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IAEA-TECDOC Application of simulation techniques for accident management training in nuclear power plants

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IAEA-TECDOC-1352 Application of simulation techniques for accident management training in nuclear power plants May 2003 The originating Section of this publication in the IAEA was: Safety Assessment Section
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IAEA-TECDOC-1352 Application of simulation techniques for accident management training in nuclear power plants May 2003 The originating Section of this publication in the IAEA was: Safety Assessment Section International Atomic Energy Agency Wagramer Strasse 5 P.O. Box 100 A-1400 Vienna, Austria APPLICATION OF SIMULATION TECHNIQUES FOR ACCIDENT MANAGEMENT TRAINING IN NUCLEAR POWER PLANTS IAEA, VIENNA, 2003 IAEA-TECDOC-1352 ISBN ISSN IAEA, 2003 Printed by the IAEA in Austria May 2003 FOREWORD Many IAEA Member States operating nuclear power plants (NPPs) are at present developing accident management programmes (AMPs) for the prevention and mitigation of severe accidents. However, the level of implementation varies significantly between NPPs. The exchange of experience and best practices can considerably contribute to the quality, and facilitate the implementation of AMPs at the plants. Various IAEA activities assist countries in the area of accident management. Several publications have been developed which provide guidance and support in establishing accident management at NPPs. These publications include Safety of Nuclear Power Plants: Design Requirements, and the Safety Report on Development and Implementation of Accident Management Programmes in Nuclear Power Plants. Separate technical documents are being prepared on methodology for severe accident analysis, the development and review of emergency operating procedures, and on training and technical support for AMPs. The safety service for review of AMPs is offered to Member States; its purpose is to perform an objective assessment of the status at various phases of AMP implementation in the light of international experience and practices. Various technical meetings and workshops are also organized to provide a forum for presentations and discussions and to share experience in the development and implementation of AMPs at individual NPPs. The Safety Report on Development and Implementation of Accident Management Programmes in Nuclear Power Plants has a special role among the IAEA s guidance documents. It provides a description of the elements that should be addressed by the team responsible for preparation, development and implementation of a plant specific AMP at an NPP and is the basis for all other related IAEA publications. The Safety Report underlines the importance of training for the successful implementation of an AMP. The use of simulators with severe accident modelling capabilities is also mentioned as an effective means for training. The applicability of simulators in the area of accident management, or simulation in a more general sense, is discussed in greater detail in the present report. It describes various approaches from graphical interfaces into severe accident analysis codes up to full scope simulators. Specific issues related to the use of simulation techniques for different training levels and different groups of personnel are discussed. An overview of existing simulators and the status of their application in a number of countries is provided. The IAEA officer responsible for this publication was J. Mišák of the Division of Nuclear Installation Safety. EDITORIAL NOTE The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries. The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA. CONTENTS 1. INTRODUCTION Background Scope and objectives Structure SPECIFIC ASPECTS OF ACCIDENT MANAGEMENT Development of an AMP Influence of accident management on risk OBJECTIVES AND REQUIREMENTS Objectives of accident management training Objectives of the use of simulators Basic requirements to be met by the accident management training tools and simulators Requirements for training in preventive and mitigative actions Requirements for the different training levels Requirements for different personnel groups Requirements concerning the simulator type APPLICATION OF SIMULATORS TO TRAINING Present simulator capabilities Methodology for verification and validation of simulators Development and use of simulators for training Use of simulators for training of preventive and mitigative actions Use of simulators for different training levels Use of simulators for training different personnel groups Further developments in simulator training Low power and shutdown modes Improvement in the use of severe accident codes Computerized accident management support Full-scope simulators in the severe accident domain Virtual reality CONCLUSIONS APPENDIX I: Overview of existing simulator types: Examples APPENDIX II: Status of application simulators to accident management training in a number of countries REFERENCES ABBREVIATIONS DEFINITIONS CONTRIBUTORS TO DRAFTING AND REVIEW... 45 1. INTRODUCTION 1.1. Background The defence in depth concept in nuclear safety [1, 2] requires that, although highly unlikely, beyond design basis and severe accident conditions should also be considered, in spite of the fact that they were not explicitly addressed in the original design of currently operating nuclear power plants (NPPs). Defence in depth is physically achieved by means of four successive barriers (fuel matrix, cladding, primary coolant boundary, and containment) that prevent the release of radioactive material. These barriers are protected by a set of design measures at three levels, including prevention of abnormal operation and failures (level 1), control of abnormal operation and detection of failures (level 2) and control of accidents within the design basis (level 3). Should these first three levels fail to ensure the structural integrity of the core, additional efforts are made at the fourth level of defence in depth in order to further reduce the risks. The objective at level 4 is to ensure that both the likelihood of an accident entailing significant core damage (severe accident) and the magnitude of radioactive releases following a severe accident are kept as low as reasonably achievable. The term accident management refers to the overall range of capabilities of a NPP and its personnel to both prevent and mitigate accident situations that could lead to severe fuel damage in the reactor core. These capabilities include the optimized use of design margins as well as complementary measures for the prevention of accident progression, its monitoring, and the mitigation of severe accidents. Finally, level 5 includes off-site emergency response measures, the objective of which is to mitigate the radiological consequences of significant releases of radioactive material. In the IAEA Safety Report on Development and Implementation of Accident Management Programmes in Nuclear Power Plants [3] accident management is defined as: the taking set of actions during the evolution of a beyond design basis accident (1) to prevent the escalation of the event into a severe accident, (2) to mitigate the consequences of a severe accident, and (3) to achieve a long term safe stable state. Similarly, severe accident management is a subset of accident management measures with the objective to: (1) terminate the core damage once it has started, (2) maintain the capability of the containment as long as is possible, (3) minimize on-site and off-site releases, and (4) return the plant to a controlled safe state. An accident management programme (AMP) comprises plans and actions undertaken to ensure that the plant and its personnel with responsibilities for accident management are adequately prepared to take effective on-site actions to prevent or to mitigate the consequences of a severe accident. The IAEA definitions are in line with the definitions of severe accident management in OECD/NEA documents as given, for example, in Ref. [4]. As stated above, accident management constitutes one of the key components of defence in depth. Provisions for accident management should be made even if all provisions within the design basis are adequate. The approaches particularly to accident management and severe accident management vary in the Member States. Some countries focus on actions aimed at defining procedures and severe accident management guidelines (SAMGs) that are based on utilizing the existing capabilities of the plant once a predetermined safety level has been achieved. Some countries require that plant modifications, including those to hardware and to instrumentation and control (I&C), as well as procedural changes should be made to significantly improve the 1 plant s capability to manage severe accidents without large releases to the environment occurring. Regardless of the approach used, most countries have already implemented or plan to implement an AMP that includes development of the procedures or SAMGs or both. Furthermore, the definition of the training programme for the plant personnel who will be involved in the severe accident management actions during an emergency is an integral part of the AMP. The main means for training are classroom training, drills and exercises as well as the respective simulator training, if available. Reference [3] also underlines the importance of training as necessary condition for the successful implementation of an AMP. The use of simulators with severe accident modelling capabilities is also mentioned as an effective means for training. The applicability of simulators, or simulation in a more general sense, is discussed in further detail in the present report. The use of simulators for training NPP personnel was already addressed in earlier IAEA publications, e.g. in Ref. [5], but without reference to specific accident management training issues. Available national standards, mainly the American National Standard ANSI/ANS on Nuclear Power Plant Simulators for Use in Operator Training and Examination [6], were taken into consideration when preparing this report. The IAEA Advisory Group Meeting on Implementation of Severe Accident/Accident Management Simulation in NPP Support Staff Training (Software, Hardware), held in Vienna, 4 8 September 2000, also provided valuable input for the report Scope and objectives This report describes simulation techniques used in the training of personnel involved in accident management of NPPs. This concerns both the plant personnel and the persons involved in the management of off-site releases. The report pertains to light water reactors (LWRs) and pressurized heavy water reactors (PHWRs), but it can equally be applied to power reactors of other types. The report is intended for use by experts responsible for planning, developing, executing or supervising the training of personnel involved in the implementation of AMPs in NPPs. It concentrates on existing techniques, but future prospects are also discussed. Various simulation techniques are considered, from incorporating graphical interfaces into existing severe accident codes to full-scope replica simulators. Both preventive and mitigative accident management measures, different training levels and different target personnel groups are taken into account Structure Based on the available information compiled worldwide, present views on the applicability of simulation techniques for the training of personnel involved in accident management are provided in this report. Apart from the introduction, this report consists of four sections and three appendices. In Section 2, specific aspects of accident management are summarized. Basic approaches in the development of an AMP and the importance for its successful implementation of various well trained groups of staff are described. The influence 2 of effective accident management on risk reduction is emphasized. The objectives of and requirements for accident management training and for the use of simulators, in particular, are given in Section 3. Simulation requirements for training in both the preventive and the mitigative domain, for different training levels and different personnel groups as well as requirements concerning the simulator type are briefly specified. Various issues related to the application of simulators in training are discussed in Section 4. The present capabilities and limitations of various categories of simulators and examples of the simulators software basis are described. Specific aspects of the methodology used for verification and validation of severe accident simulators are given. Differences in the use of simulators for various purposes and different target groups are summarized. The prospects for further development in simulator training are presented. The main conclusions with respect to the applicability, capabilities and limitations of simulation for accident management training are given in Section 5. Appendix I gives an overview of different types of existing simulators. The status of the application of simulators in accident management training and a more general description of the approach to accident management training in selected countries is presented in Appendix II. 2. SPECIFIC ASPECTS OF ACCIDENT MANAGEMENT 2.1. Development of an AMP For each individual plant, there are a wide variety of severe accident scenarios and sequence classes. The sequences start with different initiating events or precursors that may lead directly or through additional failures to core degradation. The range of the initial plant states includes power operation, the plant heat-up and cool-down phase and shutdown conditions. Once started, core degradation may proceed further, leading to melting of the reactor core, to melt-through of the pressure vessel and to a multitude of physical phenomena potentially challenging the containment integrity. The further the accident progresses into the severe accident domain, the more difficult it becomes to manage by means of the emergency operating procedures (EOPs), which usually do not include severe accidents. Therefore, the utilities tend to develop SAMGs with a structure that is more appropriate for such situations. Developing an AMP is the responsibility of the plant licensee, i.e. the plant owner and operator. There are various alternative ways of incorporating accident management in plant operations [7]. The selection of the best way in each individual case is influenced by the organizational structure and the available technical expertise of the utility as well as by the extent of the problem solving required to accomplish specific strategies. The various aspects involved are discussed in Ref. [3]. The principal ways are to: (1) develop severe accident guidelines (SAGs), (2) develop a severe accident guidance document, or (3) incorporate severe accident elements in existing procedures. The term 'procedures' refers to a set of detailed documents that prescribe specific actions in specified conditions, while guidelines refers to a general description of actions that could be effective in managing a particular situation. The personnel involved in the emergency response organization (ERO) for accident management is composed of different groups, as described in Ref. [3]. Typically, these are: (1) control room (CR) operators, (2) a possible permanent or on-call safety engineer, (3) an 3 on-site technical support centre (TSC), (4) an on-site operations support centre, and (5) offsite emergency operations staff. The implementation of the AMP should take into account that the availability of the technical expertise in these groups may vary significantly among the utilities. The organizational aspects of accident management implementation include the definition of the roles and responsibilities of the personnel involved. The tasks of the different personnel groups may vary according to the particular phase of the accident management. The implementation also varies among the utilities since the accident management organization must be integrated in the overall utility and plant organization and in the existing ERO. The personnel responsible for the plant operation under emergency conditions must make a number of critical decisions. These decisions typically concern such issues as: off-site and on-site emergency preparedness recommendations, effectiveness of in-plant mitigation measures, and prioritization of actions to recover inoperable equipment and systems. Further critical decisions regard: adding water to a degraded core, depressurization of the reactor coolant system, preventing steam generator tube creep rupture, and containment related decisions, e.g. use of sprays, flooding, filtered venting and hydrogen management. The timing of these decisions is plant specific. The timing also determines the organizational level at which the decision should be taken. In the initial phase, the plant operators should be able to take them. In a later phase, when the on-call safety engineers or the TSC are available, they can assist or take over in the decision making. The decisions are considered critical in the sense that the implementation of a decision can also include adverse effects. In such a case, the potential negative consequences have to be properly assessed before an action is carried out. In the decision making process, awareness of these possible consequences is necessary in order to balance the actions to be taken. The personnel involved have to be trained in all aspects related to the decisions to be taken. The training needs and methods are specified in Ref. [3]. Many utilities have adopted the practice of developing SAGs supported by a separate document containing the necessary technical background information. Others have chosen an approach integrating these two elements in a single background information document, often referred to as the SAM Handbook. This background material should contain all information that is deemed necessary for the operators and the technical support staff with respect to severe accident progression and phenomena and the procedures and guidelines applied for accident management. This document itself is an important part of the training material. The AMP implementation report [3] discusses the application of classroom training and exercises and emergency drills, but it does not specify in detail the needs and methods for simulator training. For the preventive phase of accident management, the full-scope replica simulators are an efficient means of training since they normally include the necessary range of phenomena. 4 The operator aids to be used during a severe accident have been developed in many countries; they range from graphs and simple hand calculation tools to more sophisticated computational tools. They are described in more detail in Ref. [3]. Two specialists meetings have been organized by the OECD where such tools were reviewed, including the development and application stage of simulators used for training [8, 9] Influence of accident management on risk When planning the accident management approach and strategies, one must take into account possible adverse effects to avoid any unwanted impact on the plant safety. The potential adverse effects are closely related to the critical decisions. A precondition for the development of a consistent accident management approach is that the severe accident phenomenology and progression as well as the consequences of accident management actions be sufficiently understood for the individual plant. If this knowledge has been duly incorporated into the accident management guidance and accident management personnel training, successful management during the accident is far more probable. The accident management approach should be simple and robust as well as easily understandable to the accident management personnel.
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