Extended Drop Tests Of DCI Casks with Artificial Flaws Demonstrating the Existing Safety Margins

Abstract

The use of spent fuel shipping and storage casks made of ductile cast iron (DCI) has been common practice for about 15 years when the development of such casks started in Germany where qualified foundries are able to produce these heavy section castings at the high quality level needed for this kind of application. To promote the discussion on safety against brittle failure a lot of research had been carried out in different countries. The two test programmes in Germany on casks with big artificial flaws under severe impact conditions is summarised in this paper. The first test object was a thick walled DCI ‘pipe’ (150 mm wall thickness) with dimensions equivalent to a 1:2.5 scale cask model. It was dropped with a 40 mm deep laser sharpended flaw from heights of up to 9 m onto rails. As a second test object a full scale CASTOR VHLW cask was used. This specimen had a flaw with a depth of 120 mm in a 260 mm thick wall. With increasing drop heights (up to 14 m) and stress intensity factors (up to material fracture toughness) this object was also dropped onto rails. For both cases the measured data (decelerations, crack opening displacement, strains, material properties) are presented. No brittle failure occurred, although in the 14 m drop of the CASTOR VHLW Cask the impact was 6.5 times higher than the impact measured in the mechanical test of the type B package design. The results demonstrate that DCI casks have significantly high safety margins even in the hypothetical case of an impact beyond type B package design requirements.     

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

Abstract

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

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.     

Safety Margins of Spent Fuel Transport and Storage Casks Considering Aircraft Crash Impacts

Abstract

The safety of spent fuel transport casks in severe accident conditions is always a matter of concern. This paper surveys German missile impact tests that have been carried out in the past to demonstrate that German cask designs for transport and interim storage are safe even under conditions of an aircraft crash impact. A fire test with a cask beside an exploding propane vessel and temperature calculations concerning prolonged fires also show that the casks have reasonably good safety margins in thermal accidents beyond regulatory fire test conditions.     

An Overview of R&D Work Performed in France Concerning the Risk of Brittle Fracture of Transport Casks

Abstract

An overview is given of the methodology followed in France to evaluate the risk of brittle fracture in transport casks. Various aspects are considered in this evaluation. First: the material characterisation: effect of strain rate and temperature on tensile properties and toughness. Secondly: evaluation of applied stress under dynamic loading and criteria to define worst cases (time, distribution). Thirdly: the global validation is presented to demonstrate that the whole methodology is conservative; consideration of representative analytical cases where initiation is obtained under controlled conditions. Results are presented concerning cast ductile iron and carbon steel materials.     

Application of IAEA TECDOC 717 to Packagings and Comparison with Reactor Vessels

Abstract

Brittle fracture evaluation is important for type B(U) packagings and packagings carrying fissile materials in conformity with IAEA Regulations. Packaging materials susceptible to brittle fracture can be evaluated by Linear Elastic Fracture Mechanics (LEFM) or other credible methods as shown in IAEA TECDOC 717. Major similarities and differences between the packagings and reactor pressure vessels are here compared in terms of the brittle fracture evaluation. Examples of brittle fracture evaluations of 100 ton class ductile cast iron (DCI) packagings (casks) under hypothetical accident conditions are discussed.     

Determination of the dynamic fracture toughness using a new stress pulse loading method

This paper presents a method for the determination of the dynamic fracture toughness K_Id of metallic materials at loading rates K_I of about . The method is derived from the known split Hopkinson pressure bar technique and uses a well-defined stress pulse for the loading of a fatigue precracked specimen. The interpretation of the experimental data is strictly based on a numerical analysis of the specimen under the given dynamic loading conditions. It is shown, that a conventional quasi-static approach would yield incorrect fracture toughness values. The results for some steels confirm, that the fracture toughness decreases with increasing loading rate. Therefore, in some sense the fracture toughness versus temperature curve determined with the presented stress pulse method can be regarded as lower bound curve.     

Influence of loading rate on the fracture toughness versus temperature curve

This paper presents recent experimental investigations on the influence of loading rate on the fracture toughness K_Ic for different structural steels. The loading rate in terms of increasing in stress intensity factor was changed from quasistatic up to dynamic conditions (

Full-size image (<1K)

). The results show significant differences in the amount of temperature shift between K_Ic -T curve obtained after static and dynamic loading for the materials investigated. Based on microscopic fracture criteria for cleavage fracture correlations were made between fracture toughness and yielding behaviour depending on temperature and strain rate. The experimental results were also compared with the predictions given by different models. The most promising results were achieved by a correlation between transition temperature shift in K_Ic caused by dynamic loading and strain rate sensitivity of steels.     

Adaptation of Approved Transport and Storage Cask Lines to Individual Customer Requirements

Abstract

Within the decommissioning programmes of the Italian nuclear power plants, the Italian multi-utility company ENEL decided to rely on on-site dry storage while waiting for the availability of the national interim storage site. SOGIN (Società Gestione Impianti Nucleari SpA, Rome, Italy), now in charge of all nuclear power plant (NPP) decommissioning activities was created in the ENEL group but is now owned by the Italian government. In 2000 it ordered 30 CASTOR® casks for the storage of its spent fuel not covered by existing or future reprocessing contracts. Ten CASTOR X/A17 casks will contain the Trino pressurised water reactor (PWR) fuel and the Garigliano boiling water reactor (BWR) fuel currently stored in pools at the nuclear power plant Trino and the Avogadro nuclear facility at Saluggia. Additionally 20 CASTOR X/B52 casks will contain the BWR fuel assemblies, which are stored in the pool at the Caorso nuclear power plant. GNB (Gesellschaft fuer Nuklear-Behaelter mbH, Essen, Germany) has completed detailed studies for the design of both types of cask. The tailored cask design is based on the well-established and proven design features of CASTOR reference casks and is responsive to the needs and requirements of the Italian fuel and handling conditions. The design of the CASTOR X/A17 for up to 17 Trino PWR fuel assemblies or 17 Garigliano BWR fuel assemblies and the CASTOR X/B52 cask holding up to 52 Caorso BWR fuel assemblies is suitable for the following conditions of use: loading of the casks in the fuel pools of the nuclear installations at Trino, Caorso and Avogadro; no upgrading of the Current on-site crane capacities; transport of the fuel assemblies, which are currently stored at the Saluggia facility to the nuclear power plant Trino; on-site storage in a vertical or horizontal position with the possibility of transfer to another temporary storage or a final repository, even after a number of years; the partial loading of mixed oxide (MOX) and failed fuel; loading and drying of bottled Garigliano fuel assemblies. On the basis of the CASTOR V/19 and CASTOR V/52 cask lines, the design of the CASTOR X/A17 and X/B52 casks aims at optimising safety and economics under the given boundary conditions. The long time for which fuel is kept in intermediate wet storage results in a reduced shielding and thermal-conduction requirement. This is used to meet the tight mass and geometry restrictions while allowing for the largest cask capacity possible.     

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.     

Transport of MOX fuel: continuous challenge

Abstract

For 45 years TN International has been involved in the radioactive materials transportation field. Since the beginning the spent nuclear fuel transportation has been its core business. During all these years TN International, now part of AREVA, has been able to anticipate and fulfil the needs for new transport or storage casks design to fit the nuclear industry evolutions. A whole fleet of casks able to transport all the materials of the nuclear fuel cycle has been developed. This paper focuses on the casks used to transport the fresh and used mix oxide (MOX) fuel. To transport the fresh MOX boiling water reactor and pressurised water reactors fuel, TN International has developed two designs of casks: the MX 6 and the MX 8. These casks are and have been used to transport MOX fuel for French, German, Swiss and in a near future Japanese nuclear power plants. A complete set of baskets have been developed to optimise the loading in terms of integrated dose and also of course capacity. Mixed oxide used fuel has now its dedicated cask: the TN 112 which certificate of approval has been obtained in July 2008. This cask is able to transport 12 MOX spent fuel elements with a short cooling time. The first loading of the cask has been performed in September 2008 in the Electricité de France nuclear power plant of Saint-Laurent-des-Eaux. By its continuous involvement in the nuclear transportation field, TN International has been able to face the many challenges linked to the radioactive materials transportation especially talking of MOX fuel. TN International will also have to face the increasing demand linked to the nuclear renaissance.     

Assessment of integrity of a dual-purpose metal cask after long-term interim storage: seal performance under transport conditions

Abstract

Spent fuels generated in nuclear power plants (NPPs) must be stored until they can be reprocessed into new energy sources in Japan. The quantity of spent fuel stored at each NPP site is increasing, and early realisation of a method of interim storage is expected. Dual-purpose metal casks will be used which will not be reopened until they are delivered to a reprocessing plant, in order to minimize radiation exposure to personnel. The Japan Nuclear Energy Safety Organization (JNES) was established on 1 October 2003 with the mission of ensuring public safety from the potential hazards of nuclear energy. JNES have also been conducting a study of dry storage technology for interim storage. The study of verification of metal cask storage technology, the results of which from a storage and subsequent transport safety point of view are presented in this paper, was originally initiated with government funds in 1999 at the Nuclear Power Engineering Corporation, before being transferred to and conducted by JNES up to the end of 2003.     

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

Abstract

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

Comparison of the Structural Integrity of Ductile Iron and Stainless Steel Casks for Transport of Radioactive Material

Abstract

The results are presented of 9 m (30 ft) drop simulations of three different types of transport casks, a monolithic ductile iron (DI) cask, a monolithic stainless steel (SS) cask, and a lead-shielded stainless steel (SS/Pb) sandwich cask. Each simulation involves two casks, one lying horizontally on an unyielding surface and the other positioned 9 m (30 ft) above the top surface of the lower cask. The top cask then free falls onto the lower cask, resulting in a more severe impact than the standard drop test required by the Nuclear Regulatory Commission (NRC). The drop tests were simulated using DYNA3D, a non-linear, explicit, three-dimensional finite element code for solid and structural mechanics. The results show that the monolithic casks are much stiffer than the stainless steel/lead sandwich cask. The largest difference was observed between the DI cask and the SS/Pb sandwich cask. Although the SS/Pb cask experiences considerable plastic deformation, none of them experiences failure by rupture, and they all perform within the requirements of Regulatory Guide 7.6, Revision 1 and IOCFR71. The better to compare the results, stress- and strain-based factors of safety were calculated for all of the simulations. These calculations show that the DI cask has a larger margin of safety than the SS/Pb sandwich cask, while the monolithic SS cask has a larger margin of safety than the monolithic DI cask. Finally, to address the concern over the brittleness of the DI casks, critical flaw sizes were calculated. All flaws required for crack propagation were larger than those detectable by current inspection techniques. Overall, the results of this study indicate that DI has sufficient strength, ductility, and fracture toughness to be considered as a structural material for transport casks.     

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.     

Dynamic fracture toughness test and master curve method analysis of IAEA JRQ material

IAEA conducted a round-robin fracture test program to test and verify the Master Curve method. One of the materials selected for the round-robin is a A-533B1 plate designated as reference material JRQ. Unnotched Charpy-size specimens were fabricated and distributed to a number of testing laboratories. The three US Owners Groups received specimens for both Charpy impact and three-point bending tests to establish fracture toughness master curves. The B&W Owners Group elected to perform a dynamic fracture toughness test under a high loading rate using the JRQ specimens. The master curve method was successfully applied to numerous fracture toughness data sets of pressure vessel steels. Joyce [Small Specimens Test Technique, ASTM STP 1329, 1997, ASTM] applied this method to high loading rate fracture toughness data for A-515 steel and showed the applicability of this approach to dynamic fracture toughness data. This paper presents the data and the resulting reference temperature shift in the Master Curves from static to dynamic fracture data. Based on earlier PTS analyses performed in 1985, an appropriate T₀ shift value is selected for nuclear power plant applications. This shift in T₀ is compared with the temperature shift between K_Ic and K_Ia curves in ASME Boiler and Pressure Vessel Code.     

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.     

Safety of transport of radioactive Material: 2004 in retrospect

Abstract

There are basically two main technologies for the intermediate storage of spent nuclear fuel in Europe: dry storage in casks or vaults and wet storage in pools. The advantage of casks is their modularity and hence investment can be phased to suit the planned dates of loading individual casks, pools and vaults usually provide longer term capacity and thus require a greater initial investment for operators. Transnucléaire has developed a range of modular dry cask solutions for customers and more than 100 examples of the TN 24 type cask have been licensed for transport and storage in Belgium, Switzerland, Italy, Germany, the United States of America and Japan. This paper compares the requirements for cask licensing in Europe and the USA and shows how two particular BWR cask designs were developed by Transnucléaire. (1) The TN 97 L cask was designed primarily for the European market and the first use is foreseen at the Leibstadt nuclear power station in Switzerland. (2) The TN 68 cask was designed by Transnuclear Inc. and its first use is foreseen at the Philadelphia Electric Company's Peach Bottom Atomic Power Station.     

Moving the Largest Capacity PWR Dual-Purpose Cask in the World from Gösgen NPP to the Zwilag Interim Storage Site

Abstract

The Swiss Gösgen nuclear power plant (NPP) has decided to use two different methods for the disposal of its spent fuel. (1) To reprocess some of its spent fuel in dedicated facilities. Some of the vitrified waste from the reprocessing plant will be shipped back to Switzerland using the new COGEMA Logistics, TN81 cask. (2) To ship the other part of its spent fuel to the central interim storage facility at Zwilag (Switzerland) using a COGEMA Logistics dual-purpose TN24G cask. The TN24G is the heaviest and largest dual-purpose cask manufactured so far by COGEMA Logistics in Europe. It is intended for the transport and storage of 37 pressurised water-reactor (PWR) spent fuel assemblies. Four casks were delivered by COGEMA Logistics to Gösgen NPP. Three transports of loaded TN24G casks between Gösgen and Zwilag were successfully pelformed at the beginning of 2002 using the new COGEMA Logistics Q76 wagon specifically designed to transport heavy casks. This article describes the procedure of operations and shipments for the first TN24G casks up to storage at Zwilag. The fourth shipment of loaded TN24G was due to take place in October 2002. The TN24G cask, as part of the TN24 cask family, proved to be a very efficient solution for Kemkraftwerk Gösgen spent fuel management.     

Probabilistic Evaluation of Brittle Failure Design Criterion of DCI Cask

Abstract

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