RESULTS OF THE LOSS OF OFFSITE POWER EVENTS ANALYSIS

This paper presents the results of statistical and engineering analysis of Loss of Offsite Power (LOOP) events registered in four reviewed databases. The paper includes events registered in IRSN (Institut de Radioprotection et de Surete Nucleaire) SAPIDE and GRS (Gesellschaft fair Anlagenund Reaktorsicherheit mbH) VERA database in time period 1992 to 2011. The Nuclear Regulatory Commission (NRC) Licensee Event Reports (LERs) database and the IAEA International Reporting System (IRS) database are screened for the relevant events registered in period 1990 to 2013. In total, 228 relevant events were identified in the IRSN database, 190 in GRS, 120 in LER and 52 in IRS. The data include events registered both during the critical (at power) and shutdown operation of the plants. The identified events were classified considering nine different categories. In the three databases (SAPIDE, VERA, IAEA-IRS) the largest numbers of events are registered for the plant centered category. The largest number of the events in the NRC-LER database is found for switchyard centered events. According to the mode of operation, most events were reported during critical power operation, in all four databases. The "Partial loss of external power" events are the most frequent type of event found in the IRSN and NRC databases while the "Physical loss of electrical busbars" is the main type in the GRS and IAEA databases. The largest number of events in all databases is identified for the switchyard failures followed by the interconnections failures (both lines and transformers). Mainly LOOP event are identified by the fault report in the control room. Electrical deficiency is detected as the main direct cause of events. Environment is registered as the main contributor for the electrical grid deficiency in the French and NRC databases. Electrical failures are dominant contributor to the electrical grid deficiency in the German and IAEA databases. The principal root cause for the LOOP events are human failures with the human errors during test, inspection and maintenance as the largest sub-group. The largest number of the LOOP events resulted in reactor trip followed by the Emergency Diesel Generator (EDG) start. The majority of the reported LOOP events lasted for more than 2 minutes. Main lessons learned from the analysed events and potential actions for decrease of the number of LOOP events are presented.