The removal of decay heat from a nuclear core can be accomplished by passive means using either an emergency heat exchanger or an isolation condenser (IC), depending on the system design. In some advanced pressurized water reactors (PWR), the emergency heat exchanger decay heat removal system consists of a closed loop that includes a shell and tube heat exchanger immersed in a large liquid pool that is elevated above the core. The relative elevation between the heat source and heat sink creates a buoyancy-driven natural circulation flow that eliminates the need for a pump. Decay heat is removed from the core by convective heat transfer from the fuel to the single-phase liquid in the reactor vessel. The heat stored in the liquid is carried by natural circulation to the emergency heat exchanger. Heat is transferred from the fluid through the emergency heat exchanger tubes into the pool by three mechanisms; single-phase convective heat transfer at the tube inside surface, heat conduction through the tube walls, and nucleate boiling at the tube outside surface. Some advanced PWRs use the steam generator as an intermediate emergency heat exchanger with a passively cooled, natural circulation feedwater loop.

Some advanced boiling water reactors (BWR) use isolation condensers as the means of removing core decay heat. The IC consists of a shell and tube heat exchanger immersed in a large liquid pool elevated above the core. In a BWR, core decay heat is removed by nucleate boiling. The steam generated by this process is condensed inside the IC tubes creating a low pressure region inside the tubes which draws in additional steam. Thus the driving mechanism for the flow is steam condensation. Heat is transferred through the IC tubes into the pool by three mechanisms; single-phase steam condensation (phase change) at the tube inside surface, heat conduction through the tube walls, and convective heat transfer at the tube outside surface. The condensate is returned as a single-phase liquid to the reactor vessel by gravity draining. Performance of the IC can be affected by the presence of non-condensable gases.

The following is a listing of the emergency heat exchanger related local phenomena:

• emergency heat exchanger loop flow resistance

• Buoyancy force

• Single-phase convective heat transfer

• Shell-side nucleate boiling heat transfer

The following is a listing of the IC related local phenomena:

• IC loop flow resistance

• Low pressure steam condensation

• Condensation heat transfer in the presence of non-condensable gases

• Shell-side convection heat transfer

• Condensate/steam countercurrent flow limitations