1. A STARTUP BYPASS FLUID FLOW CONTROL SYSTEM FOR A STEAM GENERATOR HAVING FLUID HEAT ABSORPTION CONDUITS, A FLASH TANK, A FLOW CONTROLLED FLUID BYPASS CONDUIT FROM AN INTERMEDIATE POINT BETWEEN SAID HEAT ABSORPTION CONDUITS AND DISCHARGING TO SAID FLASH TANK, THE INVENTION COMPRISING A TWO COMPARTMENTED LOWER PRESSURE VESSEL SYSTEM, EACH COMPARTMENT HAVING A VAPOR AND WATER SIDE, CONDUIT MEANS FOR EQUALIZING THE VAPOR AND WATER SIDES OF BOTH COMPARTMENTS BY GRAVITY FOR MAINTAINING A COMMON VAPOR PRESSURE AND WATER LEVEL, MEANS THEREIN CONNECTED TO ONE OF SAID COMPARTMENTS AND ADAPTED TO CONDUCT DRAINS FROM SAID FLASH TANK TO SAID ONE COMPARTMENT, A HEAT SUMP, MEANS FOR FLOWING WATER FROM SAID ONE COMPARTMENT TO SAID HEAT SUMP, MEANS FOR RETURNING WATER FROM SAID HEAT SUMP TO THE OTHER COMPARTMENT TO MAINTAIN A SET LEVEL IN SAID LOWER PRESSURE VESSEL SYSTEM, A STEAM GENERATOR REGENERATIVE FEEDWATER CYCLE, AND A FLOW CONDUIT DISCHARGING WATER FROM THE SAID OTHER COMPARTMENT TO THE REGENERATIVE FEEDWATER CYCLE.