On the Estimation and Reduction of the Frequency of the Loss of Offsite Power Event

Loss of offsite power (LOOP) contributes significantly to the estimated core damage frequency (CDF) for the majority of nuclear power plants (NPPs). Based on experience from some near-miss events around the world and especially in Japan from Fukushima Daiichi accident and several other NPPs it was proven the how important power supply is for nuclear safety. Therefore comprehensive description of the LOOP initiating event is given in this paper. Specific experience of the NPP Krško (NEK) related operation is also presented with comparison to the U.S. NPPs’ approaches and experience. Statistical data are given to explain main characteristics (frequency and duration) of the LOOP event. Important historical trends are also identified. LOOP event frequency is often calculated using generic data because of statistical data base. Thereat, many important factors that influence LOOP are neglected, such as the specific power network configuration, load profiles, climate conditions, and ageing of the equipment. Several different approaches that take these factors into consideration are therefore presented and discussed. Familiarity with the actual state of the NPP, power network and weather conditions helps in the proper planning process of the test and maintenance (T&M) activities and reducing of the risk. This is significant for better online risk estimate. Due to the Fukushima accident, it is necessary to review the safety of the NPPs from new different angles. This paper also shortly reviews planned modifications in NEK that will increase the electric power supply availability (i.e., reduce LOOP event frequency. Other, worldwide solutions that can help to avoid or mitigate LOOP effects are briefly presented