CSNI Draft Report DEFENCE IN DEPTH OF ELECTRICAL SYSTEMS AND GRID INTERACTION Final DIDELSYS Task Group Report 1 2 Forward The July 2006 Forsmark-1 event identified a number of design deficiencies related to electrical power supply to systems and components important to safety in nuclear power plants. While plant- specific design features at Forsmark-1 contributed to the severity of the sequence of events which occurred at Forsmark, a number of the design issues are of a generic nature as they relate to commonly used approaches, assumptions, and design standards for voltage protection of safety related equipment. The NEA Committee on the Safety of Nuclear Installations (CSNI) authorized formation of a task group in January 2008 to examine Defence in Depth of Electrical Systems and Grid Interaction with nuclear power plants (DIDELSYS). The task was defined based on the findings of an NEA sponsored workshop on lessons learned from the July 2006 Forsmark-1 event held in Stockholm, Sweden in 5-7 September 2007. The task group members participating in this review included: • John H. Bickel, ESRT, LLC (Sweden) - Chairman • Alejandro Huerta, OECD/NEA - Secretary • Per Bystedt, SSM (Sweden) • Tage Eriksson, SSM (Sweden) • Andre Vandewalle, Nuclear Safety Support Services (Belgium) • Franz Altkind, HSK (Switzerland) • Thomas Koshy, USNRC (United States) • David M. Ward, Magnox Electric Co. (United Kingdom) • Brigitte Soubies, IRSN (France) • Kim Walhstrom, STUK, (Finland) • Alexander Duchac, EC Joint Research Center Petten (European Commission) • Robert Grinzinger, GRS (Germany) • Ken Kawaguchi, JNES (Japan) The general objectives of the task group review were to: • Evaluate the robustness of existing safety related electrical systems in nuclear power plants (including: design standards, acceptance criteria, design bases disturbances to be coped with) ; • Evaluate the basic principles used to develop a robust safety related electrical system in terms of critical design features, redundancy, diversity, and use of proven technologies; • Evaluate methodologies used to demonstrate the robustness of safety related electrical systems, considering: definition of input transients, analytical approaches, defence in depth considerations, simulation techniques and use of computer codes (including the verification and validation of obtained results), definition of safety margins; and • Evaluate the various modes of interactions between nuclear power plants and the electrical grid and the command and control interface between operators of the electrical grid and nuclear power plants. 3 From this DIDELSYS task group review, the NEA Committee on the Safety of Nuclear Installations (CSNI) desired a summary report that would: • Provide information on the state-of-the-art regarding the robustness of safety related electrical systems (SRES), taking into account their interaction with other electrical equipment, the use of new technologies and the problems encountered when modernization of existing plants is undertaken; and • Provide guidelines for improving the communication and co-ordination between the grid (grid operator and regulator), the nuclear safety authorities and the licensees. The DIDELSYS task group has now completed this technical report which is submitted to the CNRA and CSNI for further action. 4 Table of Content Forward ......................................................................................................................................................... 3 Table of Content ........................................................................................................................................... 5 Executive Summary ...................................................................................................................................... 9 Acronyms .................................................................................................................................................... 13 1. Introduction ..................................................................................................................................... 15 1.1 Scope ........................................................................................................................................... 15 1.2 Objectives ................................................................................................................................... 16 1.3 General Features of NPP Electrical Power Systems .................................................................. 16 1.3.1 Introduction ................................................................................................................................ 16 1.3.3 The DIDELSYS Workshop, Stockholm 5-7 September 2007 ................................................... 18 1.4 Highlights from International Operating Experience .................................................................. 20 1.4.1 Incident reporting system ........................................................................................................... 20 1.4.2 Results of the Event Database Screening ................................................................................... 21 1.4.3 Contributing factors to the selected events ................................................................................ 23 2. Electrical Defence in Depth ............................................................................................................ 35 2.1 Defence in Depth Levels ............................................................................................................. 35 2.2 Robustness of Defence in Depth ................................................................................................. 36 2.2.1 Robustness of Electrical System Designs ........................................................................... 36 2.2.2 Quality Assurance Measures ............................................................................................... 36 2.2.3 Confirmatory Testing and Inspections ................................................................................ 37 2.2.4 Electric Power System Operation ...................................................................................... 37 2.2.5 Redundant Automatic Emergency Protection Systems ....................................................... 37 2.2.6 Adequate Reactor Design Margins .................................................................................... 37 3. DIDELSYS Issues .............................................................................................................................. 39 3.1 Grid Challenges .......................................................................................................................... 40 3.1.1 Introduction and General Background ....................................................................................... 40 3.1.2 Scope .......................................................................................................................................... 40 3.1.3 Grid Faults ................................................................................................................................. 40 3.1.4 Oskarshamn NPP Case Study .................................................................................................... 46 3.1.5 Conclusions and Recommendations .......................................................................................... 47 5 3.2 Communication Interface between Nuclear Power Plant and the Electrical Power Grid ........... 48 3.2.1 Introduction ................................................................................................................................ 48 3.2.2 Scope .......................................................................................................................................... 48 3.2.3 Issue specific section .................................................................................................................. 49 3.2.4 Conclusions ................................................................................................................................ 51 3.2.5 References .................................................................................................................................. 51 3.3 Advantages and Disadvantages of House Load Operation Capability ....................................... 52 3.3.1 Introduction ................................................................................................................................ 52 3.3.2 Scope .......................................................................................................................................... 52 3.3.3 General Design Structure of NPP Electric Power Systems ....................................................... 53 3.3.4 General NPP Process System Considerations ............................................................................ 55 3.3.5 Electrical System Considerations ............................................................................................... 56 3.3.6 Summary of Major Benefits and Risks ...................................................................................... 58 3.3.7 Challenges .................................................................................................................................. 59 3.3.8 Conclusions and Recommendations .........................................................................................