The IC (Isolation Condenser Cooling System) is part of the emergency core cooling system but provides an important safety role. This system provides cooling to the reactor when certain failures have happened, any one of these could cause the IC to be used:
1. Loss of access to the “ultimate heat sink” (the river, sea, or lake used for cooling).
2. Loss of AC electrical power.
Isolation condenser systems were used in some very early BWR reactors built in the 1960s. Unit 1 at Fukushima Daiichi has this type of emergency cooling system. It may have played a role in the rapid meltdown of unit 1, and raised questions about the ability of this system to function properly in a reactor accident.
“The IC system transfers residual and decay heat from the reactor coolant to the water in the shell side of the heat exchanger resulting in steam generation. The steam generated in the shell side of the heat exchanger is then vented to the outside atmosphere. The system employs natural circulation as the driving head from the reactor steam side, through the isolation condenser tubes, and back to the reactor.
The IC system is automatically initiated if a high reactor pressure condition is sustained for 15 seconds. The time delay prevents unnecessary system initiation during turbine trips. Also at most plants, the IC system automatically initiates on a low vessel water level to aid in reducing reactor pressure for small line breaks. The IC system is designed to provide core cooling regardless of whether electrical power is available.” source
Isolation Condenser System description from INL
Isolation Condenser System reliability report from INL
The Isolation Condenser system at unit 1 failed early and one train may not have worked at all. Additional information about this is available in the Unit 1 timeline.
In 2013 Japan’s nuclear regulator, NRA decided to inspect the IC system in Unit 1 themselves.
TEPCO tried to prevent this outside inspection and ultimately failed to do so.
Asahi Shimbun disclosed that TEPCO lied to the National Diet about issues at Unit 1 and the IC system.