Analysis of events with common cause failures (CCF) from the international common cause failure date exchange (ICDE)

Common-cause-failure (CCF) events can significantly impact the availability of safety systems of nuclear power plants. In recognition of this, CCF data are systematically being collected and analyzed in several countries. A comprehensive evaluation of CCF events derived only from the operating experience in German nuclear power plants is not sufficient due to the low probability of occurrence of such events. Therefore it is necessary to make use of the operating experience of other countries using similar technology. In order to be able to use the CCF operating experience from other countries in the aim to carry on the development of the bases for evaluation of CCF GRS decisively co-initiated the setting up of an international common-cause failure working group. This working group has elaborated the project ''International Common-Cause Failure Data Exchange'' (ICDE). The project's objective is to organize a broad exchange of information concerning observed events with relevance to common-cause failures. The tasks for preparation and evaluation of information of the ICDE working group serve for confirmation and extension of the common-cause failure knowledge with regards to probabilistic safety analyses, the better understanding of causes and mechanisms of common-cause failures and the evaluation of preventive measures against the occurrence of common-cause failures. The objectives of the exchange on a long term basis are to - improve the comprehension of CCF events and their causes and their prevention, - generate qualitative insights into the root causes of CCF events which can then be used to derive and assess preventive measures against the occurrence of such events or their consequences, - establish an efficient feedback of experience gained in connection with observed common-cause failure phenomena which could be used e.g. for the development of indicators for risk based inspections, - provide quantitative information regarding the occurrence of common-cause failures to analyze the effectiveness of preventive measures and to obtain safe bases for common-cause failure reliability data in the frame of PSA. The information exchange in the frame of the ICDE project covers meanwhile the operating experience for the component types ''centrifugal pumps'', ''emergency diesel generators'', ''motor operated valves'', ''safety- and relief-valves'', ''check valves'', ''batteries'', ''breakers'', ''level measurement'', ''control rods drive assemblies'' and ''heat exchangers''. German operating experience is completely included for the period 1990 to 2002. The event analysis for the period 2003 to 2010 has started. Furthermore, a first data exchange is currently being carried out for the component types ''main steam isolation valves'' and ''fans''. For the component ''digital I and C equipment'' a data exchange is currently in preparation. The established precautionary measures against CCF should be assessed whether they are sufficient to prevent recurrence of these new phenomena. For control rod drive assemblies, new phenomena arose due to bending of fuel elements, due to premature wear, due to wrong water chemistry and due to maintenance deficits. For heat exchangers, new phenomena were caused by maintenance errors which led to clogging, inner leakage or by design flaws which led to faulty flow measurements.