HIGH ENERGY ARC FAULTS (HEAF) AND THEIR IMPACT ON PRA FOR A NUCLEAR POWER PLANT - LATEST INTERNATIONAL OPERATING EXPERIENCE AND RESEARCH ACTIVITIES

摘要

The international operating experience with fires in nuclear power plants (NPP) has provided insights that high energy arc faults (HEAF) may cause significant explosions and create ensuing fires that could damage systems, structures and components (SSC) important to safety. Two so-called Topical Reports on HEAF induced fire events and on event combinations of fires and other events prepared in the frame of the OECD (Organization for Economic Cooperation and Development) Nuclear Energy Agency (NEA) Database Project OECD FIRE have demonstrated that the number of fire resulting from HEAF, even if the HEAF event was an event subsequent to another hazard, is non-negligible. Several of such events did either impair nuclear safety or has the potential for impairing safety under different boundary conditions (precursor events). For a better understanding of the potential safety significance of HEAF events to safe NPP operation an international in-depth investigation of HEAF was initiated. The objective of this project is to determine damage mechanisms, extent of areas affected, methods of protecting SSC important to safety and possible calculation methods for modeling of HEAF events as applicable to fire protection in NPP environment. As part of this effort, an experimental program was initiated investigating HEAF fire phenomena to inform future deterministic and probabilistic methods. The first phase of the experimental program with seven member countries looked at a variety of different low and medium voltage components (e.g., breakers, switchgears and bus bars). Some of the general observations into the HEAF phenomena from the data collected in the experiments are: Those experiments where electrical components were made of aluminum were much more energetic during the HEAF resulting in more severe physical damage to equipment than those involving only copper and steel at any voltage level. The most severe electrical enclosure damage was observed as a result of a low voltage HEAF in an enclosure with aluminum bus bars. Increased duration arcing events were more likely to create an ensuing fire. HEAF events involving aluminum have the potential to create a new failure mechanism, specifically the aluminum produces a conductive aluminum plasma in the smoke that coats any exposed material causing short circuits and unintended current paths in electrical systems. All medium voltage enclosures, approx. 4 kV and above, maintained the arc for more than 2 s. Experiments with arc durations less than 2 s typically did not result in ensuing fires. The paper provides valuable insights for probabilistic risk assessment (PRA) from the operating experience collected in the OECD FIRE Database as well as from the HEAF experimental program.