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Proceedings of the Eighth International Conference on Probabilistic Safety Assessment & Management (PSAM)

Michael G. Stamatelatos
Michael G. Stamatelatos
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Harold S. Blackman
Harold S. Blackman
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ASME Press
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The overall objective of the Nordic Working Group on Common Cause Failure Analysis (NAFCS) is to support safety by studying potential and real CCF events and to report conclusions and recommendations that can improve the understanding of these events, eventually resulting in increased safety. The NAFCS was founded in 2001 by the Nordic PSA Group and is financed jointly by the Nordic utilities and the Swedish Nuclear Power Inspectorate (SKI). The results from the activities within the group are intended for application in NPP operation, maintenance, inspection, and risk assessments.

Understanding the failure mechanisms is an important feature of CCF data analysis. The data analysis process itself, by concentrating on failure mechanisms and possible defenses, provides insights into the plant design and operation. The results of data analyses performed within the group indicate that a large portion of the causes of the events are related to weaknesses in work practices and work organization. With the intention to decrease the frequency of such events in the future it was recommended to develop and carry out a general training program for all plant staff, aiming at increasing the awareness of the possibilities for occurrence of dependency events (including CCF) and stressing the importance of always maintaining relevant dependency defenses.

The objectives of the training program are summarized in the following task description:

“The training shall increase the awareness regarding the importance of CCF events by presenting potential and real CCF events. The training shall improve the understanding of these events and shall present strategies for defense against various dependencies, as well as methods for identification and analysis of such events.”

The training should specifically make use of NAFCS experience to provide guidance in the defense against dependent failures. Using real CCF events as examples and considering experience from several countries opens the possibility for the course participants to relate to their own work situation and conditions. It is important to give the plant staff a wide reference base in terms of events and experiences in order to allow them to recognize occurred situations and potential problems. The training shall as far as possible be adapted to different staff categories and their different responsibilities, authorities and conditions in the daily work, and it shall focus on the specific possibility of each staff category to prevent CCF events to occur.

Plant staff usually have own experience of negative effects due to dependencies between plant equipment but may not have given it that much thought in their normal duties. The first important step is to increase the awareness and understanding by starting a mind process reflecting on dependencies and on own experiences. After the introduction part the following step is to increase the insight in how important dependencies can be in the context of deterministic and probabilistic safety analysis. An overview of experiences available in Swedish and international experience data is used as a basis and to learn from. Examples from experience data are used throughout the discussion to increase the understanding of the dependent events, their causes and actual impact on safety.

Following the insight that dependencies exist and have strong influence on safety the next step is to reflect on strategies for defense against dependencies. The purpose is to provide insight in the design of a good defense. An overview of factors constituting defenses are used as a basis for the discussion. All factors shall be applied to reveal potential dependencies as early as possible by maintaining relevant dependency defenses.

The purpose of the final step, the dependency analysis part, is to provide a common methodological guidance for the analysis of dependencies in PSA:s. The Guidance is meant to clarify the scope of the analysis of the various dependency categories, the interaction of the various analyses and their PSA context, as well as to provide guidance for the performance of the analysis of the various dependency categories.

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