JPH04194791A - How to reduce radioactive iodine - Google Patents

Abstract

PURPOSE:To reduce the density of radioactive iodine gas and prevent the discharge to the environment by spraying water in the water reservoir of controlled water quality in the case of accident and removing radioactive iodine discharged in the containment. CONSTITUTION:Radioactive iodine discharged from a reactor pressure vessel 2 exists partly in liquid phase in ion state and partly in gas phase in single gas state. So, to reduce the radioactive iodine, after controlling the pH of spray water 12 in a reservoir tank 11 to be ca. 7 to 10 by monitoring and automatically injecting for example, sodium thiosulfate from a chemical injection line 14, the solution is sprayed from a spray nozzle 7 by a containment spray pump 10. By this, the radioactive iodine density in the reactor containment 1 is reduced and therefore the discharge to the environment is prevented by a filter vent system 16.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a boiling water nuclear reactor, and in particular to a method for efficiently removing radioactive iodine released from a pressure vessel of a -i subreactor together with steam and aerosol particles. Regarding.

[Conventional technology]

In the event of a nuclear reactor accident, the pressure inside the containment vessel increases due to steam released from the pressure vessel.

Therefore, in order to cool the core and prevent damage to the containment vessel, suppression pool water is sprayed into the dry well/wet well of the pressure vessel and containment vessel using the ECC5 device. The sprayed water returns into the suppression pool through the vent pipe. The pool water is cooled by the heat exchanger of the residual heat removal system, returned to the pool, and sprayed again by the ECC8 device. However, E.C.
If the pressure within the containment vessel still increases after being sprayed by the C8 device, the isolation valve in the containment vessel is opened and the vapors released by the filter vent device are treated and released to the environment. Here, the suppression pool water is pure water (-1t
t S/an), pH 6-7, dissolved oxygen concentration approximately 1
．． It is 2 ppm.

[Problem to be solved by the invention]

In the above-mentioned conventional technology, the pressure inside the containment vessel increases due to steam released from the pressure vessel in the event of a nuclear reactor accident. Therefore, to prevent damage to the containment vessel, the suppression pool water is pumped into the pressure vessel.

It is being sprayed into the containment vessel to lower the pressure. At this time,
Radioactive iodine gas, which is a fission product released into the containment vessel from the pressure vessel, cannot be efficiently removed by spraying, so a large amount remains in the containment vessel, and no consideration is given to reducing radioactive materials. Additionally, radioactive materials released from the pressure vessel return to the suppression pool water. Radioactive iodine dissolved in water changes its form due to radiation and becomes volatile iodine, which has the problem of transferring to the gas phase when water is circulated or sprayed. Furthermore, since the oxygen dissolved in the spray water is easily transferred to the gas phase by spraying, the concentration of flammable gas in the containment vessel increases, leading to the possibility of combustion and explosion.

The purpose of the present invention is to efficiently remove radioactive iodine gas in a containment vessel and to suppress the transition of radioactive iodine dissolved in pool water to the gas phase even if its form changes due to radiation.

- [Means for Solving the Problems] In order to achieve the above object, the present invention provides a water tank outside the containment vessel, and stores water to be sprayed into the containment vessel in advance in the water tank in the event of an accident. It controls the water quality. To control the water quality in the water tank, an alkaline reagent is added to adjust the pH to 7 or higher. If sodium thiosulfate is used as the added reagent, the radioactive iodine gas remaining in the containment vessel can be removed by being dissolved in water by spraying. The transition from spray water to the gas phase can be suppressed by degassing the water in the water tank with an inert gas to lower the dissolved oxygen level in advance.

[Effect]

At the time of the accident, sodium thiosulfate was added to radioactive iodine gas, a fission product released from the pressure vessel, to bring the pH to 7-10.
Radioactive iodine can be absorbed and removed by spraying controlled water from the water tank into the containment vessel using a pump. Further, by adjusting the pH of radioactive iodine dissolved in water to 7 or more, transfer of volatile iodine to the gas phase can be suppressed. With these,
Since the radioactive iodine in the gas phase can be reduced, its release into the environment can be completely prevented by means of a filter vent device. Furthermore, the dissolved oxygen concentration of the water in the water tank is 1 or 2.
By reducing the amount to ppm or less, the transfer from spray water to the gas phase can be reduced, so the time required for the flammable gas in the containment vessel to reach its flammability limit can be extended to the same extent as suppression pool water.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. In the event of a nuclear reactor accident, steam and aerosol particles containing iodine, which is a radioactive substance of nuclear fission products, are released from the pressure vessel 2 into the dry well 3 in the containment vessel 1 . When the pressure in the dry well 3 rises due to the steam, a pressure break valve at the top of the vent pipe 4 is operated, and the steam is scrubbed into the suppression pool 6 water through the vent pipe 4 and guided to the wet well 5. Immediately after the accident, the emergency core cooling system (EC
The C8 device 8) is activated and water is sprayed from the core cooling spray nozzle 9. On the other hand, in order to prevent a pressure increase due to the steam released into the containment vessel 1, the dry well 3 and wet well 5 in the containment vessel 1 are equipped with spray nozzles 7. Was. Furthermore, as the accident progresses, the pressure inside the containment vessel 1 will rise and exceed the design pressure, causing damage to the containment vessel 1. To prevent this, the isolation valve 15 of the containment vessel 1 is opened and the filter vent device i16 is activated. After removing the radioactive materials, they are released into the environment from the exhaust tower 17. The above describes the events that occurred at the time of the accident.

The feature of this embodiment is that a water storage tank 11 for spraying is provided outside the containment vessel 1, and the water quality of the spray water 12 is controlled by the chemical injection line 14 and the gas line 13 to control the P Hr dissolved oxygen concentration. Water is sprayed from the spray nozzle 7 by the pump 10. A part of the radioactive iodine released from the pressure vessel 2 is in the form of an ion and exists in the liquid phase, and a - part is in the form of an elemental gas and exists in the gas phase. Figure 2 shows the relationship between volatile iodine transfer to the gas phase and pH.

From this, the method for reducing radioactive iodine is to control the automatic injection from the chemical injection line 13 while monitoring the water quality so that the pH of the spray water 12 in the water storage tank 11 is 7 to 10, and then to control the spray water 12 in the storage tank 11 by using the containment vessel spray pump 10. Water is sprayed from the spray nozzle 7.

For example, if the pH is adjusted and the reagent is sodium thiobenzene, the radioactive iodine gas will be removed, and the iodine in the liquid phase will change its form to volatile iodine due to radiation, and the transition to the gas phase will be prevented. It can be suppressed.

On the other hand, as shown in FIG. 3, as can be seen from the relationship between the liquid phase and the gas phase of the spray water, oxygen dissolved in the spray water is transferred to the gas phase by spraying, so the dissolved oxygen in the water storage tank 11 is By degassing and lowering the oxygen concentration using gas line 13.

Migration from spray water can be reduced. According to this embodiment, the radioactive iodine gas concentration can be reduced by injecting the reagent having the above-mentioned effect.

〔Effect of the invention〕

According to the present invention, since the concentration of radioactive iodine gas in the containment vessel can be reduced by spray water, the filter vent device has the effect of preventing release into the environment. Also,
Radioactive iodine in water changes form to volatile iodine due to radiation, reducing the amount of iodine transferred to the gas phase, which also has the effect of suppressing the concentration inside the containment vessel. Furthermore, by lowering the amount of dissolved oxygen in the spray water, the amount of oxygen transferred to the gas phase can be reduced, which has the effect of extending the time it takes to reach the flammability limit in the containment vessel.

[Brief explanation of the drawing]

Figure 1 is a system diagram of an embodiment of the present invention, Figure 2 is a characteristic diagram showing the relationship between volatile iodine transfer and pH, and Figure 3 is a characteristic diagram showing the relationship between liquid phase and gas phase transfer of spray water. It is a diagram. 1... Reactor containment vessel, 2... Reactor containment vessel, 3
... Containment vessel dry well, 4... Vent pipe, 5
... Containment vessel wet well, 6... Suppression pool, 7... Spray nozzle, 8... ECC
8 W, 9...Core cooling spray nozzle, 10...
Containment vessel spray pump, 11... Water tank, 12.
...Spray water, 13.. Gas line, 14.. Chemical injection line, 15.. Isolation valve, 16.. Filter vent device, 17.. Exhaust tower. Figure 7

Claims (1)

[Claims] 1. In a boiling water nuclear power plant consisting of a reactor containment vessel, a water storage tank, and a spray system, in the event of an accident, water whose quality is controlled in the water storage tank is sprayed into the containment vessel. A method for reducing radioactive iodine, which comprises removing released radioactive iodine. 2. In claim 1, the pH of the water in the water tank is 7 to 7.
How to reduce radioactive iodine to 10. 3. The method for reducing radioactive iodine according to claim 2, comprising adding sodium thiosulfate to the water in the water tank. 4. The method for reducing radioactive iodine according to claim 3, which reduces dissolved oxygen in the water quality of the water tank.