Experimental determination of heat transfer from transport casks with axial fins

Abstract

In order to achieve higher heat dissipation, the outer surface area of a transport and storage cask for radioactive material can be increased by the use of cooling fins. CASTOR^® casks are fitted with cooling fins machined into the cask body, which run circumferentially around the outer surface of the cask. The first-generation CONSTOR^® casks have a smooth outer surface without fins, which is made from a steel plate. This is possible because the heat capacity of their contents is relatively low. For CONSTOR^® casks to have a higher heat capacity it is necessary to develop a special solution to allow heat to be dissipated from the outer surface of the cask. For the CASTOR^® cask series it is also desirable to achieve higher rates of heat dissipation. From an economic point of view, a solution whereby separate cooling fins are attached to a smooth outer surface would be preferable to the currently machined fins. Several possible solutions are available for achieving this and one of the ideas has been investigated in detail. This concept comprises a series of aluminium profiles which are strapped to the smooth outer cask surface. This in effect provides a cask with axial fins. This solution also allows for the inclusion of moderator material within certain areas of the aluminium profiles. Several experiments have been performed to investigate this concept. A test specimen was investigated, consisting of a 2 m by 0.4 m segment of the finned profile attached to a heating plate. In order to simulate various cask orientations during transport and storage, measurements were taken for different test-piece orientations. Computational fluid dynamics simulations of the various tests were also performed.     

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

Abstract

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

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.     

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

Abstract

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

Japanese perspectives and research on packaging,transport and storage of spent fuel

Abstract

The present paper gives an overview of Japanese experimental studies of dual-purpose metal casks. The studies included: cask drop without impact limiters, drop of a heavy weight onto a cask due to building collapse, burial of a cask in debris from building collapse, tipping over of a cask during an earthquake, long-term containment of metal gaskets and transportability of casks after long-term storage. Most of the studies employed full-scale casks for the experiments.     

Fracture Toughness of DCI Casks and Prediction by Small Specimen Test

Abstract

Spent nuclear fuel transport and/or storage containers (casks) must maintain their structural integrity even when subjected to hypothetical accidents during transport or handling accidents at storage facilities. For ductile cast iron (DCI) to be used as a cask containment boundary material, adequate fracture toughness must be demonstrated at service temperatures and Impact loading conditions of concern. In Japan, comprehensive studies of the fracture toughness of heavy section DCI have been undertaken by a number of research organisations to provide the safety assurance for the DCI casks. In the present study, the fracture toughness data were used to develop a lower bound trend curve for heavy section DCI and to examine the prediction methods by small specimen tests. The fracture toughnesses K_Ic, K_IIc and K_IIIc were also obtained to study the safety assessment of DCI casks under different loading mode conditions.     

Application of Lefm Design to Nuclear Material Transport Casks

Abstract

Fracture safe design can be assured by proper application of fracture mechanics. Linear elastic fracture mechanics (LEFM) can generally be applied to a broad range of cask designs and component materials. The use of LEFM is straightforward when the linear elastic plane strain fracture toughness (K_1c) of the cask material can be directly measured. When the plane strain fracture toughness cannot be directly measured, a special form of LEFM can be used. The fracture toughness can be equivalently determined through measurement of the J_1c (elastic–plastic) fracture toughness. While this LEFM approach can only be used under specific conditions, such conditions are generally met by heavy-walled casks under severe loading conditions. The regulatory drop test, in which a subcritical flaw has been intentionally introduced into a prototype cask, can be used to demonstrate the suitability of applying LEFM design to a specific cask. This paper describes the LEFM design approach as applied to cask design for a broad class of materials. The advantages and limitations of the LEFM approach are also discussed with respect to existing regulatory acceptance criteria.     

CONSTOR® V/TC drop tests: pre-test analysis using the finite-element method

Abstract

CONSTOR^® is a family of steel–CONSTORIT–steel sandwich cask designs that have been developed with special consideration for an economical and effective method of manufacture by using conventional mechanical engineering technologies and common materials. The CONSTOR^® concept fulfils both the internationally valid IAEA criteria for transport and the requirements for long-term intermediate storage in the USA and various European countries. A full-scale prototype test cask, CONSTOR^® V/TC, of the latest CONSTOR^® design has been developed, with a heat removal capacity of up to 32 kW. A comprehensive drop testing programme consisting of five 9 m drops onto a flat unyielding target and seven 1 m drops onto a punch is to be carried out by BAM at the test facilities in Horstwalde during Autumn 2004, with the first 9 m side drop to be carried out during PATRAM 2004. The drop tests will form part of the application for a transport licence in both Germany and the USA. Extensive pre-test calculations have been performed using finite-element methods. The objectives of the analyses are as follows: (1) As an intermediate step in demonstrating the performance of the package in fulfilling the requirements of 10 CFR 71 and the IAEA transport regulations. (2) To justify the selection of drop tests. (3) To predict the performance of the V/TC in the drop tests. (4) To estimate the strain and acceleration–time history at measuring points to aid the setting up of the instrumentation. (5) To develop an analysis model that can be used in future safety analyses for transport and storage licence applications to confidently demonstrate the performance of the package. This paper will: present an overview of the analyses; discuss the methodology of the analysis, including the design and make-up of the models taking into account the behaviour of the package, the requirements of the licensing regimes and the present and future purposes of the model; discuss the modelling techniques used; present key results from the analyses; and discuss the behaviour of the package.     

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.     

Effects and Consequences of Interactions Between Containment Components and Content of Type B Packages During 9 ill Drop Tests

Abstract

Tests with different Type B casks confirm the existence of effects with stress peaks in cask components due to interactions between the cask and its contents. These effects can be caused by a delayed strike of the content onto surrounding cask components which cannot be excluded if the content is movable. Some results of the drop tests with two different Type B casks and with a model designed for the study of this problem are presented in the paper. Results of calculations performed with the ABAQUS computer code and by use of analytical methods to simulate the measured effects are discussed.     

From experiment to appropriate finite element model-safety assessment for ductile cast iron casks demonstrated by means of IAEA puncture drop test

Abstract

In the approval procedure of transport packages for radioactive materials, the competent authority mechanical and thermal safety assessment is carried out in Germany by BAM Federal Institute for Materials Research and Testing. The combination of experimental investigations and numerical calculations in conjunction with materials and components testing is the basis of the safety assessment concept of the BAM. Among other mechanical test scenarios, a 1 metre drop test onto a steel bar has to be considered for the application of the hypothetical accident conditions to Type B packages according to IAEA regulations. Within the approval procedure for the new German package design of the HLW cask CASTOR® HAW 28M, designed by GNS Gesellschaft für Nuklear-Service Germany, a puncture drop test was performed with a half-scale model of the cask at ?40°C. For independent assessment and to control the safety analysis presented by the applicant, BAM developed a complex finite element (FE) model for a dynamical ABAQUS/Explicit^TM analysis. This paper describes in detail the use of the FE method for modelling the puncture drop test within an actual assessment strategy. At first, investigations of the behaviour of the steel bar were carried out. Different friction coefficients and the material law of the bar were analysed by using a ‘rigid-body’ approximation for the cask body. In the next step, a more detailed FE model with a more realistic material definition for the cask body was developed. The validation of calculated strains was carried out by comparison with the results of the strain gauges located at the relevant points of the cask model. The influence of the FE meshing is described. Finally, the validated FE half-scale model was expanded to full-scale dimension. Scaling effects were analysed. The model was used for safety assessment of the package to be approved.     

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.     

Thermal analysis of proposed transport cask for three advanced burner reactor used fuel assemblies

Abstract

Preliminary studies of used fuel generated in the US Department of Energy's Advanced Fuel Cycle Initiative have indicated that current used fuel transport casks may be insufficient for the transportation of said fuel. This work considers transport of three 5-year-cooled oxide advanced burner reactor used fuel assemblies with a burn-up of 160 MWD kg^–1. A transport cask designed to carry these assemblies is proposed. This design employs a 7-cm-thick lead gamma shield and a 20-cm-thick NS-4-FR composite neutron shield. The temperature profile within the cask, from its centre to its exterior surface, is determined by two-dimensional computational fluid dynamics simulations of conduction, convection and radiation within the cask. Simulations are performed for a cask with a smooth external surface and various neutron shield thicknesses. Separate simulations are performed for a cask with a corrugated external surface and a neutron shield thickness that satisfies shielding constraints. Resulting temperature profiles indicate that a three-assembly cask with a smooth external surface will meet fuel cladding temperature requirements but will cause outer surface temperatures to exceed the regulatory limit. A cask with a corrugated external surface will not exceed the limits for both the fuel cladding and outer surface temperatures.     

Safety Aspects of Long-term Dry Interim Storage Of Type-b Spent Fuel and Hlw Transport Casks

Abstract

Based on the German decision to minimise transport of spent fuel casks between nuclear power plants, reprocessing plants and central storage facilities several on-site storage facilities were licensed until the end of 2003. Because of the large amount of Type B(U) transport casks which are going to be used for long-term interim storage the question of timelimited Type B(U) licence maintenance during the storage period of up to 40 years has been discussed under different aspects. This paper describes present technical aspects of the discussion. A main aspect of qualification of transport casks for interim storage is the long-term behaviour of the metallic seal–lid system. Here results are presented from current long-term experimental tests with metallic 'Helicoflex' seals in which pool water is enclosed. This series of tests has been performed by the Federal Institute for Materials Research and Testing (BAM) on behalf of the Federal Office for Radiation Protection (BfS) since 2001. Finally, the paper presents a German concept for an exchange of experience, know-how and state-of-the-art between authorities and technical experts with regard to cask dispatch in nuclear facilities. BAM has taken over a central role in this so-called 'coordinating institution for cask dispatching information' ('KOBAF') which entails management of an online database of cask-specific documents and a technical working group meeting twice a year. The goal is to keep comparable technical standards for all nuclear sites and storage facilities which are going to load and dispatch casks of the same or similar types under the responsibility of different German state governments for the coming decades.     

Long-Term Containment Performance Test Facilities for Spent Fuel Transport/Storage Casks (CRIEPI,Japan)

Abstract

Long-term containment tests of full-scale transport/storage cask models have been in progress since 1990 at CRIEPI, Japan. The results demonstrate and confirm the very reliable containment pelformance of the cask lid structure with metallic gaskets.     

Finite element analyses and verifying tests for a shock-absorbing effect of a pad in a spent fuel storage cask

A spent fuel storage cask is required to prove the safety of its canister under a hypothetical accidental drop condition which means that the canister is assumed to be free dropped on to a pad of the storage cask during the loading of the canister into a storage cask. In this paper, finite element analyses and verifying tests for a shock-absorbing effect of a pad in a spent fuel dry storage cask were carried out to improve the structural integrity of the canister under a hypothetical accidental drop condition. The pad of the storage cask was originally designed as cylindrical steel structure filled with concrete. The pad was modified by using the structure composed of steel and polyurethane-foam instead of the quarter of the upper concrete as an impact limiter. The effects of the shape and the thickness of the steel structure and the density of the polyurethane-foam which was used in between steel structures were studied. As the optimized pad of a spent fuel dry storage cask, the quarter of the upper concrete was replaced with 12 mm thick circular steel structure and polyurethane-foam whose density was 85 kg/m³. The drop tests of a 1/3 scale model for the canister on to the original pad and the optimized pad were conducted. The effect of the pad structure was evaluated from the drop tests. The optimized pad has a greater shock-absorbing effect than the original pad. In order to verify the analysis results, strains and accelerations in the time domain by the analytical methods were compared with those by a test. The numerical method of simulating the free drop test for a dry storage cask was verified and the numerical results were found to be reliable.     

Mechanical design assessment approaches of actual spent fuel and HLW transport package designs

Abstract

In recent years, BAM Federal Institute for Materials Research and Testing finalised the competent authority assessment of the mechanical and thermal package design in several German approval procedures of new spent fuel and high level waste package designs. The combination of computational methods and experimental investigations in conjunction with materials and cask components testing is the most common approach to mechanical safety assessment. The methodology in the field of safety analysis, including associated assessment criteria and procedures, has evolved rapidly over the last years. The design safety analysis must be based on a clear and comprehensive safety evaluation concept, including defined assessment criteria and constructional safety goals. In general, for new package designs, the implementation of experimental package drop tests in the approval process should be obligatory. Additionally, pre- and post-test calculations as well as components or material testing could be important. The extent to which drop tests are necessary depends on the individual package construction, the materials used and identified safety margins in the design.     

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.     

Plans for the future for the Packaging,Transport, Storage and Security of Radioactive Material Journal

Abstract

The regulatory driven design of radioactive material transportation packages leads package vendors to perform analyses that demonstrate the ability of packages to meet the regulatory requirements. For risk assessment and communication, the analysis of package response to thermal environments that are more severe than those described in the regulations is required. In general, experimental and analytical assessments of casks exposed to thermal insults other than the regulatory environment are performed in the USA by the Department of Energy national laboratories. This paper provides a brief summary of some recent thermal analyses of spent fuel transportation packages exposed to thermal environments different from regulatory standards. The analyses were performed by Sandia National Laboratories under several different projects for multiple customers. These analyses examined the response of spent fuel packages exposed to severe thermal environments different from the regulatory hypothetical accident condition. One assessment determined the response of four generic casks to very long duration engulfing fires. The results from these analyses included fire durations necessary to reach critical temperatures of the fuel and seals. In another assessment, two certified spent fuel casks were analysed for exposure to 1 h pool fires. The height of the cask above the pool was varied to study the effect of the vapour dome on the heating of the casks. Another assessment investigated the effect of offset long duration fires on rail cask performance, which showed that casks can withstand offset fires of much longer duration than the regulatory fire. Other assessments examined the response of packages to thermal environments resulting from propane fires and realistic liquid hydrocarbon fires that included various positions of the transportation rail car in the simulation.     

Description of the German Concept of Final Storage Cask

Abstract

The German storage concept for the direct final storage of spent fuel assemblies from LWR reactors is described. The final storage concept is designed in such a way that it encompasses the whole spectrum of fuel elements to be stored from German reactors, Le. U fuel assemblies and MOX fuel assemblies with a mean bumup of 55 GW.d.t^?1 heavy metal were considered. The further design requirements are defined in such a way that the cask concept satisfies the conditions for type B(U) transport, interim storage and fmal storage. The safe long-term containment of the activity is guaranteed by an inner cask welded leak-tight; the sufficient shielding and the transport packaging are ensured by a shielding cask.