Patent Number: 051280930
Section: claims

1. A hydraulic system for a control rod drive (CRD) comprising: a variable output-pressure CRD pump operable in a charging mode for providing a pressurized fluid at a charging pressure, and in a normal mode for providing said pressurized fluid at a purge pressure being lower than said charging pressure;  a charging line disposed in flow communication between said CRD pump and said CRD for channeling therethrough a first portion of said pressurized fluid as scram fluid, said charging line including an isolation valve;  a purge line disposed in flow communication between said CRD pump and said CRD, and in parallel flow with said charging line, for channeling therethrough a second portion of said pressurized fluid as purge fluid;  a hydraulic control unit (HCU) disposed in flow communication in said charging line between said isolation valve and said CRD and including in series flow in said charging line an HCU check valve and a normally closed scram valve, and a scram accumulator disposed therebetween for accumulating said scram fluid at said charging pressure during said charging mode, said HCU check valve being effective for checking backflow of said scram fluid upstream toward said isolation valve;  pressurizing means disposed in flow communication with said charging line downstream of said isolation valve and upstream of said scram valve for maintaining said scram fluid at a scram pressure greater than a predetermined minimum scram pressure between said purge pressure and said charging pressure when said CRD pump is in said normal mode;  a pressure sensor operatively connected to said charging line and responsive to said scram pressure;  control means operatively connected to said CRD pump, said isolation valve, and said pressure sensor, and being effective for:  a motor operatively connected to said CRD pump;  an adjustable speed drive operatively connected to said motor and said control means; and  said adjustable speed drive being effective for operating said CRD pump at a first speed for obtaining said purge pressure in said normal mode, and at a second speed, greater than said first speed, for obtaining said charging pressure in said charging mode.  a plurality of said CRDs;  said charging line including a charging fluid header disposed downstream from said isolation valve for receiving said scram fluid therefrom;  said purge line including a purge fluid header disposed downstream from said CRD pump, and a plurality of branches disposed in parallel flow communication between said purge fluid header and respective ones of said CRDs for channeling said purge fluid thereto; and  a plurality of said HCUs disposed in parallel flow communication between said charging fluid header and respective ones of said CRDs.  a variable output-pressure CRD pump operable in a charging mode for providing said pressurized fluid at a charging pressure, and in a normal mode for providing said pressurized fluid at a purge pressure being lower than said charging pressure;  a charging line disposed in flow communication between said CRD pump and said CRD for channeling therethrough a first portion of said pressurized fluid as scram fluid, said charging line including an isolation valve;  a purge line disposed in flow communication between said CRD pump and said CRD, and in parallel flow with said charging line, for channeling therethrough a second portion of said pressurized fluid as purge fluid;  a hydraulic control unit (HCU) disposed in flow communication in said charging line between said isolation valve and said CRD and including in series flow in said charging line an HCU check valve and a normally closed scram valve, and a scram accumulator disposed therebetween for accumulating said scram fluid at said charging pressure during said charging mode, said HCU check valve being effective for checking backflow of said scram fluid upstream toward said isolation valve;  said method comprising the steps of:  in said normal mode:  in said charging mode: 2. A CRD hydraulic system according to claim 1 wherein said pressurizing means include a pressurizing pump having a smaller power requirement than said CRD pump and effective for channeling into said charging line during said normal mode makeup fluid to offset leakage of said scram fluid from said charging line. 3. A CRD hydraulic system according to claim 1 wherein said pressuring means include a makeup accumulator for accumulating a portion of said scram fluid as makeup fluid at said charging pressure in said charging mode, and in said normal mode channeling back into said charging line said makeup fluid to offset leakage of said scram fluid from said charging line for maintaining said scram fluid at least at said minimum scram pressure. 4. A CRD hydraulic system according to claim 3 further including: 5. A CRD hydraulic system according to claim 4 further including: 6. A method of providing a pressurized fluid to a control rod drive (CRD) in a hydraulic system including: 7. A method according to claim 6 further including the step of opening said scram valve in a scram mode to allow said scram fluid stored in said scram accumulator to enter said CRD.