JP4489046B2 - How to start a natural circulation furnace - Google Patents

Description

  The present invention relates to a startup method for a natural circulation reactor that prevents or alleviates an unstable flow phenomenon that appears in the early stage of startup of a natural circulation boiling water nuclear reactor and stabilizes the startup.

  Conventionally, a natural circulation boiling water reactor (natural circulation type BWR) that naturally circulates a coolant made of light water has been developed, such as the Dutch Doddword reactor (reactor thermal output 183 MW, already closed), and currently under development Large natural circulation furnaces are known. Such a method for starting a natural circulation reactor is different from a method for starting a forced circulation boiling water reactor (BWR) having a recirculation pump.

  In the above-described method for starting a Dodward reactor, as shown in Non-Patent Document 1, first, the coolant temperature is adjusted by a Reactor Shutdown Cooling System to maintain the reactor water level. When the coolant temperature reaches about 95 ° C., the control rod is withdrawn and the reactor becomes critical. Thereafter, the temperature of the coolant is increased by a heating rate of 45 to 55 ° C. per hour by nuclear heating, the turbine and the bypass valve are closed, and the pressure adjustment of the reactor using the small vent line of the main steam line is performed.

  When the reactor pressure reaches 4 atm, the bypass valve is opened and the reactor pressure is adjusted. When the reactor pressure increases, the Reactor Water Cleanup System is started and the reactor water level is adjusted. The start-up method for the large-scale natural circulation reactor currently under development is the same as that established in the Doddward furnace.

  Non-Patent Document 2 shows each stage of the startup process. The starting process shown in this document will be described with reference to the graph of FIG. As shown in FIG. 5, in the deaeration period (A), the main steam isolation valve “MSIV” is opened, and the coolant is deaerated while the steam drain line is open. The coolant is heated up to 80-90 ° C. using an auxiliary heater or decay heat of a coolant purification system / cooling system at shutdown (Reactor Water Cleanup / Shutdown Cooling System). The reactor pressure at this point is 50-60 kPa.

  After deaeration, the main steam isolation valve (MSIV) is closed, and the control rod is pulled out (B) to start critical operation. The coolant is heated by nuclear heating while maintaining the reactor water level near the top of the separator (which is well below the main steam line). The inside of the reactor pressure vessel is pressurized by evaporation near the water level. The core is sufficiently subcooled by the chimney and the large separator head. As the reactor is heated and pressurized, the coolant purification / stop cooling system is used for downcomer temperature adjustment, coolant flow rate increase, and lower plenum stratification prevention. The main steam isolation valve is opened again at the end of the start-up process when the reactor pressure is 6.3 MPa. Thereafter, the turbine bypass valve is used for reactor pressure adjustment. The reactor power is then increased and the turbine is prepared.

  On the other hand, recently, in a test apparatus simulating a natural circulation type BWR, an unstable flow phenomenon was observed at the start of low pressure and low temperature, and it was found that two unstable flow behaviors may occur at low pressure. .

  Non-Patent Document 3 and Non-Patent Document 4 show the first unstable flow phenomenon. This phenomenon will be described with reference to FIG. As shown in FIG. 6A, with respect to the coolant flowing through the chimney unit 101, as shown in FIGS. 6B to 6D sequentially, the void 103 generated at the core outlet is in the subcooled state. As a result of growing and condensing in this case, a Geisling flow rate vibration in which the core flow rate vibrates due to the filling of the cold water as the coolant 102 occurs. This geysering flow rate vibration phenomenon is caused by repeated generation and disappearance of the void 103 in the core. It should be noted that the Geiseling flow rate vibration has never been experienced at the start of 22 times in the Doddward furnace. However, if such Gaiseling flow vibration occurs, it is assumed that the generation and disappearance of the void 103 are repeated in the core, which may lead to a large flow vibration through nuclear heat feedback.

  The second unstable flow phenomenon is the flushing flow rate vibration shown in Non-Patent Document 4 and Non-Patent Document 5. As shown in FIGS. 7A to 7D in sequence, the flushing flow rate vibration is caused by the generation of the void 103 in the upper region of the chimney unit 101, which lowers the hydraulic head of the chimney unit 101, and natural circulation in the core 102. This occurs when the core flow rate as the flow 105 increases and then the void 103 of the chimney portion 101 disappears and the core flow rate decreases again. This flushing flow rate oscillation is slow only without condensation. Even in the Doddward furnace, such a flushing flow rate oscillation phenomenon may have occurred during startup. However, the flow of the core 104 is a single phase during the period in which the flushing flow rate oscillation occurs. Therefore, no generation and disappearance of voids occur in the core 104, and no nuclear heat feedback occurs, so this is not a serious problem, but the flow oscillation itself should be suppressed as much as possible to establish a stable startup method. It is.

Conventionally, as a method for starting such a natural circulation type BWR, when starting the natural circulation type BWR that is in a cold shutdown state, the reactor core inlet subcooling is performed by degassing the inside of the reactor so that the reactor core inlet subcooling is performed. There has been proposed a method of starting output increase after being set below an unstable generation condition determined by sub-cooling (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 5-256991 Startup of the Dodewaard Natural Circulation Boiling Water Reactor (GKN-Report 92-017 / FY / R, 1992) Analysis of ESBWR Startup in Natural Circulation, ICAPP'05, (2005) Transient Behavior of Natural Circulation for Boiling Two-Phase Flow (2nd Report, Mechanism of Geysering), ICONE-1 (1991) Nuclear Coupled Flow Instability Study for Natural Circulation BWR Startup Transient, NUTHOS-6 (2004) Experimental Investigation of Natural-Circulation Flow Behavior under Low-Power / Low-Pressure Conditions in the Large-Scale PANDA Facility, Nuclear Technology, (2004)

  The above-described conventional startup method for degassing the inside of the nuclear reactor applies the degassing operation at startup, which is also adopted in the current forced circulation type BWR, to the natural circulation type BWR. Considering the head, the core inlet subcooling degree is not reduced at the time of start-up, and it is not necessarily a method for sufficiently suppressing the unstable flow phenomenon at the time of start-up of the natural circulation type BWR.

  The conditions for generating the two unstable flow phenomena of the natural circulation type BWR will be described with reference to FIG. FIG. 8 shows the enthalpy distribution in the axial direction of the reactor pressure vessel when the natural circulation type BWR is started. In the natural circulation type BWR, since the head is large, the saturation enthalpy becomes smaller toward the upper part of the reactor pressure vessel 106. At the time of startup from low temperature and low pressure, in the case of a normal temperature increase rate, the enthalpy distribution in the reactor pressure vessel 106 is first saturated at the upper part of the chimney portion 101 as shown by A or B in FIG. Therefore, there is a possibility that a flushing flow rate vibration occurs. Further, if the reactor power is large at the time of start-up, as shown as C in FIG. 8, there is a possibility that the reactor becomes saturated at the outlet of the core 104, and a geysering flow oscillation occurs.

  This invention is made | formed in view of such a situation, and it aims at providing the starting method of the natural circulation furnace which can implement | achieve the stable starting avoiding the conditions from which a coolant becomes low pressure previously.

  According to the inventor's study, in the above-described natural circulation type BWR, in the process of increasing the pressure from the atmospheric pressure to the normal operating pressure, there is a possibility that the flow rate greatly oscillates particularly at the passage of 2 to 3 atmospheres. It turned out to be a problem for unstable reactivity control at the output.

  That is, the Geisel flow rate oscillation phenomenon may occur at the time of low pressure start-up where the density ratio of water and steam is large. Therefore, it is considered that the occurrence of the Geisel flow rate oscillation phenomenon can be suppressed by increasing the coolant pressure or decreasing the density ratio of water and steam by increasing the natural circulation flow rate. For example, if the pressure is 3 atm or more and the flow rate of the coolant is 2 cm / s or more, it is considered that the Geiseling vibration phenomenon does not occur.

  The present invention has been made based on the above knowledge, and is a method of starting a natural circulation boiling water reactor, and after completing a deaeration operation, the coolant is assembled into the reactor pressure vessel. Fill up to a state where the body is completely immersed, fill the reactor pressure vessel space above the coolant level with a non-condensable gas, and fill the non-condensed gas with a gas pressure of 3 atm, preferably Provides a method for starting a natural circulation furnace in which pressurization by nuclear heating is started after pressurization to a high pressure of 10 atmospheres or more.

  According to the present invention, the natural circulation type BWR can be activated so as not to generate the Geisling flow rate vibration and to make the flushing flow rate vibration as short and gentle as possible.

  Hereinafter, an embodiment of a method for starting a natural circulation furnace according to the present invention will be described with reference to FIGS.

[First Embodiment (FIG. 1)]
FIG. 1 is a natural circulation type BWR configuration diagram for explaining a first embodiment of the present invention. As shown in FIG. 1, in this embodiment, a core 3 is provided above the lower plenum 2 of the reactor pressure vessel 1, and a chimney 4 is provided above the core 3. A separator 6 and a dryer 7 are provided above the upper plenum 5. A main steam outlet 8 is provided at the upper part of the reactor pressure vessel 1, and a water supply port 9 is provided below the main steam outlet 8.

  In such a configuration, the natural circulation furnace is activated by pulling out the control rod 11 and heating the core. As the nuclear heating starts, the power of the core 3 increases, the heat transfer to the coolant in the core 3 increases, and the natural circulation flow is reduced by reducing the density of the fluid through the hot non-boiling hot water to the bubbly flow. It comes to occur. That is, an upward flow is generated from the core 3 and a downward flow is generated in the downcomer 10.

  Here, it is assumed that a normal degassing operation is performed in the reactor pressure vessel 1 before starting the natural circulation reactor and the coolant is sufficiently degassed. In addition, it is assumed that the upper end of the fuel assembly 3a constituting the core 3 is completely filled with light water as a coolant. Under such conditions, it is considered that the gas phase space 28 above the coolant water surface 27a above the fuel assembly 3a is generally filled with water vapor.

  In this embodiment, in this state, first, nitrogen gas 12 that is a non-condensable gas is supplied from outside the furnace, and the gas above the water surface 27 a of the coolant is replaced with nitrogen gas 12. The nitrogen gas 12 is a non-condensable gas sealed in the reactor containment vessel, and it is not necessary to supply another non-condensable gas, and is an optimum gas for maintaining a high pressure.

  The gas supply is connected to the reactor pressure vessel 1 through each pipe connected to the reactor pressure vessel 1 so that the supplied nitrogen gas 12 does not leak from the reactor pressure vessel 1 to the outside. All the pipes to be isolated are closed by closing the valves connected to the pipes other than the gas supply pipe.

  When the supply of the nitrogen gas 12 is continued in the sealed state in this way, the nitrogen gas space pressure starts to rise monotonously beyond the atmospheric pressure according to the supply amount. Although the upper limit of the pressure at which the gas supply is stopped depends on the case, the operation plan is generally made so that the pressure occupied by the nitrogen gas is increased to 3 atmospheres or more and about 10 atmospheres. If it is confirmed that a high pressure of about 10 atmospheres as a guide has been reached, the supply of nitrogen gas 12 is terminated at that point. If it is confirmed that a steady state has been reached after a sufficient amount of time, the final pressure in the gas space is not only the supply gas but also the saturated vapor pressure of water filling downward.

  Thus, in this embodiment, after the deaeration operation is completed, the coolant is filled in the reactor pressure vessel 1 until the fuel assembly 3a is completely immersed, and atoms above the liquid level of the coolant are filled. Nitrogen gas 12 that is a non-condensable gas is supplied to the interior of the furnace pressure vessel to fill it, and the pressure of the nitrogen gas 12 is increased to 3 atmospheres or higher, preferably 10 pressures or higher.

  If pressurization by nuclear heating is started in this state, bubbles that foam from the surface of the fuel assembly 3a due to boiling of the coolant and the surrounding coolant are mixed by heat, and generally composed of gas and liquid. A gas-liquid two-phase flow state is formed. As for the pressure of the two-phase flow, bubbles are generated in a high pressure state in which a water head is added to an upper gas pressure (a high pressure of about 10 atm) and sent to the upper part.

  At such a high pressure exceeding 10 atmospheres, the density ratio of water and steam also changes from that under atmospheric pressure. Although the gas-liquid density ratio under atmospheric pressure is extremely different, approximately 1/1600, changes in physical properties are observed at a high pressure of 10 atmospheres, and the gas-liquid density ratio is increased by about 10 times to 1/157. Can be approached.

  The purpose of the present invention is to prevent increase / decrease in liquid level (geysering) generated by bubble condensation in the subcool region in the upper part of the core of the two-phase flow at startup, but if the method of this embodiment is adopted, It is possible to reduce the gas-liquid density ratio, which is the main cause of the occurrence of geysering, and to increase the possibility of preventing the geysering concerned. Especially when the amount of gas supplied is 10 atmospheres or more, it is possible to completely prevent geysing, and when it is 3 atmospheres or more, vibrations can be greatly reduced compared to conventional cases, even if gaisering cannot be completely prevented. It is possible to make it. Further, according to the present embodiment, the flushing flow rate vibration is short and can be made gentle.

  In the present invention, it is of course possible to apply a gas other than nitrogen gas as the non-condensable gas, but it is desirable to apply nitrogen gas. As described above, the nitrogen gas is a non-condensable gas sealed in the reactor containment vessel, and it is not necessary to supply a non-condensable gas other than the nitrogen gas again, and is an optimal gas for maintaining a high pressure.

  Further, after the pressure is increased to 10 atmospheres or more and a sufficiently high pressure is reached so that there is no concern about the occurrence of large-scale guying, the nitrogen gas becomes unnecessary, and the gas is outside the reactor pressure vessel 1. It is necessary to discharge. When the evaporation of light water gradually increases with the continuation of nuclear heating, the gate valve (main steam drain valve) with the main steam drain pipe that had been isolated until then was gradually opened, and the nitrogen gas was sequentially released into the reactor. What is necessary is just to discharge | emit outside the pressure vessel 1. FIG. The final gas discharge destination may be released to the outside air after confirming that the radiation dose is sufficiently low via the turbine building.

[Second Embodiment (FIG. 2)]
FIG. 2 is a configuration diagram for explaining a second embodiment of the present invention. In addition, about the structure same as 1st Embodiment, the code | symbol same as FIG. 1 is attached | subjected to FIG. 2, and the overlapping description is abbreviate | omitted.

  In the present embodiment, water vapor is used instead of the nitrogen gas as the non-condensable gas in the first embodiment. That is, after the deaeration operation is completed, the coolant is filled in the reactor pressure vessel 1 until the fuel assembly 3a is completely immersed, and power is generated from the pipe 31 located near the top in the reactor pressure vessel 1. By supplying the in-house steam 33 that can be used in the station, while condensing the steam at the water surface 27a of the coolant, the temperature of the coolant is raised and the vapor pressure is maintained while maintaining the saturation pressure corresponding to the liquid surface temperature. After pressurizing to a high pressure of about 3 atmospheres or more, preferably 10 atmospheres or more, pressurization by nuclear heating is started.

  As a steam supply source, an in-house steam source 30 and a steam pipe 31 used in a nuclear power plant are used. As the in-house steam source 30, a pipe connected to the gas phase space 41 above the water surface 27a in the reactor pressure vessel 1 and located near the top of the reactor pressure vessel 1, for example, a reactor vent pipe is used.

  According to this embodiment, compared with the first embodiment, the pressurization time of the upper space of the reactor pressure vessel 1 becomes longer, but the supply steam is supplied at a pressure higher than atmospheric pressure, and the deaeration is completed. However, since the saturation temperature of the water surface is 80 to 90 ° C., the supply steam from the steam pipe 31 comes into contact with the water surface 27a and condenses quickly. As a result of the continued condensation by the supplied steam, the coolant gradually increases the water temperature from the surface side. If the steam supply is continued, not only the temperature of the water surface 27a increases, but also the volume of the high-temperature water region 40 near the water surface 27a increases due to heat conduction of water. At the same time, the pressure in the gas phase space 41 is not as steep as the nitrogen gas supply, but the pressure in the space can be increased when the effect of water surface condensation becomes slow.

  In this embodiment, it is also possible to use steam generated in the deaerator 32 instead of supplying in-house steam that can be used in the power plant. That is, a deaerator 32 having a heater is disposed in the vicinity of the top in the reactor pressure vessel 1, steam is generated at the water surface 27a of the coolant, and the temperature of the coolant is increased to achieve saturation corresponding to the liquid surface temperature. While maintaining the pressure, a method of starting pressurization by nuclear heating after pressurizing the gas phase pressure to a high pressure of 3 atmospheres or more, preferably 10 atmospheres or more can be employed.

  Even in such a method, the supply steam comes into contact with the water surface 27a and quickly condenses, so that the water temperature is gradually increased from the surface side, the temperature of the water surface 27a is increased by continuing the supply of steam, and the heat conduction of water. Thus, the volume of the high-temperature water region 40 near the water surface 27a can be increased, and the pressure in the gas phase space 41 can be increased.

[Third Embodiment (FIG. 3)]
FIG. 3 is a block diagram for explaining a third embodiment of the present invention. In addition, about the structure same as 1st Embodiment, the code | symbol same as FIG. 1 is attached | subjected to FIG. 3, and the overlapping description is abbreviate | omitted.

  In this embodiment, after the deaeration operation is completed, the coolant is filled in the reactor pressure vessel 1 until the fuel assembly 3a is completely immersed, and an open end is formed on the inner surface of the core shroud 3b of the reactor pressure vessel 1. After supplying the hot water 53 deaerated from the tank 52 installed outside the reactor pressure vessel 1 to the upper part in the core shroud 3b using the supply pipe 51 having 51a, pressurization by nuclear heating is started.

  That is, high temperature water 53 is supplied from the outside in order to increase the fluid temperature in the upper part of the core shroud 3b surrounding the core 3 before starting the nuclear heating. As the supply pipe 51, a pipe having an open end 51 a on the inner surface of the core shroud 3 b, for example, a boric acid water injection line is used. After the hot water 53 is supplied to the upper part of the core shroud 3 b, pressurization by nuclear heating is started.

  In addition, an amount of cold water 55 corresponding to the supply amount of the high-temperature water 53 is discharged from the bottom of the reactor pressure vessel 1 through the pipe 54 located at the bottom of the reactor pressure vessel 1. Specifically, for example, using a drain pipe or a reactor water purification system pipe, an amount of cold water 55 that is substantially balanced with the high-temperature water 53 is drawn from the bottom. By doing so, it becomes possible to quickly fill the high temperature volume in the upper part of the core shroud 3b in the reactor pressure vessel 1 with high temperature water.

  In this way, if the nuclear heating is gradually started under the condition that the coolant in the upper part of the core is preheated, the subcooled water in the upper part of the fuel (upper plenum), which is one of the conditions that cause geelsing, suddenly drops due to vibration. The possibility of contact with the fuel assembly 3a can be avoided or reduced. According to the present embodiment, the degree of condensation is greatly reduced, and the degree of vibration (amplitude and period of fluid vibration) due to Geiselling is reduced, even if it is not completely suppressed.

[Fourth Embodiment (FIG. 4)]
In the fourth embodiment of the present invention, after the pressurization by the nuclear heating is started, the separator temperature and the reactor steam dome pressure are detected, and the separator temperature is compared with the temperature corresponding to the reactor steam dome pressure. A description will be given of a method of determining the generation of voids in the reactor and increasing the core power during the void generation period. FIG. 4 is a configuration diagram for explaining a fourth embodiment of the present invention.

  In the present embodiment, a core 3 is provided above the lower plenum 2 of the reactor pressure vessel 1, and a chimney 4 is provided above the core 3. A separator 6 and a dryer 7 are provided above the upper plenum 5. A main steam outlet 8 is provided at the upper part of the reactor pressure vessel 1, and a water supply port 9 is provided below the main steam outlet 8.

  Further, a natural circulation reactor core power meter 24, a reactor steam dome pressure gauge 21, a core inlet thermometer 25, a chimney thermometer 23, a separator thermometer 22, a core flow meter 26, and a reactor water level meter 27 are provided. . Each of these instruments performs on-line measurements. These measurement quantities, namely, the core power meter Q, the reactor steam dome pressure P, the core inlet temperature TIn, the chimney temperature Tcmn, the separator temperature Tsep, the core flow rate W and The reactor water level L is input to the output control device 60.

  Under such a configuration, the natural circulation furnace is started by pulling out the control rod 11 and heating the core. That is, the output of the core 3 is increased by the start of the nuclear heating, the heat transfer to the coolant in the core 3 is increased, and the natural circulation is achieved by reducing the density of the fluid through the hot non-boiling hot water to the bubbly flow. A flow is generated, an upward flow is generated from the core 11, and a downward flow is generated in the downcomer 10.

  Then, in the output control device 60, the separator temperature Tsep is compared with the saturation temperature corresponding to the reactor steam dome pressure P, and when both of them are at a level at which it can be determined that they are isothermal, It is determined that the coolant is saturated and voids are generated. At this time, in the output control device 60, the core output is temporarily increased only during the period in which voids occur in the separator 6.

  According to the fourth embodiment as described above, it is possible to shorten the period in which the voids are generated in the separator 6, and thus it is possible to shorten the period of flow rate oscillation (Flashing-induced oscillation) due to flushing. Further, since the core output is temporarily increased during the period in which the voids are generated in the separator 6, it is possible to shorten the startup time from the low pressure to the low temperature to the high temperature and pressure.

  In the present embodiment in which the output controller 60 temporarily increases the core output only during the period in which the voids are generated in the separator 6, if the core output is excessively increased, the flow-rate vibration due to condensation (Condensation-induced oscillation). ) May also occur. Therefore, the upper limit of the core power is limited to the power for preventing the occurrence of geysering. That is, the core output is controlled by the output control device 60 so as to satisfy the output when the number of zoomers is equal to the sum of the subcooling number and the flushing number. Thereby, it is possible to prevent the possibility of occurrence of flow-rate vibration (condensation-induced oscillation) due to condensation.

Explanatory drawing which shows 1st Embodiment of this invention. Explanatory drawing which shows 2nd Embodiment of this invention. Explanatory drawing which shows 3rd Embodiment of this invention. Explanatory drawing which shows 4th Embodiment of this invention. Explanatory drawing of the starting process of ESBWR which shows a prior art example. Explanatory drawing of the 1st unstable flow phenomenon of the natural circulation furnace which shows a prior art example. Explanatory drawing of the 2nd unstable flow phenomenon of the natural circulation furnace which shows a prior art example. Explanatory drawing of the temperature rising pattern at the time of starting of the natural circulation furnace which shows a prior art example.

Explanation of symbols

  1 reactor pressure vessel, 2 lower plenum, 3 core, 3a fuel assembly, 3b core shroud, 4 chimney, 5 upper plenum, 6 separator, 7 dryer, 8 main steam outlet, DESCRIPTION OF SYMBOLS 9 ... Water inlet, 10 ... Downcomb, 11 ... Control rod, 12 ... Nitrogen gas, 21 ... Reactor steam dome pressure gauge, 22 ... Separator thermometer, 23 ... Chimney thermometer, 24 ... Core power meter, 25 ... Core inlet Thermometer, 26 ... Core flow meter, 27a ... Water surface, 27 ... Reactor water level meter, 28 ... Gas phase space, 30 ... In-house steam source, 31 ... Steam piping, 33 ... In-house steam, 40 ... High-temperature water region, 41 ... Gas phase space 51. Supply pipe 51 a Open end 52 Tank 53 Hot water 54 Pipe 55 Cold water 60 Output control device.

Claims (6)

A method for starting a natural circulation boiling water reactor, wherein after the deaeration operation is completed, the coolant is filled in the reactor pressure vessel until the fuel assembly is completely immersed, and the coolant Supplying and filling non-condensable gas into the reactor pressure vessel space above the liquid level, pressurizing the gas pressure of this non-condensed gas to a high pressure of 3 atmospheres or more, and then start pressurization by nuclear heating A natural circulation furnace start-up method characterized by the above. A method of starting a natural circulation boiling water reactor, wherein after the deaeration operation is completed, a coolant is filled in the reactor pressure vessel until the fuel assembly is completely immersed, and the reactor pressure Corresponding to the liquid surface temperature by raising the temperature of the coolant while condensing the steam at the liquid surface of the coolant by supplying the in-house steam available in the power plant from the pipe located near the top in the container A method for starting up a natural circulation furnace, characterized by starting pressurization by nuclear heating after pressurizing a gas phase pressure to a high pressure of 3 atm or higher while maintaining a saturation pressure. A method of starting a natural circulation boiling water reactor, wherein after the deaeration operation is completed, a coolant is filled in the reactor pressure vessel until the fuel assembly is completely immersed, and the reactor pressure A deaerator having a heater is disposed in the vicinity of the top in the container to generate steam at the liquid level of the coolant, and the temperature of the coolant is raised to maintain a saturation pressure corresponding to the liquid level temperature. A method for starting a natural circulation furnace, characterized in that pressurization by nuclear heating is started after pressurizing the phase pressure to a high pressure of 3 atm or higher. A method of starting a natural circulation boiling water reactor, wherein after the deaeration operation is completed, a coolant is filled in the reactor pressure vessel until the fuel assembly is completely immersed, and the reactor pressure Using piping having an open end on the inner surface of the reactor core shroud, high-temperature water degassed from a tank installed outside the reactor pressure vessel is supplied to the upper portion of the reactor core shroud, and then pressurized by nuclear heating. A start-up method of a natural circulation furnace characterized by starting. The start-up method of the natural circulation reactor of Claim 4 which discharges the quantity of cold water corresponding to the supply amount of the said high temperature water from the bottom part of a reactor pressure vessel through piping located in the bottom part of the said reactor pressure vessel. After starting pressurization by nuclear heating, the separator temperature and the reactor steam dome pressure are measured, and the separator temperature and the temperature corresponding to the reactor steam dome pressure are compared. The method for starting a natural circulation reactor according to any one of claims 1 to 5, wherein it is determined that a void has occurred in a portion of the coolant, and the core output is temporarily increased during the void generation period.

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