Cooldown of a water-cooled reactor during the natural circulation mode using decay heat of the core and a low-power steam turbine

•Use of natural circulation and decay heat from the core of a water-cooled reactor.•Redundancy of NPP’s auxiliary needs.•Low power steam turbine in providing the NPP’s auxiliary needs during the blackout period.•Reducing economic consequences of nuclear reactor accidents. The Fukushima-1 nuclear power plant accident in 2011 confirms that NPP safety is one of the most urgent present-day problems. Moreover, a rapidly growing number of NPP units increases the risk of serious radiation accidents. The authors have developed a novel cooldown method for water-cooled reactors to be used under loss of connection with the power system using decay heat from the core and a low-power steam turbine. A distinctive feature of the method is financial viability of the steam turbine due to its possibility to generate additional electricity at the normal conditions of NPP operation. The aim of the invention is to ensure a high level of NPP safety while maintaining and increasing commercial efficiency and competitiveness of nuclear power plants. To implement the developed method, it is necessary to provide guarantee for reliability of NPP units at natural circulation of the primary coolant, since the primary circulation pumps will be shut off caused by a trip of a reactor. For this reason, the authors carried out an investigation into international practice theory and actual usage of natural circulation of water-cooled reactors. Experiments conducted at Kozloduy NPP and research of a number of scientists demonstrate a possibility to maintain constant decay heat removal that will ensure obtaining steam with stable parameters in the secondary circuit. The second part of the research deals with a new method aimed at beneficial usage of steam obtained by using the decay heat from the core under natural circulation of the primary coolant. The generated steam can be used in the additional low-power steam turbine to power supply the NPP's auxiliary needs. In our earlier research we performed a preliminary assessment of reliability of the developed system. The results of the research showed that a combination of a steam turbine and a standard three-channel emergency power supply system with diesel generators enables a safety level comparable to that of external heat exchangers in the passive heat removal systems. At the same time, passive heat removal systems require significant capital investments and maintenance costs, whereas according to the method developed by the authors, a low-