Abstract
In this paper, we propose a new methodology of identifying important research problems to be solved to improve the performance of some specific scientific technologies by the phenomena identification and ranking table (PIRT) process which has been used as a methodology for demonstrating the validity of the best estimate simulation codes in US Nuclear Regulatory Commission (USNRC) licensing of nuclear power plants. The new methodology makes it possible to identify important factors affecting the performance of the technologies from the viewpoint of the figure of merit and problems associated with them while it keeps the fundamental concepts of the original PIRT process. Also in this paper, we demonstrate the effectiveness of the new methodology by applying it to a task of extracting research problems for improving an inspection accuracy of ultrasonic testing or eddy current testing in the inspection of objects having cracks due to fatigue or stress corrosion cracking.
1. Introduction
All the artificial structures in the world are subject to aging and their functions deteriorate as time goes on. If there is a possibility of unallowable loss of function due to aging, it is necessary to inspect or monitor the artificial structure sufficiently before the expected time of its functional loss, grasp the condition of the structure at the time of inspection, evaluate and predict the progress of aging using inspection results, and manage aging to avoid the occurrence of functional loss. The inspection or monitoring techniques used in this regard are required to have high accuracy because they will play very important roles if the functional loss of the structure results in adverse effects on human lives, society, or the environment.
In response to such needs, inspection and monitoring techniques are constantly studied and developed to pursue higher accuracy. However, identification of problems and research themes for improving inspection accuracy are virtually left to individual researchers and engineers who develop techniques. As a result, there is occasionally a gap between practical needs and the specific contents of research and development. This gap often arises when research themes are selected at universities and other institutions that are remote to practical matters. Although many efforts are being made to match seeds and needs in line with the recent trend to promote academic–industrial cooperation, there are still significant gaps between academic and industrial points of view. Because available resources are limited in the current circumstances of research and development, it is very important to select appropriate research themes using a highly objective method.
Based on such backgrounds as described above, we propose a new methodology to extract important research themes by using the Phenomena Identification and Ranking Table (PIRT) process. As we describe later on, the PIRT process was developed as a method to verify the adequacy of best estimate codes in the licensing of a nuclear power plant (NPP). It has been used to identify parameters that affect various events, quantify the importance and knowledge levels of such parameters, and validate best estimate codes in large-scale, complex systems. The new methodology we propose in this study has the characteristics as follows: while preserving the fundamental concepts of the original PIRT, the methodology makes it possible to define parameter(s) of interest (i.e. figure(s) of merit), then, from the viewpoint of figure(s) of merit, develop scenarios of a series of physical phenomena that occur in the object being investigated, identify influential factors exhaustively that affect figure(s) of merit, and extract important research themes related to the influential factors. In the following, we outline the proposed methodology and demonstrate its effectiveness by showing a specific example in which we use the methodology to systematically identify influential factors that affect the accuracy of nondestructive inspection and extract important research themes necessary for improving the accuracy.
2. Application of the PIRT process
The PIRT process was developed as a systematic, documented method to validate the use of best estimate simulation codes for licensing of NPPs under rules approved by the USNRC [Citation1]. In Japan, the procedure for preparing PIRT and relevant matters are described in an appendix to the AESJ code [Citation2] that prescribes specific procedures for probabilistic safety assessment of accidents and abnormal transients during operation of light water nuclear power facilities as well as requirements on best estimate simulation codes and control methods used for the assessment.
First, typical application of the PIRT process, which has been conventionally used for identifying items that require verification of high prediction accuracy or validity of simulation codes for nuclear reactor safety analysis [Citation3,4], is shown in . The details are described in subsequent sections.
Figure 1. Illustration of typical application on the PIRT process.
2.1. Step 1 (define problem or issue)
The first step of the PIRT process is to define the problem or issue of discussion. For example, if an experimental program needs to be developed to validate the safety performance of reactor components in the case of developing a new type of nuclear reactor, the development of this program is the problem or issue to be studied in the PIRT process.
2.2. Step 2 (define PIRT objectives)
In the next step, the objective of a PIRT development is defined. The objective strongly depends on the intended use of the PIRT. The primary role of the PIRT is to define plant behavior in the context of identifying the relative importance of systems, components, processes, and phenomena, and thereby providing guidance for finding solutions to the development of tests and analytical codes for reactor safety analysis and to uncertainties in simulation codes. This step is therefore aimed at defining things to be performed in order to solve relevant problems. For example, this step is for defining the contents of investigation to get knowledge required for obtaining a certificate of a new nuclear reactor design.
2.3. Step 3 (define potential plant designs or events)
In this step, it is necessary to establish a system of interest in NPP to which the PIRT will be applied. To define systems or subsystems that play important roles in the plant behavior of interest, the system to be focused must be coupled with the selection of event scenarios discussed in the following steps. Because events and processes that occur in the system of interest depend greatly on the plant system, it is sometimes difficult to conduct specific, quantitative discussion when general plant systems are to be investigated. It is therefore necessary to focus on specific investigation items in the PIRT process by taking account of the relative importance of phenomena/processes. For example, when developing a new type of nuclear reactor, specific plant systems or subsystems, or specific components can be selected as systems of interest where unknown phenomena may occur.
2.4. Step 4 (define potential scenarios)
Scenarios of events (developing processes) that occur in the system of interest, or phenomena that occur in the event are described in this step. It should be noted that the relative importance of events or phenomena depends on the assumed scenarios. The events or phenomena considered here are, for example, events or phenomena caused by a loss of off-site power, coolant leakage, etc., which could pose greatest threat to safety of a new type of nuclear reactor in the development stage.
2.5. Step 5 (define parameter(s) of interest (figure(s) of merit))
In this step, the parameters of interest (figures of merit) are decided after all experts participating in the expert panel have common understanding about the objectives, systems, scenarios, etc. defined in Steps 1 through 4. The figures of merit are key evaluation criteria used for judging the relative importance of events and phenomena in the plant behavior of interest. Some of the examples are peak fuel clad temperature, hydrogen generation rate and so on, which are regulatory safety requirements.
2.6. Step 6 (identify, obtain, and review all available experimental and analytical data)
Relevant information on existing experimental and analytical data, etc. is collected, organized, and reviewed.
2.7. Step 7 (define high-level basic system process)
It is recommended to start from defining high-level processes and events in the system of interest (e.g., depressurization, inventory reduction, short-term dynamic core cooling, etc.).
2.8. Step 8 (partition scenario into convenient time phases)
2.9. Step 9 (partition plant design into components (subsystems))
In Steps 8 and 9, the scenario is partitioned into multiple time phases so that main processes and events do not change substantially, and the system is partitioned into multiple components or subsystems so that some important phenomena can be divided spatially. Then, a matrix of time and space zones is created, in which all possible phenomena and processes are identified.
2.10. Step 10 (identify plausible phenomena by phase and component)
Based on the results of Steps 8 and 9, the study in this step defines phenomena which are considered to be appropriate for each time phase and each component or subsystem.
2.11. Step 11 (develop ranking of component and phenomena relative importance)
2.12. Step 12 (apply AHP to determine phenomena relative importance)
2.13. Step 13 (review AHP results; modify if required)
In Steps 11 through 13, the importance of each phenomenon that occurs in the system of interest is ranked from the viewpoint of the figures of merit. The analytic hierarchy process (AHP) may be used to determine the relative importance of phenomena. The results of applying AHP are reviewed and revised as necessary.
2.14. Step 14 (perform selected PIRT confirmation sensitivity studies)
As shown by the arrows in , PIRT development is an iterative process with significant feedback between the various elements. Therefore, repeated discussions in the expert panel are made for improving preliminary results in each step.
2.15. Step 15 (finalize and document PIRT for subject scenarios and plant designs)
The PIRT development based on the created scenarios and plant design is finalized and documented.
In the PIRT process, as shown in the above procedure, multiple experts exchange opinions according to the standardized steps and the contents of discussion processes are documented, so that the discussion processes as well as data and bases used for evaluations and judgments can be rechecked or inspected at a later date. The process is a highly objective approach. It is therefore considered that this approach can be applied not only to NPP accidents, but also to any other events that can actually occur.
3. Methodology for identifying research themes using the PIRT process
In this section, we use the above-described PIRT process to discuss and propose a methodology to extract research themes necessary for the improvement of existing technologies, and present the procedure of discussion in a specific example. The example adopted here is the extraction of important research themes necessary for improving the accuracy of nondestructive inspection techniques applicable to NPPs. In other words, we look at events that occur in the object to be inspected during nondestructive inspection, systematically identify phenomena that occur in the events and influential factors affecting inspection accuracy, and then extract research themes necessary for improving inspection accuracy for each influential factor. After that, we develop the ranking of importance and other aspects of the themes.
In the following paragraphs, we describe the methodology proposed in this study in correlation with each step of the general PIRT approach described in the previous section, and explain the results obtained by this methodology. For this study, more than ten experts from the industry and more than ten experts from research institutions including universities were invited to hold expert panel meetings for discussion in each step of the PIRT process.
The specific procedure that we followed in this investigation is described below ().
Figure 2. Steps of the PIRT process in this study.
3.1. Step 1 (define issue)
This step is for defining the problem or issue of discussion. In this investigation, we defined the issue of discussion as follows: what are the influential factors of high importance among the factors that affect the performance (detectability and sizing accuracy) of nondestructive inspection to detect cracks due to fatigue or stress corrosion cracking; which factors are not understood sufficiently; and how much will the factor affect the inspection performance? Influential factors that affect the inspection performance may include inspection personnel, inspection procedures and inspection equipment (automatic or manual). However, because the objective of this investigation is to extract research themes related to the performance of nondestructive inspection, we purely focus on physical phenomena in the object to be inspected and therefore do not address issues on the skills of inspection personnel and the performance of inspection equipment.
3.2. Step 2 (define objectives)
This step is for defining the objective of developing a PIRT on the issue. The objectives of this investigation is determined as follows: identifying important influential factors that affect the performance of nondestructive inspection to detect cracks due to fatigue or stress corrosion cracking, extracting important research themes necessary for improving inspection accuracy for each influential factor, and developing the ranking of importance and other aspects of the themes.
3.3. Step 3 (define potential conditions of material to be tested)
Typical PIRT described in Section 2 states that this step is for defining the system of interest in NPP. In the investigation here, instead of defining events or phenomena that occur in a system, we defined a system in the object to be inspected, by considering it a domain in which events or phenomena such as the interaction between fatigue cracks and physical phenomena generated by nondestructive inspection technology occur. (This is the principle of detecting fatigue cracks by nondestructive inspection technology.)
Specifically, we assumed that the objects to be inspected were the following parts of pressure-retaining components in NPP where cracks due to fatigue or stress corrosion cracking occur. The size of components is specified to have a clear image on the object to be inspected, so it is not restricted. If cracks due to fatigue or stress corrosion cracking are expected to occur on the scanning surface, there are basically no restrictions on component size.
Materials
Austenitic stainless steel, stainless steel casting, nickel-base alloy, low-alloy steel
Size
Tubular components:
Small bore: Outer diameter: 38.04 mm, wall thickness: 11.4 mm (PWR: BMI housing) Outer diameter: 50.4 mm, wall thickness: 6 mm (BWR: ICM housing)
Large bore: Outer diameter: 882 mm, wall thickness: 74.6 mm (PWR: main loop piping and welds) Outer diameter: 609.6 mm, wall thickness: 33.0 mm (BWR: piping and welds in primary loop recirculation system)
Other components:
Shell plates with inner surface cladding of BWR Reactor Pressure Vessel (low-alloy steel and stainless cladding): wall thickness: 160–170 mm
Nozzle corners of BWR Reactor Pressure Vessel (In the case of PWR, inspection is conducted from inner surface of the vessel.)
Regarding to relative position relationship between the probe and cracks due to fatigue or stress corrosion cracking that occur at component surface in contact with reactor coolant, the following two cases are considered in principle ().
Figure 3. Relative position between scanning surface and probe.
The outer surface is scanned, and cracks due to fatigue or stress corrosion cracking are on the inner surface.
The inner surface is scanned, and cracks due to fatigue or stress corrosion cracking are on the inner surface.
3.4. Step 4 (define potential scenarios)
The typical PIRT described in Section 2 states that phenomena occurring in the system of interest are described in this step. Here in this investigation, we regarded this as describing an event scenario (developing process) that occurs in the object to be inspected or describing phenomena that occur in the event, and analyzed what physical phenomena occur in the inspection probe and the object to be inspected from transmission to reception of non-destructive inspection (NDI) signals along a time axis. For example, we investigated how ultrasonic wave entered a component and propagated in it, how it was reflected by a fatigue crack or stress corrosion cracking (SCC), and how crack characteristics affected the ultrasonic wave propagation process.
The investigation results for ultrasonic testing and eddy current testing are shown in and , respectively.
Table 1. Potential scenario and influential factors in ultrasonic testing phenomena.
Table 2. Potential scenario and influential factors in eddy current testing phenomena.
3.5. Step 5 (define parameters of interest)
This is a step for defining parameters (figures of merit), and the typical PIRT described in Section 2 states that this step is for defining the key evaluation criteria used to judge the relative importance of events or phenomena in the plant behavior of interest. Here in this investigation, the following parameters were identified because the objective of the investigation was to identify factors that affect inspection accuracy and extract important themes necessary for improving accuracy.
Detectability (accuracy to detect the presence or absence and the location of fatigue or SCC crack)
Sizing capability (accuracy to size the depth and surface length of fatigue or SCC crack)
3.6. Step 6 (identify, obtain, and review all available experimental and analytical data)
The typical PIRT described in Section 2 states that this step is for collecting, organizing, and reviewing existing experimental data, analytical data, and other relevant data. Here in this investigation, we collected and organized various data by conducting a preliminary survey and gathering information on ultrasonic testing and eddy current testing from experts. shows an example of the results obtained for ultrasonic testing.
Table 3. Various data on accuracy of ultrasonic testing (example).
3.7. Step 7 (define high-level basic system process)
The typical PIRT described in Section 2 states that the high-level processes and events in the system are defined in this step. Here in this investigation, this step was considered unnecessary because what we investigate are relatively simple events or phenomena related to flaw detection in the object to be inspected, but not complex events or phenomena that occur in plant systems.
3.8. Step 8 (partition scenario into convenient time phases)
3.9. Step 9 (partition plant design into components (subsystems))
Steps 8 and 9 in the typical PIRT described in Section 2 are for spatially and temporally dividing events or phenomena in plant systems into segments. However, this step was considered unnecessary in this investigation because what we investigate are events or phenomena related to flaw detection in a relatively simple object to be inspected.
3.10. Step 10 (identify plausible phenomena by phase and component)
This is a step for defining phenomena that are considered to be appropriate for each time phase and component on the basis of Steps 8 and 9. Based on the outcome of the study and discussion up to Step 6, we decided to identify assumed events or phenomena. We therefore studied which events or phenomena are likely to occur and progress in ultrasonic testing and eddy current testing on the basis of the scenarios discussed in Step 4. Specifically, we assumed events or phenomena that are likely to occur and progress, identified major factors (influential factors) that affect the events or phenomena, and studied how the influential factors would affect the figures of merit discussed in Step 5. An example of investigation on ultrasonic testing is shown in the “Scenario/scene of event/phenomenon,” “Influential factor,” and “Description of influence” columns of .
Table 4. Effect of influential factors on accuracy of ultrasonic testing (example).
To validate the events/phenomena and influential factors identified in this step, we compared them with the influential/essential parameters of ultrasonic testing and eddy current testing described in a report issued by the European Network for Inspection and Qualification (ENIQ) [Citation5]. As a result, the factors identified in the present investigation mostly matched with the parameters given in the ENIQ's list except for those factors related to the skills of inspection personnel and the performance of inspection equipment, which were not addressed in this investigation.
3.11. Step 11 (develop ranking of component and phenomena importance)
3.12. Step 12 (apply AHP to determine phenomena relative importance)
3.13. Step 13 (review AHP results; modify if required)
Steps 11 to 13 in the typical PIRT described in Section 2 are for determining the relative importance of events or phenomena that occur in the system of interest in terms of the figure of merit, by applying the AHP as necessary and reflecting its results. In this investigation, we considered that the use of the AHP was not necessary because the expert panel members determined the relative importance of research themes related to influential factors affecting the performance of nondestructive inspection, which are extracted on the basis of events or phenomena that occur in the object to be inspected.
On the basis of the result of Step 10, we determined the relative importance of each research theme based on its significance. shows an example of the results of ranking obtained for ultrasonic testing. The criteria shown in were used for determining the relative importance.
Table 5. Criteria for ranking of importance.
3.14. Step 11′ (develop ranking of knowledge level)
This step is not explicitly described in the typical PIRT described in Section 2. Because the objective of this investigation here is to identify important influential factors that affect the performance of nondestructive inspection and extract important research themes necessary for improving inspection accuracy for the influential factors, we considered that it was important to evaluate how the current knowledge was enough, and then we decided to rank the knowledge levels on the influential factors in terms of the amount of knowledge. It is reported that this step was also performed in the past development project of a new nuclear reactor design to define the contents of test and research to obtain the knowledge required for receiving a design certificate [Citation4]. An example of the results of ranking obtained for ultrasonic testing is shown in the “Importance” and “Knowledge level” columns of . The criteria shown in were used for determining the knowledge level ranking.
Table 6. Criteria for ranking of knowledge.
3.15. Step 11″ (develop ranking of self-confidence)
This step is also not explicitly described in the typical PIRT described in Section 2, as is the case with Step 11′. However, we considered that it was important to evaluate how much the expert panel members had confidence on their decisions when they ranked the importance and knowledge level described in the above steps, and then we decided to rank the confidence level of the expert panel members’ decision on importance and knowledge level. An example of the results of ranking obtained for ultrasonic testing is shown in the “Confidence” column of . The criteria shown in were used for determining the confidence ranking.
Table 7. Criteria for ranking of confidence.
3.16. Step 14 (perform selected PIRT confirmation sensitivity studies)
As shown by the arrows in , the investigation processes between steps moved to and fro and the expert panel made discussions repeatedly.
3.17. Step 15 (finalize and document PIRT)
This step is for organizing ranking results based on data prepared in the preceding steps and entering the results in the 3 × 3 matrix of to complete a PIRT. In addition, research themes that have higher priority are extracted from the PIRT. We summed up the ranking results in Steps 11, 11′, and 11″ in the PIRT. The investigation results for ultrasonic testing and eddy current testing are shown in and , respectively.
Table 8. Phenomena Identification and Ranking Table (phenomena of ultrasonic testing).
Table 9. Phenomena Identification and Ranking Table (phenomena of eddy current testing).
For the research themes categorized in the PIRT, priority ranking consisting of six levels was developed as shown in . The priority was determined according to the following criteria.
Figure 4. Priority ranking for problem solution.
Research themes of high importance (H) and low knowledge level (U) are given the highest priority. (Priority 1)
Research themes of high importance (H) and intermediate knowledge level (P) are given the priority next to the highest priority described in (1). (Priority 2)
Likewise, research themes of intermediate importance (M) and low knowledge level (U) are also given the priority next to the highest priority described in (1). (Priority 2)
Research themes of intermediate importance (M) and intermediate knowledge level (P) are given the priority next to Priority 2 described in (2) and (3). (Priority 3, Priority 4, and Priority 5)
Regardless of importance, research themes with high knowledge level (K) are given low priority because they do not need to be addressed as a matter of high priority. (Priority 6)
Although some may consider that research themes of low knowledge level (U) should be given the highest priority even if importance is intermediate (M) or low (L), we adopted the priority policy described above because this study is for extracting research themes related to the accuracy of inspection techniques, not directly relevant to safety features such as safety analysis codes. (MU: priority 2, LU: priority 4)
The confidence of the expert panel members on the judgment of “importance” and “knowledge level” was regarded as a factor of affecting priority decision. For example, research themes of high importance (H) and low knowledge level (U) may not be given the highest priority when confidence is not high (i.e. M or L). In this study, we adopted a one-rank-lower priority when confidence was not high (M or L).
4. Conclusion
In this study, we developed a new methodology to extract research themes based on the typical PIRT approach and applied it to a specific problem of extracting important research themes necessary for improving the accuracy of nondestructive inspection in order to verify its effectiveness. As a result, we confirmed that the PIRT approach we had developed was applicable to extracting research themes on nondestructive inspection. In the process, we obtained the fruitful results, namely, the systematic approach to identify and organize the phenomena/scenarios and influential factors that affect the inspection accuracy of ultrasonic testing and eddy current testing, the well-organized information such as experimental data and analytical results on the influential factors, the systematically extracted research themes related to the influential factors, and the results of ranking importance and other aspects of the research themes.
From the above discussions, we believe that the PIRT approach developed in this study would be applicable to not only other technologies such as a monitoring technology but also technologies in other technical areas.
Acknowledgements
The authors would like to thank Mr Hideki Horie and Mr Hisato Matsumiya of Toshiba Corporation and all other relevant people for providing information on the PIRT when initiating this study.
This study was conducted as part of the Aging Management Project for System Safety of Nuclear Power Plants commissioned by Nuclear Regulation Authority of Japan. The authors would like to express gratitude to all relevant people for cooperation, especially to the experts who participated in the expert panel for the development of PIRT on UT and ECT.
References
- Boyack B, Lellouche G, Griffith P, Duffey R, Levy S. Quantifying reactor safety margins: application of code scaling, applicability, and uncertainty evaluation methodology to a large-break, loss-of-coolant accident ( U.S. Nuclear Regulatory Commission Report, NUREG/CR-5249). Washington: US Nuclear Regulatory Commission; Dec 1989.
- Atomic Energy Society of Japan. Code on implementation and review of statistical safety evaluation: 2008 (AESJ-SC-001: 2008). Tokyo: Atomic Energy Society of Japan; May 2009, 25–26. Japanese.
- Wilson GE, Boyack BE. The role of the PIRT process in experiments, code development and code applications associated with reactor safety analysis. Nucl Eng Des. 1998;186:23–37.
- Horie H, Matsumiya H, Miyagi K, Nishi Y, Grenci T, Fontana MH, Moody FJ, Ninokata H, Wilson GE, Yamaguchi A. Phenomena Identification and Ranking Tables (PIRTs) for 4S -loss of offsite power, failure of a cavity can, and sodium leakage from intermediate piping scenarios. The 13th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-13) N13P1093; 2009 Sep. 27–Oct. 2; Kanazawa City, Ishikawa Prefecture, Japan.
- European Commission Joint Research Centre, European Network for Inspection and Qualification. ENIQ recommended practice 1: influential/essential parameters issue 2 ( ENIQ Report No. 24). Petten (The Netherlands): European Commission Joint Research Centre; June 2005, 13–18.
- Terachi T, Fujii K, Arioka K. Microstructural characterization of SCC crack tip and oxide film for SUS 316 stainless steel in simulated PWR primary water at 320°C. J Nucl Sci Technol. 2005;42(2):225–232.
- Horinouchi S, Ikeuchi M, Shintaku Y, Ohara Y, Yamanaka K. Evaluation of closed stress corrosion cracks in Ni-based alloy weld metal using subharmonic phased array. Jpn J Appl Phys. 2012;51:07GB15-1-5.