Structural Integrity and NDE Reliability III

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1 Structural Integrity and NDE Reliability III Concept of Total Reliability of NDT Methods for Inspection of the EB Weld of the Copper Canister Used for a Long-Term Storage of Spent Nuclear Fuel D. Kanzler, S. Milsch, M. Pavlovic, C. Müller, BAM, Germany J. Pitkänen, Posiva Oy, Finland ABSTRACT Problems of high level nuclear waste disposal have to be solved in the near future. In this matter, Finland is just building a permanent deep repository for their high-level nuclear waste. One barrier to prevent the contact of the radioactive waste with the environment is the copper canister in which the nuclear fuel will be encapsulated. To assure the functionality of the canister, it is important to make sure that there is no leakage of the canister. For this, non-destructive testing methods are applied. The estimation of the detection limit of the non-destructive testing system is an important step within the assessment of the total reliability of the deposit system. To guarantee the integrity of the canister, the size of the defect that is detected with 90% probability and 95% confidence, has to be determined. This defect size is also called a 90/95. The copper shell consists of two components. The copper lid is welded by electron beam welding to the copper tube. This by itself is one of the critical areas of the copper shell. To make sure that all defect types of the weld will be detected, inspections with four different non-destructive methods (NDT) are planned: ultrasonic, radiographic, visual and eddy current method, all of which will be evaluated. Every method is suitable for the detection of different defect types, because of their different physical operating principles. At least one of the NDT methods should be able to detect the defects appearing in the weld. Through reference defects, it is possible to verify the principle suitability of the different NDT-methods for testing the canister weld. Evaluation has to show that critical defects lie beyond the a 90/95 point to demonstrate the high POD of the defect - otherwise the NDT-methods have to be improved. Depending on the defect type, the POD of the defect will differ depending on the NDT method. INTRODUCTION Problems of high active nuclear waste disposal have to be solved in the near future. One approach is the permanent disposal for nuclear waste. Therefore Finland is already building a permanent deposit for its high-level nuclear waste. To guarantee the functionality of the copper canister, which is one of the components of the deposit, the company uses non-destructive testing (NDT) methods. This paper is about a concept to create an evaluation of NDT methods by means of the calculation of the probability of detection (POD). The first section gives an overview over the product, its importance and the weld as a critical part of the tested copper canister. Afterwards the article describes possible defects and an approach finding all critical defects in the weld of the copper canister. After a general introduction into the calculation of POD-curves, the article describes the general procedure to create a meaningful POD for all used NDT-methods and at the end an example using the procedure for eddy current testing. Long-term storage for high active nuclear waste The safety of the deposit is guaranteed by multiple independent technical and natural barriers (Figure 1a) 1). One of the barriers is the canister in which the nuclear fuel will be encapsulated (Figure 1b). The core of the canister is made of cast iron with the function to withstand mechanical loads imposed on the canister. The shell of the canister, made of copper, has the role to prevent the contact of the radioactive fuel with the environment.

a b Figure 1 - Multiple technical and natural barriers and the canister for encapsulation of nuclear fuel (POSIVA) Once the waste has been put inside the canister, the copper lid will be welded to

Even though the beam is tightly focused and the heat affected zone is narrow, it is possible that defects develop in the weld and also in the surrounding area 2).

2 a b Figure 1 - Multiple technical and natural barriers and the canister for encapsulation of nuclear fuel (POSIVA) Once the waste has been put inside the canister, the copper lid will be welded to the copper tube by electron-beam welding (EBW). EBW is the fusion joining process that produces a weld by impinging a beam of high energy electrons to heat the weld joint. Even though the beam is tightly focused and the heat affected zone is narrow, it is possible that defects develop in the weld and also in the surrounding area 2). That is the reason, why the weld will be inspected with several NDT methods, to ensure there are no defects which might jeopardize the function of the canister. This testing is done before the canister is put into the final repository. Defect types that might occur in the electron-beam welds To make sure that the range of the NDT systems will successfully detect all of them, it is important to identify defect types which might occur in the weld during the welding process: internal root defects, void or cavities caused by spiking, gas porosity, excess of penetration, cavities and defects caused by gun discharge. These types of defects can arise from the highly stressed welding process and need to be found by at least one NDT method 1). The estimation of the detection limit of the non-destructive testing system is an important step within the assessment of the total reliability of the deposit system. Non-destructive testing of the electron-beam weld To assure the functionality of the canister and to be sure that there is no leakage of the canister, nondestructive testing methods are applied. The testing process is split in three steps. In the first testing trial the defects are artificial reference defects, which have a similar structure as the real defects. Afterwards, the defects are created by intentionally changing parameters of the welding process. Finally, there is a test of a complete welded copper shell in the production of the future waste canister.

To make sure that all defect types will be detected, inspection with the following four different nondestructive methods is planned: radiographic testing (RT), ultrasonic testing (UT), eddy current

3 To make sure that all defect types will be detected, inspection with the following four different nondestructive methods is planned: radiographic testing (RT), ultrasonic testing (UT), eddy current testing (ET) and visual testing (VT) with a remote camera. The methods have different physical operating principles and are suitable for the detection of different defect types, based on the individual defect morphology, location and orientation. Figure 2 - To ensure that every critical defect will be found, the weld will be tested with different methods The practice showed that inspection systems, when applied at the extreme of their capabilities, will not detect all flaws of the same size 6). Repeated inspection of the same flaw will also not always detect it. This is the reason to introduce the concept of reliability in applications where every missed defect can lead to severe consequences. Probability of Detection In the 70th Berens 7) created a POD approach to estimate the reliability of eddy current method in finding cracks. It is the best known and widespread method and is also used since then in a lot of different industrial branches as a characteristic number for quality. The maximum amplitude is plotted versus the significant defect parameter causing the signal(figure 3a) - depth of the crack 7). The plotted graph is called â vs. a graph. For an easy interpolation and evaluation of the relation between the chosen parameter and the measured amplitude, both axes will be logarithmized to get a linear relation. With the deviation and the mean of the data it is possible to calculate the probability of detection for every parameter status. Graphically it is summed up in the POD curve with a confidence belt (Figure 3b).

4 a b Figure 3 - â vs. a-graph (a) and probability of detection curve (b) The POD curve with the lower 95% confidence band is a typical way to present the capability of the NDT system to detect a flaw 7). Those types of curves were applied for simple cases where it is assumed that the POD of the defect depends only on the defect size and no geometrical influence of the component is taken into account. To guarantee the integrity of the canister, the size of the defect that is detected with 90% probability and 95% confidence, has to be determined. This defect size is also called a 90/95. If the critical defect, which is necessary to be found, is bigger than the a 90/95 then the NDT-method is acceptable. If the critical defect is smaller than a 90/95 then it is necessary to improve the method or to use another method. In applications for a thick copper part of the canister, the POD of the defect is influenced by many parameters, and depending on the position within the inspection volume, the POD of the same defect will be different 8). Earlier studies of POD curves for the inspection of copper welds were reported in several publications 2)4)5)6). MEANINGFUL PROBABILITY OF DETECTION OF NON-DESTRUCTIVE TESTING METHODS It is important to consider that every method has its advantages and disadvantages in finding defects. While ultrasonic testing is able to find defects in the material, visual testing is only able to find surface defects. The first step in creating a POD is to define which NDT-method is used to find a certain defect. Each NDT method will show a different defect detection probability. From these single method probability curves we will create later the multi-method POD of their combination. But it is important that to make sure every kind of defect with a critical size will be found at least by one method. The most important parameter for the detection of the defect is called a. It is necessary to have a relationship between the changing of this parameter and the changing of detection-probability. For example in radiography a is the penetrated size of the volumetric defect. The interpretation of the amplitudes is the most important part for the detection of defects. In the basic POD, Berens used the amplitude 7). In this evaluation the signal to noise ratio (SNR) is used, which does not depend on as much influencing factors as the pure amplitude. The amplitude is the so-called â. With the parameter a and the amplitude â, it is possible to create a double logarithmized â vs.a graph. This graph gives a first impression of the deviation of the measurement points around the trend line. If the deviation is too large or if there are too many outliers, then it is necessary to choose another relation between a and â or question the reason for the outlier. From this graph it is easy to find out if there are enough data values and if they are distributed over an area which is important for the evaluation. For the calculation of the POD it is necessary to know a decision level. The choice of the decision level depends on the variation of the signal amplitudes and the height of the noise. It is important to choose the decision level adequate for the method. With the â vs. a graph and the decision level, it is possible to create the POD graph and to calculate the a 90/95.

The concept to create a POD for the eddy current testing method For the evaluation of the eddy current testing method at POSIVA, two different probes are used: the low frequency probe and a high

The shown data are low frequency measured data of holes on the surface. The diameter of these holes is defined as the parameter a.

5 The concept to create a POD for the eddy current testing method For the evaluation of the eddy current testing method at POSIVA, two different probes are used: the low frequency probe and a high frequency probe. The eddy current testing method is used to find surface defects and defects near the surface. Artificial defects are holes and notches on or near the surface. The shown data are low frequency measured data of holes on the surface. The diameter of these holes is defined as the parameter a. The depth of the holes, which can also have an effect on the detection, is not a part of this assessment. 24 holes with different diameters between 0.1 mm and 2 mm were used in the evaluation, where more than a half have a diameter below 1 mm. The original signal amplitude â is the measured changing of the electrical current in the probe, which is caused by a magnetic field created by the eddy current in the specimen with the flaw. The signal to noise ratio is actually used as the parameter â. The plotted SNR â over the parameter a on a logarithmized axis is shown on Figure 3a. The outliers are not part of this evaluation, because it was proved that the behaviour was caused by an influence which will not happen in the production of the copper canister later on. The deviation of the measured values is small, so the amount of 24 different holes is enough to create a meaningful POD for artificial defects for Eddy Current Testing. The decision level is set to 6 db above the measured noise level. The assessed POD-curve with 95% tolerance limit is shown at Figure 3b. The calculated a 90/95 is around 0.37 mm diameter for holes. 11 measured values are in direct neighbourhood of the calculated a 90/95, so the result is mainly based on the measured data and is meaningful. a Figure 3 - â vs. a-graph (a) and probability of detection curve (b) for eddy current testing for holes b CONCLUSION Through reference defects, it is possible to verify the principle suitability of different NDT-methods for testing the structural integrity of the copper canister. In a critical application, such as testing of a repository canister, it is necessary to conduct a reliability analysis of NDT systems by calculating the probability of detection of these methods. The evaluation has to show that critical defects lie beyond the a 90/95 point to demonstrate a high POD of the defect - otherwise the NDT-methods have to be improved. At this moment every calculated and assessed a 90/95 is smaller than the size of the defect, which was defined as critical. Therefore every assessed method can be used to detect artificial defects with a similar behaviour as the critical defects in the electron-beam weld of the copper canister.

6 Unlike the POD-approach from Berens, in this calculation the SNR was used and also the decision threshold was based on the SNR. The SNR based POD is more independent from influences, which are not directly part of the detection capability, such as gain. The procedure shows that it is important to check the statistical basis of the data. Therefore enough data values are required. The exact number of measured data depends on the deviation of the measurement values, the knowledge of the testing situation and the size of the evaluated measurement values. Additionally, if there are a large number of outliers or few data values with a big difference to the trend line, it is important to know why it happened. If the reason for the outliers is not a consequence of an influence in real production situation, it is allowed to remove the outlier from the evaluated data. Depending on the defect type, the POD of the defect will differ depending on the NDT method. Inspection of the weld with different NDT methods will increase the POD, especially for some defect types. This means that it is necessary to combine all methods together and to create a combined POD curve. REFERENCES 1) Pitkänen J, Salonen T, Sandlin S, Ronneteg U, Defect detectability in EB-welded copper disposal canister with 9 MeV accelerator, 6 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Budapest ) Müller, C., Elagin, M., Scharmach, M., Bellon, C., Jaenisch, G.-R., Bär, S., Redmer, B., Goebbels, J., Ewert, U., Zscherpel, U., Böhm, R., Brekow, G., Erhard, A., Heckel, T., Tessaro, U., Tscharntke D. & Ronneteg, U., 2006, Reliability of nondestructive testing (NDT) of the copper canister seal weld, Svensk kärnebränslehantering AB report R-06-08, 158p. 3) Raiko, H., Meuronen, I., Pitkänen, J., Salonen, T., Wikström, N-C., Ämmälä, V-M., EB-DEMO Canister Sealing Demonstration, POSIVA Working Report , Eurajoki ) Müller, C., Pavlovic, K. Takahashi, U. Ewert, M. Rosenthal, G. Brekow, R. Böhm, U. Ronneteg, J. Pitkänen, 2007, POD Evaluation of NDE Techniques for Canister-Components for Risk Assessment of Nuclear Waste Encapsulation, 6th International conference on NDE in Relation to Structural integrity for Nuclear and Pressurized Components, Budapest, ) Pavlovic M., Takahashi K. Müller, Boehm, R., Ronneteg, U., 2008, NDT reliability - Final report Reliability in non-destructive testing (NDT) of the canister components, Svensk kärnebränslehantering AB report R , 56p. 6) Pavlovic, M., Takahashi K., Müller, C., Pitkänen, J., 2009, The Total Reliability of NDT Methods for Inspection of the EB Weld of the Copper Canister used for a Long term Storage of Spent Nuclear Fuel, 7th International conference on NDE in Relation to Structural integrity for Nuclear and Pressurized Components, Yokohama 12th -14th May, 5p. 7) Berens A P, NDE Reliability Data Analysis Metals Handbook, Volume 17, 9th Edition: Nondestructive Evaluation and Quality Control, ASM International, OH, ) Pavlovic, M.. Müller,,C., Takahashi:, K.., Pitkänen, J., Ronneteg, U., 2009, Multiparameter influence on the response of the flaw to the phased array ultrasonic NDT system., 4th European- American Workshop on Reliability of NDE, ) Takahashi, K., Pavlovic, M., Ronneteg, U., Pitkänen, J., Müller, C., 2009, POD from C-Scan Data for Reliability Analysis of Ultrasonic NDT, 4th European-American Workshop on Reliability of NDE, 2009.

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