It is a passive system for cooling of some reactors (BWR/2, BWR/3 ..., and the (E)SBWR series) in nuclear production, located above containment in a pool of water open to atmosphere.
This process keeps the cooling water in the reactor, making it unnecessary to use powered feedwater pumps.
The water in the open pool slowly boils off, venting clean steam to the atmosphere.
During the accident at the Fukushima nuclear plant in 2011, the operators did not open the valve manually, and emergency system had been activated too late and could not work for long.
Operators did not know if they should have left the valves open or not when the tanks of two condensers were emptied of their water cooling.