4 whereas isolation condenser return 36 into the downcomer 24 shows no core flow decrease until about 25 seconds when the control blades began to reduce core power. Isolation condenser return 40 to the upper plenum 44 suppresses core flow earlier by increasing the density of the fluid in the upper plenum region 44. Isolation condenser return 36 to the downcomer 24 on the other hand is on the opposite side of the manometer which would tend to increase core flow although this effect appears to be insignificant. The impact that the cold isolation condenser condensate water addition to the downcomer 24 has on the core inlet enthalpy is shown in Figure 5. The decrease in core inlet enthalpy reduces voids in the core 20 and causes the power to increase. Cold water into the upper plenum 44 has no initial impact on the core inlet enthalpy but later as the water is heated by steam in the upper plenum 44 causes a surge in separator drain flow and a temporary increase in steam carry under which causes the momentary increase in core inlet enthalpy in the timeframe from 20 to 50 seconds. This increase in core inlet enthalpy helps to prevent an increase in core power long enough for the scram to become effective. The amount of condensate return is important in determining the core power, flow and enthalpy responses. [0039] The first set of results depicted in Figure 2 through Figure 5 were shown for the 3 out of the 4 isolation condenser trains functioning so that failure of one isolation condenser train could be assumed for safety analysis. A second set of results in Figure 6 through Figure 8 shows how the number of operating isolation condenser trains impact the calculated reactor pressure vessel pressure, core power, and core flow responses.