Patent Number: 
Section: claims

1. A multi stage safety injection device, comprising:a safety injection tank formed to contain coolant to be injected into a reactor vessel by a gravitational head of said coolant when an accident occurs in which the pressure or water level of the reactor vessel is decreased;a pressure balance line connected to the reactor vessel and to the safety injection tank to provide a pressure balance between the reactor vessel and the safety injection tank; anda set of safety injection lines connected to the safety injection tank at different heights that connect the safety injection tank and the reactor vessel to inject coolant to the reactor vessel in a pressure balance state between the reactor vessel and the safety injection tank, thereby passively reducing the flow rate of coolant injected into the reactor vessel step by step according to the coolant level reduction in the safety injection tank; anda plurality of orifices configured to passively form a total flow resistance being increased step by step according to the coolant level reduction of the safety injection tank to decrease the flow rate of coolant injected into the reactor vessel,wherein the pressure balance line is always maintained in an open state to maintain the pressure balance between the reactor vessel and the safety injection tank,wherein the plurality of orifices comprise:a first orifice configured to passively form a flow resistance to decrease the flow rate of coolant injected into the reactor vessel through the first safety injection line, anda second orifice configured to passively form a flow resistance to decrease the flow rate of coolant injected into the reactor vessel through the second safety injection line, and wherein the set of safety injection lines comprise:a first safety injection line connected to a lower end part of the safety injection tank to continuously provide an injection passage for coolant filled within the safety injection tank into the reactor vessel, anda second safety injection line connected to the safety injection tank at a location higher by a predetermined height from the first safety injection line to provide an injection passage for coolant until the coolant level of the safety injection tank becomes lower than said predetermined height. 2. The multi stage safety injection device of claim 1, further comprising:wherein the first safety injection line and the second safety injection line are merged together outside the safety injection tank,wherein the safety injection lines comprise branch lines disposed between the reactor vessel and a merged portion of the first safety injection line and the second safety injection line, andthe multi stage safety injection device further comprises an isolation valve installed in each of the branch lines to block the flowing of coolant from the safety injection tank to the reactor vessel in a pressure balanced state with the reactor vessel during a normal plant operation, and designed to be opened by a control signal generated from the pressure or water level reduction of the reactor vessel to implement coolant injection from the safety injection tank to the reactor vessel when an accident occurs. 3. A passive safety injection system, comprising:a core makeup tank connected to a reactor vessel to maintain a pressure balance state with the reactor vessel and inject coolant to the reactor vessel when an accident occurs in which the pressure or water level of the reactor vessel is decreased; anda multi stage safety injection device connected to the reactor vessel to inject coolant by passively reducing the flow rate of injection step by step to the reactor vessel at a pressure lower than that of the core makeup tank following the injection from the core makeup tank,wherein the multi stage safety injection device comprises:a safety injection tank formed to contain coolant to be injected into a reactor vessel by a gravitational head of said coolant when an accident occurs in which the pressure or water level of the reactor vessel is decreased;a pressure balance line connected to the reactor vessel and the safety injection tank to form a pressure balance between the reactor vessel and the safety injection tank; anda set of safety injection lines connected to the safety injection tank at different heights that connect the reactor vessel to inject coolant to the reactor vessel in a pressure balance state between the reactor vessel and the safety injection tank to reduce a flow rate of coolant injected into the reactor vessel step by step according to the coolant level reduction of the safety injection tank; anda plurality of orifices configured to passively form a total flow resistance being increased step by step according to the coolant level reduction of the safety injection tank to decrease the flow rate of coolant injected into the reactor vessel,wherein the pressure balance line is always maintained in an open state to maintain the pressure balance between the reactor vessel and the safety injection tank,wherein the plurality of orifices comprise:a first orifice configured to passively form a flow resistance to decrease the flow rate of coolant injected into the reactor vessel through the first safety injection line, anda second orifice configured to passively form a flow resistance to decrease the flow rate of coolant injected into the reactor vessel through the second safety injection line, and wherein the set of safety injection lines comprise:a first safety injection line connected to a lower end part of the safety injection tank to continuously provide an injection passage for coolant filled within the safety injection tank into the reactor vessel, anda second safety injection line connected to the safety injection tank at a location higher by a predetermined height from the first safety injection line to provide an injection passage for coolant until the coolant level of the safety injection tank becomes lower than said predetermined height. 4. The multi stage safety injection device of claim 1, wherein the flow rate through the first orifice and the flow rate through the second orifice are passively limited, andwherein the first safety injection line and the second injection line contain no isolation valves configured to be open by a signal generated from the pressure or water level reduction of the reactor vessel. 5. The multi stage safety injection device of claim 3, wherein the flow rate through the first orifice and the flow rate through the second orifice are passively limited, andwherein the first safety injection line and the second injection line contain no isolation valves configured to be open by a signal generated from the pressure or water level reduction of the reactor vessel. 6. The multi stage safety injection device of claim 1, further comprising at least one check valve in a portion of the safety injection lines which the first safety injection line and the second safety injection line are merged. 7. The multi stage safety injection device of claim 3, further comprising at least one check valve in a portion of the safety injection lines which the first safety injection line and the second safety injection line are merged.