Methods for removing decay or residual heat from a reactor core can be grouped into two general categories:
Closed Loop System. One category includes methods which circulate fluid through the reactor core in a closed loop, using some type of heat exchanger to transfer heat out of the system.
Heat Removal through Steam Generators.Steam generators can be and are usually used to remove decay heat from a reactor. They dissipate heat directly to the main condenser or via the atmospheric steam dump system. In this operational mode a some of reactor coolant pumps (RCPs) must be usually in operation or natural circulation must be remained.
Heat Removal through RHR. The residual heat removal system (RHR) is a low-pressure heat removal system. In PWRs, it takes water from one or two RCS hot legs, cools it in RHR heat exchangers, and pumps it back to the cold legs or core flooding tank nozzles. To accomplish RHR heat removal, RHR heat exchangers transfer heat to the component cooling water or service water system, which then transports heat to the ultimate heat sink (UHS).
Open System: The other category includes methods which operate in an open system, drawing in cool fluid from some source and discharging warmer fluid to some storage area or the environment.
Decay heat removal using feed-and-bleed method. The feed-and-bleed method for decay heat removal becomes necessary method in some pressurized water reactors (PWRs) if there is a loss of steam generator heat sink capability. The feed-and-bleed technique involves passing hot coolant out of the primary system through apilot-operated relief valve (PORV), while simultaneously feeding the primary system with subcooled coolant through high pressure safety injection system (HPSI), which is part of the emergency core cooling system.
Nuclear and Reactor Physics:
J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.