Patent Number: 
Section: claims

1. A cooling system of an emergency cooling tank, comprising:an emergency cooling tank configured to store cooling water therein, the cooling water receiving heat, transferred from a nuclear reactor or a containment, when an accident occurs in the nuclear reactor;a heat exchanging device installed to be exposed to an outside of the emergency cooling tank to operate in air, and configured to externally emit heat by way of a heat exchange between a fluid within the emergency cooling tank and the air such that the operation of the emergency cooling tank is continued even without refilling the cooling water; andan opening and closing unit installed at the emergency cooling tank to be located higher than a water level of the cooling water for emitting the fluid, and configured to be open by a flow of the fluid generated by an evaporation of the cooling water, the flow being formed due to a pressure difference from external air at pressure higher than a preset pressure, such that some of the fluid is externally emitted when a heat load exceeding a cooling capacity of the emergency cooling tank is transferred, whereinthe heat exchanging device is designed to have a cooling capacity that is smaller than a heat load transferred to the emergency cooling at an early stage of the accident and greater than a heat load transferred to the emergency cooling tank at a later stage of the accident,the opening and closing unit is configured to be passively opened when a heat load exceeding the cooling capacity of the heat exchanging device is transferred to the emergency cooling tank at the early stage of the accident, andthe opening and closing unit is configured to be passively closed when a heat load transferred to the emergency cooling tank drops below the cooling capacity of the heat exchanging device at the later stage of the accident,the heat exchanging device is installed in an upper side of the emergency cooling tank and comprises:a duct installed on an uppermost wall of the emergency cooling tank and extending upwardly to provide an upward flow path to steam generated by an evaporation of the cooling water within the emergency cooling tank;a heat exchanging portion configured to perform the heat exchange with air to cool and condense the steam introduced through the duct, the heat exchanging portion being connected to the emergency cooling tank to collect condensed fluid which has lost heat and flows down due to a density difference, andan air circulating unit installed vertically above and on top of the emergency cooling tank such that the uppermost wall of the emergency cooling lank is shared with a lowermost wall of the air circulating unit, the air circulating unit allowing air introduced through a lower side portion thereof to flow up therealong so as to increase a heat exchange rate of the heat exchanging device by natural convection. 2. The cooling system of claim 1, wherein the heat exchanging portion has at least part formed in a straight pipe, a curved pipe, or in a helical shape. 3. The cooling system of claim 1,wherein the air circulating unit is installed in a manner of covering at least part of the heat exchanging device. 4. The cooling system of claim 1, wherein the opening and closing unit is implemented as a type of check valve or flap valve, which is passively open at pressure higher than a preset pressure formed by the fluid within the emergency cooling tank. 5. The cooling system of claim 1, wherein the opening and closing unit prevents the emission of steam when a heat load transferred to the emergency cooling tank is reduced below the cooling capacity of the emergency cooling tank, and is passively closed at pressure lower than a preset pressure to maintain a quantity of the cooling water of the emergency cooling tank. 6. A nuclear power plant, comprising:a passive containment cooling system configured to condense steam discharged from a nuclear reactor into a containment to prevent an increase in pressure of the containment when an accident occurs in the nuclear reactor; andan emergency cooling tank cooling system configured to receive sensible heat and residual heat of the nuclear reactor, transferred from the passive containment cooling system, and externally emit the received heat,wherein the emergency cooling tank cooling system comprises:an emergency cooling tank configured to store cooling water therein, the cooling water receiving heat, transferred from a nuclear reactor or a containment, when an accident occurs in the nuclear reactor;a heat exchanging device installed at an outside of the emergency cooling tank in an exposed manner to operate in the air, and configured to externally emit heat by way of a heat exchange between fluid within the emergency cooling tank and the air such that the operation of the emergency cooling tank is continued even without refilling the cooling water; andan opening and closing unit installed at the emergency cooling tank to be located higher than a water level of the cooling water for emitting the fluid, and configured to be open by a flow of the fluid generated by an evaporation of the cooling water, the flow being formed due to a pressure difference from external air at pressure higher than a preset pressure, such that some of the fluid is externally emitted when a heat load exceeding a cooling capacity of the emergency cooling tank is transferred, whereinthe heat exchanging device is designed to have a cooling capacity that is smaller than a heat load transferred to the emergency cooling at an early stage of the accident and greater than a heat load transferred to the emergency cooling tank at a later stage of the accident,the opening and closing unit is configured to be passively opened when a heat load exceeding the cooling capacity of the heat exchanging device is transferred to the emergency cooling tank at the early stage of the accident, andthe opening and closing unit is configured to be passively closed when a heat load transferred to the emergency cooling tank drops below the cooling capacity of the heat exchanging device at the later stage of the accident,the heat exchanging device is installed in an upper side of the emergency cooling tank and comprises:a duct installed on an uppermost wall of the emergency cooling tank and extending upwardly to provide an upward flow path to the fluid within the emergency cooling tank;a heat exchanging portion configured to perform the heat exchange with air to cool or condense the fluid introduced through the duct, the heat exchanging portion being connected to the emergency cooling tank to collect the fluid which has lost heat and flows down due to a density difference, andan air circulating unit installed vertically above and on top of the emergency cooling tank such that the uppermost wall of the emergency cooling tank is shared with a lowermost wall of the air circulating unit, the air circulating unit allowing air introduced through a lower side portion thereof to flow up therealong so as to increase a heat exchange rate of the heat exchanging device by natural convection. 7. A nuclear power plant, comprising:a passive residual heat removal system configured to remove sensible heat and residual heat of a nuclear reactor by circulating cooling water when an accident occurs in the nuclear reactor; andan emergency cooling tank cooling system configured to receive sensible heat and residual heat of the nuclear reactor, transferred from the passive residual heat removal system, and externally emit the received heat,wherein the emergency cooling tank cooling system comprises:an emergency cooling tank configured to store cooling water therein, the cooling water receiving heat, transferred from a nuclear reactor or a containment, when an accident occurs in the nuclear reactor;a heat exchanging device installed at an outside of the emergency cooling tank in an exposed manner to operate in the air, and configured to externally emit heat by way of a heat exchange between fluid within the emergency cooling tank and the air such that the operation of the emergency cooling tank is continued even without refilling the cooling water; andan opening and closing unit installed at the emergency cooling tank to be located higher than a water level of the cooling water for emitting the fluid, and configured to be open by a flow of the fluid generated by an evaporation of the cooling water, the flow being formed due to a pressure difference from external air at pressure higher than a preset pressure, such that some of the fluid is externally emitted when a heat load exceeding a cooling capacity of the emergency cooling tank is transferred, whereinthe heat exchanging device is designed to have a cooling capacity that is smaller than a heat load transferred to the emergency cooling at an early stage of the accident and greater than a heat load transferred to the emergency cooling tank at a later stage of the accident,the opening and closing unit is configured to be passively opened when a heat load exceeding the cooling capacity of the heat exchanging device is transferred to the emergency cooling tank at the early stage of the accident, andthe opening and closing unit is configured to be passively closed when a heat load transferred to the emergency cooling tank drops below the cooling capacity of the heat exchanging device at the latter stage of the accident,the heat exchanging device is installed in an upper side of the emergency cooling tank and comprises:a duct installed on an uppermost wall of the emergency cooling tank and extending upwardly to provide an upward flow path to the fluid within the emergency cooling tank;a heat exchanging portion configured to perform the heat exchange with air to cool or condense the fluid introduced through the duct, the heat exchanging portion being connected to the emergency cooling tank to collect the fluid which has lost heat and flows down due to a density difference, andan air circulating unit installed vertically above and on top of the emergency cooling tank such that the uppermost wall of the emergency cooling tank is shared with a lowermost wall of the air circulating unit, the air circulating unit allowing air introduced through a lower side portion thereof to flow up therealong so as to increase a heat exchange rate of the heat exchanging device by natural convection.