the core into the upper plenum or chimney region of the boiling water reactor. 

[001 1] The isolation condenser system lowers reactor pressure vessel pressure following an isolation event. Any increase in core power that results from the condensate return into the downcomer region of the reactor pressure vessel reduces the effectiveness of the isolation condenser system. The reactor pressure vessel pressure increase can be unacceptably large especially when the isolation event occurs concurrent with a failure or delay in scram (reactor shutdown) such as in an anticipated transient without scram. Return of the isolation condenser system condensate into the region above the core, on the other hand, dramatically increases the effectiveness of the isolation condenser system and substantially lowers the peak pressure that will occur. This lowering of the pressure provides additional time for an alternate slower scram mechanism to function during an anticipated transient without scram scenario where the faster primary scram mechanism has failed or was severely degraded. 

[0012] The isolation condenser system condensate is returned to the reactor pressure vessel of the boiling water reactor inside the core barrel above the core rather than into the downcomer. This increases isolation condenser system effectiveness and makes it possible to greatly reduce the magnitude of the pressure increase that would otherwise occur consequently, the opening of safety relief valves can be avoided which retains fluid inventory in the reactor pressure vessel. The need for high pressure emergency core cooling systems can be eliminated because the 

