Station blackout and nuclear safety

Abstract

The safety of the nuclear power plant depends on the availability of the continuous and reliable source of electrical energy during all modes of operation of the plant. The station blackout corresponds to a total loss of all alternate current (AC) power as a result of complete failure of both offsite and onsite AC power sources. The electricity for the essential systems during station blackout is provided from the batteries installed in the nuclear power plant. The results of the probabilistic safety assessment show that station blackout is one of the main and frequently the dominant contributor to the core damage frequency. The accident in Fukushima Daiichi nuclear power plants demonstrates the vulnerability of the currently operating nuclear power plants during extended station blackout events. The U.S. NRC Station Blackout Rule describes procedure for the assessment of the size and capacity of the batteries in the nuclear power plant. The description of the procedure with the application on the reference plant and identified deficiencies will be presented. The implications of the extension of the station blackout coping capability on the safety of the nuclear power plant will be analysed with state-of-the-art probabilistic and deterministic methods applied on reference models of the nuclear power plants. Obtained results show large decrease of core damage frequency with the extension of the station blackout coping capability. The extension of blackout coping capability results in delay of the core heat up for at least the extension interval. Operation of the steam driven feedwater system results in no core heat up for 72 hours, even in the presence of the reactor coolant pumps leakage. The main conclusions from the analysis and recommendations considering prolonged blackout of the nuclear power plant are given.