Reducing the loss of offsite power contribution in the core damage frequency of a VVER-1000 reactor by extending the house load operation period

The availability of alternating current power via the electrical grid is essential for safe operation and accident recovery of nuclear power plants (NPP). Loss of offsite power (LOOP), as an initiating event, contributes more than 26 percent to the core damage frequency (CDF) of generation II reactors. The LOOP event dramatically affects plant operations because it influences the mitigation responses by placing demands on the onsite power systems. The objective of this paper is the improvement of NPP stability against LOOP accident by utilizing the probabilistic safety assessment (PSA) methodology. In order to ameliorate the value of CDF due to LOOP, it is possible to modify the normal process of supplying electrical power to the crucial safety operations. The ability of a NPP to reduce power to house load operations allows the reactor to stay at power during temporary grid issues. House load operation is usually acceptable only for 30-40 min because of the poor quality of the generated steam. In this paper, by proposing a reheating system implemented on the steam collector of a VVER-1000 reactor, the house load operation is extended for a considerable period of time as a result of enhancing the steam quality. The obtained PSA results show significant decrease of the CDF, indicating improvement of NPP safety against LOOP.