Containment vessel venting method

Abstract

To provide a containment vessel venting method using a rare gas-impermeable filter, which can reduce a leakage amount of rare gas leaking to the outside while improving the durability of a membrane filter.SOLUTION: A venting method in a reactor containment vessel vent system for discharging gas inside a reactor containment vessel to the outside and decompressing the reactor containment vessel, comprises providing a structure in a vent line of the reactor containment vessel that has a membrane with a permeability coefficient of water vapor larger than that of radioactive noble gas, and that separates the water vapor and the radioactive noble gas from gas passing through the vent line by the membrane, and configuring this structure to operate at a pressure above a certain level and stop at a pressure below the certain level, depending on a pressure of the reactor containment vessel, thereby performing containment vessel venting intermittently.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a containment vessel venting method using a reactor containment vessel venting system used in a nuclear power plant. [Background technology]

[0002] One of the functions of the reactor containment vessel installed in a nuclear power plant is to confine radioactive materials within the reactor containment vessel and prevent them from leaking outside, even if an incident occurs in which the reactor core located inside the reactor pressure vessel melts (hereinafter referred to as a "severe accident") and radioactive materials are released outside the reactor pressure vessel.

[0003] Even if a severe accident occurs, the accident can be resolved if sufficient water is subsequently injected and the reactor containment vessel is cooled. However, if steam production continues and the containment vessel is not sufficiently cooled, the containment vessel will become pressurized. When the reactor containment vessel becomes pressurized, the gas inside the reactor containment vessel may be released into the atmosphere to reduce the pressure inside the vessel. This operation is called containment vessel venting.

[0004] Boiling water reactors are equipped with a reactor containment vessel vent system to minimize public exposure even when venting operations are performed.

[0005] In a boiling water reactor, most of the radioactive material is first removed by scrubbing with water from the suppression pool inside the reactor containment vessel.Then, the gas inside the reactor containment vessel (hereafter referred to as "vent gas") is released into a chemical solution in a tank outside the reactor containment vessel and scrubbed to remove particulate radioactive material, inorganic iodine, and cesium. Next, a metal filter is used to remove any particulate radioactive material that could not be removed by scrubbing. Finally, gaseous radioactive materials such as organic iodines are removed by chemical reaction and adsorption in the iodine filter, and the vent gas is released into the atmosphere.

[0006] However, it is difficult to remove radioactive rare gases that have no chemical reactivity using the above-mentioned techniques. Therefore, the reactor containment vessel vent system described in Patent Document 1 uses a membrane filter that allows water vapor to pass through but not rare gases, thereby removing the radioactive rare gases that cannot be removed by the above-mentioned reactor containment vessel vent system. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2018-179693 A Summary of the Invention [Problem to be solved by the invention]

[0008] A reactor containment vessel vent system aimed at removing radioactive rare gases using a membrane filter removes radioactive rare gases by installing a membrane filter that allows water vapor and hydrogen to pass but not rare gases in the flow path through which the vent gas passes, as described in Patent Document 1.

[0009] Aerosols and iodine contained in the atmosphere inside the containment vessel during an accident may reduce the membrane filter's performance if they adhere to it, so it is necessary to remove them as much as possible. Also, the higher the temperature and pressure of the gas flowing into the membrane filter, the higher the load on the membrane filter, so reducing these temperatures and pressures as much as possible can extend the membrane filter's lifespan.

[0010] Current boiling water nuclear power plants are already equipped with a filter vent device, and although the vent gas that passes through the filter vent device contains radioactive rare gases, the pressure and temperature of the vent gas have been reduced. Therefore, by placing a membrane filter downstream of the filter vent device, the load on the membrane filter can be reduced.

[0011] Because the permeability of rare gases through the membrane filter is very low, almost no rare gases leak out. However, because a certain amount of rare gases permeates through the membrane filter in proportion to the permeability, membrane area, and the partial pressure of the rare gases inside the containment vessel, in order to prevent exposure to rare gases as much as possible, it is necessary to vent the containment vessel using a venting method that can reduce the amount of rare gases leaking out.

[0012] However, with conventional venting, the vent equipment is activated when the pressure inside the containment vessel rises above a certain level, and the gas discharge line is then kept open until the accident is resolved; this venting method is not intended to reduce the amount of rare gases released.

[0013] Since the membrane filter is almost impermeable to rare gases, when a mixture of rare gas and water vapor flows into the membrane filter section, the rare gas, which is almost impermeable, remains there, preventing the release of water vapor to the outside. In Patent Document 1, when the release of water vapor is hindered and the pressure in the containment vessel increases, a relief valve opens to release the trapped rare gas, allowing water vapor to again permeate the membrane filter. However, in the structure of Patent Document 1, the membrane filter itself needs to be installed inside the containment vessel, and it is difficult to reduce the load on the membrane filter in the downstream part of the filter vent device, so it is necessary to use a highly durable membrane, etc.

[0014] Vent gas, which is mainly composed of water vapor, flows into the membrane filter, and the membrane properties are used to preferentially allow water vapor and hydrogen to pass through and be released to the outside. In order to reduce the pressure in the containment vessel, a large amount of water vapor needs to be released, and a membrane filter mass (membrane area) commensurate with the amount of water vapor released is required. On the other hand, since the amount of radioactive rare gas leaking to the outside is proportional to the membrane area, increasing the membrane area also increases the amount of radioactive rare gas leaking to the outside. In addition, since high-temperature water vapor is ventilated, the membrane filter deteriorates depending on the ventilation time. Depending on the resistance of the membrane, a mechanism to prepare for the deterioration of the membrane filter (for example, switching the membrane filter every time aeration is performed for a certain period of time) may be required.

[0015] In consideration of the above problems, the present invention provides a containment vessel venting method using a filter that is impermeable to rare gases, which can reduce the amount of rare gas leaking to the outside while improving the durability of the membrane filter.

[0016] The above and other objects of the present invention and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings. [Means for solving the problem]

[0017] The containment vessel venting method of the present invention is a method of venting in a reactor containment vessel vent system for discharging gas inside the reactor containment vessel to the outside and reducing the pressure in the reactor containment vessel, in which a structure is provided in the vent line of the reactor containment vessel that has a membrane having a water vapor permeability coefficient greater than that of a radioactive noble gas, and in which the gas passing through the vent line is separated into water vapor and radioactive noble gas by the membrane. In the containment vessel venting method of the present invention, this structure is configured so that it operates when the pressure in the reactor containment vessel is above a certain level and stops when the pressure is below a certain level, depending on the pressure in the reactor containment vessel, thereby performing containment vessel venting intermittently. Effect of the Invention

[0018] According to the containment vessel venting method of the present invention, a structure is provided in which a membrane has a water vapor permeability coefficient greater than that of a radioactive noble gas, and the gas passing through the vent line is separated into water vapor and radioactive noble gas by the membrane, so that the amount of radioactive noble gas leaking to the outside can be reduced by the membrane. Furthermore, according to the present invention, the above-mentioned structure is configured to operate at a pressure equal to or higher than a certain level and to stop at a pressure equal to or lower than a certain level depending on the pressure in the reactor containment vessel, thereby performing containment vessel venting intermittently, thereby improving the durability of the membrane compared to conventional containment vessel venting that is performed continuously. Therefore, even in the unlikely event that gas containing radioactive materials flows out from the reactor pressure vessel into the reactor containment vessel and the reactor containment vessel becomes pressurized, it is possible to prevent the reactor containment vessel from being pressurized and to prevent radioactive materials from leaking to the outside.

[0019] Problems, configurations and effects other than those described above will become apparent from the following description of the embodiments. [Brief description of the drawings]

[0020] [Figure 1] 1 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to a first embodiment. [Diagram 2] FIG. 13 is a diagram showing an example of evaluation of the behavior of the pressure in a plant when the venting method of the present invention is performed with and without a rare gas filter. [Diagram 3] FIG. 10 is a diagram comparing the release rate of radioactive rare gas leaking to the outside by a general venting method and the venting method of the present invention, when a rare gas filter is provided and when it is not provided. [Figure 4] FIG. 11 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to a second embodiment. [Diagram 5] FIG. 11 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to a third embodiment. [Figure 6] FIG. 11 is a vertical cross-sectional view showing a schematic configuration of a reactor containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to a fourth embodiment. [Figure 7] FIG. 13 is a vertical cross-sectional view showing a schematic configuration of a reactor containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to a fifth embodiment. [Figure 8] FIG. 13 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method of a sixth embodiment. [Figure 9] FIG. 13 is a vertical cross-sectional view showing a schematic configuration of a reactor containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to a seventh embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, the embodiments and examples of the present invention will be described with reference to text and drawings. However, the structures, materials, and other specific configurations shown in the present invention are not limited to the embodiments and examples described here, and can be appropriately combined or improved without changing the gist of the invention. Elements that are not directly related to the present invention are not shown.

[0022] The containment vessel venting method of the present invention is a method for performing venting in a containment vessel vent system for depressurizing a containment vessel by discharging gas inside the containment vessel to the outside. The containment vessel venting method of the present invention provides a structure in which a membrane having a water vapor permeability coefficient greater than that of a radioactive rare gas is provided in the vent line of a reactor containment vessel, and the gas passing through the vent line is separated into water vapor and radioactive rare gas by the membrane. In the containment vessel venting method of the present invention, this structure is configured so that it operates when the pressure in the reactor containment vessel is above a certain level and stops when the pressure is below a certain level, depending on the pressure in the reactor containment vessel, thereby performing containment vessel venting intermittently.

[0023] According to the containment vessel venting method of the present invention, a structure is provided in which a membrane has a water vapor permeability coefficient greater than that of a radioactive noble gas, and the gas passing through the vent line is separated into water vapor and radioactive noble gas by the membrane, so that the amount of radioactive noble gas leaking to the outside can be reduced by the membrane. In addition, the above-mentioned structure is configured to operate at a pressure above a certain level and stop at a pressure below a certain level depending on the pressure in the reactor containment vessel, thereby performing containment vessel venting intermittently, thereby improving the durability of the membrane compared to conventional containment vessel venting that is performed continuously. Therefore, even in the unlikely event that gas containing radioactive materials flows out from the reactor pressure vessel into the reactor containment vessel and the reactor containment vessel becomes pressurized, it is possible to prevent the reactor containment vessel from being pressurized and to prevent radioactive materials from leaking to the outside.

[0024] In the above-described containment vessel venting method, a valve at the outlet from the reactor containment vessel can be opened when the pressure in the reactor containment vessel reaches or exceeds a certain level, and closed when the pressure in the reactor containment vessel falls below the certain level, thereby allowing containment vessel vent to be performed intermittently. In this configuration, the containment vessel can be vented intermittently by utilizing a valve at the outlet of the reactor containment vessel, which is conventionally installed in nuclear reactor plants, making it unnecessary to add a valve for intermittent containment vessel venting.

[0025] In the above-mentioned containment vessel venting method, an openable / closable valve may be provided after the valve at the outlet from the reactor containment vessel and upstream of the membrane, and the openable / closable valve may be opened when the pressure in the reactor containment vessel reaches or exceeds a certain level, and closed when the pressure in the reactor containment vessel falls below the certain level, thereby enabling containment vessel vent to be performed intermittently. In this configuration, a valve is added to perform intermittent containment venting, but the function of the valve (isolation valve) at the outlet from the reactor containment vessel can be maintained, and there is no need to modify the existing isolation valve.

[0026] In the above-described containment vessel venting method, by providing a valve that is configured to passively open when the pressure in the reactor containment vessel reaches or exceeds a certain level and to passively close when the pressure in the reactor containment vessel falls below the certain level, it is possible to configure the containment vessel vent to be performed intermittently. With this configuration, containment vessel venting is performed intermittently using a valve that passively opens and closes in response to the pressure in the reactor containment vessel, making it possible to omit a mechanism for detecting the pressure in the reactor containment vessel and a mechanism for opening and closing the valve.

[0027] In the above-mentioned containment vessel venting method, a scavenging mechanism for scavenging non-permeating gas that does not permeate the membrane may be provided, and the scavenging mechanism may be operated when the pressure in the reactor containment vessel reaches or exceeds a certain level, and may be stopped when the pressure in the reactor containment vessel falls below the certain level, thereby enabling containment vessel vent to be performed intermittently. With this configuration, a scavenging mechanism is added to perform intermittent containment vessel venting, but the function of the valve (isolation valve) at the outlet from the reactor containment vessel can be maintained, and there is no need to modify the existing isolation valve.

[0028] In the above-described containment vessel venting method, containment vessel vent can be configured to be performed intermittently by providing either one of a valve that opens and closes depending on the pressure in the reactor containment vessel, or a scavenging mechanism that starts and stops its operation depending on the pressure in the reactor containment vessel, or a combination of these mechanisms.

[0029] In the above method for venting a containment vessel, the membrane may be configured to be impermeable to radioactive noble gases and nitrogen, but permeable to hydrogen and water vapor. In this configuration, hydrogen and water vapor can permeate the membrane, and the hydrogen and water vapor that have permeated the membrane can be discharged to the outside, lowering the pressure in the containment vessel. On the other hand, radioactive rare gases cannot permeate the membrane, so leakage of the radioactive rare gases can be suppressed.

[0030] In the above containment vessel venting method, the membrane filter material may be a polymer membrane, a ceramic membrane, or a graphene oxide membrane. In this configuration, the filter material of the membrane is impermeable to radioactive noble gases and nitrogen, but is permeable to hydrogen and water vapor.

[0031] In the above-mentioned containment vessel venting method, the boiling water reactor may be equipped with a containment vessel vent system, which is operated at a certain pressure according to the pressure of the reactor containment vessel and stopped when the pressure is above a certain level, thereby performing the containment vessel vent intermittently. With this configuration, in a boiling water reactor, containment vessel vent can be performed intermittently, preventing pressurization of the reactor containment vessel and preventing leakage of radioactive materials to the outside.

[0032] In the above-described containment vessel vent method, the pressurized water reactor may be provided with a containment vessel vent system, which is operated at a constant pressure according to the pressure of the reactor containment vessel and stopped when the pressure is above the constant pressure, thereby performing containment vessel vent intermittently. With this configuration, in a pressurized water reactor, containment vessel vent can be performed intermittently, preventing pressurization of the reactor containment vessel and preventing leakage of radioactive materials to the outside.

[0033] In the above-mentioned containment vessel venting method, any one of a heavy water reactor, a graphite reactor, a gas reactor, a high temperature gas reactor, a supercritical pressure light water cooled reactor, a molten salt reactor, a gas-cooled fast reactor, a sodium-cooled fast reactor, and a lead-cooled fast reactor is provided with a containment vessel vent system, and the containment vessel vent system can be configured to operate at a certain pressure according to the pressure of the reactor containment vessel and stop when the pressure is above the certain pressure, thereby performing containment vessel vent intermittently. With this configuration, containment vessel vent can be performed intermittently in any of a heavy water reactor, a graphite reactor, a gas reactor, a high-temperature gas reactor, a supercritical pressure light water cooled reactor, a molten salt reactor, a gas-cooled fast reactor, a sodium-cooled fast reactor, and a lead-cooled fast reactor, thereby preventing pressurization of the reactor containment vessel and preventing leakage of radioactive materials to the outside. EXAMPLES

[0034] A specific embodiment of the containment vessel venting method will now be described.

[0035] Example 1 A containment vessel venting method according to a first embodiment, which is one of preferred embodiments for achieving the above-mentioned object, will be described with reference to FIG. 1 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to embodiment 1. The area enclosed by a dashed line in the figure is the containment vessel vent system according to the present invention.

[0036] The containment vessel venting method of this embodiment reduces the pressure inside the reactor containment vessel in the event of a severe accident such as a break in the reactor pressure vessel, and removes as much radioactive material as possible during the reduction in pressure.

[0037] The reactor containment vessel vent system shown in FIG. 1 is an example of an application to an advanced boiling water reactor (ABWR), and has the system configuration described below. A reactor pressure vessel 3 containing a reactor core 2 is installed inside a reactor containment vessel 1. A main steam pipe 4 is connected to the reactor pressure vessel 3, which sends steam generated inside the reactor pressure vessel 3 to a turbine (not shown).

[0038] The inside of the reactor containment vessel 1 is divided into a dry well 5 and a wet well 7 by a diaphragm floor 12 made of reinforced concrete. The wet well 7 refers to the area in which pool water is stored. The pool in this wet well 7 is called the suppression pool 8. The dry well 5 and the wet well 7 are interconnected by a vent pipe 11, and the vent pipe exhaust section 11a opens below the water surface of the suppression pool 8 in the wet well 7.

[0039] In the unlikely event that a pipe rupture accident occurs in which part of the piping is damaged and steam is released into the reactor containment vessel 1 (commonly known as a LOCA, occurring in the drywell 5 through which the piping passes), the pressure in the drywell 5 will rise due to the steam flowing out from the rupture. At that time, the steam released into the drywell 5 will be guided through the vent pipe 11 into the water of the suppression pool 8 in the wetwell 7 due to the pressure difference between the drywell 5 and the wetwell 7. The water in the suppression pool 8 condenses the steam, suppressing the pressure rise inside the reactor containment vessel 1. At this time, if the steam contains radioactive materials, the scrubbing effect of the water in the suppression pool 8 will remove most of the radioactive materials.

[0040] As described above, if a pipe rupture accident occurs in the dry well 5, steam flowing out from the rupture will pass through the vent pipe 11 and be condensed in the suppression pool 8. Similarly, if the pressure in the reactor pressure vessel 3 or the main steam pipe 4 increases, steam will be released into the suppression pool 8 to lower the pressure in the reactor pressure vessel 3 and the main steam pipe 4. At the same time, the released steam is condensed in the suppression pool 8 to mitigate the pressure rise in the reactor containment vessel 1. As a device for this purpose, in the ABWR, a steam relief valve 6 is installed in the area of ​​the dry well 5 in the reactor containment vessel 1. The steam released through the steam relief valve 6 passes through a steam relief valve exhaust pipe 9, and is finally released from a quencher 10 into the suppression pool 8, where it is condensed by the pool water in the suppression pool 8.

[0041] By condensing the steam into liquid water in the suppression pool 8, the volume of the steam is significantly reduced, making it possible to suppress a rise in pressure in the reactor containment vessel 1. Furthermore, if the steam contains radioactive materials, the scrubbing effect of the water in the suppression pool 8 removes most of the radioactive materials.

[0042] By condensing the steam in the suppression pool 8 and cooling the pool water in the suppression pool 8 using a residual heat removal system (not shown), it is possible to prevent an increase in temperature and pressure in the reactor containment vessel 1 and to resolve the accident. However, if the residual heat removal system loses function, although this is an extremely unlikely possibility, the temperature of the water in the suppression pool 8 will rise. As the temperature of the pool water rises, the partial pressure of the steam in the reactor containment vessel 1 will rise to the saturated vapor pressure at the temperature of the pool water, causing the pressure in the reactor containment vessel 1 to rise.

[0043] If such a pressure rise occurs, it can be suppressed by spraying cooling water inside the reactor containment vessel 1. This spray can also be operated from the outside by connecting a fire pump or the like. However, if this spray also fails to work, the pressure in the reactor containment vessel 1 will rise, although this is an even more remote possibility. If such a pressure rise in the reactor containment vessel 1 occurs, the pressure rise in the reactor containment vessel 1 can be suppressed by releasing the gas inside the reactor containment vessel 1 to the outside. This operation is called the vent operation. In a boiling water reactor, this vent operation is performed by releasing the gas inside the wet well 7, and the gas can be released to the outside after removing as much radioactive material as possible with the water in the suppression pool 8.

[0044] In carrying out this venting operation, a reactor containment vessel vent system is used as a device for further removing radioactive materials from the gas being released to the outside. Next, a conventional general vent system will be described with reference to FIG.

[0045] The vent pipe 13 is connected to the dry well 5 and the wet well 7 of the reactor containment vessel 1, and vent operation valves 14a, 14b are provided on this vent pipe 13. That is, the vent operation valve 14a on the wet well 7 side is provided for the wet well 7, and the vent operation valve 14b on the dry well 5 side is provided for the dry well 5.

[0046] The venting operation is usually performed by opening the vent operation valve 14a on the wet well 7 side. The vent pipe 13 is connected to an inlet pipe 17 of a filter vent container 16 in a filter vent device 15. The tip side of this inlet pipe 17 opens into the filter vent container 16.

[0047] Pool water 18 for scrubbing is stored in the lower portion of the filter vent vessel 16 . A metal mesh filter 19 is installed on the upper side of the filtered vent container 16. One end of an outlet pipe 20 of the filtered vent container 16 is connected to this metal filter 19. The other end of the outlet pipe 20 penetrates a partition wall 21 surrounding the filter vent container 16 and is led out of the partition wall 21. Finally, the gas is discharged from an exhaust tower 22 to the outside. In addition, an iodine filter 38 is installed midway along the outlet pipe 20 between the filter vent container 16 and the shielding wall 21 .

[0048] When the pressure in the reactor containment vessel 1 rises and a pressure detection mechanism 40 (such as a general pressure gauge) detects that the pressure has risen above a certain pressure setting, the vent valve 14a on the wet well 7 side is usually opened. At this time, the released gas is scrubbed with water from the suppression pool 8, thereby removing most of the radioactive material. This is a safety feature of boiling water reactors.

[0049] The released gas that has entered the filter vent device 15 is further scrubbed by the scrubbing pool water 18 in the filter vent vessel 16, removing most of the radioactive material, mainly in the form of aerosol. Furthermore, the metal filter 19 and the iodine filter 38 remove gaseous radioactive material such as iodine. When the pressure detected by the pressure detection mechanism 40 falls below a certain level, the vent operation valve 14a on the wet well 7 side is closed, thereby stopping the vent operation and also stopping the ventilation to the rare gas filter 23 described below.

[0050] By the above operation, most of the radioactive materials are removed, and the release gas from which the radioactive materials have been removed is released from the exhaust tower 22. However, while this system removes most of the radioactive material, radioactive noble gases are not removable by typical filter vent systems due to their low reactivity, and therefore current venting operations must wait until the radioactive noble gases decay before they can be performed, which is a relatively short time after a reactor scram.

[0051] Therefore, in the containment vessel vent system according to this embodiment, a rare gas filter 23 is installed in the outlet piping 20 at a position downstream of the filtered vent vessel 16 of the filtered vent device 15 . The radioactive rare gas is trapped by this rare gas filter 23, and a filter material that allows steam to pass through is used for this rare gas filter 23. This allows the steam to be released to the outside, thereby reducing the pressure in the reactor containment vessel 1.

[0052] In a typical venting method, when the pressure exceeds a certain value, the vent operation valve 14a on the wet well 7 side is kept open. In contrast, in the venting method of the present invention, when the pressure exceeds a certain value, the vent operation valve 14a on the wet well 7 side is opened, and when the pressure falls below the certain value, the vent operation valve 14a on the wet well 7 side is closed. In the venting method of the present invention, by performing such opening and closing venting operations, water vapor is sufficiently released to the outside, similar to the general venting method, thereby reducing the pressure in the reactor containment vessel 1 and protecting the reactor containment vessel 1. For this reason, in the venting method of the present invention, the membrane area (material amount) of the rare gas filter 23 is larger than in the general venting method.

[0053] This rare gas filter 23 can remove radioactive rare gases no matter where it is installed inside the reactor containment vessel 1 or on the vent piping 13. 1, it is preferable to install the rare gas filter 23 at a downstream position within the filter vent device 15. This makes it possible to prevent deterioration of the filter performance caused by adhesion of aerosol-like radioactive materials to the rare gas filter 23, and to prevent exposure to the effects of molten fuel that may occur in the event of an accident, thereby improving the reliability of the reactor containment vessel vent system.

[0054] The rare gas filter 23 must be permeable to vapors. Furthermore, in order to prevent pressurization of the reactor containment vessel 1, it is desirable to allow the passage of hydrogen, which may be generated when the reactor core 2 melts. Water vapor and hydrogen, which should be permeable, have small molecular diameters of 0.3 nm or less, while radioactive rare gases (mainly krypton and xenon), which should not be permeated, have molecular diameters that are significantly larger than these. Therefore, in order to selectively allow steam and hydrogen, which have small molecular diameters, to pass through, it is conceivable to use a membrane that can separate them using a molecular sieve.

[0055] In the case of a boiling water reactor, the gas inside the reactor containment vessel 1 is replaced with nitrogen. When selecting gases using a molecular sieve based on molecular size, nitrogen, which has a molecular size similar to that of krypton and xenon, may not pass through. However, this does not pose a problem from the viewpoint of reducing the pressure inside the reactor containment vessel 1. As a filter material suitable for such applications, it is desirable to use a membrane capable of separation by molecular sieving, such as a polymer membrane mainly composed of polyimide, a ceramic membrane mainly composed of silicon nitride, or a graphene oxide membrane mainly composed of carbon. These filter materials are generally used in filters used for purifying hydrogen. In addition, other membranes that do not transmit krypton or xenon but transmit hydrogen and water vapor may also be used.

[0056] The rare gas filter 23 using the above-mentioned filter material is permeable to steam and hydrogen but not to nitrogen and radioactive substances, and is therefore able to remove radioactive rare gases while releasing the steam and hydrogen that cause pressurization of the reactor containment vessel 1. However, over time, non-permeable nitrogen and radioactive rare gases accumulate immediately upstream of the rare gas filter 23, and the partial pressure of these gases increases, reducing the amount of steam and hydrogen that permeates and reducing the function of lowering the pressure in the reactor containment vessel 1.

[0057] 1, the upstream part of the rare gas filter 23 is connected to the containment vessel 1 by a return pipe 24, and a blower 25 installed on the line of the return pipe 24 returns the gas that does not permeate the rare gas filter 23 to the containment vessel 1. This makes it possible to maintain the vapor permeability of the rare gas filter 23.

[0058] 1, a check valve 26 is installed on the return pipe 24. This makes it possible to prevent gas containing radioactive materials from reaching the rare gas filter 23 from the containment vessel 1 without passing through the filter vent device 15.

[0059] Moreover, in the unlikely event that the blower 25 does not operate, the permeation flow rate of the rare gas filter 23 decreases, and the pressure reduction of the reactor containment vessel 1 may become insufficient. For this reason, as shown in FIG. 1, a bypass pipe 27 is installed that bypasses the rare gas filter 23 from the immediately upstream portion of the rare gas filter 23 and connects to the downstream portion of the rare gas filter. Furthermore, a rupture disk 28 is installed on the line of the bypass pipe 27, and the valve opens by breaking a partition plate when the pressure exceeds a certain level. As a result, in the unlikely event that the blower 25 does not operate and the pressure in the reactor containment vessel 1 rises, the rupture disk 28 opens, and the reactor containment vessel 1 can be depressurized.

[0060] Rupture disk 28 may be a blowout valve or other valve. Furthermore, the rare gas filter 23 itself may be designed to break when a certain pressure is applied thereto, thereby replacing the function of the rupture disk 28.

[0061] Here, the substances passing through each pipe (vent pipe 13, outlet pipe 20, return pipe 24) in the filter vent device 15 in Fig. 1 are indicated by the symbols (1) to (5). (1) indicates aerosol-like radioactive substances, (2) indicates radioactive rare gases, (3) indicates water vapor, (4) indicates hydrogen, and (5) indicates nitrogen and other gases, respectively. All of (1) to (5) pass through the vent pipe 13, but (1) aerosol-like radioactive material is removed by scrubbing pool water 18, and (2) to (5) pass through the outlet pipe 20. Then, by being separated by the rare gas filter 23, (3) water vapor and (4) hydrogen that pass through the rare gas filter 23 head toward the exhaust tower 22, while (2) radioactive rare gases and (5) nitrogen and other gases that do not pass through the rare gas filter 23 return to the reactor containment vessel 1 through the return pipe 24.

[0062] Next, the effect obtained by opening and closing the containment vessel vent in response to pressure in the containment vessel vent method of the present invention will be described with reference to FIGS.

[0063] Fig. 2 is a diagram showing an example of evaluation of the behavior of pressure in a plant when the venting method of the present invention (intermittent venting that opens and closes at a constant pressure) is performed with and without a rare gas filter 23. In Fig. 2, in order to conservatively evaluate the behavior of the pressure, the evaluation is performed assuming that the pressure suppression function of the wet well 7 described above is lost. In Fig. 2, the upper pressure limit for protecting the reactor containment vessel 1 is indicated by a dashed line. In the evaluation shown in Figure 2, it is assumed that the membrane area is five times larger than that required for the conventional general venting method (which maintains the open position once opened) (the amount of rare gas leakage per unit time is five times larger).

[0064] As can be seen from FIG. 2, by appropriately designing the membrane area of ​​the rare gas filter 23, even when the rare gas filter 23 is provided, the plant pressure can be appropriately controlled to a certain level or lower to prevent the containment vessel from becoming overpressurized. In the pressure change shown in FIG. 2, the rare gas leaks only when the pressure decreases.

[0065] FIG. 3 is a graph comparing the release rate of radioactive rare gas leaking to the outside by a general venting method and the venting method of the present invention, in the cases where a rare gas filter 23 is provided and where it is not provided.

[0066] As shown in FIG. 3, if a general venting method is used without providing the rare gas filter 23, the rare gas held within the furnace will flow out almost entirely. In addition, when the venting method of the present invention is adopted without the rare gas filter 23, it may be thought that the leakage of rare gas to the outside is greatly reduced by opening and closing, but since rare gas is distributed almost uniformly in the gas to be released to the outside, when a large amount of gas containing water vapor is released to depressurize the plant, most of the rare gas is also released to the outside. And, as shown in Figure 3, more than half of the rare gas is released especially during the first opening. Next, when the rare gas filter 23 is provided and a general venting method is used, it can be seen that the rare gas filter 23 reduces the leakage of the rare gas to the outside. However, the rare gas that is prevented from escaping by the rare gas filter 23 is returned to the containment vessel, but is mixed again with the vent gas and attempts to leak to the outside when the venting operation is performed. And, although the rare gas filter 23 is small, the amount of rare gas that permeates is not zero, so the rare gas circulates and is supplied to the rare gas filter 23 many times, causing the rare gas to gradually leak as shown in FIG. 3. Next, in the case where the rare gas filter 23 is provided and the venting method of the present invention is adopted, the rare gas flows into the rare gas filter 23 only during the time when the vent valve 14a on the wet well 7 side is open. In order to adopt the venting method of the present invention, it is assumed that the membrane area is five times larger than usual, so the amount of leakage per unit time is five times larger. However, since the venting time of the rare gas filter 23 is short in the venting method of the present invention, the amount of rare gas flowing out to the outside is greatly reduced. In addition, since the venting time of the rare gas filter 23 is reduced, deterioration of the rare gas filter 23 due to the passage of high-temperature water vapor can also be reduced.

[0067] In this example, the membrane area was evaluated as five times the normal area, but the optimal membrane area varies depending on the plant and the assumed accident scenario. In the plant design stage, the membrane area is set to an optimal value taking into consideration the effect of reducing radiation exposure and ventilation time, the amount of membrane material to be installed, etc.

[0068] According to this embodiment, a rare gas filter 23 is provided in the outlet pipe 20, and the membrane filter material of this rare gas filter 23 separates water vapor and hydrogen from the radioactive rare gas and nitrogen, thereby reducing the amount of radioactive rare gas leaking to the outside. In addition, the vent operation valve 14a on the wet well 7 side opens at a pressure equal to or higher than a certain level and closes at a pressure equal to or lower than a certain level according to the pressure in the reactor containment vessel 1, thereby performing containment vessel vent intermittently. This makes it possible to improve the durability of the filter membrane of the rare gas filter 23 compared to the conventional case in which containment vessel vent is performed continuously. Therefore, even in the unlikely event that gas containing radioactive materials flows out from the reactor pressure vessel 3 into the reactor containment vessel 1 and the reactor containment vessel 1 becomes pressurized, it is possible to prevent the reactor containment vessel 1 from being pressurized and to prevent radioactive materials from leaking to the outside.

[0069] Furthermore, according to this embodiment, containment vessel vent can be performed intermittently by utilizing the vent operation valve 14a on the wet well 7 side at the outlet from the reactor containment vessel 1, which has been conventionally installed in nuclear reactor plants. This makes it unnecessary to add a valve or the like for intermittently performing containment vessel vent.

[0070] In the first embodiment, the containment vessel vent is intermittently performed by the vent operation valve 14a on the wet well 7 side. However, the valve that intermittently performs the containment vessel vent may be the vent operation valve 14b on the dry well 5 side or both the vent operation valves 14a, 14b.

[0071] Example 2 A method for venting a containment vessel according to a second embodiment, which is one of preferred embodiments for achieving the above-mentioned object, will be described with reference to FIG. 4 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to the second embodiment. The area enclosed by a dashed line in the figure is a containment vessel vent system according to the present invention. In the second embodiment, the arrangement of the filter vent device 15 and the rare gas filter 23 is similar to that of the first embodiment, and only the differences from the first embodiment will be described here.

[0072] In this embodiment, when the pressure reaches or exceeds a certain level, the vent valve 14 opens and remains open. In the case of existing vent systems using typical venting methods, the function of this vent operation valve 14 is performed by the containment vessel isolation valve, and by maintaining its existing function as an isolation valve, no modifications to the existing isolation valve are made and modifications to the existing vent system are minimized.

[0073] In this embodiment, the opening and closing function provided in the first embodiment is realized by providing a vent gas opening and closing valve 41 provided upstream of the rare gas filter 23, and by opening and closing this vent gas opening and closing valve 41, the same function and effect as in the first embodiment are achieved. That is, in this embodiment, when the pressure in the containment vessel 1 detected by the pressure detection mechanism 40 reaches or exceeds a certain level, the vent gas on-off valve 41 is opened, and thereafter, when the pressure in the containment vessel 1 reaches or exceeds the certain level, the vent gas on-off valve 41 is closed. After that, the vent gas on-off valve 41 is repeatedly opened and closed as the pressure in the containment vessel 1 increases and decreases.

[0074] Example 3 A containment vessel venting method according to a third embodiment, which is one of preferred embodiments for achieving the above-mentioned object, will be described with reference to FIG. 5 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to the third embodiment. The area enclosed by a dashed line in the figure is a containment vessel vent system according to the present invention. In the third embodiment, the arrangement of the filter vent device 15 and the rare gas filter 23 is similar to that in the second embodiment, and only the differences from the second embodiment will be described here.

[0075] In this embodiment, when the pressure reaches or exceeds a certain level, the vent valve 14 opens and remains open. In the case of existing vent systems using typical venting methods, the function of this vent operation valve 14 is performed by the containment vessel isolation valve, and by maintaining its existing function as an isolation valve, no modifications to the existing isolation valve are made and modifications to the existing vent system are minimized.

[0076] In this embodiment, the opening and closing function provided in the second embodiment is operated by a vent gas opening and closing valve provided upstream of the rare gas filter 23, but this operation is realized by providing a passive vent gas opening and closing valve 42 that opens and closes passively by spring pressure. In the passive vent gas on-off valve 42, the on-off valve opens when a pressure above a certain level is applied, and the valve closes due to spring pressure when the pressure falls below a certain level. This eliminates the need for a pressure detection mechanism 40 or a mechanism for opening and closing the valve in this embodiment.

[0077] Example 4 A containment vessel venting method according to a fourth embodiment, which is one of preferred embodiments for achieving the above-mentioned object, will be described with reference to FIG. 6 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to the fourth embodiment. The area enclosed by a dashed line in the figure is a containment vessel vent system according to the present invention. In the fourth embodiment, the arrangement of the filter vent device 15 and the rare gas filter 23 is similar to that in the second embodiment, and only the differences from the second embodiment will be described here.

[0078] In this embodiment, when the pressure reaches or exceeds a certain level, the vent valve 14 opens and remains open. In the case of an existing vent system using a general venting method, the containment vessel isolation valve performs the function of this vent operation valve 14. In this embodiment, the existing function as an isolation valve is maintained as is, so that no modification is made to the existing isolation valve and modifications to the existing vent system are minimized.

[0079] In this embodiment, the opening and closing function provided in the second embodiment is virtually simulated by the operation of the blower 25. When the pressure detection mechanism 40 detects a pressure above a certain level, the blower 25 is operated to continue ventilating the rare gas filter 23, promoting the release of water vapor to the outside and reducing the pressure in the reactor containment vessel 1. On the other hand, when the pressure falls below a certain level, the blower 25 is stopped. When the blower 25 is stopped, the rare gas and nitrogen that do not pass through the rare gas filter 23 remain in the rare gas filter 23. As a result, new vent gas is not supplied, and venting to the outside is not performed.

[0080] As described above, it is also possible to virtually simulate the opening and closing of a valve by the operation of the blower 25. However, compared to the operation of the valve, the start and stop of the vent operation by the operation of the blower 25 is a slow operation, so it is necessary to allow for a certain amount of time for the operation, and there is a possibility that the amount of rare gas leaking to the outside may increase slightly.

[0081] Example 5 A containment vessel venting method according to a fifth embodiment, which is one of preferred embodiments for achieving the above-mentioned object, will be described with reference to FIG. 7 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to embodiment 5. The area enclosed by a dashed line in the figure is a containment vessel vent system according to the present invention. In embodiment 5, only the configuration of the filter vent device 15 is changed from embodiment 1, and only the difference will be described.

[0082] Generally, there are wet and dry filter vent devices, and the device that removes aerosols using scrubbing pool water 18 in a container as in Example 1 is a wet filter vent device.

[0083] On the other hand, as shown in Fig. 7, the filter vent device 15 of the fifth embodiment is a filter vent device 15 in which sand filters 32 for removing radioactive materials are laid inside the filter vent container 16, and the radioactive materials are removed by the sand filters 32. This is a dry-type filter vent device, and unlike a wet-type filter vent device, there is no need to manage the water quality of the scrubbing pool water 18, but the device needs to be heated in the event of an accident.

[0084] Since the dry filter vent device 15 cannot remove radioactive rare gases, the rare gas filter 23 of the present invention is necessary, and the configuration thereof is the same as that of the first embodiment. Moreover, the configuration for carrying out the open / close vent method of the present invention may be configured as in the second to fourth embodiments.

[0085] Example 6 A containment vessel venting method according to a sixth embodiment, which is one of preferred embodiments for achieving the above-mentioned object, will be described with reference to FIG. 8 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to a sixth embodiment. The area enclosed by a dashed line in the figure is a containment vessel vent system according to the present invention. In the sixth embodiment, only the configuration of the filter vent device 15 is changed from that of the fifth embodiment, and only the difference will be described.

[0086] The dry filter vent device also includes a type in which a metal filter 19 and zeolite 34 for removing iodine are installed instead of the sand filter 32 for removing radioactive materials in the fifth embodiment. 8, in this embodiment, a metal filter 19 and zeolite 34 are installed between the vent pipe 13 and the outlet pipe 20 in the filter vent device 15. The zeolite 34 is installed downstream of the metal filter 19. In addition, an orifice 43 is installed between the metal filter 19 and the zeolite 34.

[0087] Zeolite 34 adsorbs organic iodine, particularly by silver ions arranged within the structure of zeolite 34. Since there is a possibility that the capacity of zeolite 34 may be reduced by moisture, the orifice 43 arranged immediately before the zeolite 34 reduces the pressure of the vent gas and turns it into superheated steam, thereby preventing the capacity of zeolite 34 from being reduced by moisture.

[0088] In this filter vent device 15 as well, in order to remove the radioactive rare gas, the rare gas filter 23 of the present invention is required, and the configuration thereof is similar to that of the first embodiment and the like.

[0089] Example 7 A containment vessel venting method according to a seventh embodiment, which is one of preferred embodiments for achieving the above-mentioned object, will be described with reference to FIG. 9 is a vertical cross-sectional view showing a schematic configuration of a containment vessel and a containment vessel vent system for carrying out a containment vessel vent method according to embodiment 7. The area enclosed by a dashed line in the figure is the containment vessel vent system according to the present invention.

[0090] The seventh embodiment is an example in which the reactor containment vessel filter vent system according to the present invention is applied to a pressurized water reactor.

[0091] 9, in this embodiment, a pressurized water reactor is configured by disposing a pressurizer 35, a steam generator 36, and a recirculation pump 37 inside a reactor containment vessel 1. The pressurizer 35, the steam generator 36, and the recirculation pump 37 are connected to piping that is connected to a reactor pressure vessel 3. Then, the water from the reactor pressure vessel 3 (water in the primary system) is pressurized to a high pressure in the pressurizer 35 to become high-temperature, high-pressure water, and in the steam generator 36, this high-temperature, high-pressure water is used to heat the secondary cooling water to generate steam. The steam generated in the steam generator 36 is sent to the outside of the reactor containment vessel 1 through the main steam pipe 4. In addition, the high-temperature, high-pressure water used in the steam generator 36 is returned to the reactor pressure vessel 3 by a recirculation pump 37 . In FIG. 9, piping through which steam passes beyond the main steam pipe 4 and piping for supplying secondary cooling water from the outside of the reactor containment vessel 1 to the steam generator 36 are omitted. The other configurations are the same as those in the first embodiment.

[0092] Since the pressurized water reactor of this embodiment does not include the dry well 5 and the wet well 7 shown in FIG. 1 and the like, the vent operation valve 14 is provided at only one location in the containment vessel 1 . Therefore, in this embodiment, the containment vessel venting method of the present invention is carried out by opening and closing this one vent operation valve 14 in response to an increase or decrease in pressure in the reactor containment vessel 1.

[0093] In addition, the configuration for carrying out the containment vessel venting method of the present invention may be changed from the vent operation valve 14 shown in FIG. 9 to the same configuration as in Examples 2 to 4 (vent gas on-off valve, passive vent gas on-off valve, blower).

[0094] Since a pressurized water reactor does not have a wet well 7 and a suppression pool 8 for suppressing a pressure rise in the reactor containment vessel 1, it is not possible to expect removal of radioactive materials by scrubbing using the suppression pool 8. Therefore, in this embodiment, removal of radioactive materials occurs primarily in the scrubbing pool water 18 within the filter vent vessel 16 .

[0095] Furthermore, the containment vessel filter vent system according to the present invention may be provided in a boiling water reactor that does not have a wetwell.

[0096] (Modification) Although the first to seventh embodiments are examples in which the method for venting a containment vessel of the present invention is applied to a light water reactor (a boiling water reactor or a pressurized water reactor), the method for venting a containment vessel of the present invention may also be applied to a heavy water reactor, a graphite reactor, or a gas reactor. Furthermore, the method for venting a nuclear reactor containment vessel of the present invention may be applied to other types of reactors, such as so-called fourth generation nuclear reactors, such as high-temperature gas reactors, supercritical-pressure light-water-cooled reactors, molten-salt reactors, gas-cooled fast reactors, sodium-cooled fast reactors, and lead-cooled fast reactors.

[0097] The present invention is not limited to the above-described embodiments and examples, and includes various modified examples. For example, the above-described embodiments and examples have been described in detail to easily explain the present invention, and the present invention is not necessarily limited to those having all of the configurations described. [Explanation of symbols]

[0098] 1... reactor containment vessel, 2... reactor core, 3... reactor pressure vessel, 4... main steam pipe, 5... dry well, 6... steam relief valve, 7... wet well, 7a... wet well gas phase, 8... suppression pool, 9... steam relief valve exhaust pipe, 10... quencher, 11... vent pipe, 11a... vent pipe exhaust section, 12... diaphragm floor, 13... vent piping, 14... vent operation valve, 14a... wet well side vent operation valve, 14b... dry well side vent operation valve, 15... filter vent device, 16... filter vent container, 17... filter vent inlet piping, 1 8...Scrubbing pool water, 19...Metal filter, 20...Outlet piping, 21...Shielding wall, 22...Exhaust tower, 23...Rare gas filter, 24...Return piping, 25...Blower, 26...Check valve, 27...Bypass pipe, 28...Rupture disk, 29...Hydrogen recombiner, 30...Turbine, 31...Power transmission mechanism, 32...Sand filter for removing radioactive materials, 33...Baffle plate, 34...Zeolite, 35...Compressor, 36...Steam generator, 37...Recirculation pump, 38...Iodine filter, 40...Pressure detection mechanism, 41...Vent gas on-off valve, 42...Passive vent gas on-off valve, 43...Orifice

Claims

1. 1. A containment vessel venting method, comprising: discharging gas inside a reactor containment vessel to the outside and performing venting in a reactor containment vessel vent system for depressurizing the reactor containment vessel, the method comprising: a structure is provided in a vent line of the reactor containment vessel, the structure having a membrane having a water vapor permeability coefficient greater than a radioactive rare gas permeability coefficient, and the gas passing through the vent line is separated into water vapor and a radioactive rare gas by the membrane; The structure is configured to operate at a pressure equal to or higher than a certain level and to stop at a pressure equal to or lower than a certain level depending on the pressure of the reactor containment vessel, thereby performing containment vessel venting intermittently. A method for venting a containment vessel comprising:

2. 2. The containment vessel venting method according to claim 1, wherein a valve at an outlet from the reactor containment vessel is opened when the pressure in the reactor containment vessel reaches or exceeds a certain level, and closed when the pressure in the reactor containment vessel falls below the certain level, thereby performing the containment vessel vent intermittently.

3. 2. The containment vessel vent method according to claim 1, further comprising providing an openable / closable valve subsequent to a valve at an outlet from the reactor containment vessel and upstream of the membrane, the openable / closable valve being opened when the pressure in the reactor containment vessel reaches or exceeds a certain level, and being closed when the pressure in the reactor containment vessel falls below the certain level, thereby intermittently venting the containment vessel.

4. 2. The containment vessel venting method according to claim 1, further comprising a valve configured to passively open when the pressure in the reactor containment vessel reaches or exceeds a certain level and to passively close when the pressure in the reactor containment vessel falls below the certain level, thereby performing the containment vessel vent intermittently.

5. 2. The containment vessel vent method according to claim 1, further comprising the steps of: providing a scavenging mechanism for scavenging a non-permeating gas that does not permeate the membrane; and operating the scavenging mechanism when the pressure in the reactor containment vessel reaches or exceeds a certain level; and stopping the scavenging mechanism when the pressure in the reactor containment vessel falls below the certain level, thereby intermittently venting the containment vessel.

6. 2. The containment vessel venting method according to claim 1, wherein the containment vessel venting is performed intermittently by providing either one of a valve which opens and closes depending on the pressure of the reactor containment vessel, or a scavenging mechanism which starts and stops its operation depending on the pressure of the reactor containment vessel, or a combination of these mechanisms.

7. 2. The method for venting a containment vessel according to claim 1, wherein the membrane is configured to be impermeable to radioactive noble gases and nitrogen, but permeable to hydrogen and water vapor.

8. 2. The method for venting a containment vessel according to claim 1, wherein the filter material of the membrane is a polymer membrane, a ceramic membrane, or a graphene oxide membrane.

9. 2. The containment vessel vent method according to claim 1, further comprising the steps of: providing a containment vessel vent system for a boiling water reactor; and operating the reactor containment vessel vent system at a constant pressure in accordance with a pressure of the reactor containment vessel, and stopping the reactor containment vessel vent system when the pressure is equal to or higher than the constant pressure, thereby intermittently performing the containment vessel vent.

10. 2. The containment vessel vent method according to claim 1, further comprising the steps of: providing a containment vessel vent system for a pressurized water reactor; and operating the reactor containment vessel vent system at a constant pressure according to the pressure of the reactor containment vessel, and stopping the reactor containment vessel vent system when the pressure reaches or exceeds the constant pressure, thereby intermittently performing the containment vessel vent.

11. 2. The containment vessel vent method according to claim 1, further comprising the steps of: providing the containment vessel vent system in any one of a heavy water reactor, a graphite reactor, a gas reactor, a high temperature gas reactor, a supercritical pressure light water cooled reactor, a molten salt reactor, a gas cooled fast reactor, a sodium cooled fast reactor, and a lead cooled fast reactor; and operating the containment vessel vent system at a constant pressure according to the pressure of the reactor containment vessel and stopping the system when the pressure is equal to or higher than the constant pressure, thereby intermittently performing the containment vessel vent.

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