JPH07209488A - Radioactivity emission reducing device - Google Patents

Abstract

PURPOSE:To suppress the release of a radioactive material emitted into a reactor housing vessel at reactor accident to the environment. CONSTITUTION:A scrubber tank 1 and a collecting tank 26 are connected to each other through a second bent pipe 6. A water scrubber 2 and a stainless steel fiber filter 5 are provided in the scrubber tank 1, and a venturi nozzle 4 connected to a first bent pipe 3 is submerged in the water scrubber 2. The collecting tank 26 has a moisture removing layer 8 and an activated charcoal filter 9. A drain line is connected to the moisture removing layer 8 and the water scrubber 2 through a valve 27. The moisture is condensed into dew and removed by cooling the stainless steel fiber filter 5, and this filter 5 is improved in removing performance of fission product by containing water therein.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for reducing radioactivity released to the environment in the event of an accident at a boiling water nuclear power plant.

[0002]

2. Description of the Related Art An emergency gas treatment system (hereinafter referred to as SGT) is used as equipment for reducing radioactivity released to the environment in the event of a nuclear reactor accident.
(Abbreviated as S) is known. This equipment is a countermeasure equipment for the events (hereinafter referred to as "design standard accidents") that should be considered in the safety design and evaluation of the reactor facilities among the events that may lead to abnormal states of the reactor facility, Inorganic and organic iodine are highly efficiently collected by using iodide-impregnated activated carbon.

On the other hand, FIG. 5 shows a main configuration of a filter vent system as equipment for an accident (hereinafter referred to as a "severe accident") that greatly exceeds a design standard accident in a boiling water reactor. FIG. 5 shows a MARKII improved containment vessel 20 and a filter system 24 of a boiling water reactor, and a vent line 23.
The system connected by is schematically shown.

In a severe accident in which the core 21 and the reactor pressure vessel 22 are damaged, a large amount of fissionable products are released from the core into the containment vessel 20, and the temperature and pressure inside the containment vessel 20 increase as the accident progresses. To do.

The gas in the storage container 20 is released (vented) to the environment in order to prevent the storage container 20 from overheating and overpressure damage.
At this time, in order to minimize the exposure of the public due to the release of radioactive material in the containment vessel 20, the gas containing the radioactive material is emitted through the vent line 23 through the filter system 24 such as a water scrubber or gravel bed. It is released from stack 25 into the environment.

[0006]

[Problems to be Solved by the Invention]
GTS is required to have an efficiency of removing inorganic and organic iodine of 95% or more (decontamination factor DF> 20), and in reality, it can be removed with a high efficiency of 99.99%. However, an activated carbon filter for removing iodine cannot withstand pressure application under severe accident conditions that exceed the design criteria, so it cannot be used as a countermeasure against severe accidents.

In a boiling water reactor filter vent system comprising a water scrubber capable of repairing water-soluble and aerosol-like radioactive materials and a stainless steel fiber filter in the subsequent stage, the stainless steel fiber filter portion is an aerosol. The purpose is to collect the particles and prevent the water-soluble component from being released by the splash of water droplets from the water scrubber.

This filter system does not take into account the collection of gaseous radioactive substances that are not water soluble. Further, when only the gravel bed is used as the filter system, there is a possibility that the gaseous radioactive substance having a small adsorption force and the decay heat cannot be sufficiently removed.

Therefore, in such a structure, a gaseous radioactive substance such as organic iodine that cannot be expected to be collected in a water scrubber or a gravel bed may be released to the environment, resulting in public exposure during a nuclear reactor accident. It is necessary to collect these gaseous radioactive substances in order to minimize.

The present invention has been made to solve the above problems, and suppresses the release of radioactive materials released into the reactor containment vessel into the environment at the time of a nuclear reactor accident to prevent design standard accidents and severe accidents. It is intended to provide a device for reducing emitted radioactivity capable of coping with both.

[0011]

The present invention comprises a scrubber tank to which a first vent pipe is connected, and a collection tank connected to this scrubber tank via a second vent pipe.
The scrubber tank has a water scrubber, a venturi nozzle connected to the first vent pipe is immersed in the water scrubber, and a stainless steel fiber filter is provided above the water scrubber. The collecting tank is characterized by including a moisture removal layer and an activated carbon filter.

[0012]

[Function] Most of the radioactive substances released from the containment vessel at the time of the nuclear reactor accident are water-soluble and aerosol-like, and are mostly collected by the water scrubber and water drop removal filter.
Organic iodine and the like pass without being collected.

Water scrubbers also provide some cooling of the gas released at high temperatures and removal of decay heat.
Since the water drop removal filter is heated by the decay heat of the collected radioactive material, it is considered that the humidity is increased as the relative humidity is low while passing through the filter, the water drop removal filter is cooled while proceeding through the pipe thereafter.

Depending on the type of scavenger used in the gaseous radionuclide scavenging layer, the ability to scaveng the organic iodine may decrease at high humidity. It is necessary to provide. Activated carbon, impregnated carbon, silver theodolite or the like can be used as the organic iodine scavenger.

Collection with activated carbon utilizes adsorption as in the case of inorganic iodine. In the case of impregnated carbon, there are one in which isotope exchange is performed by impregnating sodium iodide or potassium iodide (formula (1)) and one in which a reaction with an organic amine such as triethylenediamine is used (formula (2)).

CH ₃ I ^* + I (activated carbon) → CH ₃ + I ^* (activated carbon) (1) CH ₃ I ^* + RNH ₂ (activated carbon) → [RN (CH ₃ ) ₃ ⁺ I ^{* +} (activated carbon) ... (2) Here, I ^* : radioactive iodine I: non-radioactive iodine R: alkyl group.

The silver zeolite uses the reaction of silver and iodine. (Equation (3)) Ag (zeolite) + I ^* → AgI ^* (zeolite) (3)

The organic iodine can be collected by using it in combination with the moisture removal layer in accordance with the characteristics of these scavengers, thereby reducing the emission of radioactive substances generated in the event of a nuclear reactor accident.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a method and apparatus for reducing emitted radioactivity according to the present invention will be described with reference to FIG. FIG. 1 schematically shows a device for reducing emitted radioactivity according to the present invention in a longitudinal section. That is, reference numeral 1 in the figure is a scrubber tank,
A water scrubber 2 is provided in the scrubber tank 1, and a stainless steel fiber filter 5 is provided above the water scrubber 2. A venturi nozzle 4 is provided which is immersed in the water scrubber 2 and connected to the first vent pipe 3. First
The vent pipe 3 is attached to penetrate the side wall of the scrubber tank 1.

An end of a second vent pipe 6 is connected to the upper end of the scrubber tank 1, and an orifice 7 is provided in the second vent pipe 6. The other end of the second vent pipe 6 is connected to a collection tank 26 containing a moisture removal layer 8, a guard filter 11 and an activated carbon filter 9. The outlet side of the collection tank 26 is connected to the stack 25. A drain line 10 is connected between the moisture removal layer 8 and the scrubber 2 via a valve 27.

Thus, in the above-mentioned emission activity reducing apparatus, the gas containing the radioactive substance emitted from the reactor containment vessel is released from the first vent pipe 3 through the Venturi nozzle 4 into the water scrubber 2, and the water solubility and Aerosol-like radioactive material is collected in water. Aerosol and water droplets that cannot be collected by the water scrubber 2 are collected by the stainless fiber filter 5.

The gas that has passed through the scrubber tank 1 is introduced into the gaseous radioactive substance collection tank 27 through the second vent pipe 6. At that time, due to the orifice 7 in the second vent pipe 6,
The flow rate of the gas introduced into the collection tank 27 is controlled. Since the ability of activated carbon to adsorb organic iodine is remarkably reduced at high temperatures, it is necessary to sufficiently cool and dehumidify the gas before it is introduced into the activated carbon filter 9.

The second vent pipe 6 has a certain distance to cool the gas. However, water droplets are generated again in the cooled gas, and when they enter the activated carbon filter 9, the pores of the activated carbon are covered with water, the adsorption sites are reduced, and the collection efficiency is reduced.

The moisture removal layer 8 has a structure such as a stainless fiber filter 5 for preventing the inflow of water droplets, a sand bed or a steam dryer used in a nuclear reactor in a small size. The gas containing organic iodine after passing through the moisture removal layer 8 is introduced into the activated carbon filter layer 9 to collect the organic iodine.

Although the proportion of iodine in the form of organic iodine out of the core is considerably low according to the US renewal source term, the loss of coolant accident, which is a design standard accident in the current license analysis in Japan ( Virtual hypothesis) conservatively assumes that 10% of iodine is organic iodine.

Assuming that about 10% or less of iodine is methyl iodide even in a severe accident, it is assumed that more than 10 kilograms of iodine inventory in the reactor are all released into the PCV. About 1.5 kilograms of methyl will be generated.

If the gas temperature is cooled to 50 ° C. before being introduced into the activated carbon filter, the equilibrium adsorption amount of methyl iodide per gram of activated carbon at 50 ° C. is 10 ⁻¹ gram or more. If it is adsorbed up to the equilibrium amount, the amount of activated carbon required for adsorbing methyl iodide is about 15 kilograms.

It is also possible to use impregnated carbon as an organic iodine scavenger with the same structure as this. However, in the case of impregnated carbon, it is collected by using isotope conversion or chemical reaction, and it seems that the reaction proceeds more quickly when a certain amount of heat energy is given, so the ignition temperature of activated carbon is If the temperature is sufficiently lower than a certain temperature of 250 ° C, it is not necessary to cool the gas when using activated carbon.

A second embodiment will be described with reference to FIG.
In the second embodiment, the scrubber 2 and the stainless steel fiber filter 5 are the same as in the first embodiment, and the trapping layer for gaseous radioactive substances is used as a cold trap.

In the second embodiment shown in FIG. 2, the guard filter 11 and the activated carbon filter 9 of the collection tank 26 are provided in the cooling tank 12 to form a cold trap. In the cooling tank 12, the cryogen 13 is stored. It is stored. Further, a pressure adjusting line 14 and a cold material injection line 15 are connected to the side surface of the coolant tank 12.

According to the second embodiment, by cooling the activated carbon filter 9, it is possible to collect the gaseous radioactive substance more efficiently. If cooled activated carbon (so-called cold trap) is used, it is possible to adsorb even gases that are difficult to adsorb at room temperature.

For example, rare gases such as neon and helium, which are usually very difficult to be adsorbed, are also adsorbed on the activated carbon cooled to the liquid nitrogen temperature. The ease with which a gas is adsorbed by activated carbon mainly depends on the boiling point and the critical temperature of the gas, and the lower the boiling point or the critical temperature, the less likely the gas is adsorbed on the activated carbon.

The main radioactive gases that pass through the water scrubber and flow into the activated carbon filter 9 at the time of a design standard accident or a severe accident are organic iodine and a noble gas, and as a non-radioactive gas, the inside of the containment vessel is purged. There are inflows of nitrogen, oxygen generated by radiolysis of water, and hydrogen generated by metal-water reaction.

When a non-radioactive gas is collected at the same time when the gas is collected by the cold trap, more activated carbon is needed to collect the radioactive gas with high efficiency.
In particular, nitrogen is present in a large amount as compared with other gases, so it should be avoided to collect it simultaneously with radioactive gas. The boiling points and critical temperatures of these gases are shown in Table 1. Table 1 shows the boiling points and critical temperatures of gases that may flow into the activated carbon filter.

From Table 1, it can be seen that xenon, krypton and methyl iodide have considerably higher boiling point critical temperatures than non-open nitrogen, oxygen and hydrogen. Therefore,
There is an activated carbon temperature where xenon and the like are adsorbed and nitrogen and the like are not adsorbed.

After adsorbing xenon and krypton together with air to activated carbon at −183 ° C. and then raising the temperature to −78 ° C., nitrogen and oxygen were evaporated, but xenon and krypton remained on the activated carbon. is there.

From this, it is expected that methyl iodide is selectively adsorbed at about −78 ° C. at a low temperature such that xenon and krypton are adsorbed because it is a gas that can be adsorbed on activated carbon even at room temperature. To be done. Therefore, it is possible to selectively collect the radioactive gas by cooling the activated carbon near -78 ° C.

[0038]

[Table 1]

Dry ice (achievement temperature: -78.5 ° C) is a freezing agent that can be cooled to around -78 ° C. An example of a method of cooling using a cryogen will be described with reference to FIG. Cooling tank
The activated carbon filter 9 is stored in 12 and the cryogen is injected from the cryogen injection line 15 in the event of an accident.

When dry ice is used, it is possible to produce dry ice by injecting CO ₂ gas into the tank from the injection line 15 and adiabatically expanding it when an accident occurs. In this case, when the gas from the storage container enters the tank 12 and is rapidly cooled, the moisture freezes. Therefore, the moisture removal layer 8 in the preceding stage of the cooling tank 12 removes the moisture as much as possible, and the generated ice pieces are A guard filter 11 is provided so as not to damage the activated carbon filter 9.

Next, a third embodiment will be described with reference to FIG. In this third embodiment, silver zeolite is used as a collector for organic iodine. The third embodiment is that the silver zeolite layer 16 is installed above the stainless fiber filter 5 in the scrubber tank 1 in the first embodiment, and the other parts are the same as in the first embodiment.

Similar to impregnated carbon, the collection of organic iodine by silver zeolite is due to the chemical reaction of silver and iodine, and the application of heat energy promotes this reaction to improve the collection efficiency. According to the experimental results, silver zeolite is 100
Best collection efficiency at ~ 150 ° C.

If the silver zeolite layer 16 is installed immediately after the stainless fiber filter 5 as shown in FIG. 3, the silver zeolite layer 16 is also heated and the collection efficiency of organic iodine is improved.
Similar to the activated carbon, when the water adheres to the silver zeolite layer 16, the contact surface with iodine is deprived and the collection efficiency decreases, so it is necessary to take measures against moisture. In this case, the scrubber tank 1
Since the gas temperature inside is high, vapor does not condense even in the silver zeolite layer 16.

The adsorption capacity of methyl iodide on silver zeolite is 1
It is about 85 mg / g at 30 ° C (although it is a value under different conditions from the time of the accident). Using this value and the above-mentioned assumption of iodine release, it is necessary to adsorb methyl iodide. The amount of silver zeolite is about 20 kilograms.

In the third embodiment, as shown in FIG. 4, the surface of the stainless fiber 19 is coated with silver 18 so that the silver-coated stainless steel has the functions of both a water drop removing filter and an organic iodine scavenger. The filter 17 can be provided after the water scrubber.

[0046]

According to the present invention, by connecting a water scrubber that collects water-soluble and aerosol-like radioactive materials and a collection layer of gaseous radioactive materials, the inside of the reactor containment vessel can be stored in the case of a reactor accident. It is possible to suppress the release of various radioactive substances released into the environment, and
Both severe accidents can be dealt with.

[Brief description of drawings]

FIG. 1 is a vertical sectional view schematically showing an embodiment of a device for suppressing emitted radioactivity according to the present invention.

FIG. 2 is a longitudinal sectional view showing a second embodiment of the present invention.

FIG. 3 is a vertical cross-sectional view showing a third embodiment of the present invention.

FIG. 4 is a partial cross-sectional view showing another example of the silver-coated stainless steel filter in the embodiment shown in FIG.

FIG. 5 is a system diagram schematically showing a filter vent device for explaining a conventional method of reducing emitted radioactivity.

[Explanation of symbols]

1 ... Scrubber tank, 2 ... Water scrubber, 3 ... 1st vent pipe, 4 ... Venturi nozzle, 5 ... Stainless steel fiber filter, 6 ... 2nd vent pipe, 7 ... Orifice, 8 ... Moisture removal layer, 9 ... Activated carbon Filter, 10 ... Drain line, 11 ... Guard filter, 12 ... Cooling tank, 13 ... Cryogen, 14 ... Pressure adjusting line, 15 ... Cryogen injection line, 16 ... Silver zeolite layer,
17… Silver coated stainless steel fiber filter, 18…
Silver, 19 ... Stainless steel fiber, 20 ... Reactor containment vessel, 21 ...
Core, 22 ... Reactor pressure vessel, 23 ... Vent line, 24 ... Filter system, 25 ... Stack, 26 ... Collection tank, 27 ... Valve.

Claims (5)

[Claims] 1. A scrubber tank to which a first vent pipe is connected, and a collection tank connected to this scrubber tank via a second vent pipe, wherein the scrubber tank has a water scrubber, A venturi nozzle connected to the first vent pipe is immersed in the water scrubber, and a stainless steel fiber filter is provided above the water scrubber. The collection tank has a moisture removal layer and an activated carbon filter. A device for reducing emitted radioactivity, comprising: 2. The drain line is provided between the water scrubber of the scrubber tank and the moisture removal layer in the collection tank via a valve. Radioactivity reduction device. 3. The emitted radioactivity reducing device according to claim 1, wherein the activated carbon filter in the collection tank is covered with a cooling tank to form a left cold trap. 4. The emission activity reducing apparatus according to claim 1, wherein a silver zeolite layer is provided above the stainless steel fiber filter in the scrubber tank. 5. The emitted radioactivity reducing apparatus according to claim 4, wherein a silver-coated stainless steel fiber filter in which the surface of stainless steel fibers is coated with silver is used in place of the silver zeolite layer.