These results are all based on a 120 second scram time where the isolation condenser return is into the upper plenum 44. The scram time longer than 30 seconds is not shown for isolation condenser return 36 into the downcomer 24 because it leads to an unacceptably high reactor pressure vessel 12 pressure greater than 14 MPa (-2000 psia). 

[0040] The power responses in Figure 6 show that the three trains of isolation condenser 30 are most effective in limiting the core power prior to the scram starting to become effective around 80 seconds. The amount of condensate from two isolation condenser trains 30 is not sufficient to limit the reactor pressure vessel 12 pressure increase as shown in Figure 8 so the increasing pressure collapses core voids and partially counters the full benefit of returning the isolation condenser 30 condensate to the upper plenum 44. When all four isolation condenser trains are used, the core flow is so suppressed that it reverses as shown in Figure 7 whereas three isolation condenser trains suppress the core flow but do not cause it to reverse. The addition of too much water into the upper plenum 44 (four isolation condenser trains) later cause a resurgence in core flow as it is heated, expands, and is circulated through the separator drains back to the downcomer 24. The subsequent flow surge will produce an unfavorable resurgence in core power as shown by the curve for four isolation condenser trains shown in Figure 6. The resurgence in core power will also result in an increase in reactor pressure vessel 12 pressure as shown by the curve for four isolation condenser trains in Figure 8. Note that the reactor pressure vessel peak pressure is lowest for three isolation condenser trains as shown by the curve in Figure 8. Together, Figure 5 

