On the possibility of cooling the high-power VVER reactor pressure vessel using an air-droplet spray in the event of a beyond design basis accident

For reactors with high decay power (VVER-1000, VVER-1200, VVER-TOI, etc.), no reliable reactor vessel cooling is achieved via the concept of retaining the corium inside the reactor pressure vessel only by means of heat transfer to water in the process of its boiling within the reactor vessel vault since high heat fluxes (density above 10 6 W/m 2 ) cause boiling evolving into film boiling and to a heat transfer crisis. A comparative analysis of different cooling systems shows that the most reliable and effective way to prevent the heat transfer crisis is an intensification of heat transfer on the cooled surface by using an air-droplet spray device (ADSD), which can be used to retain the core melt inside the reactor pressure vessel. The paper provides an overview of studies on gas-droplet cooling of high-temperature surfaces, and presents preliminary calculations using correlations of the non-wetting cooling mode as being of greatest interest for the ADSD. Methods have been considered for simulating gas-droplet cooling in system codes. The qualitative agreement of the calculation results with experimental data is shown. The peculiarities of spray cooling of the large VVER reactor vessel are identified. A diagram of the possible locations of spray nozzles within the reactor vessel vault is demonstrated. Due to the lack of a single verified spray cooling model for high-temperature surfaces, experimental studies are considered to be the key approach in the scientific problem under consideration, that is cooling of the large VVER reactor vessel in the event of a beyond design basis accident.