The operating experience from nuclear installations worldwide has provided a non-negligible number of mostly explosive plant internal fire events due to high energy arcing equipment faults (HEAF). These typically occur at higher voltage electrical components such as switchgears and circuit breakers, or at high cables.

In some of the events, the electric arcs have led to partly significant consequences to the environment of the affected components exceeding typical fire effects. In-depth investigations have indicated failures of those fire barriers and protection features not designed against such impacts induced by the rapid pressure increase.

The potential safety significance of HEAF events has caused the OECD Nuclear Energy Agency (NEA) to initiate a task on “HEAF” for in-depth investigations on this type of events in member states, their damage mechanisms and root causes as an important part of better understanding fire risk at NPP which is better accomplished by an international group to pool knowledge and research means.

Major goal is to develop deterministic correlations to predict damage and to establish a set of input data and boundary conditions for more detailed modeling. The output may directly support development of improved treatment methods in fire PRA for NPP applications.

One input into this OECD task is an in-depth analysis of the German operating experience with HEAF in nuclear power plants based on a questionnaire specifically developed for collecting the necessary data and information on these events. This survey has provided more than 30 events. The investigations demonstrate that HEAF only occur at few specific components such as switchgears and transformers, but also at cables and distribution connections on typical voltage levels between 0,4 kV and 400 kV.

All high energy arcing faults reported from German nuclear power plants were detected within a short time period and signaled via fault indications. In case of a relevant release of smoke there was a fire alarm by the fire detection system. The separation of redundant trains was ensured in case of all events. Neither the required function of fire protection means nor additional components were impaired by the explosive failure.

Technical causes have been found to be the major root causes for the high energy arcing faults. Other causes are human failure, ageing effects and faulty procedures in combination with other root causes. A variety of reasonable preventive measures have already or are to being taken in German nuclear power plants for improving nuclear safety.

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