RU2197762C2 - Apparatus for decontaminating steam-gas mixtures flowing during excess pressure relief from under containments of nuclear power plants - Google Patents

Abstract

FIELD: accident confinement at nuclear power plants. SUBSTANCE: apparatus used for treatment of materials contaminated with radioactive nuclides and also for filtering gaseous wastes at radiochemical factories has high-pressure tank filled with detergent that accommodates jet-type direct-contact devices immersed in detergent, with devices for distributing steam-gas mixture over sectional area of tank, mounted on their top, entrainment traps, pipe unions for inlet and outlet of steam-gas mixtures being decontaminated, and filter using loose filter sorbent. Filter is disposed outside high-pressure tank above detergent level and communicates with pipe union for inlet of steam-gas mixtures to be decontaminated. Entrainment traps are mounted inside high-pressure tank above detergent level. Direct- contact devices are made in the form of ejectors. Filter elements of entrainment trap mounted upstream of pipe union for outlet of decontaminated steam-gas mixtures are made of glass fiber. EFFECT: facilitated entrapping of all forms of volatile iodine and aerosol residue by ejectors and entrainment traps; preventing unwanted wetting of sorbent and maintaining its sorbing capacity. 3 cl, 3 dwg

Description

 The invention relates to devices for processing materials with radioactive contamination in order to eliminate this contamination and can be used primarily for localizing the consequences of accidents at a nuclear power plant.

 One of the measures to manage the accident, to prevent damage to the protective shell of a nuclear reactor is to relieve pressure from the protective shell by filtering the resulting radioactive media. During filtration, the vapor-gas mixture is cleaned of radioactive aerosols and volatile forms of radioactive molecular iodine and radioactive organic iodine to prevent the release of radioactive substances into the environment.

 Currently, in connection with the large-scale commissioning of nuclear power plants, Russian specialists have been tasked with developing a highly efficient fission product capture system (FFP).

 The analysis of scientific, technical and patent documentation, the results of studies of Russian models of UPD systems, joint Russian and foreign systems in Germany and the USA showed that the devices that are equipped with several stages of cleaning are the most effective, satisfying the requirements for cleaning gas-vapor mixtures from radioactive substances.

 Typically, a device is a tank filled with a washing solution, under the level of which there are installed jet-type mixing devices (Venturi pipes, nozzles) and distributors placed above them, used to crush gas bubbles and equalize the vapor-gas mixture flow over the tank section. Drop separators (dehumidifiers) with filter elements made of metal fiber or metal mesh are placed above the level of the washing solution.

 At the same time, the Russian UPD system also contains a filter with a loose filtering sorbent "Thermoxide", with the help of which the main capture of volatile forms of iodine (molecular and organic), as well as the capture of fine aerosols.

 Let us consider in more detail the solutions known from the world level of technology, the technical essence of which is based on the structural elements described above.

A device for cleaning flows of gas-vapor mixtures formed during the discharge of excess pressure from under the protective shells of nuclear power plants (see USSR patent 1718740, class G 21 F 9 / 02,14.04.89, applicant company "Siemens" Germany), including a high pressure tank with a washing solution, Venturi washing devices located inside the high pressure tank in the volume of the washing solution, droplet separators and fiber filters, while the vents of the Venturi washing devices are located above the level of the washing solution in the zone of the greatest isontal section of the pressure vessel, and overhead vents of the venturi washing devices are fitted with drop plates for droplet separation, and filters made of metal fibers are located above the surface of the washing solution, each of which consists of three layers, and the thickness of the fibers in the outer layers of each filter is from 8 up to 20 microns, in the inner layer from 8 to 7 microns, and the thickness of each layer of filters is 10-20 mm:
In accordance with the dependent claims of the invention under consideration, it can be improved, namely:
- in the high pressure tank with the washing solution, two differently inclined pipes relative to the horizontal are placed on which Venturi washing devices are located at right angles, having different lengths and ending in the same horizontal plane;
- Venturi washing devices in the volume of the washing solution can have two inlet zones located at a distance from one another in the direction of the gas-vapor mixture, the distance between the zones being at least twice the neck width of the Venturi washing device;
- Filters made of metal fibers are combined with each other in pairs.

 In an accident accompanied by melting of the nuclear reactor zone, the pressure in the protective shell rises to a dangerous value, and the device for cleaning the vapor-gas mixture emerging from the protective shell is automatically connected to the shell and the ventilation pipe. The vapor-gas mixture flows through its inlet into the tank and, under the influence of the pressure difference at the inlet (pressure in the containment) and the outlet (atmospheric pressure) from the device, passes through the Venturi washing devices. In them there is an effective capture of aerosols and volatile fission products by dispersed washing liquid, which is sucked through nozzles into washing devices. The resulting vapor-liquid mixture leaves their Venturi washing devices and passes through the washing liquid in the high pressure tank, where it is also cleaned of aerosols and volatile fission products. Then, when the vapor-gas mixture passes through metal-fiber filters, aerosols that are not trapped in the washing liquid of the high-pressure tank, as well as aerosols generated by the bursting of air bubbles on the surface of the liquid, and droplets of liquid trapped in the vapor-gas mixture when leaving the washing liquid in the tank are captured.

 This device, operating in sliding pressure mode, has shown good results in the capture of aerosols and iodine.

 However, it should be noted that some structural disadvantages of the known device do not allow the most efficient implementation of the cleaning process coming from the protective shell of the vapor-gas mixture.

 Thus, the openings made in the side wall of the Venturi washing devices limit the flow rate of the injected washing solution from the tank, and therefore limit the ratio of the combined gas mixture of the washing solution to the washing solution in the direction of decreasing the latter. This does not contribute to a more intensive transfer of aerosols and volatile forms of iodine from a gaseous medium to a liquid one.

 The proof of the existence of such a problem is that in the device according to the USSR patent 1718740 it was necessary to increase the number of holes - "Venturi washing devices have two inlet zones located at a distance from the vapor-gas mixture in the direction of movement" (we mean holes in the side wall of the Venturi washing device )

 The operation of making holes is laborious and expensive.

 In addition, filters made of metal fibers having a micron lateral size located above the washing solution are corroded by the fumes of the washing solution during the entire period of operation (30-50 years) in a trouble-free mode.

 In the known device there is no filter with a loose filtering sorbent, which means that all forms of volatile iodine are less effectively captured.

 Therefore, in the event of an accident, a known filter cannot, due to the above-mentioned shortcomings, provide the necessary degree of purification of the gas-vapor mixture before being discharged into the environment, and it is necessary to provide for additional purification.

 Closest to the claimed and adopted as a prototype is a device for the capture of fission products developed by Sulzer, Switzerland, used for “ventilation of containment” (see the collection of reports “Improvement of the system for capturing products during accidents at WWER nuclear power plants,” Project: R 2.08 / 95. Results, Projects, Edition: Siemens Power Generation under the auspices of EUROPEAN COMISSION, namely task 2. "Assessment of the status of the development of fission products capture systems in the West", p.14, 15, Fig. 11).

 The device or filter according to the text of the report includes a high-pressure tank (vessel - in the Report) with washing solution, jet-type mixing devices (a set of Venturi nozzles - in the Report) located inside the high-pressure tank in the volume of the washing solution and devices for distributing gas-vapor mixtures above them ( Sulzer’s mixing elements located in two layers in the middle of the tank along its section - in the Report), droplet separators (other layers of mixing elements located in the upper part of the body separate water droplets from yhodyaschego gas-vapor stream - the Report) located above the level of the cleaning solution, and inlet and outlet fittings to be cleaned gas-vapor mixture. The nozzles are surrounded by diffusion plates located above the nozzle exits to destroy the water jet and convert the flow into a rapidly moving foam. At the same time, aerosols from the vapor-gas mixture are captured by the washing solution.

 The vapor-gas mixture, having converted into nozzles into the vapor-liquid mixture, moves upward and enters the Sulzer mixing elements immersed in the washing solution. Sulzer's mixing elements with open, self-intersecting flow channels scatter gases that carry aerosols in the liquid. The separating forces arising in the mixing elements break the gas stream into small bubbles, as a result of which the phase boundary is continuously renewed by the combination of bubbles and the formation of new bubbles. And since energy dispersion also takes place in the stream jet in the mixing element, the stream of bubbles and the mass transfer surface are preserved. Increased fluid turbulence increases mass transfer.

 The advantage of the constructive implementation of the considered means of production is that it is equipped with a larger than the analogue described above, the number of steps for cleaning gas-vapor and vapor-liquid mixtures from radioactive substances, which ensures high efficiency of the device. But at the same time, the constructive implementation of the steps causes deficiencies in the operation of the device as a whole. The implementation in the side wall of each Venturi nozzle of the inlet channels for the suction of the washing solution inside the nozzle limits the flow rate of the injected washing solution, thereby limiting the ratio of the combined gas-vapor mixture and the washing solution in the direction of decreasing the latter. This in turn reduces the intensity of the transfer of aerosols and volatile forms of iodine from a gaseous medium to a liquid one.

 Droplet separators made of corrugated sheet steel (mixing elements located above the level of the washing solution in the middle of the tank and in the upper part of it) are subject to destruction under conditions of constant evaporation of the washing solution. The absence in the device of a filter with a loose filtering sorbent, which could provide at least 100-fold purification of the vapor-gas mixture from each form of iodine, together with the above disadvantages, will not allow the known device to provide the necessary degree of purification of the vapor-gas and vapor-liquid mixtures before discharge into the environment .

 The authors of this application have created such a device that eliminates the disadvantages of the objects of the same purpose known from the prior art and described above.

 The claimed invention is a device for cleaning flows of gas-vapor mixtures formed during the discharge of excess pressure from under the protective shells of nuclear power plants, as the prototype contains a high pressure tank with a washing solution, located inside the high pressure tank in the volume of the washing solution, jet-type mixing devices and above them devices for distributing the vapor-liquid mixture, droplet separators placed in the tank above the level of the washing solution, and fittings for the input and output of the cleaned steam flows gas mixtures.

 The inventive device differs from the prototype in that it is equipped with a filter with a bulk filter sorbent located outside the high pressure tank above the level of the washing solution and connected to the inlet of the cleaned gas-vapor mixture streams, the jet-type mixing devices are made in the form of ejectors, and the filter elements of the droplet separator installed in front of the outlet fitting of the cleaned streams of gas-vapor mixtures made of fiberglass.

 In accordance with the dependent paragraph 2 of the claims, the device is also characterized in that check valves are installed in the inlet openings of the ejector nozzles, and the dependent clause 3 of the claims provides for supplying the outlet openings of the ejector nozzles with locking devices.

Improving the design of the device in accordance with the claims, and especially with the features of the distinctive part of the formula, involves achieving the following advantages over the prototype:
1. Providing a filter cleaning device with a loose filter sorbent located outside the high pressure tank and connected to the inlet fitting will allow the steam-gas mixture to be cleaned before it enters the tank. Due to mass transfer in the filter, the main part of the radioactive molecular and organic iodine will be absorbed by the Thermoxide filtering sorbent, which will facilitate the capture of residues of all forms of volatile iodine, as well as aerosols in ejectors and droplet separators.

 Placing the filter above the level of the washing solution in the high pressure tank will prevent unwanted wetting of the sorbent in the event of an inevitable rise in the level of the washing solution due to the vapor-gas mixture entering the tank and ensure that the sorption capacity of the sorbent is preserved.

2. The use of ejectors, and not Venturi nozzles, will significantly change the ratio of the detergent and steam-gas mixture entering into the interaction towards an increase in the detergent solution. This will lead to a more intensive transfer of the mass (aerosols, iodine) from the vapor-gas mixture to the washing solution;
3. The implementation of the filter elements of the droplet separator of fiberglass eliminates their corrosion, while the likelihood of corrosion destruction of metal fibers is high. This probability is due to the micron size of the fiber across. However, on the other hand, such a minimum size of metal fiber is necessary to achieve a given degree of purification of the vapor-gas mixture from the smallest particles of aerosols and drops of washing solution.

 The experimental tests of the claimed invention carried out by the authors revealed that iodine sorption occurs on the fiberglass filtering elements, which increases the purification coefficient of gas-vapor mixtures.

 It should be noted that metal fiber is much more expensive compared to fiberglass; moreover, metal fiber is not produced in Russia.

 4. Installing check valves in the inlet nozzles of the ejectors and supplying the outlet nozzles with shut-off devices prevent the bottom cavity of the high pressure tank and the inlet of the vapor-gas mixture from filling up in standby mode. Due to this, hydraulic shocks that occur during the initial period of operation of the cleaning device are excluded, when, in the absence of check valves and shut-off devices, the cold washing solution located in the inlet of the gas-vapor mixture and in the bottom cavity of the tank contacts the gas-vapor mixture having the maximum flow rate, temperature and pressure.

 Hydraulic shocks can also occur with significant fluctuations, either in the direction of decreasing or increasing the consumption of the vapor-gas mixture supplied during the emergency release from the containment. Check valves, adjusting the flow rate of a gas-vapor mixture, eliminate this negative phenomenon.

 The absence of vibrations of the cleaning device accompanying hydraulic shocks ensures stable operation, its strength and stability.

 The claimed invention meets all the criteria of patentability.

 It is new because at the moment, the applicant has not identified any known solutions from the prior art characterized by the same set of features.

 The invention meets the criterion of "inventive step", because the applicant has not identified solutions whose signs would coincide with the distinguishing features of the invention.

 The invention is industrially applicable, because it is characterized by specific design features, each of which is reproducible and does not contradict the use of the claimed device in an industrial environment. Moreover, the entire set of features and each feature individually are aimed at achieving the expected technical result - highly efficient cleaning of gas-vapor mixtures formed in the protective shells of nuclear reactors during an accident at nuclear power plants, from radioactive aerosols and volatile forms of radioactive iodine while ensuring reliable operation of the device design and its constituent parts.

 To confirm the above, we present a description of the design of the device and its operation.

 The invention is illustrated by drawings. Figure 1 shows a General view of the device for cleaning in section; figure 2 and 3 - the design of the nozzle of the ejector according to claims 2 and 3 of the formula of the utility model.

 The device comprises a high pressure tank 1 with a washing solution 2 filling the tank 1 to level A. Inside the high pressure tank 1 in the volume of the washing solution 2 there are jet-type mixing devices 3. Above the devices 3 also in the volume of the washing solution 2 are devices 4 for distributing the gas-vapor mixture over the cross section of the tank 1, made of corrugated stainless steel mesh. Above level A of the washing solution 2, droplet separators 5 and 6 are placed in the pressure vessel 1.

 At the bottom 7 and cover 8, respectively, a fitting 9 of the input of the gas-vapor mixture and a fitting 10 of the exit of the gas-vapor mixture are installed.

 The cleaning device is equipped with a filter 11 with a loose filter sorbent 12. The filter 11 is placed outside the tank 1 above the level A of the washing solution 2 in the tank 1 and is connected to the nozzle 9 by a pipe 13. The filter 11 has a nozzle 14 for connection to a protective sheath.

 The mixing devices 3 of the jet type are made in the form of ejectors, in each of which between the nozzle 15 and the mixing chamber 16 there is an annular gap 17 for injecting the washing solution 2 from the tank 1 into the mixing chamber 16.

 The droplet separator 5 is made of corrugated mesh, and the filter elements 18 of the droplet separator 6 installed in front of the nozzle 10 are made of fiberglass.

 A check valve 19 made in the form of a ball or a piston can be installed in the inlet of the nozzle 15 of each ejector 3, and the outlet of the nozzle 15 of each ejector can be equipped with a shut-off device 20, for example, in the form of a membrane.

 The purification of steam-gas mixture flows during the release of excess pressure from under the protective shells in the event of an accident at a nuclear power plant will occur as follows.

 The flows of gas-vapor mixtures from the protective shell enter through the nozzle 14 into the filter 11 with the same thermodynamic parameters as the conditions in the protective shell. When steam-gas mixtures pass through a layer of filter sorbent 12, volatile forms of iodine are sorbed in its pores. Part of the aerosols will be retained by the sorbent layer 12, but the main part of them, as well as the non-absorbable part of the volatile iodine, are carried away from the filter 11 by the vapor-gas mixture flow through the pipe 13 to the cleaning device. Then the vapor-gas mixture enters the nozzles 15 of the ejectors 3, where the reduced pressure arising due to the accelerated movement of the vapor-gas mixture inside the nozzles 15 is used to suck the cleaning solution 2 into the high-speed medium inside the ejector 3 through the annular gap 17. The suction solution 2 forms a droplet mist, which traps aerosols and volatile iodine.

 Gas and drops of solution 2, leaving the ejectors 3, fall into the washing solution 2 in the tank 1. When passing through the layers of solution 2 upwards, most of these drops are captured by it, therefore, small aerosols remain in solution 2, however, a certain amount of drops are released from the solution at bursting gas bubbles on its surface. The size of the droplets is very small, so the capture of submicron aerosols and volatile iodine trapped in the droplets is carried out at two stages of cleaning located above the level of the washing solution: first on a drop catcher 5 from a corrugated mesh, and then on a drop catcher 6 with filtering elements 18 made of ultrafine fiberglass, which able to capture not only aerosols, but also volatile iodine.

 After passing through the drip trap 6, the vapor-gas mixture purified by 99.99% from radioactive aerosols and by 99.9% from all forms of iodine is released into the atmosphere.

 When installing check valves 19 in the inlet openings of the nozzles 15 of the ejectors 3 and supplying locking devices 20 to the outlet openings of the nozzles 15, the initial period of operation of the cleaning device can be characterized as follows.

 The flows of the gas-vapor mixture, passing through the nozzle 9 into the ejector 3, lift up the check valves 19, after which they break the membrane 20 and enter the washing solution 2 in the tank 1. And then the process is carried out as described above.

 As soon as the flow rate of gas-vapor mixtures is significantly reduced, the check valves 19 are lowered, closing the inlet openings of the nozzles 15, preventing the "solution" of the washing solution 2 from the tank 1 to the bottom cavity of the tank 1, and hence hydraulic shock and vibration. The cleaning device is in stable mode.

Claims (3)

 1. A device for cleaning flows of vapor-gas mixtures resulting from the discharge of excess pressure from under the protective shells of nuclear power plants, containing a high pressure tank with a washing solution, jet-type mixing devices located inside the high pressure tank in the volume of the washing solution and devices for distributing vapor-liquid above them mixtures over the cross section of the tank, droplet separators placed in the high pressure tank above the level of the washing solution, and fittings for the inlet and outlet of the cleaned flow steam-gas mixtures, characterized in that it is equipped with a filter with a loose filter sorbent located outside the high pressure tank above the level of the washing solution and connected to the inlet of the cleaned gas-vapor mixture flows, the jet-type mixing devices are made in the form of ejectors, and the filter elements of the droplet separator installed in front of the nozzle the output of the cleaned streams of gas-vapor mixtures made of fiberglass.  2. The device according to claim 1, characterized in that check valves are installed in the inlet openings of the nozzles of the ejectors.  3. The device according to paragraphs. 1 and / or 2, characterized in that the outlet openings of the nozzles of the ejectors are equipped with locking devices.

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