Patent Number: 
Section: claims

1. A safety injection tank (SIT) system pressurized by inert gas and steam, for injecting emergency core coolant into a reactor vessel of a nuclear reactor system comprising:a safety injection tank (SIT) into which the inert gas is charged and in which the emergency core coolant is housed, the SIT fluidly connected to the reactor vessel through an emergency coolant injecting pipe;a pressurizer in which the pressurized steam is housed, and on which a safety valve pipe is mounted to discharge the pressurized steam; anda pressure equalization pipe which is selectively openable and closable, and which connects an upper portion of the SIT and an upper portion of the pressurizer to equalize the pressure between the pressurizer and the SIT when the pressure equalization pipe is open, further wherein when the pressure equalization pipe is opened, the SIT shifts from a first pressure operation mode charged with the inert gas to a second pressure operation mode, wherein the second pressure operation mode has increased pressure over the first pressure operation mode provided by the pressurized steam during an accident that pressurizes the reactor system, so that the emergency core coolant is introduced into the reactor vessel. 2. The SIT system according to claim 1, further comprising an inert gas exhaust pipe which connects the upper portion of the SIT to the safety valve pipe, wherein the safety valve pipe includes a safety valve configured discharge the inert gas when the pressurized steam of the pressurizer is introduced into the safety injection tank. 3. The SIT system according to claim 1, wherein the inert gas is nitrogen gas and the first pressure is approximately 4.3 MPa, and the second pressure is approximately 17 MPa. 4. The SIT system according to claim 2, further comprising:an injection isolation valve mounted on the emergency coolant injection pipe;a pressure equalization valve mounted on the pressure equalization pipe; andan inert gas exhaust isolation valve mounted on the inert gas exhaust pipe, wherein the injection isolation valve, the pressure equalization valve, and the inert gas exhaust isolation valve are motor operated valves configured to be opened and closed by a separate battery power during station blackout event even when there is a loss of off-site and on-site power generation. 5. The SIT system according to claim 2, wherein the inert gas is nitrogen gas and the first pressure is approximately 4.3 MPa, and the second pressure is approximately 17 MPa. 6. The SIT system according to claim 4, wherein the inert gas is nitrogen gas and the first pressure is approximately 4.3 MPa, and the second pressure is approximately 17 MPa. 7. A safety injection tank (SIT) system pressurized by inert gas and steam, for injecting emergency core coolant into a reactor vessel of a nuclear reactor system comprising:a safety injection tank (SIT) into which the inert gas is charged and in which the emergency core coolant is housed, the SIT fluidly connected to the reactor vessel through an emergency coolant injection pipe, the SIT configured to shift between a plurality of emergency core coolant injection modes, the emergency core coolant injection modes including at least a first pressure operation mode and a second pressure operation mode,a pressurizer for storing the pressurized steam,a pressure equalization pipe comprising a pressure equalization valve that connects the SIT to the pressurizer, wherein the pressure equalization valve is selectively openable to equalize the pressure between the pressurizer and the SIT, further wherein opening of the pressure equalization valve causes the SIT to shift from the first pressure operation mode to the second pressure operation mode,wherein in the first pressure operation mode the inert gas charged SIT is configured to inject the emergency core coolant into the reactor system under a first pressure condition of the reactor system, and wherein in the second pressure operation mode, the pressure equalization valve is opened to provide increased pressure by the pressurized steam to the SIT to inject the emergency core coolant into the reactor system under a second pressure condition of the reactor system; andone or more valves, comprising at least the pressure equalization valve, wherein the one or more valves are driven by a backup emergency power source during a station blackout accident with a loss of off-site and on-site power including emergency generators of a nuclear power plant due to malfunction, to thereby enable injection of the emergency core coolant into the reactor system. 8. The SIT system according to claim 7, further comprising:an emergency coolant injecting pipe connecting the SIT to the reactor vessel;wherein the pressurizer comprises a safety valve pipe to discharge the pressurized steam; andwherein the SIT is configured to shift from the first pressure operation mode to the second pressure operation mode such that the emergency core coolant in the SIT is capable of being introduced into the reactor vessel even when the pressure in the reactor system exceeds the pressure of the SIT in the low pressure operation mode. 9. The SIT system according to claim 7, wherein the inert gas is nitrogen gas and the first pressure is approximately 4.3 MPa, and the second pressure is approximately 17 MPa. 10. The SIT system according to claim 8, further comprising an inert gas exhaust pipe which connects an upper portion of the SIT to the safety valve, wherein the safety valve is configured to selectively discharge inert gas when the pressurized steam of the pressurizer is introduced into the SIT. 11. The SIT system according to claim 8, wherein the inert gas is nitrogen gas and the first pressure is approximately 4.3 MPa, and the second pressure is approximately 17 MPa. 12. The SIT system according to claim 10 wherein the one or more valves comprise:an injection isolation valve mounted on the emergency coolant injection pipe;the pressure equalization valve mounted on the pressure equalization pipe; andan inert gas exhaust isolation valve mounted on the inert gas exhaust pipe, wherein the injection isolation valve, the pressure equalization valve, and the inert gas exhaust isolation valve are motor operated valves configured to be opened and closed during a station blackout event even when there is a loss of off-site and on-site power generation. 13. The SIT system according to claim 10, wherein the inert gas is nitrogen gas and the first pressure is approximately 4.3 MPa, and the second pressure is approximately 17 MPa. 14. The SIT system according to claim 12, wherein the inert gas is nitrogen gas and the first pressure is approximately 4.3 MPa, and the second pressure is approximately 17 MPa.