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atw Vol. 64 (2019) | Issue 11/12 ı November/December FEATURE | ENVIRONMENT AND SAFETY 516 Task Task Information and Requirements Task to Consider Working Time Teamwork and Communication Workload Task Support Workplace Factors Hazard Identification Expected PSFs 2 Task Analysis Task Analysis (TA) is the initial step in the case of human error assessment. It provides the characteristics of different tasks, their vulnerabilities, and properties by understanding the objectives, performance methods, scopes of the tasks, and procedures involved . In addition, TA helps eliminate the conditions that can cause errors before they occur by providing detailed information about the tasks as well as other information for predicting and preventing errors. 2.1 TA Method Although several TA methods exist, considering the physical characteristics (complexity) and Human Machine Interface (HMI) aspect (using remote devices) of the decommissioning activities, we utilized the Hierarchical TA (HTA) method. HTA is a systematic and detailed TA method that is used to achieve task objectives. It is appropriate for not only identifying detailed task configuration and conditions, but also expressing complex task steps as a simple, hierarchical structure. Furthermore, the HTA method was used because it can easily describe the characteristics of the work involved and identify significant information about the Title | Tab. 1. Details of tasks to be performed for TA. • Task influencing factors (cutting size, number of cutting operations, precision, etc.) • Task output requirements • Record feedback to indicate adequacy of action taken • Alarms and warnings • Actions to be taken • Equipment needed (type, size, performance, required utility, equipment usage constraint, etc.) • Task frequency and required accuracy • Physical position of the operator (standing, sitting, squatting, etc.) • Biomechanics • Movement (lifting, pushing, rotating, pulling, swaying, etc.) • Force required • Unit working time based on “work contents” • Additional work hours taking into account “work support” and “workplace environmental conditions” • Number of work shifts and workers per work shift • Cooperation required between the teams performing the work • Personal communication for monitoring or taking control actions • Cognitive workload • Physical workload • Overlap of task requirements (serial versus parallel task elements) • Special and protective clothing for work • Job aids, procedures, or reference materials needed • Required auxiliary tools and equipment • Ingress and egress paths to the work site • Workspace required to perform the task • Typical workplace environmental conditions (e.g. lighting, temperature, noise, etc.) • Work breaks taking into account “work support” and “ workplace environmental conditions” • Identify work-related hazards, e.g. potential personal injury hazards Examples include: • Stress • Time pressure (critical path operations) • Extreme environmental conditions • Reduced staffing HMI, communication and decision making processes, as well as possible accidents. The HTA method involves describing the manner in which tasks need to be performed after establishing their overall objectives and classifying them into their sub-tasks . Using a tabular format to perform HTA allows one to express complex tasks that require significant skill in a suitable manner, because one can include detailed notes, if necessary. In this study, we comprehensively reviewed various items, such as HMI, Communication, Time, and Accident, for the decommissioning activity in a tabular format; this is shown in Table 1 . 2.2 Target Decommissioning Activity for TA In general, one of the most challenging tasks during plant decommissioning is believed to be the removal of the highly radioactive internal components of the reactor pressure vessel (RPV); this is true for Kori Unit 1 as well. In addition, another reason that this is one of the most difficult activities is because these radioactive components must be dismantled and cut underwater owing to the severe radiological conditions of the RPVI components [7-8]. Therefore, it is recommended that the reactor internals be removed as early as possible in the plant dismantling sequence, so that these water systems and their associated support systems can be released for decommissioning, which minimizes the costs associated with maintaining these systems in operation after permanent plant shutdown . The cross-section of the RPV with the primary internal components at Kori Unit 1 is shown in Figure 2. As can be seen from the figure, the internal structures adjacent to the core barrel active region are the most highly activated, and in most cases, include intermediate level waste components that might require removal prior to the disposal of the remainder of the RPV and reactor internal components . Thus, in this study, this RPV internal segmentation activity is selected from among the various dismantling activities in Kori Unit 1. Furthermore, in this study, the TA was performed for the most complex and highly radioactive RPVI cutting task among various disassembly activities by using the HTA method. The sequence of operations for each sub-activity in this target task is listed in Table 2. 2.3 TA Results The TA for the RPVI Dismantling Activity was performed as per the activities listed in Table 2 based on the items listed in Table 1. In our study, this analysis was performed for each of the 10 sub-activities. The summary of the TA for the RPVI Dismantling Activity is given as follows. p In general, in the decommissioning of nuclear power plants, the cutting of parts in the RPVI is the most complicated and difficult task during the dismantling process. Therefore, the influence on the internal factors of the workers was evaluated in order to have a relatively high value in terms of operator internal response. p This kind of dismantling operation, which is complex and requires accuracy and reliability, is significantly influenced by the internal and external characteristics of the worker. Therefore, sufficient education and training is required. However, as the driving principle and operation method of these cutting equipment and accessories ( cutting equipment, remote control device, display, auxiliary equipment, etc.) are not complicated and operation is relatively simple, an operator is not required to have considerable experience in using them. Feature Development of Performance Shaping Factors for Human Error Reduction during Reactor Decommissioning Activities through the Task Analysis Method ı Byung-Sik Lee, Hyun-Jae Yoo and Chang-Su Nam
atw Vol. 64 (2019) | Issue 11/12 ı November/December No Sub activity Activity Description 1 Control Rod Guide Tube Upper Area Cutting and Packaging 2 Control Rod Guide Tube Lower Area Cutting and Packaging 1) Lift up the Upper Internal Assembly and fix it to the turntable 2) Cut the control rod guide tube upper area 3) Transfer and package the cut section to a storage container 1) Cut the control rod guide tube lower area 2) Transfer and package the cut section to a storage container 3 Upper Plate Cutting and Packaging 1) Move and release the Upper Support Plate in an empty space in the reactor tank 2) Cut the Upper Core Plate fixed to the turntable 3) Fix the Upper Core Plate to the turntable and cut the Upper Support Plate 4) Transfer and package the cut pieces to a storage container 4 Baffle Fixed Bolt Head Cutting 1) Install the mechanical drill for Baffle Separation 2) Cut the Baffle Fixing Bolt Head by placing the mechanical drill inside the RPV 5 Baffle Cutting and Packaging 1) Lift the Baffle and remove it from the Former 2) After fixing it to the turntable, cut the Baffle 3) Transfer and package the cut Baffle pieces to a storage container 6 Core Barrel Lower Area Cutting 1) Lift up the Lower Internal Assembly and fix it to the turntable 2) Rotate the turntable to cut the Lower Internal Assembly 3) Lift the upper area of the Lower Internal Assembly into the Vessel 7 Lower Internal Structure Assembly Cutting and Packaging 8 Thermal Shield Separation, Cutting and Packaging 1) Cut the Instrument Nozzle from the Core Support Structure Assembly 2) Transfer and package the cut nozzle to a storage container 3) Cut and package the tie plate fixed to the Turntable 4) Fix the Lower Core Plate to the Turntable and cut it 5) Transfer and package the cut pieces to a storage container 1) Lift the upper area of the Lower Internal Assembly and fix it to the turntable 2) Separate the Thermal Shield by cutting the Bolt Head 3) Release the removed Core barrels from the Thermal Shield inside the vessel 4) After fixing the Thermal Shield to the Turntable, remove the Irradiation Specimen Guide 5) Cut the Turntable Thermal Shield Upper and Lower Panels 9 Former Separation 1) Lift the Core Barrel to the turntable and fix it 2) Cutting the Former fixing bolt head outside the Core Barrel 3) Separate the Former from the Core Barrel 4) Transfer and package the separated Former to a storage container 10 Core Barrel Cutting and Packaging 1) Fix the Core Barrel to the turntable and cut it 2) Temporarily release the cut Upper Core Barrel in the Vessel 3) Segment the Lower part of the Core Barrel fixed to the turntable 4) Transfer and package the cut pieces to a storage container 5) Repeat the procedure for the cutting the remaining Core Barrel FEATURE | ENVIRONMENT AND SAFETY 517 | Tab. 2. Task Description of RPVI Dismantling Activity. p Considering the characteristics of the work (equipment and facilities, the object to be cut, and clothing for radiation protection in the work environment), a detailed work plan must be established in advance. Further, as this cutting work is time-consuming, the psychological and physical influences that the supervisor and the worker can receive are considerable. p If the cutting activity is dangerous, takes a long time, and has associated time constraints, it should be performed during the day/night time. In this case, various difficulties (such as break time, clothing discomfort, and physio logical factors) are generated. Because these difficulties have a significant impact on the internal and external factors affecting the worker, much cooperation and communication is required between the worker and the supervisor in this working environment. p The radiation and the physical environment of the workplace are the major risk factors for the workers, and the influence of these working environments on the internal and external factors of the workers was con siderable. p Nuclear decommissioning work is not a frequently performed task. Therefore, workers may have insufficient experience and education/training. Therefore, after the decommissioning activity has been carried out, it is necessary to feedback the results of the work to be reflected in the necessary work procedures and to be managed as experience data. | Fig. 2. Internal components for the RPV in Kori Unit 1 . 3 PSFs in the Decommissioning Activities Because the workers’ activities are the fundamental factors that renders the system vulnerable, it is necessary to identify, model, and quantify the possibility of human error using HRA . In particular, the nominal Human Error Probability (HEP) used in the HRA is estimated Feature Development of Performance Shaping Factors for Human Error Reduction during Reactor Decommissioning Activities through the Task Analysis Method ı Byung-Sik Lee, Hyun-Jae Yoo and Chang-Su Nam