AN INTEGRATED SUITE OF TOOLS TO SUPPORT HUMAN FACTORS ENGINEERING

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1 INL/CON PREPRINT AN INTEGRATED SUITE OF TOOLS TO SUPPORT HUMAN FACTORS ENGINEERING American Nuclear Society Jacques V. Hugo August 2014 This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint should not be cited or reproduced without permission of the author. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, or any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party s use, or the results of such use, of any information, apparatus, product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. The views expressed in this paper are not necessarily those of the United States Government or the sponsoring agency.

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3 AN INTEGRATED SUITE OF TOOLS TO SUPPORT HUMAN FACTORS ENGINEERING Jacques V Hugo Idaho National Laboratory Idaho Falls, Idaho, 83415, USA [email protected] ABSTRACT Human factors engineering (HFE) work for the nuclear industry imposes special demands on the practitioner in terms of the scope, complexity and safety requirements for humans in nuclear installations. Unfortunately HFE lags behind other engineering disciplines in the development and use of modern, powerful tools for the full range of analysis and design processes. HFE does not appear to be an attractive market for software and hardware developers and as a result, HFE practitioners usually have to rely on inefficient general-purpose tools like standard office software, or they have to use expensive special-purpose tools that offer only part of the solution they require and which also does not easily integrate with other tools. There have been attempts to develop generic software tools to support the HFE analyst and also to achieve some order and consistency in format and presentation. However, in spite of many years of development, very few tools have emerged that have achieved these goals. This would suggest the need for special tools, but existing commercial products have been found inadequate and to date not a single tool has been developed that adequately supports the special requirements of HFE work for the nuclear industry. This paper describes an integrated suite of generic as well as purpose-built tools that facilitate information solicitation, issues tracking, work domain analysis, functional requirements analysis, function allocation, operational sequence analysis, task analysis and development of HSI design requirements. In combination, this suite of tools supports the analytical as well as the representational aspects of key HFE activities primarily for new nuclear power plants, including capturing information from subject matter experts and various source documents directly into the appropriate tool and then linking, analyzing and extending that information further to represent detailed functional and task information, and ultimately human-system interface design requirements. The paper also describes a tool developed especially for functional requirements analysis, function allocation, and task analysis. Key Words: Human Factors Engineering, Advanced Nuclear Power Plants, Human Factors Tools 1 INTRODUCTION Ever since the Three Mile Island (TMI-2) accident [14], there has been general agreement in the nuclear industry that human factors principles and requirements should be incorporated in the engineering process for new nuclear power plants (NPPs). The industry has recognized that a systematic, integrated process was needed to identify and track performance and safety issues to ensure a balanced development of both technical and human aspects of systems, throughout the life cycle of the system. This has led to the nuclear industry requirement for defense-in-depth (that is, definition of levels and allocation of the safety functions within these levels), to ensure plant safety. This approach was pragmatic and quickly yielded useful insights and the results of some of this are seen in regulatory guidance documents like NUREG ( Human Factors Engineering Program Review Model ) [12] and NUREG-0700 ( Human System Interface Design Guidelines ) [11]. The extent to which license applicants have complied with these twelve elements in their NPP design is evaluated by the U.S. Nuclear Regulatory Commission (NRC) in terms of NUREG However, the choice of specific methods and tools is left to the designers. In most

4 other industries one would have expected a parallel effort to develop methods and tools to support the human factors engineering (HFE) process. This has not happened in the nuclear industry and the main reason for this appears to be the hiatus in the construction of new NPPs since the commission of Watts Bar in 1996, the last NPP to come on line in the U.S. There simply has not been sufficient incentive or pressure in the nuclear industry to keep up with HFE developments in other industries. Now, however, the resurgence of interest in nuclear energy, accompanied by advances in design, technologies, materials and construction techniques, is imposing new challenges on human factors engineers because the capabilities of the few existing tools no longer match the demands of new engineering processes. The HFE activities, as described by NUREG-0711, start with the development of an HFE Program Plan, which becomes the roadmap for the actual HFE process for engineers and designers. This process includes functional requirements analysis, function allocation, operating experience review, task analysis, analysis of important human actions, procedure development, training program development, humansystem interface design, design implementation, integrated system validation, and human performance monitoring. The objective of these HFE activities is to ensure that all human tasks that are needed to accomplish the safe and efficient execution of plant functions (process control, monitoring, maintenance, diagnosis, calibration and testing, and event recovery) are identified, analyzed and described in sufficient detail to determine the design requirements and to assess risk. The analyses also allow the organization to compare the demands that the system makes on the operator with the capabilities of the operator, and if necessary, to alter those demands, thereby reducing the probability of error and achieving successful performance. The current growing interest in advanced nuclear reactors and the emphasis on efficient engineering methods will inevitably include an emphasis on efficient tools in all engineering disciplines. It will soon become obvious that the lack of effective tools and methods will have an effect on the quality, productivity and cost of the HFE elements of engineering projects. All of this suggests that the need for up-to-date tools and integrated methods has now become pressing. 2 WHY THE HFE ANALYST NEEDS HELP A large part of HFE analysis and design activities has to do with the visual presentation of operational and task information. Current practice is to a large extent influenced by the visual conventions associated with methods like FRA, FA, and TA. Conventions like textual, tabular and hierarchical representations have become well established in the human factors community and there is little evidence of attempts to investigate the usability of such conventions, with or without dedicated tools. With reference to the practice in the armed forces, Hone and Stanton (2004) stated that "The most popular tool for use when conducting hierarchical task analysis (HTA) appears to be Microsoft Excel - and this for representational purpose only" [2]. They also mention other tools like Microsoft PowerPoint and OrgChart (organizational charting software) and mention that such tools do not offer any assistance to the analyst in the conduct of task analysis or any of the other HFE activities. The experience at INL has confirmed that trivial FRA, FA, and TAs could indeed be could be done with general purpose tools like spreadsheets or normal text documents. However, most people would probably agree that HFE work for a new NPP is a non-trivial undertaking and that analysts need all the help they can get. It is certainly not cost effective to simply add more analysts when the task is extensive. This is where tools are needed. The main purpose of a tool should be to enable the analyst to really understand the nature and structure of a task. If the tool requires the user to attend to the complexities and functionality of the software itself, it will detract from the analysis process and this may lead to errors and omissions. Thus, any HFE analysis software should assist the users by forcing them to think about the relationship between different pieces of information without burdening them with needless complexity. For example, if the structure of the information is linear, hierarchical or a network, the tool should make that structure visible. Similarly, if there is a relationship between different pieces of information (for example, precedence, dependency, input to, output from, part of, and so on), then that relationship should not only be visible, but it should be possible to specify and also interrogate that relationship. Page 2 of 12

5 The same requirements apply to all other HFE processes and especially where there is a need to integrate and link information between the processes. Hone and Stanton (2004) emphasize that an effective software tool must provide the necessary features for representation, interaction and usability. Indeed, experience has shown that there are very specific requirements for HFE tools. In order to support key processes like functional requirements analysis, function allocation, and task analysis, and also to translate functional and task information into HSI and control room design requirements, an effective HFE tool needs to: 1. Support the elicitation, capture, analysis, modification and reporting of data required for the development of the system, function and task hierarchies for a new NPP or for plant modification projects. 2. Support editing, verification and extended analysis of the information at all levels of goal, function, system and task hierarchies and enable further extensions of the analysis to be added through each of the iterations and phases of a project. 3. Support manipulation of information in formats suitable for effective visual representation, for example, tabular, graphical, outline or textual. 4. Produce professional-quality reports 5. Support peer-review of results These objectives are valid for the HFE process overall. However, the scope and complexity of data collection, organization, analysis, modeling and simulation activities change over the analysis continuum; some of these activities require complex judgment, while others may be almost mechanical. The data collection and organization phases of HFE require tools that help the analyst to understand and manipulate the structure of the raw data and to identify where additional data need to be collected, whereas the analysis and modeling phases are more dependent on a deep understanding of the concepts, principles and rules underlying the tasks being analyzed. The last phase, simulation, is normally only performed for complex tasks, and here the analyst is dependent on specialized tools like discrete event simulation hardware and software systems, which fall outside the scope of the tools discussed in this paper. Figure 1 below is a simplified illustration of the HFE process merged with the generic systems engineering process (Hugo, 2012 [4]). The highlighted blocks indicate specific HFE activities that can be effectively supported by various tools, as will be described later. Page 3 of 12

6 Figure 1: HFE Process merged with Systems Engineering While no tool can replace the analyst s insight, experience and skill, one should not underestimate the value of a dedicated tool that makes information structures and relationships visible and also allows the analyst to create or discover relationships that may not be obvious at first. Ultimately, a good HFE tool can be described as a tool to think with [7]. 3 A SUITE OF GENERAL-PURPOSE AND SPECIAL SOFTWARE TOOLS Like all other engineering disciplines, HFE requires special methods and tools to ensure costeffective, efficient, accurate and reliable development of design solutions that comply with industry requirements for safety and productivity. A range of tools (both paper and computer-based) have been developed for various HFE activities, including requirements analysis, task analysis, human reliability analysis, prototyping and human performance modeling. Most of these tools have been developed inhouse by the users and are not commercially available, or are aimed at a specific work domain, such as defense, maritime, or aviation (see for example, O Hara, 2009 [10]). The few HFE tools that are commercially available, such as MicroSaint (Alion Science and Technology), TaskArchitect (Kern Technology Group), JACK (Tecnomatix), or The Observer XT (Noldus Software) are special-purpose tools that in most cases do not offer everything an analyst needs for all the complexities of the various phases of HFE, especially for the nuclear industry. For most human factors practitioners, task analysis (TA) and human system interface (HSI) design are two processes right at the center of their activities. In a previous related paper (Hugo, 2009 [3]), the use of a general-purpose tool (MindManager by Mindjet) for hierarchical task analysis (HTA) was described. The usefulness of this tool for functional analysis and task analysis has been demonstrated in several projects over the years. However, experience has also shown that none of the various HFE methods can be regarded as stand-alone methods and any tool used for HFE is best used in conjunction with other generalpurpose or specialized tools, like checklists, databases, normal documents, modeling and simulation systems, and graphical modeling systems. Page 4 of 12

7 Ryerson University s Inventory of Human Factors Tools and Methods (Neuman et al [9]) identifies a large number of tools for all phases of the HFE process, but most of them focus on TA. These tools include video observation tools, usability labs, discrete event simulation, and anthropometric measurement tools, but none of them is designed for the kind of analyses required for NPP operators of new, advanced designs. Some phases of HFE that intersect with systems engineering, like functional requirements analysis (FRA), could be supported by software tools that are typically used for large-scale engineering projects. A typical example is Rational DOORS, which is used for requirements management. Although it could be used to capture some HFE information, it is large and complex and not suited to the analytical aspects of human factors engineering. This lack of tools has lead to many practitioners using labor-intensive tools like paper-based forms, Excel spreadsheets and even PowerPoint and Visio for processes like requirements analysis, function allocation, work domain analysis, task analysis and HSI prototyping. This may be acceptable for smallscale projects, but one of the big challenges in HFE for large-scale projects has always been integration of design requirements and results across functions and systems. For example, operator roles, functions and tasks must match the technical requirements of systems, in particular the functional and system breakdowns and other design information obtained from systems engineering (see IEEE [6] and IEEE [5]). Similarly, HSI design decisions must be traceable to the task analysis, which in turn must be linked to human performance requirements from standards, guidelines, specifications and risk analyses. If these traceable links are not built into both the Systems Engineering and the HFE processes, it is likely to lead to several practical problems, such as poor integration of human factors requirements in system design, lack of consistency of format and presentation in HSI designs, and many other problems that may ultimately lead to expensive rework, or worse, operator error, equipment damage, or safety incidents. In addition, inefficient tools used for the key processes will either affect the quality of the results, or extra effort will be required to produce good results and this will inevitably affect schedules and cost. Several general-purpose commercial tools, including those mentioned above, are available to support most of the HFE processes and it is possible to compile a suite of tools made up of off-the-shelf software utilities as well as customized general-purpose software. These range from simple textual and graphical representation tools, to software development platforms that can be used to develop special-purpose applications, one of which will be described in the next section. The emphasis should be on the ability of these tools to exchange and integrate information in a way that supports the stringent requirements of NPP projects. The following is a short list of the tools most commonly used by the INL HFE group to support specific project phases and HFE activities: Table 1: Example of HFE Tools by Project Phase Typical project phase Tools used Description Functional Requirements Analysis HFE Issues Tracking FRA/FA/TA Assistant MindManager Excel Spreadsheets FRA/FA/TA Assistant HFE Issues Register (MS Access Database) In combination, these tools are used to develop traceability tables, check original user requirements, engineer s interpretation, applicable standards and codes, and revised requirements. MindManager is particularly suitable for breakdown of functions, structures, systems and components and can be linked to requirements databases and spreadsheets. An HFE Issues Register is a control process to capture, analyze and track HF issues, assign their resolution to responsible persons, and record resolutions of the issues. A relational database is the ideal tool for this purpose. Page 5 of 12

8 Typical project phase Tools used Description Cognitive Work Analysis (CWA) and Work Domain Analysis (WDA) Function Allocation Task Analysis Human Performance Modeling and Simulation Detail Design Verification & Validation FRA/FA/TA Assistant Excel Spreadsheets Visio Access database MindManager Concept mapping software, e.g. Cmap Tools, Smart Ideas, The Brain. Access relational database Excel MindManager (Mindjet) TaskArchitect FRA/FA/TA Assistant (INL) IPME (Discrete event simulation) Sequence Diagram Editor (Efexis Software) Mobile eye tracker hardware and software Discrete event simulation Ergonomics Checklists Prototyping software HSI development hardware and software (part of automation system development software) FRA/FA/TA Assistant Excel Spreadsheets No commercial tools exist for CWA and WDA, but most concept mapping software and databases can be adapted to the classical methods. In addition, graphical tools like Visio may be useful for some of the classical WDA diagrams. A relational database and spreadsheets are very useful for high-level identification of the role of the humans and systems, and linking to functional requirements. This is progressively refined as more functional information becomes available. There are many diverse tools for task analysis, ranging from simple hierarchical decomposition tools like MindManager, to Sequence Diagram Editor for Operational Sequence Analysis, Integrated Performance Modeling Environment (IPME) for task network and human performance modeling, and eye tracking technology for assessment of situation awareness. This is an extension of task and interaction analysis and simulations, using the task network method of IPME. It supports evaluation of personnel accessibility to equipment and work areas, physical constraints of tasks and layouts. Also evaluation of the effects of tasks and the specific environment on humans. Development of prototype HSI mimics, HSI evaluation, and user interface management and control. Evaluation of designs against functions and requirements. A comprehensive discussion of all possible software and hardware tools that can be used in HFE is beyond the scope of this paper. The following is a short list of additional tools that practitioners may consider: General-purpose Software this includes all of the typical office software normally used for textual, numerical and graphical information, including Microsoft Access, Excel, Word and Visio. In addition, general-purpose concept mapping tools like Mindjet MindManager, The Brain, Cmap Tools, and SmartIdeas can be very effective in developing and capturing early conceptual information. Graphical Tools a range of tools are available for the development of diagrams, HSI prototypes and control room designs, including SketchUp, Sequence Diagram Editor, and Visio. Special Hardware this includes technologies such as Eye Tracking for physiological measures of mental workload, and other physiological measurement tools used to assess human performance, including Heart Rate Variability (HRV) and electroencephalograms (EEG). Even more sophisticated tools include haptic interfaces that Page 6 of 12

9 use neumatic stimulation, vibrotactile stimulation, electrotactile stimulation, and functional neuromuscular stimulation to facilitate human-machine interaction in complex environments. Usability Labs a range of static and portable systems are available to support observation and evaluation of human performance in the use of specific applications. Typical software used in usability labs are The Observer and Morae, as well as hand-held observation devices. Simulation and simulators. Physical simulators and discrete event simulation software are among the most complex technologies used to model and evaluate human performance. Systems that are specifically designed for human factors engineering include Micro Saint and the Integrated Performance Modeling Environment (IPME), included in the table above. 4 A TOOL FOR INTEGRATION OF FRA, FA AND TA DATA In any engineering project the functionality, quality and safety of the end product is only as good as the quality of the analysis that went into it. The same applies to the extent to which the engineering organization has considered human abilities and limitations in the NPP design. The discussion so far suggests that FRA, FA and TA are the most complex and error-prone phases of HFE and would therefore demand a tool that allows the integration and verification of a large amount of information. With the possible exception of in-house tools that are not generally available to the public, we can confidently state that no current tool, or combination of tools, adequately support the integration of FRA, FA and TA. Therefore, in response to the increasing emphasis on the complex demands of these analyses for new-build projects, an integrated software application was developed to satisfy as many of the requirements described before as possible. The resulting tool is used to capture, analyze and control a large amount of functional, system and task information for the HFE program during the early phases of a project. It is used to generate inputs for subsequent HFE processes, particularly the human-centered requirements for automation and HSI design. The same application is also used throughout the project for verification and updating of design requirements. The relationships and links between the data elements of FRA, FA and TA are illustrated in the entity-relationship model in Figure 2. Page 7 of 12

10 Figure 2: FRA, FA, TA Entity Relationship Model The data capture, analysis and reporting functions have been implemented in a Microsoft Access application called the FRA/FA/TA Assistant. The Access platform was chosen because of the large number of dependencies and relationships among the data, as shown in Figure 2. These relationships require the use of a relational database that avoids duplication of data and allows cross-referencing and linking of a large number of data items. The FRA/FA/TA Assistant is used (in conjunction with other tools such as spreadsheets, checklists and normal documents where necessary, as indicated in Table 1 above) to collect data from subject matter experts and other sources. This information is then used to develop system breakdown structures and functional breakdowns of the intended power plant design. This initial information captured in the tool is expanded by developing extensive descriptions of all functions, as well as performance parameters, operating limits and constraints, function start and end states. Once these have been verified, the human factors elements are added to each function, including intended operator role, function allocation considerations, performance considerations, prohibited actions, primary task categories, and primary work station in the plant. For example, the hierarchical functional and system breakdowns are an important source of information for Work Domain Analysis (see Naikar, 2013 [8]), which can be directly linked to a Goals-Means Task Analysis (Ainsworth & Kirwan, 1992 [1]). In addition to the data capture functions, the system includes a computational method to assess a number of factors that may shape operator performance, in accordance with the FA principles described in NUREG/CR-3331 [13]. This results in a semi-quantified allocation of functions to three or more levels of automation for a conceptual automation system. The aggregate of all this information is then linked to the TA section of the tools where the existing information on all operator functions is transformed into task information and ultimately into design requirements for HSI and control rooms. This final step includes assessment of methods to prevent potential operator errors. The functional requirements for the tool that were identified before are all aimed at enabling the HFE analyst to capture and analyze the following data, either from source documents or directly in interviews with subject matter experts. The following data items are recorded in the FRA/FA/TA Assistant: 1. Function identification number (this is a unique number for each function in the system). Page 8 of 12

11 2. System name (the system that is most closely associated with the function being described). 3. Related Main or Sub-system(s) (one or more systems related to the main system and that are also required for the function). 4. Operational mode (one of the operational modes identified from available plant design information). 5. Operational function (a short description of the system function in the context of the operational mode). 6. Performance Parameters (characteristics of the system performance and that may suggest requirements for operator actions or automation. Only parameters that can be monitored or controlled are identified). 7. Operating Limits, Constraints and Dependencies (operational characteristics of the plant, environment or the system itself that may influence the effectiveness or efficiency of the function, e.g., limits of pressure, temperature, tolerance, speed, size, etc.). 8. Start and end state of the function (numerical value of process condition: pressure, temperature, reactivity, speed, etc.) 9. Initiating event or condition (a description of the condition, command or action that triggers the function). 10. Function allocation considerations (description of the operational conditions or human abilities and limitations that may influence the allocation). 11. Rating of the function allocation criteria per function (a numerical rating for each criterion on the table on a semantic differential scale). 12. Operator role concept (description of the conceptual role of the operator for this function). 13. Prohibited or Mandatory actions (actions that the operator would not be permitted to perform, or actions that are mandatory for this function). 14. Assigned operator (the most likely operator, selected from a list of possible roles). 15. Control Station (the location where the function is most likely to be performed). 16. Control Section (a selection of the conceptual section in the Main Control Room where the function would be performed). 17. Recommended Allocation (an allocation to system, human, or both, calculated from the ratings and weights). 18. Function Allocation Justification (the reason and further considerations for the recommended allocation). Each of the items listed above has a corresponding data field and associated controls in the software, as shown in the screen captures below (Figure 3: Main Data Capture and Lookup Form, and Figure 4: FRA/FA report example). Page 9 of 12

12 Figure 3: FRA/FA/TA Assistant Main Data Capture and Lookup Form The next figure illustrates one of the reports that may be obtained from the tool: Figure 4: FRA/FA report example Page 10 of 12

13 5 CONCLUSION There is little argument among human factors practitioners about the validity and usefulness of the various activities of the HFE process. However, there has not been much consensus on the best tools to use for the various analytical and representational phases of the process. It is not suggested that there should be consensus; requirements differ among individuals, projects, environments and organizations. But ultimately every practitioner needs a toolbox with specialized as well as generic tools. As shown in this paper, several of the standard HFE processes could be significantly easier with software support. However, a tool would need very special features if it were to actually help an analyst carry out an analysis. It is true that tools for depiction purpose only" offer very little assistance to the analyst. Although generic tools can support the representation of FRA, FA and TA information, complex analyses in particular will benefit from the ability to input and manipulate dynamic and changing information structures. Generic graphing tools do not provide this capability. But even without the features of specialized analytical tools described before, the value of a tool that helps the analyst to capture, organize and control NPP functional, operational and task information in a coherent, consistent, yet flexible manner, and also eases the task of presenting information for reporting purposes, should not be underestimated. HFE for the mission-critical industries is complex and challenging and there can be little argument about the need for a tool that can reduce the drudgery, support the analyst s cognitive processes, and improve the quality and reliability of the analysis and design results. 6 REFERENCES [1] Ainsworth, L.K. and Kirwan, B. (1992). A Guide to Task Analysis. London:Taylor and Francis. [2] Hone, G. & Stanton, N. (2004). HTA: The development and use of tools for Hierarchical Task Analysis in the Armed Forces and elsewhere. Human Factors Integration Defense Technology Centre. HFIDTC/WP.2.21/1. [3] Hugo, J. (2009). Practical Tools for Task Analysis. In: Proceedings of the Sixth American Nuclear Society International Topical Meeting on Nuclear Plant Instrumentation, Control, and Human- Machine Interface Technologies, Knoxville, Tennessee, April 5-9, [4] Hugo, J. (2012). Towards a Unified HFE Process for the Nuclear Industry. In: Proceedings of the Eighth American Nuclear Society International Topical Meeting on Nuclear Plant Instrumentation, Control, and Human-Machine Interface Technologies, San Diego, California, July 22-26, [5] IEEE , Recommended Practice for the Application of Human Factors Engineering to Systems, Equipment, and Facilities of Nuclear Power Generating Stations and Other Nuclear Facilities. The Institute of Electrical and Electronics Engineers, Inc., New York, NY. [6] IEEE IEEE Standard for Application and Management of the Systems Engineering Process. The Institute of Electrical and Electronics Engineers, Inc., New York, NY. [7] McIntosh B.S., Seaton, R.A.F. and Jeffrey, P. (2007). Tools to Think With? Towards Understanding the Use and Impact of Model-Based Support Tools. In Environmental Modelling & Software, 22:5 (2007) Elsevier. [8] Naikar. N. (2013) Work Domain Analysis: Concepts, Guidelines and Cases. Boca Raton:CRC Press. [9] Neuman, W.P. (Ed) (2007). An Inventory of Human Factors Tools and Methods. Ryerson University, Toronto, Canada. [10] O Hara, J. (2009). Trends in HFE Methods and Tools and Their Applicability to Safety Reviews. Brookhaven National Laboratory. [11] O. Hara, J.M., Brown, W.S., Lewis, P.M. and Persensky, J.J. (2002) Human-System Design Review Guidelines, (NUREG-0700, Revision 2). Washington DC: U.S. Nuclear Regulatory Commission. Page 11 of 12

14 [12] O Hara, J., Higgins, J.C., Fleger, S.A and Pieringer, P.A. (2012). Human Factors Engineering Program Review Model, (NUREG-0711, Revision 3). Washington DC: U.S. Nuclear Regulatory Commission. [13] Pulliam, R., Price, H. E., Bongarra, J., Sawyer, C. R., and Kisner, R. A. (1983). A methodology for allocating nuclear power plant control functions to human or automatic control (NUREG/CR- 3331). Washington DC: U.S. Nuclear Regulatory Commission. [14] US NRC (2013) Three Miles Island Accident Backgrounder. NRC Office of Public Affairs. Washington DC: U.S. Nuclear Regulatory Commission. Page 12 of 12