Facility for reducing radioactive material and nuclear power plant having the same

The present invention provides a facility for reducing radioactive material comprising: a cooling water storage unit installed inside a containment and formed to store cooling water; a boundary unit forming a boundary of radioactive material inside the containment and surrounding a reactor coolant system installed inside the containment to prevent a radioactive material from releasing from the reactor coolant system or a pipe connected with the reactor coolant system to the containment; a connecting pipe connected with an inner space of the boundary unit and the cooling water storage unit to guide a flow of a fluid caused by a pressure difference between the boundary unit and the cooling water storage unit from the boundary unit to the cooling water storage unit; and a sparging unit disposed to be submerged in the cooling water stored in the cooling water storage unit and connected with the connecting pipe to sparge the fluid that has passed through the connecting pipe and the radioactive material contained in the fluid to the cooling water storage unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(a), this application claims the benefit of earlier filing date and right of priority to Korean Application No. 10-2013-0102649, filed on Aug. 28, 2013; Korean Application No. 10-2014-0036321, filed on Mar. 27, 2014; Korean Application No. 10-2014-0083848, filed on Jul. 4, 2014, the contents of which are incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Disclosure

This specification relates to a safety system for securing safety of a nuclear power plant, and in particular, to a facility that may decrease the concentration of a radioactive material in a containment by a passive principle when an accident occurs in the nuclear power plant and a nuclear power plant having the same.

2. Background of the Disclosure

Depending on the position of installation, nuclear reactors are classified into loop-type reactors (e.g., commercial reactors, Korea) with main components (steam generators, a pressurizer, reactor coolant pumps, etc.) installed outside the reactor vessel and integral reactors (e.g., SMART reactor, Korea) with the main components installed in the reactor vessel.

Further, nuclear reactors are classified into active reactors and passive reactors depending on how the safety system is implemented. The active reactors are reactors that use an active component, such as a pump, which is powered by an emergency diesel generator in order to operate the safety system, and the passive reactors are reactors that use a passive component which is powered by a passive force such as gravity or gas pressure in order to operate the safety system. In the passive reactors, the passive safety system may safely maintain the reactors only with a natural force embedded in the system without a safety-grade AC power source such as an emergency diesel generator or an operator's action at least for a time (72 hours) required by the regulations when an accident occurs, and after 72 hours, the passive safety system may be treated by the operator or assisted by a non-safety system.

A containment (containment building, reactor building, containment vessel or safeguard vessel) that plays a role as a final protection barrier to prevent radioactive materials from releasing to the external environment are classified into the containment building (or reactor building) formed of reinforced concrete and the containment vessel and safeguard vessel formed of steel depending on the material constituting a pressure boundary. The containment vessel is a large vessel that is designed to have a low pressure like the containment building, and the safeguard vessel is a small vessel designed to be rendered to have a small size and having the higher design pressure. Unless mentioned specially, as used herein, the terms “containment building,” “reactor building,” “containment vessel,” or “safeguard vessel” are collectively referred to as a containment.

Various forms of active and passive systems, such as a containment spray system, a containment cooling system, a suppression tank or suppression pool, are put to use in order to decrease the density of radioactive material, the pressure and temperature in the containment at accidents. Hereinafter, such facilities are described below one by one.

The active containment spray system (Korean commercial reactor, SMART reactor, etc.) sprays a large amount of cooling water using containment spray pumps when an accident occurs, recollects the cooling water to an in-containment refueling water storage tank or sump, and re-sprays the cooling water to decrease the pressure and temperature of the containment and the concentration of radioactive material for a long time. The active containment spray system may perform a long-term spraying function and requires a power system to be available for activating the pumps.

The passive containment spray system (Canadian CANDU, etc.) has a cooling water storage tank at an upper side of the containment and sprays a large amount of cooling water when an accident takes place to decrease the pressure and temperature inside the containment and the concentration of the radioactive material. Since the passive containment spray system has a limited storage capacity of cooling water, and thus, cannot be operated more than a predetermined time. Accordingly, the cooling water storage tank needs to be periodically made up using a pump for long-term use of the passive containment spray system. This means that the passive containment spray system also needs to use a pump and a power system for activating the pump in order for a long-term operation.

The suppression tank (commercial BWR, CAREM: Argentina, IRIS: Westinghouse, U.S. et. al.) guides the steam discharged into the containment to the suppression tank using a difference in pressure between the containment and the inside of the suppression tank and condenses the steam to decrease the pressure and temperature in the containment and the concentration of the radioactive material. The suppression tank operates only when the pressure in the containment is higher than the pressure in the suppression tank.

The passive containment cooling system has heat exchangers and a cooling water tank installed in or outside the containment and condenses the steam in the containment using the heat exchangers to decrease the pressure and temperature in the containment and the concentration of the radioactive material. The passive containment cooling system uses the natural circulation in the containment and thus has a lower performance in reducing the pressure and temperature and concentration of radioactive material as compared with the active containment spray system.

Besides, there is a sort of passive containment cooling system (AP1000: Westinghouse, U.S.) that applies a steel containment vessel to cool (spray, air) the external wall and that condenses the steam in the containment vessel on the internal wall of the containment vessel to thus decrease the pressure and temperature in the containment vessel and the concentration of radioactive material. This system uses the natural circulation in the containment similarly to the passive containment cooling system and thus shows a relatively low performance in reducing pressure and temperature and the concentration of radioactive material as compared with the active containment spray system.

Most of the above-described systems show a relatively excellent performance in decreasing the pressure and temperature inside the containment. However, among the radioactive materials that may spread to the external environment when an accident occurs in the nuclear power plant, iodine may have a highest proportion of concentration. Iodine, when contacts water, is mostly dissolved in the water (solubility 0.029 g/100 g(20° C.)). Accordingly, among the containment-related safety systems, the active containment spray system (which is adopted for the Korean commercial reactors), which uses an active pump to spray a great amount of cooling water and to recirculate the cooling water for a long time, may show the most excellent performance in decreasing the concentration of radioactive material in the containment. However, the active safety system necessarily requires supply of emergency AC power for operating the active components such as pumps when an accident occurs in the nuclear power plant, and without supply of emergency AC power, does not operate.

In this point of view, demand for the passive safety system with relatively high safety is on the rise. This is why the passive safety system does not require a power system nor continuous operation of the active components. However, in case the passive safety system is adopted as safety system of the containment, the concentration of radioactive material in the containment would be relatively higher due to a lower performance in containment cooling as compared with the active safety system.

Further, an exclusion area boundary (EAB) is set for the public safety to restrict the public access in preparation for an accident that may occur in the nuclear power plant. In case the passive safety system is applied to the nuclear power plant, the safety of nuclear power plant may be increased relatively further than the active safety system is applied, but it needs to secure a relatively broader EAB. The expansion of EAB may result in a significantly increased cost of constructing the nuclear power plant.

Accordingly, an increasing need exists for a facility for reducing radioactive materials, which allows for application of a passive safety system to enhance the safety of nuclear power plant by resolving the problem of an expanding EAB.

SUMMARY OF THE INVENTION

Therefore, an aspect of the detailed description is to provide a facility for reducing radioactive material in a containment, which may contribute to increasing safety of a nuclear power plant. In particular, an aspect of the detailed description proposes a facility for reducing radioactive material, which may reduce the concentration of radioactive material that is discharged in the containment when an accident occurs in the nuclear power plant.

Another aspect of the detailed description is to provide a facility for reducing radioactive material which is configured to suppress an increase in the number of valves that may occur due to an introduction thereof and to prevent re-volatilization of radioactive material and a nuclear power plant having the same.

Still another aspect of the detailed description is to provide a facility for reducing radioactive material, which may resolve the problem of an increasing EAB that may be caused as a passive safety system is adopted in a nuclear power plant and a nuclear power plant having the same.

To achieve these and other advantages and in accordance with the purpose of this specification, as embodied and broadly described herein, there is provided a facility for reducing radioactive material. The facility comprises a cooling water storage unit installed inside a containment and formed to store cooling water; a boundary unit forming a boundary of radioactive material inside the containment and surrounding a reactor coolant system installed inside the containment to prevent a radioactive material from releasing from the reactor coolant system or a pipe connected with the reactor coolant system to the containment; a connecting pipe connected with an inner space of the boundary unit and the cooling water storage unit to guide a flow of a fluid caused by a pressure difference between the boundary unit and the cooling water storage unit from the boundary unit to the cooling water storage unit; and a sparging unit disposed to be submerged in the cooling water stored in the cooling water storage unit and connected with the connecting pipe to sparge the fluid that has passed through the connecting pipe and the radioactive material contained in the fluid to the cooling water storage unit.

According to an embodiment of the present invention, the cooling water storage unit may include an inlet through which the connecting pipe passes, and the highest part of the connecting pipe may be formed at a predetermined height from a bottom of the cooling water storage unit to prevent the cooling water stored in the cooling water storage unit from flowing back to an inside of the boundary unit.

According to another embodiment of the present invention, the facility may further comprise a check valve formed to allow for a flow only in one direction and installed at the connecting pipe to prevent the cooling water in the cooling water storage unit from flowing back to the boundary unit through the connecting pipe.

To achieve these and other advantages and in accordance with the purpose of this specification, as embodied and broadly described herein, there is provided a facility for reducing radioactive material. The facility comprises a boundary unit forming a boundary of a radioactive material inside a containment and surrounding a reactor coolant system installed inside the containment to prevent the radioactive material from releasing from the reactor coolant system or a pipe connected with the reactor coolant system to the containment; a discharging unit installed at the boundary of the radioactive material to form a fluid path that runs from the boundary unit to the containment and configured to guide a flow of a fluid caused by a pressure difference between the containment and the boundary unit from the containment to the boundary unit through the fluid path; and a filter facility installed in the fluid path of the discharging unit to capture the radioactive material contained in the fluid passing through the discharging unit in the boundary unit.

According to an embodiment of the present invention, at least a portion of the boundary unit may be expanded to a region adjacent to the containment while surrounding a penetration pipe penetrating the containment to prevent a loss-of-coolant accident from occurring due to breakage of the penetration pipe in a region between the containment and the boundary unit.

According to another embodiment of the present invention, the boundary unit may form a sealing structure around the reactor coolant system to prevent release of the radioactive material.

According to another embodiment of the present invention, at least a portion of the boundary unit may be formed by a concrete structure inside the containment or a coating member installed on the concrete structure.

According to another embodiment of the present invention, the boundary unit may comprise a barrier formed to surround the reactor coolant system; and a cover formed to cover an upper part of the reactor coolant system and coupled with the barrier.

According to another embodiment of the present invention, the filter facility may comprise at least one of: a filter configured to form iodic silver by reacting silver nitrate with iodine contained in the fluid and formed to remove the iodic silver from the fluid; and an absorbent configured to remove the iodine contained in the fluid through chemisorption that is performed by charcoal.

According to another embodiment of the present invention, the facility may further comprise a cooling water storage unit installed inside the containment, the cooling water storage unit formed to store cooling water for dissolving the radioactive material.

According to another embodiment of the present invention, the discharging unit may be extended from the boundary unit to an inside of the cooling water storage unit to discharge the fluid into the cooling water storage unit.

According to another embodiment of the present invention, the facility may further comprise a cooling water recollecting portion forming a fluid path that runs from the containment to the cooling water storage unit to recollect cooling water present inside the containment to the cooling water storage unit; and an opening portion formed by opening at least a portion of the cooling water storage unit to maintain pressure balance between the cooling water storage unit and an inside of the containment.

According to another embodiment of the present invention, the facility may further comprise an additive injection unit supplying an additive for maintaining a pH of cooling water to a predetermined value or more to prevent volatilization of the radioactive material dissolved in the cooling water storage unit.

According to another embodiment of the present invention, the additive injection unit may be installed at a predetermined height inside the cooling water storage unit to be submerged in the cooling water as a water level of the cooling water storage unit increases, and as the additive injection unit is submerged in the cooling water, the additive may be dissolved in the cooling water.

According to another embodiment of the present invention, the additive injection unit may be installed on a fluid path of the cooling water recollecting portion to dissolve the additive in the cooling water recollected to the cooling water recollecting portion.

According to another embodiment of the present invention, the facility may further comprise a sparging unit installed at an end of the discharging unit to be submerged in the cooling water of the cooling water storage unit and configured to sparge a fluid that has passed through the discharging unit, to condense steam and to dissolve soluble radioactive materials in the discharged air contained in the fluid.

According to another embodiment of the present invention, the sparging unit may have a flow resistance therein to induce an even distribution of the fluid into a plurality of fine fluid paths.

According to another embodiment of the present invention, the facility may further comprise a pressure balance line passing through at least a portion of the boundary unit and extended to an inside of the containment to form a fluid path of atmosphere passing through the boundary of the radioactive material, and the pressure balance line, when a pressure inside the containment is higher than a pressure inside the boundary unit, introduces atmosphere inside the containment to the inside of the boundary unit to prevent the cooling water in the cooling water storage unit from flowing back to the inside of the boundary unit.

According to another embodiment of the present invention, the facility may further comprise a check valve formed to allow for a flow only in one direction and installed at the pressure balance line to prevent the atmosphere inside the boundary unit from being discharged to the inside of the containment through the pressure balance line.

According to another embodiment of the present invention, the cooling water storage unit may be connected with a pipe forming a fluid path that runs to a safety injection line of a safety injection system to inject the cooling water stored in the cooling water storage unit to the inside of the reactor coolant system.

DETAILED DESCRIPTION OF THE INVENTION

Description will now be given in detail of the exemplary embodiments, with reference to the accompanying drawings. For the sake of brief description with reference to the drawings, the same or equivalent components will be provided with the same reference numbers, and description thereof will not be repeated. In describing the present invention, if a detailed explanation for a related known function or construction is considered to unnecessarily divert the gist of the present disclosure, such explanation has been omitted but would be understood by those skilled in the art. The accompanying drawings are used to help easily understood the technical idea of the present invention and it should be understood that the idea of the present disclosure is not limited by the accompanying drawings.

FIG. 1Ais a concept view illustrating a facility1100for reducing radioactive material and a nuclear power plant110having the same according to an embodiment of the present invention.

The nuclear power plant110includes a containment112, a reactor coolant system111, and a core111a. In addition to the components shown inFIG. 1A, the nuclear power plant110may include a reactor coolant pump, a pressurizer, a steam generator, other systems for normal operation of the nuclear power plant110and various systems for securing safety of the nuclear power plant110.

The containment112is installed outside the reactor coolant system111to prevent release of radioactive material. The containment112serves as a final barrier to prevent the radioactive material from releasing from the nuclear power plant110to the external environment. Containments112may be classified into a containment building (or also referred to as a reactor building) formed of reinforced concrete, a containment vessel formed of steel, and a safeguard vessel formed of steel depending on the material constituting the pressure boundary. The containment vessel is a large vessel designed to have a low pressure like the containment building, and the safeguard vessel is a small vessel designed to have a small size and having the higher design pressure. Unless mentioned otherwise, as used herein, the term “containment112” includes all of the containment building, the reactor building, the containment vessel, and the safeguard vessel.

The reactor coolant system111is installed in the containment112. The reactor coolant system111is a coolant system that delivers and conveys heat energy generated by nuclear fission of fuel in the core111a. A primary fluid fills the inside of the reactor coolant system111. When an accident, such as a loss of coolant accident, occurs, steam may be discharged from the reactor coolant system111to an atmosphere of the containment112, and an isolation system of the containment112shuts off the external release of the atmosphere and the radioactive material contained in the atmosphere.

A reactor coolant pump (not shown) induces the circulation of the primary fluid, and a pressurizer (not shown) maintains a pressurized state that exceeds a saturated pressure in order to control the pressure of the coolant at normal plant operation.

The facility1100for reducing radioactive material is installed inside the containment112. The facility1100for reducing radioactive material is configured to sparge into the cooling water, i) steam discharged from the reactor coolant system111installed in the containment112or a pipe113connected with the reactor coolant system111when an accident occurs, ii) an atmosphere in a boundary unit1120, and iii) radioactive material contained in the steam and the air. The facility1100for reducing radioactive material includes a cooling water storage unit1110, a boundary unit1120, a connecting pipe1130, and a sparging unit1140.

The cooling water storage unit1110is installed in the containment112. The cooling water storage unit1110is formed to store cooling water dissolving the radioactive material therein. The cooling water storage unit1110may be configured as a tank or pool.

The cooling water storage unit1110may be shared by the cooling water storage unit1110and other systems of the nuclear power plant110. For example, the facility1100for reducing radioactive material and a passive safety injection system (not shown) and a passive residual heat removal system (not shown) share the cooling water storage unit1110.

The cooling water storage unit1110may be installed at an upper side or lower side of the containment112. The cooling water storage unit1110may be installed at an upper side of the containment112to receive cooling water that is condensed and falls in the containment112as shown inFIG. 1A. In case a containment spray system (not shown) is installed in the nuclear power plant110, the cooling water storage unit1110may be installed at an upper side or lower side of the containment112to receive the sprayed cooling water.

A cooling water recollecting portion1110aand an opening portion1110bmay be installed in the cooling water storage unit1110. The cooling water recollecting portion1110aforms a fluid path that runs from the containment112to the cooling water storage unit1110to recollect the condensed water generated in the containment112to the cooling water storage unit1110. The opening portion1110bis formed as at least a portion of the cooling water storage unit1110is opened to maintain the pressure balance between the cooling water storage unit1110and the containment112. The cooling water recollecting portion1110aand the opening portion1110bmay share the same fluid path.

The cooling water storage unit1110has an inlet1111through which the connecting pipe1130passes. The highest part of the connecting pipe1130may be formed at a predetermined height from the bottom of the cooling water storage unit1110to prevent backflow of the cooling water retained in the cooling water storage unit1110.

The boundary unit1120is installed between the reactor coolant system111and the containment112to form a radioactive material boundary. The boundary unit1120surrounds the reactor coolant system111to prevent radioactive material from releasing from the reactor coolant system111or pipe113connected with the reactor coolant system111to the containment112.

The boundary unit1120forms a sealing structure around the reactor coolant system111to prevent the radioactive material from releasing along a path other than the connecting pipe1130. The boundary unit1120is designed to have a design pressure that may withstand the pressure of a head difference or more between the cooling water storage unit1110and the sparging unit1140. At least a portion of the boundary unit1120may be formed by a concrete structure inside the containment112and a coating member (1123) such as a steel liner et. al. installed on the concrete structure.

The boundary unit1120may include a barrier1121formed to surround the periphery of the reactor coolant system111and a cover1122formed to cover an upper part of the reactor coolant system111and may form a sealing structure around the reactor coolant system111by i) the bottom surface or dual bottom surface of the containment112, ii) the barrier1121, and iii) the cover1122.

The connecting pipe1130is connected with an inner space of the boundary unit1120and the cooling water storage unit1110to guide a flow of the fluid generated by a pressure difference between the boundary unit1120and the cooling water storage unit1110from the boundary unit to the cooling water storage unit1110. The connecting pipe1130forms a fluid path that runs from an inner space of the boundary unit1120to the cooling water storage unit1110. If the pressure in the boundary unit1120is larger than the pressure in the cooling water storage unit1110, the fluid in the boundary unit1120flows through the fluid path of the connecting pipe1130to the cooling water storage unit1110.

The cooling water storage unit1110has an inlet1111that allows the connecting pipe1130to pass therethrough. The connecting pipe1130extends through the inlet1111of the cooling water storage unit1110to the inside of the cooling water storage unit1110to form a fluid path that runs to the sparging unit1140and is connected with the sparging unit1140. The atmosphere (steam and air) and radioactive material in the boundary unit1120are delivered to the sparging unit1140through the connecting pipe1130.

The sparging unit1140is disposed to be submerged in the cooling water contained in the cooling water storage unit1110and is connected with the connecting pipe1130to sparge the fluid that has passed through the connecting pipe1130and the radioactive material contained in the fluid to the cooling water storage unit1110.

The sparging unit1140may have a plurality of sparging holes1141formed to sparge the fluid and radioactive material finely. The sparging unit1140may have a plurality of fine fluid paths (not shown) that run the plurality of sparging holes1141. The sparging unit1140may have a flow resistance therein, to allow the fluid to be evenly distributed through the plurality of fine fluid paths.

The nuclear power plant110may include pipes113for connecting the systems operated as the nuclear power plant110is in normal operation, other than the facility1100for reducing radioactive material, to the reactor coolant system111. The pipe113may pass through the containment112and the boundary unit1120of the facility1100for reducing radioactive material. The pipe113may have a plurality of isolation valves113a,113b,113c,113e,113f,113g, and113hor a check valve13darranged to be spaced apart from each other to close both sides of a broken line when a break occurs.

The facility1100for reducing radioactive material, contrary to when double containments112are installed, does not form a high-pressure boundary with the containment112, thus minimizing an increase in the economical expense due to added facilities. The facility1100for reducing radioactive material is a low-pressure facility.

Hereinafter, the operations of the facility1100for reducing radioactive material when the nuclear power plant110is in normal operation and when an accident occurs are described with reference toFIGS. 1B to 1F.

FIG. 1Bis a concept view illustrating a normal operation state of the nuclear power plant110shown inFIG. 1A.

When the nuclear power plant110is in normal operation, the isolation valves113a,113b,113c,113e,113f,113g, and113hinstalled on the pipe113connecting the systems (not shown) for normal operation of the nuclear power plant110with the reactor coolant system111may remain opened. The fluids circulating for normal operation of the nuclear power plant110may flow through the pipe113.

The facility1100for reducing radioactive material is a facility passively operated by a pressure difference formed between the boundary unit1120and the cooling water storage unit1110, and since there is little pressure difference between the boundary unit1120and the cooling water storage unit1110when the nuclear power plant110is in normal operation, the facility1100for reducing radioactive material remains in the standby state.

Hereinafter, the operations of the facility1100for reducing radioactive material i) when pipe breakage occurs in the facility1100for reducing radioactive material and ii) when pipe breakage occurs between the facility1100for reducing radioactive material and the containment112are described separately from each other.

FIG. 1Cis a concept view illustrating the operation of the facility1100for reducing radioactive material when an accident occurs in the nuclear power plant110shown inFIG. 1A.

When an accident such as pipe breakage occurs in the facility1100for reducing radioactive material, the reactor coolant and radioactive material may be discharged through the broken line113ito the inside of the boundary unit1120.

When the accident occurs, the isolation valves113a,113b,113c,113e,113f,113g, and113hinstalled on the pipe113passing through the boundary unit1120are closed by a related signal. In case a check valve113dforming a fluid path is installed towards the reactor coolant system111, the flow in an opposite direction is shut off, and the facility1100for reducing radioactive material maintains the sealing structure. The isolation valves113a,113b,113c,113e,113f,113g, and113hmay share the operation signal, and thus, the facility1100for reducing radioactive material may be operated even without a separate signal.

The nuclear power plant110may include a passive residual heat removing system114to remove sensible heat in the reactor coolant system111and residual heat of the core111aand a passive safety injection system15to inject cooling water to the inside of the reactor coolant system111to maintain the water level of the reactor coolant system111. The passive residual heat removing system114and the passive safety injection system115start their operation to secure safety of the nuclear power plant110when an accident occurs like the facility1100for reducing radioactive material.

If steam is discharged from the broken line113i, the radioactive material, together with the steam, is discharged into the boundary unit1120, and the pressure inside the boundary unit1120gradually increases. As the pressure inside the boundary unit1120increases, a pressure difference of H1 or more is generated between the boundary unit1120and the cooling water storage unit1110, and the fluid is rendered to flow by the pressure difference from the boundary unit1120, which has a relatively high pressure, to the cooling water storage unit1110, which has a relatively low pressure.

The connecting pipe1130guides the flow caused by the pressure difference to the inside of the cooling water storage unit1110, and the fluid (steam, air, and radioactive material) that has passed through the cooling water storage unit1110is sparged into the cooling water through the sparging unit1140submerged in the cooling water. Accordingly, the steam is sparged into the cooling water and is condensed, and the air is cooled to go up. The soluble radioactive material is dissolved in the cooling water and is collected in the cooling water storage unit1110.

As the steam is condensed and the radioactive material is collected into the cooling water, a limited amount of the radioactive material discharged from the broken line113iis discharged to the containment112. A small amount of radioactive material discharged to the inside of the containment112is suppressed from releasing to the external environment by the containment112.

In particular, iodine that may be spread to the external environment with the highest concentration among radioactive materials is soluble and is mostly dissolved in the cooling water. The facility1100for reducing radioactive material stays in operation if the amount of cooling water in the cooling water storage unit1110is maintained to be a predetermined value or more and the pressure difference between the facility1100for reducing radioactive material and the inside of the containment112is not less than H1.

The cooling water storage unit1110receives the condensed water introduced through the cooling water recollecting portion1110a. Accordingly, the facility1100for reducing radioactive material may maintain the cooling water level that is required for its operation.

The sparging unit1140sparges the steam that may cause the pressure inside the containment112to increase into the cooling water storage unit1110and condenses the steam. Accordingly, the cooling water storage unit1110may suppress an increase in the pressure inside the containment112and may decrease the design pressure of the containment112.

FIG. 1Dis a concept view illustrating a state in which a nuclear power plant110operates when an accident occurs at a location different from the position shown inFIG. 1C.

The pipe passing through the boundary unit1120may be broken in the boundary unit1120as described above in connection withFIG. 1C, but may be broken in a space between the containment112and the boundary unit1120as described above in connection withFIG. 1D. If pipe breakage occurs in the space between the containment112and the boundary unit1120, steam is discharged from the broken line113ito result in the pressure in the containment112increasing.

However, since the steam stops being discharged when all of the isolation valves113a,113b,113c,113e,113f,113g, and113hand the check valve113dare closed by a related signal, the pressure inside the containment112does not steadily go up. Accordingly, when an accident occurs to cause the pressure outside the facility1100for reducing radioactive material to increase, the accident is suppressed early.

As the pressure inside the containment112increases, the cooling water in the cooling water storage unit1110may be pressurized to cause the water level of the connecting pipe1130to partially go up. However, the highest part of the connecting pipe1130of the cooling water storage unit1110is formed at a predetermined height from the bottom of the cooling water storage unit1110so as to prevent backflow of the cooling water despite the increased pressure in the containment112. Accordingly, in case there is no significant pressure difference between the inside of the containment112and the boundary unit1120(<H2), the cooling water retained in the cooling water storage unit1110does not flow back to the inside of the boundary unit1120.

FIGS. 1E and 1Fare concept views illustrating a nuclear power plant110including passive safety systems other than a facility1100for reducing radioactive material.

The nuclear power plant110includes a passive containment cooling system116that reduces pressure inside the containment112through cooling. The passive containment cooling system116has a heat exchanger (not shown). The cooling fluid passing through the heat exchanger exchanges heat with the fluid inside the containment112. Accordingly, heat is delivered from the inside of the containment112to the cooling fluid, and the cooling fluid is discharged to the outside along the fluid path connected with the heat exchanger. Such process is repeated to suppress the increasing pressure inside the containment112.

Referring toFIG. 1E, the heat exchanger (not shown) provided in the passive containment cooling system116may be installed inside the cooling water storage unit1110. If the cooling fluid passing through the heat exchanger exchanges heat with the atmosphere or cooling water and/or atmosphere inside the cooling water storage unit1110, the cooling water storage unit1110is cooled. Since the cooling water storage unit1110and the inside of the containment112are formed to communicate with each other through the cooling water recollecting portion1110aor opening portion1110b, cooling the cooling water storage unit1110leads to the containment112being cooled, and the increasing pressure inside the containment112may be suppressed.

Referring toFIG. 1F, the heat exchanger (not shown) provided in the passive containment cooling system116may be installed at an upper side of the containment112. The cooling fluid flowing through the inner fluid path of the heat exchanger exchanges heat with the atmosphere inside the containment112.

If the facility1100for reducing radioactive material and the passive containment cooling system116are both adopted in the nuclear power plant110, the steam discharged to the inside of the containment112is cooled and condensed by the passive containment cooling system116. Since the condensed water formed as the steam is condensed may be recollected to the cooling water storage unit1110, the water in the cooling water storage unit1110may be maintained at a proper level or more.

Another embodiment of the present invention is now described.

FIG. 2is a concept view illustrating a facility1200for reducing radioactive material and a nuclear power plant120having the same according to another embodiment of the present invention.

The facility1200for reducing radioactive material includes a cooling water storage unit1110, a boundary unit1120, a sparging unit1140, and a pressure balance line1250. The description of the cooling water storage unit1210, the boundary unit1220, the connecting pipe1230, and the sparging unit1240is not repeated and replaced with the above description thereof.

The boundary unit1220forms a boundary of radioactive material. The pressure balance line1250passes through at least a portion of the boundary unit1220to form a flow path of atmosphere passing through the boundary of radioactive material and extends to the inside of the containment122.

The pressure balance line1250, in case the pressure inside the containment122is higher than the pressure inside the boundary unit1220, introduces the atmosphere inside the containment122to the inside of the boundary unit1220. By doing so, the pressure balance line1250prevents backflow of the cooling water in the cooling water storage unit1210to the inside of the boundary unit1220. The inflow of atmosphere through the pressure balance line1250is passively made by the pressure difference between the containment122and the boundary unit1220.

The pressure balance line1250may be split from the connecting pipe1230as shown inFIG. 2. The pressure balance line1250may pass through an upper side of the cooling water storage unit1210and may extend to the inside of the containment122. The atmosphere inside the containment122is introduced to the inside of the boundary unit1220through the pressure balance line1250. Since the pressure balance line1250suppresses an increase in the pressure difference in an opposite direction of the boundary unit1220, the mechanical integrity of the boundary unit1220may be more safely maintained.

The check valve1251is installed in the pressure balance line1250. The check valve1251is formed to allow for a flow only in one direction. The check valve1251prevents the atmosphere inside the boundary unit1220from being discharged to the inside of the containment122through the pressure balance line1250. According to the conditions of accident, the pressure inside the boundary unit1220may be higher than the pressure inside the containment122. In such case, the atmosphere inside the boundary unit1220may be discharged to the inside of the containment122through the pressure balance line1250to lose the unique functions of the facility1200for reducing radioactive material. The check valve1251cuts off the flow to the pressure balance line1250to thus prevent the atmosphere inside the boundary unit1220from being discharged to the inside of the containment122.

FIG. 3is a concept view illustrating a facility1100for reducing radioactive material and a nuclear power plant130having the same according to still another embodiment of the present invention.

The facility1300for reducing radioactive material includes a cooling water storage unit1310, a boundary unit1320, a connecting pipe1330, and a sparging unit1340. The description of the similar components is not repeated and replaced with the above description thereof.

The cooling water storage unit1310of the facility1300for reducing radioactive material may be installed at a lower region in the inner space of the containment132. The cooling water storage unit1310has a cooling water recollecting portion1310aand an opening portion1310b. A space is formed between the outer wall of the containment132and the inner structure of the containment132. The fluid inside the containment132may flow to the cooling water storage unit1310through the space between the outer wall and the structure, the cooling water recollecting portion1310a, and the opening portion1310b. Likewise, the fluid inside the cooling water storage unit1310may flow to the inside of the containment132through the cooling water recollecting portion1310a, the opening portion1310b, and the space between the outer wall and the structure.

Comparison between the facility1100for reducing radioactive material shown inFIG. 1Aand the facility1300for reducing radioactive material shown inFIG. 3shows that the positions where the cooling water storage units1110and1310are installed may vary depending on the requirements for the internal design of the containments112and132. Even when the positions where the cooling water storage units1110and1310are installed differently, the facilities1100and1300for reducing radioactive material may be configured to not cause a deterioration of their functions.

FIG. 4is a concept view illustrating a facility1400for reducing radioactive material and a nuclear power plant140having the same according to still another embodiment of the present invention.

The facility1400for reducing radioactive material includes a cooling water storage unit1410, a boundary unit1420, a connecting pipe1430, a sparging unit1440, and a pressure balance line1450.

Unlike the facility1200for reducing radioactive material shown inFIG. 2, the pressure balance line1450is not split from the connecting pipe1430but is formed independently. The pressure balance line1450passes through a boundary of the radioactive material, which is formed by the boundary unit1420, and extends up to the inside of the containment142. The pressure balance line1450, in case the pressure inside the containment142is higher than the pressure of the boundary unit1420, introduces atmosphere to decrease the pressure inside the containment142. The pressure balance line1450prevents the cooling water in the cooling water storage unit1410from flowing back to the inside of the boundary unit1420.

The pressure balance line1450has a check valve1451. The check valve1451is formed to allow for a flow only in a direction. The check valve1451prevents atmosphere from being discharged from the inside of the boundary unit1420to the inside of the containment142. The mechanical integrity of the boundary unit1420may be more safely maintained by the pressure balance line1450and the check valve1451.

FIG. 5is a concept view illustrating a facility1500for reducing radioactive material and a nuclear power plant150having the same according to yet still another embodiment of the present invention.

The facility1500for reducing radioactive material includes a cooling water storage unit1510, a boundary unit1520, a connecting pipe1530, and a sparging unit1540.

The connecting pipe1530has a check valve1531to prevent the cooling water inside the cooling water storage unit1510from flowing back to the boundary unit1520through the connecting pipe1530. The check valve1531allows for only flow that is formed from the boundary unit1520to the cooling water storage unit1510and cuts off flow in an opposite direction. Even when the pressure inside the cooling water storage unit1510is higher than the pressure inside the boundary unit1520due to an accident, the check valve1531may prevent the cooling water retained in the cooling water storage unit1510from flowing back to the boundary unit1520.

FIG. 6is a concept view illustrating a facility1600for reducing radioactive material and a nuclear power plant160having the same according to yet still another embodiment of the present invention.

The facility1600for reducing radioactive material is installed inside the containment162, and when an accident occurs, is configured to sparge, to the cooling water storage unit1610, the radioactive material discharged from a reactor coolant system161or pipes163,163′, and165cconnected with the reactor coolant system161.

The cooling water storage unit1610is installed inside the containment162. The cooling water storage unit1610may be formed as a tank or pool to store cooling water therein. Further, as the cooling water storage unit1610, an in-containment refueling water storage tank may be used as well. When an accident occurs, the atmosphere (steam and air) inside the boundary unit1620is sparged into the cooling water as the facility1600for reducing radioactive material operates.

The cooling water storage unit1610may be shared by other safety systems of the nuclear power plant160than the facility1600for reducing radioactive material. For example, the facility1600for reducing radioactive material and a safety injection system165may share the cooling water storage unit1610. As another example, the facility1600for reducing radioactive material and a passive residua heat removal system (not shown) may share the cooling water storage unit1610.

The cooling water storage unit1610may be installed at an upper side or lower side of an inner space of the containment162. Condensed water may be formed inside the containment162and may fall. The cooling water storage unit1610may be installed at an upper side of the inner space of the containment162to collect the falling condensed water. In case the nuclear power plant160has a containment spray system (not shown), the cooling water storage unit1610may be installed at an upper or lower side of the containment162to receive the sprayed cooling water.

The cooling water storage unit1610has an inlet1611through which a connecting pipe1630to be described below passes. The highest part of the connecting pipe1630may be formed at a predetermined height from the bottom of the cooling water storage unit1610to prevent backflow of the cooling water retained in the cooling water storage unit1610.

The boundary unit1620is installed between the reactor coolant system161and the containment162to form a boundary of the radioactive material. The boundary unit1620surrounds the reactor coolant system161to prevent release of the radioactive material from the pipes163,163′, and165cconnected with the reactor coolant system161to the containment162.

The boundary unit1620forms a sealing structure around the reactor coolant system161to prevent release of the radioactive material along a path other than the connecting pipe1630to be described below. The pipes163,163′, and165cpassing through the boundary unit1620has isolation valves163a,163b,163a′, and165c′ and check valves163b′,163b″,165c″. The isolation valves163a,163b,163a′, and165c′ and the check valves163b′,163b″,165c″ are closed when an accident occurs to maintain the sealing structure. The boundary unit1620is formed to have a design pressure to withstand a pressure of a head difference or more between the cooling water storage unit1610and the sparging unit1640. At least a portion of the boundary unit1620may be formed by a concrete structure inside the containment162and a coating member (1623) such as steel liner et. al. installed on the concrete structure.

The boundary unit1620may include a barrier1621and a cover1622. The barrier1621is formed to surround the periphery of the reactor coolant system161. The cover1622is formed to cover an upper portion of the reactor coolant system161. The barrier1621, the cover1622, and the bottom surface (or dual bottom surface) of the containment162may form a sealing structure around the reactor coolant system161.

The nuclear power plant160includes penetration pipes163and163′ passing through the containment162. The penetration pipes163and163′ are connected with the reactor coolant system161or a secondary system. The penetration pipes163and163′ may include a plurality of isolation valves163a,163b, and163a′ or check valve163b′ arranged to be spaced apart from each other to close both sides of the broken line when breakage occurs.

In case the boundary unit1620and the containment162are spaced apart from each other and the penetration pipes163and163′ pass through a region between the boundary unit1620and the containment162, a loss-of-coolant accident may occur in the region between the boundary unit1620and the containment162. In case a loss-of-coolant accident occurs in the region between the boundary unit1620and the containment162, the radioactive material might not be trapped in the inside of the facility1600for reducing radioactive material. Accordingly, in case a loss-of-coolant accident occurs in the region between the boundary unit1620and the containment162, the penetration pipes163and163′ should have isolation valves to prevent additional release of the radioactive material.

However, since the isolation valves have the mechanism to be opened or closed by a related safety system signal, the isolation valves may abnormally operate or might not operate. Addition of isolation valves is not preferred in view of simplifying the facility. To resolve such issue, the present invention has a structure that may prevent release of radioactive material even without installation of additional isolation valves.

Specifically, at least a portion of the boundary unit1620is expanded up to a region adjacent to the containment162while wrapping around the penetration pipes163and163′ in order to prevent the loss-of-coolant accident that may occur due to breakage of the penetration pipes163and163′ between the boundary unit1620and the containment162. As a result, the penetration pipes163and163′ passing through the containment162up to the inside of the containment162is caused to be positioned inside the boundary unit1620. Accordingly, the present invention may significantly lower the possibility that a loss-of-coolant accident occurs, e.g., by breakage of the pipes163and163′ in the region between the boundary unit1620and the containment162and may prevent release of radioactive material even without installation of additional isolation valves.

The connecting pipe1630is connected with the boundary unit1620and the cooling water storage unit1610to guide the fluid flow caused in the boundary unit1620to the cooling water storage unit1610. The atmosphere inside the boundary unit1620includes steam or air, and when a loss-of-coolant accident occurs, may be caused to contain radioactive material. If an accident that causes the pressure inside the boundary unit1620to rise occurs and thus the difference between the pressure inside the boundary unit1620and the pressure inside the containment162is increased to H1 or more, the atmosphere inside the boundary unit1620is passively caused to flow to the cooling water storage unit1610through the connecting pipe1630.

The connecting pipe1630passes through the inlet1611of the cooling water storage unit1610up to the inside of the cooling water storage unit1610to deliver the atmosphere inside the boundary unit1620and radioactive material contained in the atmosphere to the sparging unit1640.

The sparging unit1640is connected with the connecting pipe1630to receive the atmosphere inside the boundary unit1620and the radioactive material contained in the atmosphere from the connecting pipe1630. At least a portion of the sparging unit1640is submerged in the cooling water of the cooling water storage unit1610so that the sparging unit1640sparges the atmosphere and the radioactive material contained in the atmosphere to the cooling water.

The sparging unit1640has a plurality of sparging holes1641formed to sparge the atmosphere inside the boundary unit1620and the radioactive material contained in the atmosphere finely. Further, the sparging unit1640has a plurality of fine fluid paths (not shown) that run to the plurality of sparging holes1641. The sparging unit1640has a flow resistance in its inner fluid path to evenly distribute the fluid into the plurality of fine fluid paths.

The steam sparged through the sparging unit1640to the cooling water storage unit1610is condensed, and the air is cooled to rise. The soluble radioactive material is mostly dissolved in the cooling water. In case the cooling water in the cooling water storage unit1610maintains a predetermined water level, and the pressure difference between the boundary unit1620and the containment162is H1 or more, the facility1600for reducing radioactive material remains in steady operation.

In case a single connecting pipe1630and a single sparging unit1640are provided, the facility1600for reducing radioactive material may be caused to be impossible to operate as the connecting pipe1630or the sparging unit1640is blocked. Accordingly, it is preferable to provide a plurality of connecting pipes1630and a plurality of sparging units1640considering redundancy.

The facility1600for reducing radioactive material may further include a cooling water recollecting portion1610aand an opening portion1610b. The opening portion1610bprevents overpressure in the cooling water storage unit1610. In contrast, the cooling water recollecting portion1610arecollects the steam discharged from the cooling water storage unit1610.

The cooling water in the cooling water storage unit1610is evaporated as its temperature goes up, turning into steam. The steam may be discharged through the opening portion1610bto the inside of the containment162. The steam discharged to the inside of the containment162is cooled, turning into condensed water. The cooling water recollecting portion1610aforms a fluid path at an upper part of the cooling water storage unit1610to recollect the condensed water to the cooling water storage unit1610. The connection between the cooling water recollecting portion1610aand the cooling water storage unit1610may be made by way of a pipe or structure.

As shown inFIG. 6, the cooling water recollecting portion1610aand the opening portion1610bmay be formed at an upper part of the cooling water storage unit1610. More specifically, a portion of the upper structure forming the cooling water storage unit1610may form the cooling water recollecting portion1610aand the opening portion1610b. The cooling water recollecting portion1610aand the opening portion1610bare installed at separate regions from each other. However, the cooling water recollecting portion1610aand the opening portion1610bmay be formed to share the same fluid path.

The nuclear power plant160may have various safety systems other than the facility1600for reducing radioactive material. For example, as shown inFIG. 6, a passive safety injection system165may be installed in the nuclear power plant160. The passive safety injection system165is a system form maintaining the water level of the reactor coolant system161by injecting a coolant to the inside of the reactor coolant system161when an accident, such as loss of coolant accident, occurs.

The passive safety injection system165may include various types of tanks such as a core makeup tank165aor safety injection tank165b. The core makeup tank165aor the safety injection tank165bis connected with the reactor coolant system161by way of a safety injection line165cand the pressure balance line165d.

The coolant is injected from the tanks165aand165bthrough the safety injection line165cto the reactor coolant system161. In case the facility1600for reducing radioactive material and the passive safety injection system165are both installed in the nuclear power plant160, the passive safety injection system165may be installed in the inside of the boundary unit1620to prevent release of radioactive material.

The facility1600for reducing radioactive material proposed herein, unlike when double containments162are installed, does not form a high-pressure boundary with the containment162and thus may minimize an increase in the economical expense that may occur due to added facilities. The facility1600for reducing radioactive material may minimize an increase in the number of isolation valves.

FIG. 7Ais a concept view illustrating a facility1700for reducing radioactive material and a nuclear power plant170having the same according to yet still another embodiment of the present invention.

An opening portion1710bis formed to protrude from an upper part of a cooling water storage unit1710to the inside of a containment172. The opening portion1710bforms a fluid path by way of a pipe or structure. A filter facility1770is disposed on the fluid path to capture the radioactive material that is about to exit the cooling water storage unit1710.

If the pressure in the cooling water storage unit1710goes up, the steam or air inside the cooling water storage unit1710is discharged through the opening portion1710b. During the course, some of the radioactive materials dissolved in the cooling water storage unit1710are re-volatilized, and together with the steam or air, may be thus discharged through the opening portion1710bto the containment172. If the radioactive materials are discharged to the containment172, the concentration of the radioactive material in the containment172may increase.

The filter facility1770is disposed on the fluid path of the opening portion1710bto capture the radioactive material that, together with the steam, is about to be discharged to the containment172. The filter facility1770includes at least one of a filter and an absorbent. The filter and the absorbent are adapted to pass steam or air while capturing the radioactive material.

As the filter, a high-efficiency particulate air (HEPA) filter may be adopted. The gaseous radioactive material contained in the steam or air is filtered out when passing through the filter. For example, in case the radioactive material is iodine, iodine is combined with silver nitrate while passing through the filter to thus turn into iodic silver, and is thus removed from the steam or air.

As the absorbent, charcoal may be employed. Organic iodine compounds are combined with the materials impregnated in the charcoal to turn into quaternary ammonium salt and are absorbed into the charcoal. Molecular iodine is combined with the charcoal through chemisorption. The charcoal is typically utilized as an absorbent material thanks to its large internal surface area.

Either or both of the filter and the absorbent may be disposed. However, the above-described filter and absorbent are offered merely as an example, and according to the present invention, the type of the filter and absorbent is not necessarily limited thereto.

The cooling water recollecting portion1710a, like the opening portion1710b, has a fluid path formed by a pipe or structure. The fluid path of the cooling water recollecting portion1710amay be formed to be submerged into the cooling water storage unit1710. However, the cooling water storage unit1710and the cooling water recollecting portion1710a, rather than separated from each other, are connected with each other.

Hereinafter, the normal operation of the nuclear power plant170and the operation under accident of the nuclear power plant170are described with reference toFIGS. 7B and 7C.

FIG. 7Bis a concept view illustrating a normal operation state of the nuclear power plant170shown inFIG. 7A.

The pipes173and173′ connected with a system (not shown) relating to the normal operation of the nuclear power plant170have isolation valves173a,173b, and173a′ or a check valve173b′. When the nuclear power plant is in normal operation, the isolation valves necessary for the normal operation remain opened. When the nuclear power plant170is in normal operation, the water in the reactor coolant system171remains at a normal level. Accordingly, the passive safety injection system175remains in the standby state.

The facility1700for reducing radioactive material is a facility that is passively operated by a pressure difference between a boundary unit1720and a containment172. When the nuclear power plant170is in normal operation, little pressure difference is created between the boundary unit1720and the cooling water storage unit1710, and thus, the facility1700for reducing radioactive material remains in the standby state.

FIG. 7Cis a concept view illustrating the operation under accident of the nuclear power plant170shown inFIG. 7A.

If an accident such as a loss-of-coolant accident occurs in the nuclear power plant170due to, e.g., pipe breakage, steam and radioactive material are discharged through the broken line173f. A number of safety systems installed in the nuclear power plant170start operations.

When an accident occurs, the isolation valves173a,173b, and173a′ relating to the normal operation of the nuclear power plant170are closed by a related signal. In case check valves173b′ and175c″ are installed to form a fluid path in a direction toward the reactor coolant system171, the flow in the direction coming from the reactor coolant system171is shut off, and the boundary unit1720of the facility1700for reducing radioactive material maintains a sealing structure. The isolation valves173a,173b,173a′, and175c′ may share an operation signal. Accordingly, even when no separate signal is applied for the operation signal of the facility for reducing radioactive material, the operation of the isolation valves173a,173b,173a′,175c′ may allow the facility1700for reducing radioactive material to be operated.

The nuclear power plant170may include a passive residual heat removing system174and a passive safety injection system175. The passive residual heat removing system174removes sensible heat in the reactor coolant system171and residual heat in the core171a. The passive safety injection system175injects a coolant into the reactor coolant system171to maintain the water level of the reactor coolant system171.

The passive safety injection system175is first described. The pipe connected with the core makeup tank175ahas an isolation valve175a′ and a check valve175a″. If the isolation valve175a′ and the check valve175a″ are opened, the coolant in the core makeup tank175ais swiftly injected into the reactor coolant system171.

If the isolation valve175d′ installed in the pressure balance line175dis opened, steam is introduced from the high-pressure reactor coolant system171through the pressure balance line175dto the safety injection tank175b. As time goes by, the reactor coolant system171and the safety injection tank175bform a pressure balance. If the reactor coolant system171and the safety injection tank175bform the pressure balance, the coolant in the safety injection tank175bis also injected into the reactor coolant system171by gravity water head. The coolant in the core makeup tank175aand the safety injection tank175bis injected through the safety injection line175cto the reactor coolant system171.

Next, the passive residual heat removing system174is described. The passive residual heat removing system174may remove sensible heat from the reactor coolant system171and residual heat from the core171a. A steam generator (not shown) is installed at the boundary between the primary system and the secondary system. The passive residual heat removing system is configured to circulate the coolant to the steam generator. As the coolant circulates, the sensible heat from the reactor coolant system171and the residual heat from the core171aare removed to the outside.

The nuclear power plant170, as necessary, may further include other systems than the above-mentioned safety systems.

If steam is discharged from the broken line173f, the radioactive material, together with the steam, is discharged to the inside of the boundary unit1720. As the steam and radioactive material are continuously discharged from the broken line173f, the pressure inside the boundary unit1720is gradually increased. As the pressure inside the boundary unit1720is increased to H1 or more, a flow of the fluid (including steam, air, and radioactive material) is caused by a pressure difference from the boundary unit1720that has a relatively high pressure to the cooling water storage unit1710that has a relatively low pressure.

The connecting pipe1730guides the flow of the fluid caused by the pressure difference to the cooling water storage unit1710. The atmosphere that has passed through the connecting pipe1730is sparged into the cooling water through the sparging unit1740submerged in the cooling water storage unit1710. Accordingly, the steam is sparged into the cooling water and is condensed. The air is cooled to rise. The soluble radioactive material is dissolved in the cooling water and is collected. Accordingly, the facility1700for reducing radioactive material may suppress the radioactive material from releasing from the containment172to the external environment.

In particular, iodine, which has the highest concentration among the radioactive materials spread to the external environment, is soluble and thus is mostly dissolved in the cooling water. The facility1700for reducing radioactive material remains in steady operation when the amount of cooling water in the cooling water storage unit1710maintains a predetermined value or more and the pressure difference between the boundary unit1720and the containment172is H1 or more.

The sparging unit1740sparges the steam that may lead to an increase in the pressure inside the containment172to the cooling water storage unit1710to condense the steam. Accordingly, the facility1700for reducing radioactive material may suppress the increasing pressure inside the containment172and may reduce the design pressure in the containment172.

As time goes by, the steam may be discharged from the cooling water storage unit1710through the opening portion1710b. However, the radioactive material contained in the steam is captured while passing through the filter facility1770and is not discharged to the containment172. A portion of the steam discharged to the inside of the containment172is re-condensed and is recollected to the cooling water storage unit1710through the cooling water recollecting portion1710a.

FIG. 8is a concept view illustrating a facility1800for reducing radioactive material and a nuclear power plant180having the same according to yet still another embodiment of the present invention.

The opening portion1810band the cooling water recollecting portion1810ashare the same fluid path. The steam or air in the cooling water storage unit1810is discharged overtime through the opening portion1810bto the containment182. The condensed water created in the containment182is recollected to the cooling water storage unit1810through the cooling water recollecting portion1810a.

The filter facility1870, as shown inFIG. 8, is disposed inside the cooling water storage unit1810. Specifically, the filter facility1870is installed at an upper part of the inner space in the cooling water storage unit1810. Accordingly, the radioactive material contained in the steam or air is captured while passing through the filter facility1870and is restricted for being discharged to the containment182.

FIG. 9is a concept view illustrating a facility1900for reducing radioactive material and a nuclear power plant190having the same according to yet still another embodiment of the present invention.

The facility1900for reducing radioactive material may further include an additive injection unit1980. The additive injection unit1980supplies the cooling water storage unit1910with an additive to maintain the pH of the coolant to a predetermined value or more (typically pH 7 or more) so as to prevent volatilization of the radioactive material dissolved in the cooling water storage unit1910. The additive injection unit1980may be installed in the fluid path of the cooling water recollecting portion1910aas shown inFIG. 4.

Radioactive iodine dissolved in the cooling water exists in the form of negative ions. In case the pH of the cooling water in which iodine is dissolved is low, the amount of radioactive iodine that is to be re-volatilized may be significantly increased. This is why the amount of radioactive iodine that is converted into volatilizable elemental iodine (12) is sharply increased in the cooling water of pH 7 or less.

The additive injection unit1980injects an additive to the cooling water (or condensed water) to prevent the radioactive material dissolved in the cooling water from being re-volatilized. For example, the additive may be sodium phosphate. Sodium phosphate adjusts the pH of the cooling water to prevent corrosion inside the containment192and re-volatilization of a radioactive nuclide. However, the type of additives according to the present invention is not limited thereto. The additive may include materials to passively manage the water quality of the cooling water storage unit1910. For example, boric acid to suppress reactivity of the core191aor other additives for suppressing corrosion of the device may be added.

Referring toFIG. 9, the condensed water in the containment192is recollected through the cooling water recollecting portion1960to the cooling water storage unit1910. The additive injection unit1980may be installed in the fluid path of the cooling water recollecting portion1910ato dissolve the additive in the recollected condensed water. Accordingly, if the additive is dissolved in the condensed water introduced to the cooling water recollecting portion1910a, the additive increases the pH of the condensed water to prevent re-volatilization of the radioactive material. If the condensed water is introduced into the cooling water storage unit1910and is mixed with the cooling water, the mixture of the cooling water and the condensed water may be kept at a pH of 7 or more.

FIG. 10Ais a concept view illustrating a facility2000for reducing radioactive material and a nuclear power plant200having the same according to yet still another embodiment of the present invention.

The nuclear power plant200may have a passive containment cooling system along with the facility2000for reducing radioactive material. The passive containment cooling system is a system for cooling the inside of the containment202to suppress a rise in the pressure inside the containment202. The passive containment cooling system includes a heat exchanger206a. The atmosphere inside the containment202and the cooling water in the cooling water storage unit2010are cooled by the heat exchanger206a. The steam and air contained in the atmosphere inside the containment202may be condensed or cooled. If the temperature inside the containment202is decreased, a portion of the steam inside the containment202is decreased. Accordingly, the rise in the pressure inside the containment202may be suppressed by the passive containment cooling system.

The heat exchanger206aof the passive containment cooling system may be installed in an inner space of the containment202. Unlike this, the heat exchanger206amay be installed to be submerged in the cooling water in the cooling water storage unit2010. The heat exchanger206amay be installed in both side an inner space of the containment202and the cooling water storage unit2010. Referring toFIG. 10A, a portion of the heat exchanger206ais disposed in the inner space of the containment202and another portion of the heat exchanger206ais disposed inside the cooling water storage unit2010.

FIG. 10Bis a concept view illustrating an example where an accident occurs in the nuclear power plant200shown inFIG. 10A.

If pipe breakage occurs in a pipe connected with the reactor coolant system201, steam and radioactive material are discharged through the broken line203″. The passive safety injection system205installed inside the boundary unit2020injects a coolant into the reactor coolant system201. The passive residual heat removing system204removes sensible heat in the reactor coolant system201and residual heat in the core201a.

As steam is discharged, the pressure inside the boundary unit2020is increased to be higher than the pressure inside the containment202, and a fluid flow is created due to the pressure difference between inside the boundary unit2020and inside the containment202. The connecting pipe2030guides the fluid flow to the cooling water storage unit2010. The sparging unit2040sparges, into the cooling water, the fluid and the radioactive material contained in the fluid delivered from the connecting pipe2030. The soluble radioactive material is collected in the cooling water storage unit2010. The passive containment cooling system206cools at least one of the containment202and the cooling water storage unit2010.

As time goes by, the steam or air in the cooling water storage unit2010is discharged to the inside of the containment202through the opening portion2010b. However, the radioactive material is captured by the filter facility2070installed in the fluid path of the opening portion2010band is not discharged to the containment202. A portion of the steam that has been discharged to the containment202is re-condensed to form condensed water. The condensed water is recollected to the cooling water storage unit2010through the cooling water recollecting portion2010a.

FIG. 10Cis a concept view illustrating a variation to the nuclear power plant200shown inFIG. 10B.

The passive containment cooling systems206and206′ are formed to cool the atmosphere in the containment202and the cooling water in the cooling water storage unit2010. The heat exchanger (not shown) of the passive containment cooling system may be installed in an inner space of each of the cooling water storage unit2010and the containment202. When an accident occurs, the operation of the facility2000for reducing radioactive material, the passive safety injection system205, and the passive residual heat removing system204is the same as that described above in connection withFIG. 10B.

The passive containment cooling systems206and206′ cool the atmosphere in the containment202. Accordingly, the steam evaporated from the cooling water storage unit2010to the containment202or the atmosphere inside the containment202may be cooled or condensed. The condensed water generated as the steam is condensed is collected through the cooling water recollecting portion2010a, and this has been described above.

FIG. 10Dis a concept view illustrating another variation to the nuclear power plant200shown inFIG. 10B.

The passive containment cooling system206″ is formed to cool the atmosphere in the containment212and the cooling water in the cooling water storage unit2010. The heat exchanger (not shown) of the passive containment cooling system206″ is formed to penetrate an upper structure of the cooling water storage unit2010to simultaneously cool the containment202and the cooling water storage unit2010. Other configurations are the same as those described above in connection withFIG. 10C.

FIG. 11is a concept view illustrating a facility2100for reducing radioactive material and a nuclear power plant210having the same according to still another embodiment of the present invention.

The cooling water storage unit2110may be installed at a lower region of an inner space in the containment212. As in the embodiments described above, the connecting pipe2130passes through the inlet2111of the cooling water storage unit2110and extends to a lower part of the cooling water storage unit2110. The sparging unit2140is connected with the connecting pipe2130to receive the radioactive material that has passed through the connecting pipe2130.

The opening portion2110bis formed to project to an inner space of the containment212. A filter facility2170is installed in a fluid path of the opening portion2110b. The cooling water recollecting portion2110ais formed to collect condensed water. The heat exchanger216cof the passive containment cooling system is installed in the cooling water storage unit2110to cool the cooling water in the cooling water storage unit2110.

FIG. 12is a concept view illustrating a facility2200for reducing radioactive material and a nuclear power plant220having the same according to yet still another embodiment of the present invention.

The facility2200for reducing radioactive material further includes a pressure balance line2250. The pressure balance line2250of the facility2200for reducing radioactive material needs to be distinguished from the pressure balance line215dof the passive safety injection system215. The pressure balance line2250of the facility2200for reducing radioactive material forms a fluid path that runs from the inside of the containment222to the inside of the boundary unit2220. In case the pressure inside the containment222is higher than the pressure inside the boundary unit2220, the pressure balance line2250introduces the atmosphere inside the containment222to the inside of the boundary unit2220. Accordingly, the cooling water in the cooling water storage unit2210may be prevented from flowing back to the inside of the boundary unit2220. The pressure balance line2050may be branched from the connecting pipe2230and may extend up to the inside of the containment222. The pressure balance line2250, as shown, may pass through the upper structure of the cooling water storage unit2210.

The pressure balance line2250may have a check valve2251that allows for a flow only in one direction. The check valve2251prevents the atmosphere inside the boundary unit2220from being discharged through the pressure balance line2250to the inside of the containment222.

FIG. 13is a concept view illustrating a facility2300for reducing radioactive material and a nuclear power plant230having the same according to yet still another embodiment of the present invention.

The pressure balance line2350forms a fluid path that runs from the inside of the containment232to the inside of the boundary unit2320. The inner space of the boundary unit2320and the inner space of the containment232are connected with each other by way of the pressure balance line2350. The pressure balance line2350, rather than branched from the connecting pipe2330, is formed independently from the connecting pipe2330. In this point of view, the pressure balance line2350shown inFIG. 13differs from the pressure balance line2250shown inFIG. 12. The pressure balance line2350passes through the upper part of the boundary unit2220and may extend to the inside of the boundary unit2220. The check valve2351may be installed in the pressure balance line2350, and the function of the check valve2351is the same as that described above in connection withFIG. 7.

FIG. 14is a concept view illustrating a facility2400for reducing radioactive material and a nuclear power plant240having the same according to yet still another embodiment of the present invention.

The connecting pipe2430has a check valve2431that allows for a flow only in one direction. The check valve2431prevents the cooling water in the cooling water storage unit2410from flowing back to the boundary unit2420through the connecting pipe2430.

FIG. 15is a concept view illustrating a facility for reducing radioactive material and a nuclear power plant having the same according to yet still another embodiment of the present invention.

The cooling water storage unit2500may be connected with the safety injection line255c. The pipe2532connecting the cooling water storage unit2500with the safety injection line255chas an isolation valve2532aand a check valve2532b. If the isolation valve2532aand the check valve2532bare opened, the cooling water stored in the cooling water storage unit2510is injected into the reactor coolant system251.

FIG. 16is a concept view illustrating a facility2600for reducing radioactive material and a nuclear power plant260having the same according to yet still another embodiment of the present invention.

The passive safety injection system265may be installed selectively in or outside the boundary unit2620. Referring toFIG. 16, the passive safety injection system265is installed outside the boundary unit2620.

The safety injection line265cmay have an isolation valve265c1. The isolation valve265c1may be installed inside the boundary unit2620.

The pressure balance line265dmay also have isolation valves265d1and265d2. The isolation valves265d1and265d2, respectively, may be installed in and outside the boundary unit2620. Further, isolation valves or check valves may be added to the inside or outside of the boundary unit2620.

FIG. 17is a concept view illustrating a facility2700for reducing radioactive material and a nuclear power plant270having the same according to yet still another embodiment of the present invention.

The nuclear power plant270includes a feed water system277and a feed water supply line277a. The feed water supply line277ahas an isolation valve277b. Further, the nuclear power plant270includes a turbine system278and a steam line278a. The steam line278aalso has an isolation valve278b. When the nuclear power plant is in normal operation, water is supplied through the water supply line277ato the reactor coolant system271. The water receives heat from the core271awhile passing through the steam generator271b, and generates steam. The steam may be supplied through the steam line278ato the turbine system278.

The feed water supply line277aand the steam line278aalso pass through the boundary unit2720and the containment272. Accordingly, the feed water supply line277aand the steam line278aare also examples of the penetration line described above.

The boundary unit2720extends up to a region adjacent to the containment272while surrounding the steam line278a, the feed water supply line277a, and the pipes273and273′ penetrating the containment. Accordingly, even when pipe breakage occurs in the boundary unit2720, the radioactive material cannot exit the boundary unit2720. Further, the boundary unit2720is expanded to the region adjacent to the containment272, and the possibility that an accident such as feed line or steam line break accident occurs in the region between the boundary unit2720and the containment272may be significantly lowered. Accordingly, no isolation valve needs to be installed in the region between the boundary unit2720and the containment272. Resultantly, the present invention may reduce the number of isolation valves for closing the pipe line when an accident occurs.

FIG. 18is a concept view illustrating a facility2800for reducing radioactive material and a nuclear power plant280having the same according to yet still another embodiment of the present invention.

Additive injection units2880include a first additive injection unit2881and a second additive injection unit2882. The first additive injection unit2881is installed inside the cooling water storage unit2810. The second additive injection unit2882may be installed in a fluid path of the cooling water recollecting portion2810a.

The first additive injection unit2881may be installed at a predetermined height from the bottom of the cooling water storage unit2810to be submerged in the cooling water as the water level of the cooling water increases. If the fluid in the boundary unit2820is continuously sparged into the cooling water storage unit2810, the water level of the cooling water storage unit2810gradually increases. If the water level of the cooling water storage unit2810is higher than the water level of the additive injection unit2881, the additive injection unit2881is submerged in the cooling water. As the additive injection unit2881is submerged in the cooling water, the additive is dissolved in the cooling water.

Further, the second additive injection unit2882dissolves the additive in the condensed water recollected through the cooling water recollecting portion2810aas described above.

FIG. 19is a concept view illustrating a facility2900for reducing radioactive material and a nuclear power plant290having the same according to yet still another embodiment of the present invention.

The facility2900for reducing radioactive material is formed to configure a boundary of radioactive material between the containment292and the reactor coolant system291. The facility2900for reducing radioactive material is configured to capture radioactive material that may be discharged to the containment292when an accident occurs in the nuclear power plant290. The facility2900for reducing radioactive material includes a boundary unit2920, a discharging unit2930, and a filter facility2970.

The boundary unit2920is installed inside the containment292. The boundary unit2920forms a boundary of radioactive material in the containment292. When an accident occurs, radioactive material may release from the reactor coolant system291or pipes293,293′, and295cconnected with the reactor coolant system291to the inside of the containment292. The boundary unit2920wraps around the reactor coolant system291and the pipes293,293′ and295cto prevent release of radioactive material to the containment292.

The design pressure for radioactive material formed by the boundary unit2920is designed to withstand the pressure difference of a flow discharged from the discharging unit2930when an accident occurs. At least a portion of the boundary unit2920may be formed by a concrete structure inside the containment292. Further, at least a portion of the boundary unit2920may be formed by a coating member such as a steel liner et. al. installed on the concrete structure.

The boundary unit2920may include a barrier2921and a cover2922. The barrier2921is formed to wrap around the reactor coolant system291. As shown inFIG. 19, the barrier2921is configured to wrap around the remaining part except the upper part of the reactor coolant system291at a position spaced apart from the reactor coolant system291. The cover2922is formed to cover the upper part of the reactor coolant system291and is coupled with the barrier2921. Accordingly, at the time the reactor coolant system291disposed inside the boundary unit2920needs maintenance, the cover2922may be separated from the barrier2921to expose the reactor coolant system291.

The nuclear power plant290includes penetration pipes293,293′ and295cpenetrating the containment292. The terms “penetration pipes293,293′ and295c” may be used to denote all the pipes that have the feature of penetrating the containment292. For example, if a pipe used to make a primary fluid flow and a pipe used to make a secondary fluid flow penetrate the containment292, the pipes belong to the penetration pipes293,293′ and295c. Further, the safety injection line295cthat runs to the reactor coolant system291to form a safety injection fluid path also belongs to the penetration pipes293,293′ and295c. The penetration pipes293,293′ and295care connected to the reactor coolant system291or connected to a secondary system.

The penetration pipes293and203′ may have isolation valves293a,293b, and293a′ or check valves293b′ at positions spaced apart from each other to doubly close the containment292and the boundary unit2920when breakage occurs. If the boundary unit2920and the containment292are spaced apart from each other and the penetration pipes293and293′ pass through the region between the boundary unit2920and the containment292, an accident may occur due to breakage of the penetration pipes293and293′ in the region between the boundary unit2920and the containment292. In such case, the radioactive material might not be trapped in the reactive boundary unit2920. The isolation valves293a,293b, and293a′ have a mechanism to be opened and closed in response to a related safety system signal and thus may be likely to malfunction or halt. The check valve293b′ has a moving part and thus it is impossible to remove the possibility of malfunctioning or halting.

For the above reasons, the possibility of occurrence of a single failure may be granted an exception for some high-reliability devices, but the nuclear power plant290is basically designed to assume occurrence of a single failure when an accident occurs. Accordingly, considering a single failure, the isolation valves293a,293b, and293a′ or check valves293b′ should be installed at the portions of the penetration pipes293and293′ disposed between the containment292and the boundary unit2920to prevent additional release of radioactive material.

However, addition of the isolation valves293a,293b, and293a′ or check valves293b′ is not preferred in view of simplifying the facility. To address such issue, the present invention provides a structure that may prevent release of radioactive material even without installation of additional isolation valves293a,293b, and293a′ or check valves293b′. Hereinafter, the structure is described in detail.

At least a portion of the boundary unit2920is expanded up to a region adjacent to the containment292while surrounding the penetration pipes293and293′ to prevent an accident from occurring due to breakage of the penetration pipes293and293′ in a region between the boundary unit2920and the containment292. Due to such expanded structure of the boundary unit2920, the portions of the penetration pipes293and293′, which pass through the containment292to the inside of the containment292are mostly positioned inside the boundary unit2920. Accordingly, the present invention may significantly lower, by the expanded structure of the boundary unit2920, the possibility that a loss-of-coolant accident, feed line break accident or steam line break accident occurs due to, e.g., breakage of the penetration pipes293and293′ in the region between the boundary unit2920and the containment292.

The penetration pipes293and293′ may have a portion (first portion) disposed outside the containment292, a portion (second portion) disposed inside the boundary unit2920, and a portion (third portion) disposed between the containment292and the boundary unit2920. Under accident, as a combination of valves for isolating the containment292from the boundary unit2920, isolation valves293a,293b, and293a′ or check valve293b′ may be selectively adopted considering the direction of a flow in the penetration pipes293and293′ and flow resistance according to the characteristics of the nuclear power plant. The expanded structure of the boundary unit2920is configured to minimize the gap between the containment292and the boundary unit2920. Accordingly, the expanded structure of the boundary unit2920, even without additional installation of the isolation valves293a,293b, and293a′ at the third portion, may exclude the possibility that the penetration pipes293and293′ are broken at the third portion.

The discharging unit2930is installed at the boundary of radioactive material to form a fluid path that runs from the boundary unit2920to the containment292. If a pressure difference is created between the containment292and the boundary unit2920, the fluid flows from a place with a relatively high pressure to a place with a relatively low pressure. For example, when a loss-of-coolant accident occurs due to, e.g., pipe breakage, steam may be discharged from the reactor coolant system291or pipe connected with the reactor coolant system291. In such case, the pressure inside the boundary unit2920is rendered to be higher than the pressure inside the containment292. Accordingly, the fluid inside the containment2920is caused to flow to the containment292. As used herein, the term “pressure inside the containment292” refers to the pressure in the remaining space except the inner space in the containment292of the boundary unit2920.

The discharging unit2930is configured to guide the fluid flow caused by the pressure difference between the containment292and the boundary unit2920from the boundary unit2920through the fluid path to the containment292. The boundary unit2920forms a sealing structure around the reactor coolant system291to prevent the fluid from flowing from the boundary unit2920to the containment292through a path other than the fluid path formed by the discharging unit2930. For example, the boundary unit2920may be configured to surround the reactor coolant system291at the position spaced apart from the reactor coolant system291. Accordingly, the fluid inside the boundary unit2920may be discharged into the containment292only through the fluid path formed by the discharging unit2930but cannot be discharged via other paths. As used herein, the term “inside the containment292” refers to the remaining space in the containment292other than the inner space of the boundary unit2920.

The filter facility2970is installed in the fluid path of the discharging unit2930to capture the radioactive material contained in the fluid passing through the discharging unit2930in the boundary unit2920. The filter facility2970is configured to capture radioactive material in the boundary unit2920while the atmosphere inside the boundary unit2920is discharged through the fluid path of the discharging unit2930to the inside of the containment292.

The filter facility2970includes at least one of a filter and an absorbent. The term “additive” may be interchangeably used with the term “absorbent.”

As the filter, a high-efficiency particulate air (HEPA) filter may be adopted. The gaseous radioactive material contained in the fluid is removed while passing through the filter. For example, in case the radioactive material is iodine, iodine is combined with silver nitrate while passing through the filter to thus turn into iodic silver. Iodic silver may be separated from the fluid. Accordingly, the filter is configured to allow silver nitrate react with iodine contained in the fluid to form iodic silver. The filter is formed to eliminate iodic silver from the fluid.

As the absorbent, charcoal may be employed. Organic iodine compounds are combined with the materials impregnated in the charcoal to turn into quaternary ammonium salt and are absorbed into the charcoal. Molecular iodine is combined with the charcoal through chemisorption. The charcoal is utilized as an absorbent material thanks to its large internal surface area. Accordingly, the absorbent is configured to remove iodine contained in the fluid through chemisorption that is made by charcoal.

However, the above-described filter and the absorbent are merely an example, and the type of filter and absorbent according to the present invention is not limited thereto.

In order to prevent damage to the containment292that may occur due to a significant increase in the pressure inside the containment292and occurrence of an accident and to decrease the concentration of radioactive material discharged to the external environment, AREVA, France, and Westinghouse, U.S., have developed a filtered containment ventilation system (FCVS). The FCVS has a filter facility at the boundary between the inside and outside of the containment292and opens the boundary (using a breaking plate or valve) when an accident occurs to significantly increase the pressure inside the containment292, and discharges the atmosphere inside the containment292through the filter facility.

In case a beyond design basis accident (the beyond design basis accident refers to an accident that causes the pressure inside the containment292to be significantly increased to a design pressure or more) occurs in the nuclear power plant290adopting the FCVS, the breaking plate or valve installed between the inside of the containment292and the filter facility is opened, and a flow is caused by the pressure difference between the inside and outside of the containment292(between the high pressure created inside the containment292and the atmospheric pressure outside the containment292). The flow causes the atmosphere (air and steam) inside the containment292to pass through the filter facility and to be then discharged to the outside of the containment292.

However, the above-described, conventional FCVS is not operated when the design basis accident occurs, and the radioactive material is directly discharged to the inside of the containment292. Accordingly, the conventional FCVS, upon occurrence of a design basis accident, cannot lower the concentration of radioactive material inside the containment292and cannot resultantly suppress a certain amount of radioactive material releasing to the outside of the containment292.

In contrast, the present invention is configured to operate even when all types of accidents occur including a design basis accident and beyond design basis accident. The present invention is configured to force radioactive material in the boundary unit2920and to discharge a fluid having a low concentration of radioactive material to be discharged to the containment292. The radioactive material is captured in the boundary unit2920while passing through the filter facility2970. The present invention may reduce the concentration of radioactive material in the containment292in a very efficient manner, thus leading to a significant reduction in the amount of radioactive material releasing to the outside of the containment292.

The nuclear power plant290may further include a passive safety injection system295configured to inject a coolant into the reactor coolant system291using a natural force when an accident occurs. The passive safety injection system295may include a core makeup tank295aand a safety injection tank295b.

The core makeup tank295ais formed to store a coolant such as low-temperature boric acid solution. The core makeup tank295ais installed to have a height gap from the reactor coolant system291. The core makeup tank295aand the reactor coolant system291may be connected with each other by the pressure balance line295d. The pressure balance line295dis configured to form a pressure balance between the reactor coolant system291and the core makeup tank295aand is for allowing for coolant injection from the core makeup tank295aby gravity.

The safety injection tank295bis formed to store a coolant such as low-temperature boric acid solution. The safety injection tank295band the reactor coolant system291may be connected with each other through the pressure balance line295d. The safety injection tank295bmay be filled with some gas (typically, nitrogen gas). The pressure of the gas is set to be lower than the pressure of the reactor coolant system291that is in normal operation. When the nuclear power plant290is in normal plant operation, the safety injection tank295bis isolated by the check valve, so that the coolant inside the safety injection tank295bis not injected to the reactor coolant system291.

The passive safety injection system295includes a safety injection line295cconnected with the reactor coolant system291to form an injection fluid path for coolant. The core makeup tank295aand the safety injection tank295bare connected with the reactor coolant system291through the safety injection line295c. The safety injection line295cforms a fluid path for the coolant injected from the core makeup tank295aand the safety injection tank295bto the reactor coolant system291.

The safety injection line295cmay penetrate the containment292. Accordingly, the safety injection line295cmay be configured of an example of the above-described penetration pipes293and293′. The expanded structure of the boundary unit2920may apply to the safety injection line295cas well. At least a portion of the boundary unit2920may be expanded up to a region adjacent to the containment292while surrounding the safety injection line295cto prevent a loss-of-coolant accident that may occur due to breakage of the safety injection line295cbetween the boundary unit2920and the containment292. The other description of the expanded structure of the boundary unit2920is replaced by what has been described above therefor.

FIG. 19is a view illustrating a normal operation state of the nuclear power plant290. Accordingly, the isolation valves293a,293b, and293a′ installed on the pipes293and293′ for normal operation of the nuclear power plant290stay opened. In the normal operation of the nuclear power plant290, no steam is discharged from the reactor coolant system291, and thus, the pressure balance is maintained between the boundary unit2920and the containment292.

FIG. 20is a concept view illustrating a facility3000for reducing radioactive material and a nuclear power plant300having the same according to yet still another embodiment of the present invention.

The facility for reducing radioactive material includes a boundary unit3020, a discharging unit3030, and a filter facility3070. The facility for reducing radioactive material further includes a cooling water storage unit3010, a cooling water recollecting portion3010aand an opening portion3010b.

The cooling water storage unit3010is installed inside the containment302. For example, the cooling water storage unit3010may be installed in an upper or lower part of an inner space of the containment302. The cooling water storage unit3010is formed to store cooling water and may be formed as a tank or pool.

Among other radioactive materials spread to the external environment when an accident occurs in the nuclear power plant300, iodine may have a highest concentration. Iodine, when contacting water, is mostly dissolved in the water. The cooling water storage unit3010retains cooling water that may dissolve iodine.

Most of the radioactive materials are captured in the boundary unit3020by the filter facility3070while passing through the discharging unit3030. However, a small amount of radioactive material is not captured by the filter facility3070and may be discharged to the containment302or a small amount of radioactive material may leak from the boundary unit3020. However, a small amount of radioactive material discharged to the containment302, if dissolved by sprayed or condensed water of other containment302safety systems (for example, a containment spray system or cooling system) that may be employed as per the characteristics of the nuclear power plant300to be captured in the cooling water of the cooling water storage unit3010, may be cut off from releasing to the external environment. Accordingly, the cooling water storage unit3010may support the function of the filter facility3070.

The cooling water recollecting portion3010aforms a fluid path that runs from the containment302to the cooling water storage unit3010to recollect the condensed water created from the fluid discharged through the discharging unit3030to the containment302to the cooling water storage unit3010. However, in case the safety system is configured in combination with the spray system (not shown), the sprayed cooling water is also recollected to the cooling water recollecting portion3010a. For example, the cooling water recollecting portion3010amay be disposed to be adjacent to the inner wall of the containment302so that the condensed water flowing down the inner wall of the containment302is collected to the cooling water storage unit3010. However, the shape of the cooling water recollecting portion3010may be selectively adopted according to the characteristics of the nuclear power plant300. According to the present invention, the cooling water recollecting portion3010ahas a structure of introducing the cooling water inside the containment302such as sprayed water or condensed water to the cooling water storage unit3010and is not limited to a special shape of the cooling water recollecting portion3010a.

A portion of the fluid discharged through the discharging unit3030to the containment302is condensed to form condensed water. The concentration of boric acid in the condensed water is low, and the condensed water may contain a small amount of radioactive material. The condensed water is recollected from the containment302through the cooling water recollecting portion3010ato the cooling water storage unit3010.

The opening portion3010bis formed by opening at least a portion of the cooling water storage unit3010to maintain a pressure balance between the inside of the containment302and the cooling water storage unit3010. If a pressure difference is created between the cooling water storage unit3010and the containment302, the cooling water storage unit3010and the containment302may re-form a pressure balance by the opening portion3010b.

The cooling water storage unit3010may be configured of a single facility for the facility3000for reducing radioactive material only, but may be shared with other systems (for example, passive safety injection system305, residual heat removing system, etc.). Hereinafter, an example where the facility3000for reducing radioactive material and the passive safety injection system305share the cooling water storage unit3010is described.

The cooling water storage unit3010is connected to the safety injection line305cto inject the cooling water retained therein to the inside of the reactor coolant system301. The cooling water storage unit3010is installed at a higher position than the reactor coolant system301. The pipe3012connecting the cooling water storage unit3010with the safety injection line305chas an isolation valve3012aand a check valve3012b. If the isolation valve3012ais opened by a related signal when an accident occurs, a cooling water flow is generated from the cooling water storage unit3010to the reactor coolant system301. The check valve3012bis opened by the flow of cooling water, and the cooling water is injected through the safety injection line305cto the reactor coolant system301.

FIG. 21is a concept view illustrating a facility3100for reducing radioactive material and a nuclear power plant310having the same according to yet still another embodiment of the present invention.

The facility3100for reducing radioactive material further includes an additive injection unit3180.

The additive injection unit3180supplies an additive to increase the pH of the coolant to a predetermined value or more (typically pH 7 or more) to prevent volatilization of the radioactive material dissolved in the cooling water storage unit3110. As illustrated, the additive injection unit3180may be installed in a fluid path of the cooling water recollecting portion3110ato dissolve the additive in the cooling water, such as sprayed or condensed water, to the cooling water storage unit3110.

Radioactive iodine dissolved in the cooling water exists in the form of negative ions. In case the pH of the cooling water in which iodine is dissolved is low, the amount of radioactive iodine that is to be re-volatilized may be significantly increased. This is why the amount of radioactive iodine that is converted into volatilizable elemental iodine (12) is sharply increased in the cooling water of pH 7 or less. Besides, the amount that turns into elemental iodine is associated with the temperature of the cooling water and the concentration of iodine in the solution. The elemental iodine may be re-volatilized in the atmosphere according to a separation coefficient defined as a ratio in concentration of iodine in the atmosphere to iodine in the cooling water. According to related regulations, in case the pH of the cooling water is higher than 7.0, the amount that turns into elemental iodine is significantly reduced, so that re-volatilization may be negligible.

The additive injection unit3180supplies an additive to the cooling water, such as sprayed or condensed water, recollected to the cooling water storage unit3110to prevent re-volatilization of radioactive material. As the additive, sodium phosphate may be adopted. Sodium phosphate adjusts the pH of the cooling water to prevent re-volatilization of the radioactive nuclide or corrosion of the inside of the containment312upon accident. However, the type of additive according to the present invention is necessarily limited thereto. Boric acid to suppress the reactivity of the core311aor other additives to suppress corrosion of the device may be added so that the water quality of the cooling water storage unit3110is passively managed.

FIG. 22is a concept view illustrating a facility3200for reducing radioactive material and a nuclear power plant320having the same according to yet still another embodiment of the present invention.

The nuclear power plant320includes a steam generator321b. The steam generator321bis installed at the boundary between the primary system and the secondary system and generates steam through heat transfer of the primary fluid and secondary fluid. The steam generator321bforms a pressure boundary between the fluid path of the primary fluid and the fluid path of the secondary fluid path and thus the primary fluid and the secondary fluid are not mixed with each other.

In the normal operation of the nuclear power plant320, the feed water system327supplies water (secondary fluid) through the feed water supply line327ato the steam generator321b. The heat generated in the core321ais transferred to the primary fluid, and the primary fluid transfers heat to the secondary fluid while passing through the steam generator321b. The supplied water receives heat from the primary fluid while passing through the steam generator321b, and turns into steam. The steam discharged from the steam generator321bis delivered through the steam line328ato the turbine system328. In normal operation of the nuclear power plant320, the isolation valves327band328binstalled on the feed water supply line327aand the steam line328aremain opened.

The feed water supply line327aand the steam line328amay pass through the containment322. Accordingly, the feed water supply line327aand the steam line328amay be configured as examples of the above-described penetration pipes323and323′. At least a portion of the boundary unit3220may be expanded up to a region adjacent to the containment322while surrounding the feed water supply line327aand the steam line328aas well as the penetration pipes323and323′ to prevent an accident from occurring due to breakage of the feed water supply line327aand the steam line328abetween the boundary unit3220and the containment322.

Accordingly, the present invention may significantly lower the possibility that a steam line break accident or feed line break accident occurs due to breakage of the water supply line327aor steam line328ain the region between the boundary unit3220and the containment322by way of the expanded structure of the boundary unit3220. The facility3200for reducing radioactive material may minimize the gap between the boundary unit3220and the containment322by the expanded structure of the boundary unit3220to exclude the possibility that the penetration pipes323and323′ occur at the portion therebetween.

Unlike described above, various tanks325aand325bof the passive safety injection system325, rather than positioned inside the boundary unit3220, may be disposed between the boundary unit3220and the containment322. The safety injection line325cmay be split into a portion disposed inside the boundary unit3220, a portion disposed between the boundary unit3220and the containment322, and a portion disposed outside the containment322. Since the passive safety injection system325is disposed outside the boundary unit3220, check valves325fand325f′ are added to the safety injection line325c, and isolation valves325eand325e′ are added to the pressure balance line325d. However, the check valves325fand325f′ or isolation valves325eand325e′ may be selectively adopted considering the conditions such as direction of flow and flow resistance.

FIG. 23is a concept view illustrating a facility3300for reducing radioactive material and a nuclear power plant330having the same according to yet still another embodiment of the present invention.

The nuclear power plant330further includes a containment cooling system configured to suppress a rise in pressure inside the containment332. The containment cooling system may be a passive containment cooling system that suppresses a rise in the pressure inside the containment332using natural circulation.

The passive containment cooling system has a heat exchanger336b. The heat exchanger336b, as shown inFIG. 23, may be installed in the atmosphere of the containment332. However, the position of the heat exchanger336bis not necessarily limited thereto, and may be disposed in the cooling water storage unit3310. The cooling fluid is heat-exchanged with the atmosphere inside the containment332while passing through the heat exchanger336band is heated. The density of the heated cooling fluid is reduced, and the cooling fluid goes up along the fluid path of the heat exchanger336b. The cooling fluid is discharged from the heat exchanger336bto the outside of the containment332.

The steam discharged from the boundary unit3320to the containment332is condensed in the heat exchanger336bby natural circulation. The phenomenon that the steam is condensed to turn into condensed water reduces the steam partial pressure inside the containment332and thus functions to suppress a rise in the pressure inside the containment332. Typically, the passive containment cooling system shows a lower efficiency of reducing radioactive material as compared with the typical active containment spray system. However, in case the facility3300for reducing radioactive material proposed herein is adopted along with the passive containment cooling system, the concentration of radioactive material discharged by the facility3300for reducing radioactive material to the inside of the containment332may be remarkably reduced to solving the problems of the passive containment cooling system.

The cooling water recollecting portion3310ais disposed at a lower part of the heat exchanger336bto recollect the condensed water created by the operation of the heat exchanger336bto the cooling water storage unit3310. The condensed water generated in the heat exchanger336bmay be dropped and recollected to the cooling water storage unit3310through the fluid path of the cooling water recollecting portion3310a. In the process of recollecting the condensed water, the condensed water may be supplied with an additive from the additive injection unit3380. Accordingly, the pH of the condensed water may be adjusted, and the condensed water may be prevented from re-volatilization.

FIG. 24Ais a concept view illustrating the normal operation of a facility3400for reducing radioactive material and a nuclear power plant340having the same according to yet still another embodiment of the present invention.

The heat exchanger346aof the passive containment cooling system may be formed to cool both the cooling water in the cooling water storage unit3410and the atmosphere in the containment342. At least a portion of the heat exchanger346ais submerged in the cooling water storage unit3410and may be extended up to the inner space of the containment342from the cooling water storage unit3410.

Among the pipes343,343′ and345cpenetrating the containment342, the pipes343and343′ for normal operation of the nuclear power plant340allow the fluid to flow therethrough. The isolation valves343a,343b, and343a′ and the check valve343b′ installed on the pipes343and343′ are required for normal operation of the nuclear power plant340. During the normal operation of the nuclear power plant340, the isolation valves343a,343b, and343a′ and the check valve343b′ installed on the pipes343and343′ remain opened.

FIG. 24Bis a concept view illustrating an example in which an accident occurs in a facility3400for reducing radioactive material and a nuclear power plant340having the same according to yet still another embodiment of the present invention.

If an accident occurs in the nuclear power plant340, the nuclear power plant340may remain in safe shutdown condition by the operation of various safety systems. The passive residual heat removing system344removes sensible heat in the reactor coolant system341and residual heat in the core341a. The passive containment cooling system suppresses a rise in the pressure inside the containment342. The passive safety injection system345maintains the water level of the reactor coolant system341. The facility3400for reducing radioactive material captures radioactive material in the boundary unit3420.

If an accident such as a loss-of-coolant accident occurs, steam is discharged from the broken line343f. The discharged steam may be mixed with the atmosphere present inside the boundary unit3420. Since upon accident the isolation valves343a,343b, and343a′ and the check valve343b′ are closed, the fluid does not flow any longer through the pipes343and343′ for normal operation of the nuclear power plant340.

As steam is continuously discharged from the reactor coolant system341, the pressure inside the boundary unit3420is gradually increased, and a pressure difference is generated between the inside of the boundary unit3420and the inside of the containment342. The fluid created as the atmosphere and steam are mixed with each other forms a flow by the pressure difference. The fluid is discharged from the boundary unit3420through the discharging unit3430to the containment342. The radioactive material contained in the fluid is captured in the boundary unit3420while passing through the filter facility3470installed in the discharging unit3430.

The remaining fluid except the radioactive material is discharged to the inner space of the containment342. The fluid discharged to the containment342is mixed with the atmosphere in the containment342. Accordingly, the pressure and temperature of the containment342are gradually increased. However, the heat exchanger348of the passive containment cooling system is operated to suppress a rise in the pressure of the containment342. The atmosphere in the containment342(including the fluid discharged to the containment342) and the fluid supplied from the outside of the containment342exchange heat with each other while flowing through different fluid paths from each other. Accordingly, the atmosphere in the containment342is cooled and condensed in the heat exchanger346aby natural circulation.

The atmosphere in the containment342is cooled and condensed by the operation of the heat exchanger346a. The air contained in the atmosphere of the containment342is discharged back to the inside of the containment342, and the condensed water generated as the steam is condensed is recollected to the cooling water storage unit3410through the cooling water recollecting portion3410a. In this process, the condensed water is supplied with an additive for preventing re-volatilization from the additive injection unit3480. Accordingly, the condensed water is recollected to the cooling water storage unit3410, and the condensed water may be prevented from re-volatilization.

The cooling water stored in the cooling water storage unit3410is cooled by the heat exchanger346aby natural circulation.

The passive safety injection system345injects cooling water to the reactor coolant system341.

If upon accident a phenomenon such as reduction in pressure of the reactor coolant system341occurs, the isolation valve345a′ installed on the pipe connecting the core makeup tank345awith the safety injection line345cis opened in response to a related signal. A flow of the cooling water is caused by gravity water head from the core makeup tank345ato the reactor coolant system341, and the check valve345a″ is opened by the flow of the cooling water. The cooling water is injected from the core makeup tank345athrough the safety injection line345cto the reactor coolant system341.

If, upon accident, a phenomenon in which the pressure inside the reactor coolant system341is reduced to a predetermined value or less, for example, the isolation valve345d′ installed on the pressure balance line345dis opened by a related signal. The steam supplied from the reactor coolant system341is injected to the safety injection tank345bthrough the pressure balance line345d, and the pressure inside the safety injection tank345bincreases. If the pressure balance is formed between the reactor coolant system341and the safety injection tank345b, the cooling water inside the safety injection tank345bis injected to the reactor coolant system341by gravity water head. The check valve345b′ is opened by the flow of the cooling water, and the cooling water is injected to the reactor coolant system341through the safety injection line345c.

The cooling water retained in the cooling water storage unit3410may be used for safety injection. The isolation valve3412ainstalled on the pipe3412connecting the cooling water storage unit3410with the safety injection line345cis opened by a related signal, and as the reactor coolant system341is cooled after accident, the pressure inside the reactor coolant system341and the pressure inside the cooling water storage unit3410form a pseudo-balanced state, a flow of the cooling water is caused by gravity from the cooling water storage unit3410. The term “pseudo-balance” refers to a state that is not the theoretically complete balanced state but is close to the balanced state enough to form a flow of the cooling water. As the cooling water flows, the check valve3412bis opened, and the cooling water may be injected to the reactor coolant system341.

FIG. 25Ais a concept view illustrating the normal operation of a facility3500for reducing radioactive material and a nuclear power plant350having the same according to yet still another embodiment of the present invention.

The nuclear power plant350further includes an extended path3531and a circulation enhancement facility359.

The extended path3531is extended from the discharging unit3530up to an upper part of the heat exchanger356bto discharge the fluid from the discharging unit3530to the heat exchanger356b. The fluid in the boundary unit3520flows along the extended path3531and is discharged through the outlet of the extended path3531.

The circulation enhancement facility359is installed at the outlet of the extended path3531. The fluid is discharged through the circulation enhancement facility359. The circulation enhancement facility359may be configured in the form of a jet pump, for example. The circulation enhancement facility359is configured to introduce the atmosphere included in the containment352by a pressure decrease that is caused as the fluid is discharged with high velocity. The circulation enhancement facility359is configured to inject the introduced atmosphere together with the fluid.

The circulation enhancement facility359includes a zet nozzle unit359aand an atmosphere entrainment unit359b.

The zet nozzle unit359ais connected with the outlet of the extended path3531to receive the fluid from the extended path3531. The zet nozzle unit359ais formed to inject the received fluid to the heat exchanger356b.

The atmosphere entrainment unit359bwraps around the zet nozzle unit359aat the position spaced apart from the zet nozzle unit359ato form an atmosphere inlet space around the zet nozzle unit359a. For example, the atmosphere entrainment unit359bmay form a ring-shaped atmosphere inlet space around the zet nozzle unit359a. The atmosphere entrainment unit359bis configured to inject the atmosphere introduced through the atmosphere inlet space, together with the fluid.

The atmosphere in the containment352may be circulated more actively by the circulation enhancement facility359. This means that a small amount of the remaining radioactive material and the steam released to the inside of the containment352may be guided to the heat exchanger356b. Accordingly, the steam may be condensed, and the soluble radioactive material may be dissolved in the condensed water and recollected to the cooling water storage unit3510.

Further, the circulation enhancement facility359mitigates a decrease in the efficiency of the heat exchanger356bthat occurs due to accumulation of a noncondensable gas (air) around the heat exchanger356b. The performance of the heat exchanger356bmay be enhanced through forced circulation by the circulation enhancement facility359. Further, the circulation enhancement facility359may increase the speed of flow at the periphery of the heat exchanger356bto assist in enhancing the heat transfer coefficient.

FIG. 25Bis a concept view illustrating an example in which an accident occurs in a facility3500for reducing radioactive material and a nuclear power plant350having the same according to yet still another embodiment of the present invention.

When an accident occurs in the nuclear power plant350, the nuclear power plant350may stay in safe shutdown condition by the operation of various safety systems. The passive residual heat removing system removes sensible heat in the reactor coolant system351and residual heat in the core351a. The passive containment cooling system355maintains the water level of the reactor coolant system351. The facility3500for reducing radioactive material captures radioactive material in the boundary unit3520.

The fluid inside the boundary unit3520flows along the extended path3531connected with the discharging unit3530and is injected to the heat exchanger356bthrough the zet nozzle unit359a. If the fluid is injected with a high speed, a pressure drop phenomenon locally occurs. Accordingly, the atmosphere inside the containment352is introduced to the atmosphere entrainment unit through the atmosphere inlet space, and the atmosphere entrainment unit359binjects the introduced atmosphere, together with the fluid, to the heat exchanger356b.

The atmosphere and fluid are cooled and condensed in the heat exchanger356b. The air is discharged, and the condensed water created by the operation of the heat exchanger is recollected to the cooling water storage unit3510through the cooling water recollecting portion3510a. Since the additive injection unit3580injects an additive to the condensed water during the process of recollecting the condensed water, the condensed water may be prevented from re-volatilization.

The description of the others is replaced with what has been described above.

FIG. 26Ais a concept view illustrating the normal operation of a facility3600for reducing radioactive material and a nuclear power plant360having the same according to yet still another embodiment of the present invention.

The discharging unit3630is extended from the boundary unit3620up to the inside of the cooling water storage unit3610to discharge the atmosphere inside the boundary unit3620to the cooling water storage unit3610. The outlet of the discharging unit3630is submerged in the cooling water of the cooling water storage unit3610. Accordingly, the fluid in the boundary unit3620is not directly discharged to the containment362and is discharged to the cooling water in the cooling water storage unit3610.

The facility3600for reducing radioactive material further includes a sparging unit3640. The sparging unit3640is installed at an end of the discharging unit3630to be submerged in the cooling water of the cooling water storage unit3610. The sparging unit3640sparges the fluid that has passed through the discharging unit3630. The fluid contains steam and air, and the sparging unit3640is configured to sparge the air while condensing the steam. In case the facility3600for reducing radioactive material includes the sparging unit3640, the design pressure for the boundary of radioactive material formed by the boundary unit3620is designed considering water head. The sparging unit3640may have a flow resistance in its inner fluid path to induce an even distribution of the fluid to the plurality of fine fluid paths. The fluid may be relatively evenly distributed to each fine fluid path by the flow resistance. As the steam is condensed, the pressure inside the containment362may be suppressed from increasing.

Since the non-condensed air inside the boundary unit3620is discharged to the inside of the containment362, the pressure inside the containment362may increase. However, since the volume of the inside of the boundary unit3620is relatively smaller than the volume of the inside of the containment362, the pressure inside the containment362is not greatly increased.

The containment362and the boundary unit3620may be connected with each other via a pressure balance line (not shown). The pressure balance line may have a check valve (not shown), and the sparging unit3640, unlike shown, may be installed on the pressure balance line. In case as long-term cooling or a loss-of-coolant accident occurs outside the boundary unit3620, the pressure inside the containment362is higher than the pressure inside the boundary unit3620, the check valve of the pressure balance line is opened, and the containment362and the boundary unit3620form a pressure balance. Since the atmosphere inside the containment362is introduced to the inside of the boundary unit3620through the pressure balance line, the pressure balance line may prevent the cooling water in the cooling water storage unit3610from flowing back to the inside of the boundary unit3620.

The additive injection unit3680may be installed in each of the cooling water storage unit3610and the cooling water recollecting portion3610a. The first additive injection unit3681is installed in the cooling water storage unit3610. The second additive injection unit3682is installed in the cooling water recollecting portion3610a.

The first additive injection unit3681is installed at a predetermined height of the cooling water storage unit3610to be submerged in the cooling water by a rise in the water level of the cooling water storage unit3610. As the first additive injection unit3681is submerged in the cooling water, the additive is dissolved in the cooling water, and thus, the first additive injection unit3681may prevent the radioactive material from volatilizing.

The second additive injection unit3682injects an additive to the condensed water recollected through the cooling water recollecting portion3610ato the cooling water storage unit3610. The description of the functions of the second additive injection unit3682is replaced by what has been described above.

FIG. 26Bis a concept view illustrating an example in which an accident occurs in a facility3600for reducing radioactive material and a nuclear power plant360having the same according to yet still another embodiment of the present invention.

When an accident occurs, the fluid inside the boundary unit3620is sparged through the fluid path of the discharging unit3630to the inside of the cooling water storage unit3610. As the fluid is sparged from the sparging unit3640, the steam is condensed and the air is cooled. The air may be discharged through the opening portion3610bto the inner space of the containment362. The atmosphere inside the containment362is introduced into the heat exchanger366aby way of natural circulation.

The atmosphere in the containment362is cooled and condensed by the heat exchanger366a. The air is discharged back to the inner space of the containment362, and the condensed water is introduced through the cooling water recollecting portion3610ato the cooling water storage unit3610. While passing through the cooling water recollecting portion3610a, an additive is supplied from the second additive injection unit3682. As the water level of the cooling water storage unit3610gradually increases, the first additive injection unit3681is submerged in the cooling water, and the additive is dissolved in the cooling water. The condensed water and the cooling water may be prevented from re-volatilizing by the first additive injection unit3681and the second additive injection unit3682.

The above-described facility for reducing radioactive material has been proposed to solve the problems with expanding exclusion area boundary (EAB) that may occur when a passive safety system is introduced. In case an accident occurs in the nuclear power plant (except some limited quantities of leakage), a majority of radioactive materials discharged from the reactor coolant system or pipe line connected with the reactor coolant system is configured to be sparged into a cooling water storage unit such as a large pool or tank through a sparging unit, thus significantly decreasing the concentration of the radioactive material in the containment. Further, release of the radioactive material to the external environment may be minimized.

Use of the facility for reducing radioactive material may resolve the issue of expanding EAB that may be caused by adopting the passive safety system in the nuclear power plant and allows for easy introduction of a passive safety system with excellent effects in enhancing safety. A reduction in the EAB may save economical expense, and the facility for reducing radioactive material may maintain the function of reducing radioactive material for a long time as long as the cooling water storage unit maintains a predetermined water level or more, thus contributing to enhanced safety of the nuclear power plant.

According to the present invention, when a loss-of-coolant accident occurs, a filter facility may be used to capture the radioactive material in the boundary unit and may suppress a rise in the concentration of the radioactive material in the containment.

Further, according to the present invention, the concentration of the radioactive material in the containment is suppressed from increasing to remarkably reduce the EAB, and release of radioactive material to the external environment may be minimized. Accordingly, the nuclear power plant may enjoy significantly enhanced safety, as well as savings in the economical expense. According to the present invention, further, the problem with expanding EAB may be resolved, and a passive safety system with excellent safety enhancing effects may be applied to the nuclear power plant.

Further, according to the present invention, the pH of the cooling water in the cooling water storage unit may be controlled by a passive manner to suppress re-volatilization of radioactive material while hardly increasing the number of isolation valves, and even when the radioactive material is re-volatilized, the radioactive material may be suppressed from being discharged to the inside of the containment.