[en] When the 13"t"h amendment of the Atomic Energy Act came into force, eight Germ an nuclear power plant units had their power operating licences revoked and are now in the so-called post operation phase. Of the remaining nuclear power plants, one have by now also entered the post operation phase, with those left in operation bound for entering this phase sometime between now and the end of 2022. Therefore, failure mechanisms that are particularly relevant for post operation were to be identified and described in the frame of the present project. To do so, three major steps were taken: Firstly, recent national and international pertinent literature was evaluated to obtain indications of failure mechanisms in the post operation phase. It turned out that most of the national and international literature deals with the general procedure of the transition from power operation to decommissioning and dismantling. However, there were also some documents providing detailed indications of possible failure mechanisms in post operation. This includes e.g. the release of radioactive materials caused by the drop of containers, chemical impacts on systems important to safety in connection with decontamination work, and corrosion in connection with the storage of the core in the spent fuel pool, with the latter leading to the jamming of the fuel assemblies in the storage racks and a possible reduction of coolant circulation. In a second step, three safety analyses of pressurised water reactors prepared by the respective plant operators were evaluated to identify failure mechanisms based on systems engineering. The failure mechanisms that were found here include e.g. faults in the boric acid concentration of the reactor coolant, damage to the equipment airlock upon the unloading of Castor casks, leakages in connection with primary system decontamination, and the drop of packages holding radioactive residual materials or waste with subsequent mobilisation of radioactive aerosols. Finally, national and international operating experience was evaluated with the aim to derive general failure mechanisms from events that have occurred. For this purpose, the database of German reportable events (VERA), the database of the OECD/NEA ''International Common-cause Data Exchange (ICDE)'' project as well as the database of the ''International Reporting Systems for Operating Experience (IRS)'' of the IAEA was searched. As these data sources comprise a total number of about 12,000 events, initially the evaluation focused on the 309 events that occurred in plants that are in the post operation or decommissioning phase. In order to capture events from power operation whose damage phenomena have special relevance for post operation, too, attempts were made to develop automatic searches that would deliver a pre-selection of relevant results. To evaluate this method, 900 events from a German twin unit plant over a period from their construction until the present were analysed manually. It turned out that an automated search did not deliver enveloping results. All in all, 12 events from German plants in post operation or decommissioning and 36 events from plants with power operation licences as well as 6 events from international operating experience (IRS and ICDE) which showed a failure mechanism with special relevance for the post operation phase were identified. The events were presented individually, with special emphasis on the description of the failure mechanism that was particularly relevant for post operation. The failure mechanisms identified can be allocated to the following three general categories: special operating modes; changed operating or ambient conditions; organisational and personnel boundary conditions. The failure mechanisms assigned to these categories are presented in a summarized form and discussed.

[en] In the frame of this research project, a method to quantify common cause failures ex-ceeding present common cause failure (CCF) groups using existing operating experience has been developed. For this purpose, a procedure for the determination of generalized component groups, for which CCF and common cause failures exceeding present CCF groups (X-CCF) are modelled, has been researched. Different models to map events to generalized component groups have been developed. To use existing expert assessments as far as possible, a procedure has been developed to adapt existing expert assessments and other input variables of the GRS coupling model for events classified as X-CCF. Using the GRS coupling model, the CCF data of the German operating experience, and the above-mentioned methods, reliability parameters have been calculated for X-CCF affecting the emergency diesel generators and the additional backup diesel generators of the backup emergency power supply system. The results show that firstly the models developed are appropriate to reduce unwanted conservativeness and secondly that the application of the GRS coupling model does not generally lead to more conservative results than alternative CCF models like e. g. the alpha factor model. A novel approach for the modeling of X-CCF affecting a large number of components has been developed and tested using simple examples. This approach consists of supplementing the list of minimal cut sets with additional elements representing minimal cuts including X-CCF. An approximate quantification algorithm which minimizes the numerical effort by disregarding minimal cuts with low statistical weight has also been developed and tested.