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EXPERIENCES AND STATUS IN SAFETY ASSESSMENT METHODS WITHIN THE SCOPE OF LICENSING PROCEDURES FOR MULTI-PURPOSE FLASKS IN GERMANY H. Vo¨lzke, B. Droste and R. Ro¨del Bundesanstalt fu¨r Materialforschung und-pru¨fung (BAM) D-12200 Berlin, Germany Abstract — The Federal Institute for Materials Research and Testing in Germany (BAM) is the German competent authority for safety assessment and design testing of flasks for transport and storage of radioactive waste. Referring to the IAEA regulations (Safety Standards Series No ST-1) four different methods for demonstration of compliance with the performance standards are possible: (1) tests with prototype flasks, (2) tests with scaled models, (3) reference to previous safety demonstrations, and (4) calculation. The advantages and disadvantages of these methods are described with reference to the nearly 20 years experience of BAM in design testing under mechanical accident conditions. Drop testing with prototype flasks and scaled model flasks was the basic method at the beginning of flask development. With better knowledge of flask behaviour under such impact conditions methods (3) and (4) were increasingly used for various flask designs. The present trend is to use numerical calculation methods also for new flask designs because of the high potential of modern computer technology. Some actual examples are given and the necessary requirements for the acceptance of this method are discussed. During recent years numerous flask designs were granted transport licences as well as licences for interim storage at the Gorleben and Ahaus sites. Because of different actions of opponents the licenses for the interim storage sites were the object of legal proceedings with special reference to the flask design testing methods. Finally, the experience obtained from these legal disputes and their relevance to acceptable safety assessment methods are discussed.

SAFETY ASSESSMENT METHODS Referring to the IAEA regulations (1) four different methods for demonstration of compliance with the performance standards are possible: (1) tests with prototype flasks, (2) tests with scaled models, (3) reference to previous safety demonstrations, and (4) calculation. Methods 1 and 2 are experimental and 3 and 4 analytical methods. These methods, which are described in the transport regulations, are also valid for safety assessment under the storage site specific accident conditions. A flask for transport and storage has to fulfil the same basic requirements under accident conditions, which are subcriticality, limitation of activity release and shielding. Only the level of acceptable values differs depending on the storage site specific safety requirements. The paper focuses especially on the limitation of activity release which has to be guaranteed by the integrity and tightness of the flask and its lid system. This means that a mechanical safety assessment has to demonstrate that a flask design withstands the most critical accident scenario without breaking or cracking of the wall and the lid system and maintaining an acceptable leakage rate of at least one lid system. From the more than 30 years experience of BAM it is obvious that an appropriate combination of experimental and analytical methods is necessary for a complete safety assessment today. Even a test with an original prototype flask must be complemented by,

INTRODUCTION Safety assessment methods for flasks for transport, interim storage and final disposal of radioactive waste are well established as an important part of the different licensing procedures under the international and national transport regulations and the national atomic energy legislation. The Federal Institute for Materials Research and Testing in Germany (BAM) deals especially with the mechanical and thermal accident behaviour of such flasks. Based on extensive experience, for nearly 20 years BAM has been preparing numerous opinions on safety assessment for transport and storage flasks. These opinions are the basis of numerous licenses for many different flask designs for transport and interim storage. This paper focuses on the safety assessments for Type B transport and storage flasks under mechanical accident conditions (Figure 1). Different requirements come from the transport regulations and the storage site specific safety assessment. The transport regulations require a 9 m drop test onto an unyielding target. Consequently, the flasks are equipped with impact limiters which lead to acceptable mechanical stresses of the flask structure under the 9 m drop. On the other hand, there are the storage site specific accident conditions which come from the respective safety analysis. Normally the most critical mechanical accident scenario is the drop of a flask from a crane or any other lifting gear from the maximum lifting height onto the real ground of the facility. 39

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e.g. analytical reflections on activity release depending on a measured volumetric leakage rate because tests with radioactive contents are normally impossible. On the other hand analytical calculations of the mechanical flask behaviour under drop test conditions have to be verified, e.g. by a preceding comparison of calculation results with representative test results from other but similar configurations.

(i)

a 1:2 model of a CASTOR lla for 9 PWR spent fuel elements, (ii) an original prototype of a CASTOR lc for 16 BWR spent fuel elements, (iii) a 1:3 model of a TN 1300 for 12 PWR spent fuel elements, (iv) an original prototype of a TN 900 for 21 BWR spent fuel elements, and (v) a 1:3 model of a TS 28V for vitrified high active waste from reprocessing,

DROP TESTS WITH PROTOTYPE AND MODEL FLASKS

were of great importance. A good overview of the extended mechanical tests with many different flask designs is given elsewhere (2). With the experience and knowledge of these extensive experimental investigations and in addition several research projects, BAM was able to make positive decisions for transport and storage licenses. Nevertheless, this was not possible without additional analytical investigations of model laws and stress–strain behaviour as well as separate material investigations.

Drop tests with prototype flasks demonstrate the flask behaviour under the test conditions directly. After some test series with French flasks like TN 8/9 and TN 12, in the mid-1970s BAM started testing the new CASTOR flask design developed by GNS (Gesellschaft fu¨ r Nuklear-Service mbH) in 1978. In contrast to the TN flask design with steel-lead-steel walls the new CASTOR flasks were made of monolithic ductile cast iron. The first six drop tests were performed with a 1:2 model of a CASTOR la (for four PWR spent fuel elements) with a mass of about 8 t. Because of the totally new material BAM required four additional drop tests with an original prototype flask with a total mass of about 65 t. These were the first tests worldwide with such a big flask. They became necessary because the quality of the ductile cast iron depends on the wall thickness and the mass of the casting. In particular, a greater wall thickness leads to lower important material properties like fracture strength and fracture toughness. The test procedure for the qualification of the new material required drop tests at the lowest temperature of ⫺40°C as well as additional investigations of material specimens from the prototype flask. The CASTOR la was at first equipped with only one lid for transport. But with the development of transport and storage casks with long-term resistant two-lid systems with metallic gaskets, new drop tests became necessary. In this field the following drop tests with:

SAFETY ASSESSMENT BY NUMERICAL CALCULATION METHODS With the increasing quality of computer technology, their use for the numerical calculation of the material stresses of flasks as a result of an impact grew up. It was evident that this was an opportunity to reduce expensive drop test series. The first big project in this field within German licensing procedures was the POLLUX flask design which was developed by GNS for transport, interim and final storage. The packaging consists of two flasks, the inner containment manufactured from low-alloy ferritic steel and the outer shielding flask manufactured from ductile cast iron. Two lids are used for the closure of the inner flask. A primary lid is secured by 24 bolts and a secondary lid is welded between the lid rim and the inner flask. The outer flask lid with a thread at its periphery is screwed into the flask. Because of this totally new flask design an extensive

Figure 1. Safety assessment of flasks under mechanical accident conditions. 40

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drop test series was performed by BAM including 9 m drop tests with impact limiters onto an unyielding target as well as 5 m drop tests onto a concrete foundation without impact limiters representing the most critical accident scenario inside a storage facility. More detailed information is given elsewhere (3,4). Additionally, the applicants took the opportunity to establish the numerical calculation method by the use of finite element codes for the analysis of the mechanical behaviour of the flask as a result of such impacts. This project was very suitable because the new flask design required an extensive test programme which could give the necessary experimental data for verification of calculation results. Before safety assessment for mechanical accident scenarios by numerical calculation can be accepted by the competent authority the qualification of the users as well as the verification (benchmarking) of the codes are necessary prerequisites to achieve reliable results. In the case of the POLLUX flask, firstly pre-test computer analyses for all different drop test orientations were made. After the drop tests were carried out post-test calculations were made for a detailed comparison of test and calculation results. This verification had to discuss and to explain all relevant differences between the two methods and required additional parametric studies in order to modify modelling assumptions which are, e.g. mesh structure, material behaviour, boundary conditions and coupling effects. The paper of Zeisler et al (2) gives

two representative examples for the problems that have arisen: the first one was a significant difference of the drop duration in the case of a 9 m horizontal drop which was influenced by the shock aborbers and the trunnions. The second one was a big difference of time dependent strains in the weld of the secondary lid in the case of a 9 m corner drop onto the lid shock absorber. Another important example in this field is the mechanical analysis of a cubic ductile cast container for transport, interim storage and final disposal of radioactive waste as described by Vo¨ lzke et al (5). In this case the topic was the 5 m drop onto the ground of the repository (Figure 2). It was shown that modelling, material behaviour and contact conditions between container bottom and target are very important for getting representative and realistic calculation results. Since that time the investigations have been proceeding and additional results are presented in this meeting (6). EXPERIENCE FROM GERMAN LEGAL PROCEEDINGS Licensing of flasks for the dry interim storage of spent fuel and highly active waste from reprocessing is governed by German atomic legislation. When the safety assessments for a storage facility including different flask designs are accepted by technical expert organ¨ V Hannover/Sachsenisations such as BAM or TU Anhalt, the Federal Institute for Radiation Protection

Figure 2. FE modelling and calculation for a cubic waste container. 41

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(BfS) as the competent authority gives the license. The part of safety assessment for mechanical accident conditions is particularly the subject of BAM’s experts opinion. In 1995 and 1996 opponents brought actions against the licenses for the interim storage facilities at Ahaus and Gorleben. One of the most important topics of the legal proceedings was the flask design testing methods. It was maintained by the opponents that the new CASTOR V/19 design had never been qualified by a prototype drop test which would have been necessary in such a case (7). During the legal proceedings the experts from BAM had to explain very precisely the use and the acceptance conditions for safety assessment methods other than drop tests. Reference to previous safety demonstrations and the justification for acceptable model correlations were especially discussed over a wide range. BAM explained that a new flask design is not necessarily totally different, and especially in the case of the CASTOR V design the basic structure was very similar to other CASTOR flask designs. With reference to the numerous drop tests BAM has been performing for at least 20 years and the additional research projects it was possible to demonstrate that the application of model relations and analytical and numerical calculation methods were appropriate methods for safety assessment as well as drop tests with prototype flasks. Moreover it has to be considered that one or a few drop tests have to stand for the most critical drop orientation in combination with the lowest level of relevant flask material properties which also need additional investigation. Finally, the BAM experts convinced the juries of the

correctness of their work. As a result, the actions were refused in both cases in 1996 by the highest administrative tribunals (‘Niedersa¨ chsisches Oberverwaltungsgericht’ and ‘Oberverwaltungsgericht fu¨ r das Land NordrheinWestfalen’ (8,9)) and the written opinions explained all important details. CONCLUSIONS The current status of BAM’s experience with the different methods for demonstration of compliance with the performance standards in the case of safety assessment for transport and storage flasks under mechanical impact conditions show the following advantages and disadvantages, listed in Table 1. This overview shows that method 4 offers a wide range of opportunities for a better understanding of mechanical effects and the quantification of safety margins. This can be used by the applicants for the optimisation of flasks designs as well as by the expert organisations for detailed analyses. However, experience shows that the development of numerical calculation methods without verification by reference to experimental investigations is impossible. This includes material investigations to determine relevant material properties as well as prototype and model drop tests with complete flasks. Only a wide experience in this field leads to a sufficient understanding of the complex flask behaviour which is the basis for a well qualified interpretation of numerical calculation results (Figure 3). In practice it is obvious that for completely new flask designs like POLLUX or cubic containers, the safety assessment needs a specific combination of methods 1– 4. In our opinion it is not possible today

Table 1. Advantages and disadvantages of different methods for mechanical safety assessments of transport and storage flasks. Advantages

Disadvantages

1. Tests with prototype flasks

Direct demonstration of mechanical integrity and tightness

High effort and costs for prototype manufacturing and testing No quantification of safety margins

2. Tests with scaled models

Lower effort compared to tests with prototype flasks

Difficult model laws have to be verified Different material structure because of different manufacturing process Different scaling factors for different flask components

3. Reference to previous safety demonstrations

No additional drop tests necessary

Only for similar flask designs with small parameter variations

4. Calculation

Detailed parameter studies possible Analyses of the inner flask structure Good knowledge of mechanical effects Quantification of safety margins

Much effort on verification of the model structure, the material modelling and the calculation method No results for tightness of sealing systems Much data for interpretation

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to accept only numerical safety assessments in such cases. Nevertheless, method 4 is acceptable with a wide range of advantages for all flasks or components which are similar to former constructions with well known properties and behaviour. That means that much experience and a good knowledge of safety rel-

evant material properties, flask properties and mechanical effects by any applicant and technical expert organisation is the basis for an increasing use of that safety assessment method. Additionally, only this leads to an acceptance by the public as the legal proceedings in Germany have shown.

User know-how and experience Previous drop tests with similar objects

Geometry: Element type Material models Substructures (screws, trunnions, etc)

Flask (with or without impact limiters)

Finite element code

Drop height

Experimental investigation of Material properties: (static/dynamic)

Target (IAEA or Storage Facility)

Figure 3. Basic requirements for numerical drop test calculation.

REFERENCES 1. IAEA. Regulations for the Safe Transport of Radioactive Material Safety Standards Series No ST-1, 1996 Edition (Vienna; Austria: IAEA) (1996). 2. Zeisler, P., Droste, B. and Ro¨ del, R. Current Approval Status and Test Procedures for Large Type B Packages in Germany. Int. J. Radioact. Mater. Transp. 8(1), 53–62 (1997). 3. Quercetti, T., Droste, B. and Gogolin, B. Integrity of the German POLLUX Cask for Final Disposal: Experimental Results of the Mechanical Tests. In: Proc. PATRAM 1995, Las Vegas, NV, USA (1995). 4. Gogolin, B., Droste, B. and Quercetti, T. Drop Test Program with the German POLLUX Cask for Final Disposal of Spent Fuel. In: Proc. PATRAM 1995, Las Vegas, NV, USA (1995). 5. Vo¨ lzke, H., Wieser, G., Zencker, U. and Droste, B. Mechanical and Thermal Safety Analyses and Demonstrations for Cubic DCI Multipurpose Containers. Int. J. Radioact. Mater. Transp. 8(3–4), 247–252 (1997). 6. Zencker, U., Zeisler, P. and Droste, B. Dynamic Fracture Mechanics Assessments for Cubic Ductile Cast Iron Containers. Int. J. Radioact. Mater. Transp. 11(1–2), 113–118 (This issue) (2000). ¨ ko-Institut, Freiburg) (1998). 7. Schlich, E. (Ed.) CASTOR-Schwarzbuch (Werkstattreihe 106, O 8. Niedersa¨ chsisches Oberverwaltungsgericht, Lu¨ neburg. Beschlu␤ vom September 1996, 7 K 4357/95 (1998). 9. Oberverwaltungsgericht fu¨ r das Land Nordrhein-Westfalen, Mu¨ nster. Bechlu␤ vom 30. Oktober 1996, 21 D 2/89.AK (1996).

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