JPS6048716B2 - How to prevent corrosion of reactor piping - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing corrosion of nuclear reactor piping, and is particularly suitable for preventing corrosion of water supply and condensate system piping of a boiling water reactor during reactor startup. It is about the method.

An overview of a boiling water reactor will be explained based on FIG.

First, before starting the reactor, drive the condensate pump 6, open the water supply brushing pipe isolation valve 15, and close the water supply pipe isolation valve 17.
is closed, and the water supply and condensate system piping 3 is brushed through the water supply brushing piping 16. Since this brushing is performed with the condenser 5 at room temperature and atmospheric pressure, the dissolved oxygen concentration in the brushing water is about 8 pμm. As a result, the corrosion products inside the water supply and condensate system piping 3 are mechanically removed, and an oxide film is formed on the inner surface of the piping to some extent, but it is very unstable and easily peels off, so the corrosion rate of the piping is high. . When the nuclear reactor is started, steam generated in the station boiler 18 is guided to the bearing section of the turbine 4 through the ground steam pipe 22. The bearing portion of the turbine 4 is sealed by the above steam. For this reason, vacuum pump 1
9 and the ejector 20, the required vacuum level in the condenser 5 is approximately 700wrx, as shown in FIG.
LHg is ensured. As a result, the dissolved oxygen concentration in the water supply and condensate system piping 3 is significantly reduced to 1 to 2 ppb, and the oxide film formed on the inner surface of the piping during the above-mentioned brushing is destroyed. As the reactor is started up in such a state, the amount of crud carried into the reactor pressure vessel layer increases in a spike-like manner. On the other hand, when the nuclear reactor enters a normal operating state, steam generated within the reactor pressure vessel layer is sent to the turbine 4 through the main steam pipe 2. After driving the turbine 4, the steam is condensed in the condenser 5 and returned to water. This water is supplied from the condenser 5 to a condensate pump 6, a condenser 21, a condensate demineralizer 7, a condensate boost pump 9, a feed water heater 10, a feed water pump 11, a feed water control valve 14, and a feed water piping isolation valve 17. The water is returned to the reactor pressure vessel layer through the water supply and condensate system piping 3 that connects sequentially. As a method of preventing corrosion of the water supply and condensate system piping 3, dissolved oxygen in the water supply and condensate system piping 3 is reduced to 0.02% during reactor operation.
~0.2ppm (U.S. Pat. No. 3,663,725)
In Japan, good results have been obtained by injecting oxygen from the outlet of the condensate demineralizer 7. However, as described above, since the degree of vacuum in the condenser 5 is high, a large amount of iron cladding (including ions) is generated between the condenser 5 and the inlet of the condensate demineralizer 7. Particularly during atomic startup, the frequency of regeneration of the condensate demineralizer 7 increases, increasing the load on the waste treatment system. An object of the present invention is to eliminate the above-described drawbacks of the prior art and to prevent corrosion of pipelines during startup of a nuclear reactor.

J The feature of the present invention is that, in order to achieve the above-mentioned object, before starting up the nuclear reactor, a water supply condensate system pipe connecting a condenser and a reactor vessel and a water supply condensate pipe installed in the water supply condensate pipe The coolant present in the water supply and condensate system pipe is circulated in a closed loop consisting of a branch pipe connected to the condenser, and the inside of the condenser is made to have a negative pressure, thereby dissolving oxygen in the coolant. When the concentration of dissolved oxygen is controlled in the range of about 50 to 200 ppb, and when the dissolved oxygen concentration in the coolant is insufficient because it is difficult to adjust the dissolved oxygen concentration by the negative pressure in the condenser, the dissolved oxygen concentration is about 5 ppb.
Oxygen is injected into the coolant in a range of 0 to 200 ppb.

Before starting the reactor, the degree of vacuum in the condenser 5 is set to 600~
70077Z77ZHg) We note that the corrosion rate of carbon steel can be minimized by keeping the water temperature between 35 and 55°C and the dissolved oxygen concentration in the water between 50 and 200 ppb (see Figures 3 and 4). .

There are two main methods for oxygen injection: hydrogen peroxide and oxygen gas, but the oxide film formed by hydrogen peroxide is less affected by flow rate and temperature. It is excellent in that respect. Therefore, when the temperature of the water supply and condensate system increases as the reactor starts up, the effect of hydrogen peroxide injection is significant. A preferred embodiment of the present invention will be described in a first embodiment.
This will be explained based on the diagram.

Water supply and condensate system piping 3 before reactor startup. After brushing, the turbine 4 is first sealed by the in-house boiler 18. The steam generated in the in-house boiler 18 is further transferred to the piping 23.
It is supplied to the engineering sector 20 through the engine, and is used as a driving source for the engineering sector 20. By driving the working sector 20, the degree of vacuum in the condenser 5 increases. The dissolved oxygen concentration in the cooling water flowing through the water supply and condensation system piping 3 is measured by the oxygen concentration detector 27 attached to the water supply and condensation system piping 3 .

The measured oxygen concentration is communicated to controller 28. That is, when the oxygen concentration is higher than 200 ppb, the degree of opening of the control valve 24 provided in the pipe 23 is increased, and the degree of vacuum in the condenser 5 is increased, and the degree of vacuum is adjusted to 70-Hg. Therefore, oxygen is degassed from the cooling water flowing into the condenser 5 via the water supply and condensate system piping 3 and the water supply brushing piping 16, and the dissolved oxygen concentration decreases. The cooling water whose dissolved oxygen concentration has become 200 ppb or less is returned from the condenser 5 to the water supply and condensate system piping 3. On the other hand, when the dissolved oxygen concentration in the cooling water becomes less than 50pp, the controller 2
8, the opening degree of the control valve 24 becomes smaller, and the degree of vacuum in the condenser 5 decreases to 600TW.
It is adjusted to around LHg. By adjusting the degree of vacuum in the condenser 5 in this way, the dissolved oxygen concentration in the cooling water can be adjusted to 50 to 200 ppb. but,
Since it is difficult to finely adjust the degree of vacuum in the condenser 5 using the mechanical sector 20, it may be difficult to finely adjust the dissolved oxygen concentration in the cooling water. Therefore, when adjusting the dissolved oxygen concentration in the cooling water to a predetermined concentration of 50 to 200 ppb by finely adjusting the dissolved oxygen concentration in this way, oxygen is injected into the cooling water. That is, by the action of the controller, the oxygen injection pipe 25 connected to the water supply and condensation system piping 3
The valve 26 provided in the condensate demineralizer 7 is opened, and oxygen gas is injected into the outlet side of the condensate demineralizer 7. As described above, by adjusting the degree of vacuum in the condenser 5 according to the detected dissolved oxygen concentration or by injecting oxygen into the cooling water, The dissolved oxygen concentration in the cooling water flowing through the cooling water is adjusted to a range of 50 to 200 ppb. Isolation valves 12 and 29, as well as isolation valve 17, are closed, and feed water control valve 14 and isolation valve 15 are open. A stable oxide film is formed on the inner surface of each pipe by circulating cooling water with an adjusted dissolved oxygen concentration in a closed loop consisting of the water supply and condensate pipe 3 and the water supply brushing pipe 16 for two days. This stable oxide film has the effect of suppressing the progress of corrosion on the inner surface of the pipe. After a stable oxide film is formed on the inner surface of the water supply and condensate system piping 3, the reactor is started.

At startup, isolation valve 15 is closed and isolation valve 17 is closed.
can be opened. Furthermore, the aforementioned oxygen gas injection is switched to hydrogen peroxide injection. Hydrogen peroxide can be detected using an oxygen concentration detector 27.
Oxygen is injected from the oxygen injection pipe 25 into the water supply and condensate system piping 3 according to the detected value. Water supply and condensate system piping 3 formed before reactor startup due to the action of hydrogen peroxide injected into cooling water
Even if the stable oxide film inside the reactor peels off, spontaneous healing will occur quickly and the stable oxide film will be regenerated. ). Therefore, the crud is rapidly carried into the reactor pressure vessel 1 through the water supply and condensate system piping 3, which is experienced when the power increases to the rated power after reactor startup (at the time of reactor startup). Decrease. At the same time, the frequency of regeneration of the feed and condensate water demineralizer 7 is reduced. According to the present invention, a stable oxide film can be formed on the inner surface of the water supply and condensate system pipes before the reactor is operated, so that the oxide film can be peeled off from the inner surface of the water supply and condensate pipes at the time of reactor startup. The amount of crud carried into the reactor vessel is significantly reduced.

Furthermore, the frequency of regeneration of the purifying means provided in the water supply and condensate pipes can be significantly reduced.

[Brief explanation of the drawing]

Fig. 1 is a schematic system diagram of a boiling water reactor plant to which a preferred embodiment of the present invention is applied, and Fig. 2 shows changes over time in condenser vacuum degree and reactor pressure at reactor startup. The schematic diagram, FIG. 3, is a solubility characteristic diagram of oxygen, and FIG. 4 is a characteristic diagram showing the corrosion rate of carbon steel. 1... Reactor vessel, 3... Water supply and condensate system piping, 5... Condenser, 8... Condensate bypass piping, 13... ...Water supply minimum flow piping, 16...Water supply brushing piping, 17.
... Water supply piping isolation valve, 19 ... Vacuum pump, 20 ... Engineering sector.

Claims (1)

[Claims] 1. Before starting the reactor, a closed loop consisting of a water supply and condensate system pipe connecting the condenser and the reactor vessel and a branch pipe that is attached to the water supply and condensate pipe and connected to the condenser. The coolant present in the water supply and condensate system pipes is circulated, and the pressure inside the condenser is set to negative pressure to control the dissolved oxygen concentration in the coolant to be in the range of about 50 to 200 ppb. When the dissolved oxygen concentration in the coolant is insufficient because it is difficult to adjust the dissolved oxygen concentration by the negative pressure inside the vessel, oxygen is injected into the coolant so that the dissolved oxygen concentration is in the range of about 50 to 200 ppb. A method for preventing corrosion of reactor piping.