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This report describes the methodology of the reliability investigation performed on the ultrasonic
phased array NDT system, developed by SKB in collaboration with Posiva, for inspection of the
canisters for permanent storage of nuclear spent fuel.
The canister is composed of a cast iron insert surrounded by a copper shell. The shell is composed
of the tube and the lid/base which are welded to the tube after the fuel has been place, in the
tube. The manufacturing process of the canister parts and the welding process are described.
Possible defects, which might arise in the canister components during the manufacturing
or in the weld during the welding, are indentified.
The number of real defects in manufactured components have been limited. Therefore the reliability
of the NDT system has been determined using a number of test objects with artifical defects.
The reliability analysis is based on the signal response analysis. The conventional signal response
analysis is adopted and further developed before applied on the modern ultrasonic phased-array
NDT system. The concept of multi-parameter a, where the response of the NDT system is dependent
on more than just one parameter, is introduced. The weakness of use of the peak signal response
in the analysis is demonstrated and integration of the amplitudes in the C-scan is proposed as an
alternative. The calculation of the volume POD, when the part is inspected with more configurations,
is also presented. The reliability analysis is supported by the ultrasonic simulation based on the
point source synthesis method.
One of the aims of considering human factors in the reliability of non-destructive testing (NDT) – for the purposes of the disposal of spent nuclear fuel – is to identify factors that can lead to errors in the completion of the NDT task and to determine how to prevent these errors. The current study is the first to examine mechanised NDT from the human factors perspective. A number of risks were identified and further analysed. Among a number of influencing factors, the NDT inspection procedure was identified as a potential error source and, therefore, was examined in this study. NDT inspection procedures and instructions are, without question, some of the most important tools in the everyday life of an NDT inspector. Experience and research have shown that NDT procedures and instructions, despite being written according to requirements and by certified personnel, are not always used as foreseen and may need to be optimised. The suggested approach for developing and optimising NDT procedures and instructions consists of applying human factors principles by adopting a user-centred approach. The user-centred approach refers to involving users in the process of instruction development by learning from their errors during the data evaluation task and connecting those errors to the shortcomings in the instructions.
The first study assessed the quality of the selected NDT instruction by observing four experienced inspectors during the evaluation of data that were collected with the ultrasonic testing (UT) method while the participants followed the instruction. Through the use of an eye-tracking methodology, the participants’ eye movements across the screen were observed and further analysed. This analysis, together with discussions with the individual users, led to the identification of a number of errors and error sources. Such error identification, combined with theoretical considerations, led to the creation of a new version of the instruction. This new instruction was evaluated using the same method in a follow-up study, resulting in the creation of a third (and final) NDT instruction. Throughout this iterative process, version 4.0 of the instruction was replaced first by version 5.5 and then by version 6.0. As a result of these studies, changes were introduced to the instruction content (e.g. adding missing content) and to the instruction format (e.g. changing the layout, highlighting, presenting the information differently, altering the writing style). The participants reported higher satisfaction with the instruction after the changes were implemented. The main changes included using consistency in writing, highlighting information (e.g. number values, reporting levels, exceptions, reminders), improving navigation throughout the instruction (e.g. through the use of titles and subtitles), and presenting information more uniformly (e.g. all tasks being listed stepwise following the “one action per step” rule).
Considering the benefits of including the user in the development of the NDT instructions and procedures, the main lesson that was learned in this study is that the review process must be carried out through action rather than by solely reading the instructions. This study exposed a number of problems of which the writer and the reviewer were unaware.
After the new instruction was developed, two empirical studies with 20 participants were conducted to evaluate the instruction information by investigating 1) whether the instruction content was understood by the users (the understanding study) and 2) whether the new format supported a more efficient (i.e. less effortful), effective (i.e. more accurate and complete), and satisfying use of the instruction (the usability study). The aim of these studies was to increase the understanding of the factors that contribute to a high-quality NDT instruction.
In the understanding study, the participants were asked to read the NDT instruction and then to answer a number of questions. Based on their answers, specific problems that were related to their understanding of the written content were identified. The results showed that the understanding of the information can be affected by the information order, information organisation, logics and clarity in the writing, and cognitive demands. The study also emphasised the relevance of placing information in a logical order.
This report describes the progress in understanding and describing the detectability of the ultrasonic inspection technique developed by SKB for the inspection of copper tubes used for the final disposal of the Swedish spent nuclear fuel. In former research activities dedicated to the different parts of the canister, the probability of detection (POD) evaluation technique, as developed for thin aircraft components, was further developed for the application on complex hick-walled components. The result of this development was the introduction of a “multi-parameter” POD framework. In contrast to taking only the defect size into consideration (as for aircraft components) additional influencing factors, relevant for thick components, such as depth position, orientation and part geometry, were included. Furthermore, the variation of ultrasonic attenuation due to various material properties was included.