the graphite tiles on the blade can withstand max 5 MW/m^2 heat load with two long cooling channels. The long shaft of the transfer components connect with blade to make it can move along the axis of the limiter. The movable blade could adjust a wide variety of plasma major and minor diameters. The drive components provide motion from step motor which can be remote controlled in the control room far away from the scene. The rotation is translated to slide motion by a screw shaft. The screw shaft can push the slide which connected with the long shaft to at last move the blade. To introduce the motion from outside to the vacuum vessel, the limiter has a long bellow and special structure as dynamical seal. This seal structure proved very reliable. The remote control system has been also equipped the distance sensor to provide the information of the position of the limiter. The control system is based on PC so it can be easily operated. The two movable limiters have been installed on the EAST. During past twice discharges, the movable limiter proved to be a useful tool for EAST to control the plasma boundary.     

The application of limiter target electrostatic measurement system in J-TEXT tokamak

The limiter target electrostatic measurement system including limiter grounding current sensors and Langmuir probes have been newly developed for the measurement of the limiter target area on the Joint-Texas Experimental tokamak(J-TEXT).Current sensors fixed between graphite limiters and the vacuum vessel walls are used to measure the currents between limiters and vessel wall.Simultaneously,a rectangular poloidal array containing 54 Langmuir probes is embedded in the graphite tiles of limiters for a more localized measurement.Based on this system,the effect of both the plasma's inherent behavior,including plasma motion and the 2/1 tearing mode,and the electrode biasing on probe and sensor signals have been observed and analyzed in the experiments.     

Primary research of heat flux deposition pattern on the surface of HT-7 movable limiter

This paper presents a method of characterizing the heat flux deposition pattern on HT-7 movable limiter, a new component in the spring experimental campaign of year 2006. A new modeling establishment combines with heat transfer simulation with ANSYS code, and a shot of long pulse discharge is chosen with small radius 265 mm of movable limiter, which is 5 mm smaller than that of the main toroidal limiters. Both parallel and perpendicular heat flux are taken into account and different ratios of them are also tested in simulations. The simulation temperature values match well with the results of the IR-camera. Temperature distribution shows that ?_||/?_⊥ = 5 is quite suitable in HT-7 device, and the maximum heat flux is about 10 MW/m² and average heat flux is about 5 MW/m² on the movable limiter. This method can be also used in other tokamak devices with limiter configuration and circular cross-section shape.     

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

Abstract

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

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.     

Investigation of the impact behaviour of wooden impact limiters

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

Progress in development and application of lithium based components for Tokamak

Experiments with lithium plasma facing components (PFCs) show promising results for the operation of hot plasma facilities and the general improvement of plasma parameters. The design and development of new tokamak plasma facing material (PFM) based on lithium capillary porous systems (CPS) are described in this paper.The recent progress in the development of limiters with different kinds of CPS is relevant for protection of tokamak PFCs from damage under normal operation, ELMs and disruptions. New PFM eliminates the lithium flux into plasma, its pollution and lithium accumulation.Here we present an overview of the design and the experimental tests of the liquid lithium limiters. These limiters are based on CPS with hard matrix from stainless steel mesh, molybdenum and tungsten. Different types of limiter have been taken into account: the horizontal and vertical rail type limiters with passive and active cooling for investigation the possibility to provide the closed lithium circulation in tokamak chamber; the ring CPS-based limiter for investigation of lithium behavior in limiter scrape-off layer (SOL).Here we also present the preliminary results of the application of the cryogenic techniques for lithium removal from the chamber wall after operation in hot plasma.     

Experimental testing of impact limiters for RAM packages under drop test conditions

In context with new cask designs and their approval procedure, the experimental testing of impact limiters under drop test conditions becomes more and more important in order to assess the damage mechanics behaviour and safety margins for validation reasons. In recent years, various designs of impact limiters have been tested by the Federal Institute for Materials Research and Testing within specific component testing and particularly with regard to type B package design approval procedures. The paper focuses on the experimental realisation of impact limiter tests and presents implemented measurement techniques to determine the amount of deformation and to explain the impact behaviour by means of photogrammetric metrology and three-dimensional fringe projection method, high speed motion analysis and adjusted deceleration measurements.     

Combustion of wood equipping package shock absorbers: after regulatory fire test

According to IAEA regulation, type B packages are designed to withstand a fire test following the mechanical tests, simulating accident conditions during transport. The wood of the shock absorber located near the damage zones produced by these tests may ignite and the conditions of air accessibility inside the absorber could mean that wood smouldering persists during the cooling period. IRSN has conducted an experimental study in which a real scale shock absorber specimen has been exposed to a large scale heptane pool fire test. A dedicated facility makes it possible to simulate the thermal power of the package contents and to measure in real time the heat transfer at the rear surface of the absorber. After a pool fire of 35 min duration, wood combustion continued for four days. Significant values were measured on the contact surface between lid and shock absorber for heat flux (up to 8 kW m^?2) and temperatures (reaching 460°C). The impact of wood smouldering induced heat fluxes on typical lid gaskets has also been evaluated.     

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

Abstract

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